No //^
0 r p 2-
ACKNOWLEDGMENTS
I would like to express my appreciation to Drs. Kristiansen and Hatfield for their
support and for serving on my committee. Appreciation also goes to Lawrence
Livermore National Lab and its people (Dave Cummings, Roh Kihara, and Wayne
Hofer) for their help, advice, and funding. I would also like to thank the Texas Tech
University Center for Energy Research for its funding. Very special thanks goes to
Anton "Tony" Shulski at Richardson Electronics, without whom none of this would have
been possible, for providing the many special ignitions used for our research and for
many hours of advice and help. Heartfelt thanks also goes to the devoted people of the
Texas Tech University Pulsed Power Laboratory and especially to: Lonnie Stephenson,
Kim Zinsmeyer, Dan Reynolds, Donna Srader, Marie Byrd, Danny Garcia, and Jason
Mallonee. Very special appreciation goes to Dr. Michael Giesselmann for his suppon,
his endless energy, and for serving on my committee. I would also like to thank my
parents who were always behind me in this endeavor. Most of all, I thank my husband,
Ellis Leo Loree, for his patience and full suppon throughout the previous years and on
into the future.
11
CONTENTS
ACKNOWLEDGMENTS
ii
ABSTRACT
v
UST OF TABLES
vi
UST OF FIGURES
vii
CHAPTER
I.
n.
m.
IV.
INTRODUCTION
1
Basic Ignitron Components
1
Basic Ignitron Characteristics
8
Practical Aspects
14
Ignitron Uses
18
Theoretical Considerations of Magnetic Field Effects
20
EXPERIMENTAL APPARATUS
22
Test Ignitrons
22
Major Test S tand Components
26
Triggering and Non-Optical Diagnostics
32
Optical Diagnostics
37
TESTING AND RESULTS
41
Voltage Holdoff Tests
41
Conduction Tests
46
Risetime Tests
64
INTERPRETATION OF RESULTS/CONCLUSIONS
Voltage Holdoff Tests
65
65
•• •
111
Conduction Tests
^8
Risetime Tests
80
Glass-Walled Ignitron Failure
81
Closing Remarks
81
REFERENCES
83
APPENDICES
85
A.
SCHEMATICS
86
B.
DYNAFAX PHOTOGRAPHS
92
G.
RELATED INFORMATION
101
IV
ABSTRACT
This report describes research on the effects of axial magnetic fields on ignitron
switches. Investigations of effects of the field on the arc resistance, holdoff voltage, and
visible plasma parameters were carried out on three different types of ignitrons; a
commercial NL-2909 ignitron, two glass-walled ignitrons, and a specially built
demountable ignitron. All measurements were made in a critically damped configuration
utilizing either a 2.56 mF, 10 kV capacitor bank or a 1.89 |xF, 60 kV capacitor.
Diagnostics included voltage and current monitoring along with high speed photography
performed by a Dynafax framing camera. Model 326, and a TRW image converter
camera. The photographic techniques were performed on the glass-walled tubes alone.
Effects of the insertion of a dielectric shield into the demountable ignitron were studied.
Additional measurements of risetime dependence of the arc voltage were performed on
the demountable ignitron.
LIST OF TABLES
Table
Page
1.
GOMMERCL\L IGNITRON SIZES
2.
HIGH CURRENT, HIGH COULOMB TEST DATA FOR
IGNTFRONS
VI
15
107
LIST OF FIGURES
Figure
Page
1.
ELEMENTARY IGNITRON
2
2.
BASIC IGNirOR DESIGN
5
3.
IGNITRON WITH HOLDING ANODE, BAFFLES, AND GRIDS
7
4.
TRACINGS OF CATHODE SPOT MOTIONS
12
5.
VOLTAGE AND CURRENT OF TYPICAL IGNHOR PULSE
16
6.
NL-2909 IGNHRON
23
7.
GLASS-WALLED IGNITRON
24
8.
DEMOUNTABLE IGNHRON (DIG)
25
9.
McLEOD GAUGE
27
10.
PICTURE OF PUMPING STATION
28
11.
MAGNION ELECTROMAGNET
29
12.
FOUR-MAGNET STAND AND MAGNETIC FffiLD RELATIONSHIP 31
13.
FOUR COIL IGNITRON STAND MAGNETIC HELD vs. SPACING
PLOT
14.
DEMOUNTABLE STAND AND MAGNETIC FffiLD RELATIONSHIP 34
15.
DEMOUNTABLE IGNITRON STAND MAGNETIC FffiLD vs.
SPACING PLOT
35
16.
BLOCK DL\GRAM OF DIAGNOSTICS AND TRIGGERING
36
33
SYSTEM
17.
DYNAFAX OPTICAL TIMING DL\GRAM
39
18.
DYNAFAX PHOTOMULTIPLffiR SIGNAL
39
19.
TEST CIRCUII FOR HIGH VOLTAGE HOLDOFF TESTS
42
20.
NL.2909 HOLDOFF VOLTAGE GRAPH
43
21.
GLASS-WALLED IGNITRON HOLDOFF VOLTAGE GRAPH
43
vii
22.
DEMOUNTABLE IGNHRON HOLDOFF VOLTAGE GRAPH
44
23.
PICTURE OF DffiLECTRIC CYLINDER
47
24.
BASIC CONDUCTION TEST CIRCUII
49
25.
CURRENT AND VOLTAGE PLOT FOR FOUR-MAGNET STAND
50
26.
CURRENT AND VOLTAGE PLOT FOR TWO-MAGNET STAND
51
27.
Vare and Rare OF NL-2909 IGNHRON
52
28.
Varc and Ra^ OF GLASS-WALLED IGNHRON
54
29.
Varc and R^ vs. B FOR GLASS-WALLED IGNITRON
55
30.
PLOT OF VOLTAGE AND CURRENT FOR B=0 T SHOT
56
31.
PLOT OF VOLTAGE AND CURRENT FOR B=0.05 T SHOT
57
32.
PLOT OF VOLTAGE AND CURRENT FOR B=0.075 T SHOT
58
33.
PLOT OF VOLTAGE AND CURRENT FOR B=0.1 T SHOT
59
34.
CAMERA TIMING OSCILLOSCOPE TRACES: TRW, DYNAFAX,
60
CURRENT
35.
CAMERA TIMING OSCILLOSCOPE TRACES: TRW, CURRENT
60
36.
37.
TRW FRAMING MODE PHOTOGRAPHS
Varc and Rg^ OF DEMOUNTABLE IGNITRON
62
63
38.
CURRENT AND VOLTAGE PLOT FOR TWO-MAGNET STAND
WITH TWO BANKS
65
39.
Varc and R^ OF DEMOUNTABLE IGNITRON WUH 2 AND 4
BANKS
66
40.
SKETCH OF ELECTRON TRAJECTORffiS IN CROSSED
ELECTRIC AND MAGNETIC FffiLDS
68
41.
EQUIPOTENTTAL LEVELS OF THE FffiLD DISTRIBUTION IN
THE 2909 TUBE
69
42.
EQUIPOTENTIAL LEVELS OF THE FffiLD DISTRIBUTION IN
THE DEMOUNTABLE TUBE
70
Vlll
43.
EQUIPOTENTTAL LEVELS OF THE FffiLD DISTRIBUTION IN
71
THE GLASS-WALL TUBE
44.
CLOSE VffiW OF SECTION OF DEMOUNTABLE TUBE
74
45.
CLOSE VffiW OF SECTION OF DEMOUNTABLE WUH SPACED
DffiLECTRIC CYLINDER
CLOSE VffiW OF SECTION OF DEMOUNTABLE WUH FLUSH
75
46.
76
DffiLECTRIC CYLINDER
47.
DEMOUNTABLE - CYLINDER GEOMETRY
77
48.
PLOT OF VOLTAGE AND CURRENT FOR B=0 T AND 0.05 T
79
SHOTS
A-1.
IGNITRON FIRING SCHEMATIC
87
A-2.
DIFFERENTIAL PROBE AMPLIFffiR
88
A-3.
DUAL FIBER OPTIC TRANSMHTER
90
A-4.
B-1.
DUAL FIBER OPTIC RECEIVER
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0 TESLA
SHOT
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0.01 TESLA
SHOT
91
93
B-3.
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0.02 TESLA
SHOT
95
B-4.
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0.035 TESLA
SHOT
96
B-5.
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0.05 TESLA
SHOT
97
B-6.
DYNAFAX PHOTO SEQUENCE FOR Ip= 40 kA, B = 0.06 TESLA
SHOT
98
B-7.
DYNAFAX PHOTO SEQUENCE FOR Ip= 100 kA, B = 0.04 TESLA
SHOT
99
B-8.
DYNAFAX PHOTO SEQUENCE FOR Ip= 100 kA, B = 0.06 TESLA
SHOT
100
B-2.
ix
94
C-1.
PASGHEN CURVES FOR MERCURY
103
G-2.
CURRENT-COULOMB TRADE-OFFS AT CONSTANT LIFE
104
C-3.
CIRCUII MODEL AND OPERATING MODES
105
C-4.
HIGH CURRENT, HIGH COULOMB TEST DATA FOR
IGNITRONS
106
CHAPTER I
INTRODUCTION
Mercury pool tubes have been used as rectifying devices for more than fifty
years. The first practical use was to supply dc for series-string street lights. In the early
years, ignition of the arc in an ignitron was by mechanical means. The auxiliary anode
was used to keep the arc continuously alive. An initial arc was formed by tilting the tube
or withdrawing an energized electrode from the pool cathode. This requirement made the
tube hard to use on a repetitive basis. However, work by Slepian and Ludwig (1933)
revolutionized the ignition method [1]. They found that cathode spots on the mercury
pool could be initiated by applying a low, positive voltage to a relatively high resistance
material immersed in the mercury. This allowed the spots to be restarted every cycle by a
low voltage pulse. The transition from the initial development of the ignitor to the
successful ignitor lasted many years and required many people. During the last 25 years,
there has been very little work done on improving the ignitron for future applications.
Basic Ignitron Components
Looking at the drawing of the basic ignitron in Fig. 1, the imponant components
are the cathode pool, the anode, the envelope (or wall), and the ignitor. The mercury
pool serves the dual purpose of both a source of electrons for the discharge and a supply
of vapor through which the discharge is conducted. Some good points about mercury
besides its massive coulomb transfer ability are that it is relatively inexpensive, and it
doesn't react with many materials (among these are most glasses, ceramics, steel, nickel,
chromium, molybdenum, titanium, tungsten, silicon carbide, boron carbide and many
others). On the other hand, it does react with aluminum, gold, silver, lygon, and PVC.
1
c3
ANODE SEAL
(INSULATOR)
ANODE
TERMINAL
•
D
^ANODE
METAL
ENVELOPE
(WALL)
IGNITOR
MERCURY POOL
IGNITOR TERMINAL
FIG. 1
ELEMENTARY IGNITRON
The vapor pressure of mercury is strongly dependent on temperature (it doubles with
every 10^ C temperature rise), therefore the use of either forced air or water cooling is
necessary in some cases. In general, when the tube temperature is below lO^ C ignition
becomes difficult; above 50° C, most tubes will not hold off maximum voltage.
In past and present ignitrons, various materials have been used for the anode.
Among these materials are graphite, molybdenum, titanium, and stainless steel. Some
major factors utilized in selecting anode materials are their outgassing, their holdoff
ability, and their heat conduction capability. The ignitron is a vacuum device and the only
gas desirable in the vacuum is mercury vapor. The mercury vapor gives the conduction
plasma of the ignitron a bluish glow. However, when an arc (or cathode spot) reaches
the surface of the wall, anode, or an insulator, outgassing from that material will occur.
This outgassing contaminates the vacuum and alters the tube's characteristics. This
outgassed material could possibly condense into the mercury and form a sludge which
will coat the ignitor (or other areas) and ruin the tube. Thus, some materials are chosen
for low outgassing and erosion capabilities. Some materials also have the ability to
chemically combine with impurities in the vacuum and thus remove the impurities
(gettering). The actual smoothness of the material is a factor which affects the holdoff
voltage. Heat conduction ability is another factor which separates anode materials. In an
oscillatory mode, the anode will have to function as the cathode on every other cycle.
This means that the anode will operate with a cathode arc spot which could cause a great
deal of erosion and outgassing. In the case of some anode materials, the energy input to
the cathode spot can be absorbed by the heat conducted into the anode and it does not
have to be balanced by the evaporating anode material.
Now the advantages and disadvantages of the four major anode materials can be
presented. Graphite is inexpensive and easy to use. However, it is suspected of
releasing gases; it does not conduct heat as well as the other materials (which makes it a
unidirectional igiutron component); and it vaporizes easier than the others and sublimes
onto critical places like the throat, which could cause breakdown across the insulators
and to the wall. Graphite is also very rough and does not hold off voltage as well as
some other materials. Molybdenum is a good heat absorber and it does not erode as
seriously as graphite, thus making it usable for a good bidirectional switch.
Furthermore, when it does vaporize, it vapor-plates onto the wall with a smooth finish
(much better than the graphite case). Titanium behaves much the same as molybdenum,
and it makes a good bidirectional switch. A good factor is that titanium has good gas
gettering properties. Both of these materials arc smoother than graphite and thus usually
hold off more voltage. Stainless steel provides a good bidirectional capability along with
less outgassing than graphite. Initially, stainless steel can be machined extremely
smooth, which allows for high holdoff capability. However, this smoothness could
change after many high current discharges.
The ignitor is the means by which the ignitron can be triggered on a single shot or
repetitive basis. The ignitor dips into the mercury-pool cathode and presses down the
surface of the mercury to form a meniscus. Since the mercury does not wet the ignitor
(in a good mbe), a resistance of 20 to 100 ohms (or more) exists between the pool and
the ignitor (this value can be as low as 1 Q at higher ignitor temperatures). As seen in
Fig. 2, the ignitor is usually a molybdenum support rod connected to a graphite shank
andtippedwith a boron carbide compound. To initiate an arc, a cathode spot is fomied
by passing a short, intense, positive pulse of current through the ignitor into the mercury
pool. This pulse must be unidirectional because the ignitor is easily damaged by reverse
current flow. After formation of the cathode spot by the ignitor, the ionization of the
mercury vapor permits the fuing current to flow directiy form the top section of the
ignitor to the pool (bypassing the tip) and the ignitor-to-caihode voltage assumes an
arc-drop value of around 12 volts. Plasma then diffuses into the tube volume and a
IGNITOR
CURRENT
FLOW
SUPPPORT
yROD (MOLYBDENUM)
1
^^
MERCURY POOL
^
GRAPHITE SHANK
IGNITOR TIP
w///////M^^mmw/////////M
tip is of a boron carbide compound
FIG. 2
BASIC IGNITOR DESIGN
glow-to-arc transition takes place between the anode and the mercury pool cathode (if
enough of a potential difference exists). The arc discharge consists of three pans: a
positive ion sheath just above the cathode spot called the catiiode sheatii, a positive
column made up of approximately equal numbers of ions and electrons, and the anode
sheath which is an electron space charge adjacent to the anode. Most of the tube drop
occurs across the cathode sheath.
The envelope of an ignitron is usually stainless steel with glass insulators
separating electrodes of different potentials. The primary function of the envelope is to
maintain a vacuum tight enclosure around the operating elements. Other materials have
been used for the envelope (wall) material (ceramics, glass), with limited success.
Commercial ignitrons have a double envelope design with a copper water cooling coil
brazed to the inner envelope. Sometimes there is no coil and the cooling water flows
between the inner and outer envelopes. A last note about ignitron design is the
anode-insulator junction. Since glass-metal junctions easily produce corona when
subjected to high voltage, the anode is brought several centimeters into the tube to shield
the junction from high electric fields.
Figure 3 affords a view of a more complicated ignitron. It contains all the
components which have been previously described along with grids, a baffle, and an
auxiliary (or holding) anode. The control grid is the most important grid and is used
more often than the other grids. It is placed less than one electron mean free path from
the anode (or gradient grid) and held at zero (or negative) potentialrelativeto the cathode.
Qosure of the tube is delayed from the ignitor pulse until this grid is pulsed positive.
Afterwards, this grid loses control over the discharge because of the positive ion sheath
formed around the grid. A very important use of this grid is for low jitter, repetitive
firing of the ignitron. The shield grid can be used as an aid in deionizarion. It surrounds
the control grid and is biased negatively with respect to the cathode. It helps pick up the
ANODE
TERMINAL
ANODE SEAL
(INSULATOR)
SHIELD
GRID TERMINAL
CONTROL
GRID
TERMINAL
GRADIENT
GRID TERMINAL
GRID SEAL
SHffiLD
GRID —
CONTROL
GRID
GRADIENT
GRID
METAL
ENVELOPE
(WALL)
vf.t 'iANODE ^•
»•"
LLLLLLU
111111II111111II l i e n
II I I I I I I I I I II H I
lITi
MERCURY
SPLASH
BAFFLE
(GRAPHITE)
IGNITOR
HOLDING ANODE
MERCURY POOL
•HOLDING
ANODE
TERMINAL
no. 3
IGNirOR TERMINAL
IGNITRON WITH HOLDING ANODE,
BAFFLES, AND GRIDS
8
arc during the initiation of conduction and shields the anode structure from the residual
ionization after conduction. This grid also acts as a baffle to protect the control grid and
anode from mercury droplets sprayed by the cathode spots. The gradient grid is used as
an electrostatic shield or as a voltage divider where the holdoff voltage of the elementary
ignitron may be exceeded. Its purpose is to divide the potential gradients during the
non-conducting periods of the tube to reduce the voltage stress between the anode and the
control grid. The gradient grid is usually maintained at intermediate anode-to-cathode
potentials by means of potential dividers. The mercury splash baffle helps to protect the
upper ignitron components from mercury droplets sprayed by the cathode spots.
Problems sometimes occur because the arc is attracted to the baffle and the baffle
outgasses. The auxiliary or holding anode is used when current may fall below the level
required to maintain the cathode spot A separate excitation circuit providing more than
10 A is applied from the holding anode to the cathode to maintain the cathode spot. This
feature is most often seen in rectifier use because the ignitron sometimes has to carry very
low current andremainon. As an added note, the auxiliary anode has been used in some
experiments as a probe.
Basic Ignitron Characteristics
The ignitron is characterized byreasonablyhigh standoff voltages. The voltage
capability is basically affected by the electrode separation, geometry, and the mercury
vapor pressure.
In the ignitron, the pressure-spacing product (pd) is 0.001 to
0.01 Torr-cm which is well to the left of the minimum on the Paschen curve. The
holdoff voltage is also affected by mercury condensation on the anode and insulators
which may cause a glow-to-arc transition or breakdown along the walls. Peak and
average cunent depend on a great many external tube factors (i.e., repetition rate,
temperature, etc.). Plasma instabilities probably determine the ultimate current limitation.
The pulse width under high current conditions is limited by the tendency of the cathode
spots to migrate to the confining wall of the mercury pool. Another limit on the pulse
width is the tube loss (pulse current times tube voltage drop). Current rise (di/dt) is
determined by the plasma formation time and its density. In addition, the time required
for cathode spot formation may be a limiting factor. Pulse repetition rate is mostiy
affected by the deionization time (timerequiredafter conduction before sizeable voltage
can bereappliedto the ignitron) and the tube dissipation. Repetition rate in higher and/or
longer current cases can also be limited by the vapor recondensation time. Recovery time
is determined by the peak current conducted, the tube temperature, physical dimensions,
etc., as well as how the tube is operated. Delay time (time between trigger and switch
closure) is determined by the plasma formation time, which in turn is affected by the
ignitor voltage. Delay time is also influenced by the anode-cathode voltage. Jitter
(standard deviation from the mean value) in closure time (delay) is determined by the
ionization level in the tube prior to the trigger pulse. Jitter can be expected to be larger at
lower mercury pressures, thus it is temperature dependent. In general, a higher cathode
temperature offers lower jitter.
The life of an ignitron (number of shots) is a very strong function of the condition
under which it operates. Tubes that run near peak current and/or voltage do not last as
long (several thousand shots) as tubes in phase controlled applications which may
function for years (several hundreds of thousands of shots). The main failure mode is
the inability to trigger the mbe due to the wetting of the ignitor by the mercury. A
rule-of-thumb guide to life expectancy is that these tubes will provide around 1000 shots
when maximum ratings are applied simultaneously. Reduction of one key parameter by
50% will increase the life by a factor of ten. This is only one assumption for the device
life. There are other formulas for life which utilize peak and actual current, voltage, and
energy as an exponential factor. In actuality, some tubes have lasted only a very few
10
shots at levels below their maximum ratings. Another factor affecting life is that some
mechanical damage may bring the tube down to air pressure. One method that has been
used to extend ignitor (and thus tube) lifetime is to allow time for tiie catiiode spot to
migrate awayfixjmthe igiutor before the current between the anode and cathode occurs
[2], [3], [4].
As previously stated, the control grid loses control once the discharge is
established because of the positive ion sheath formed around it. The importance of the
recovery time is clearly seen in therepetitivemode of mbe operation. Experiments have
shown that the deionization time is affected by many factors. Just a few of these are:
(1) deionization time increases with increasing grid resistance, (2) deionization time
decreases with increasing grid bias voltage, (3) deionization time increases with
increasing repetition rate, (4) deionization time increases with increasing water
temperature, (5) deionization time increases with increasing pulse current, and
(6) deionization time is altered by the number of internal mbe components [5]. The first
factors are of concern only in ignitrons which have grids. Most of the factors can be
explained by noting the various parameters at which the mbe operates. The deionization
time increases with increasing pulse current. The reason for this is that greater pulse
current increases the ion density and therefore, the control grid requires a longer time to
extract the increased number of ions from the grid region. Reference 5 states that the
deionization time increases with increasing water temperature. This is because with
increasing water temperamre, the vapor pressure of mercury goes up. The reference
theorizes that the increased pressure decreases the mean free path of the particles in the
gas which in turn decreases the drift velocity of the electrons. To carry the same current
requires an increased number of electrons, and hence ions and these added ions cause the
deionization time to increase. Therestof the parameters can be thought of the same way.
11
A mathematical formula derived for an ignitron with multiple grids, as an approximation
for the deionization time is given as
Deionization Time =
,^'^^"
M,/cR g
A = cross sectional area of the ignitron
d = spacing between grids
n = ion density
(D
\^ = bias voltage
Rg = grid resistance
e = electron charge
It is seen that the deionization time is inversely proportional to the volume of the grid
region, and directiy proportional to the ion density in the region, the spacing between the
grids, and the value of the grid bias and gridresistance[5].
Cathode spots and their motions arc other aspects of ignitron conduction that
affect operation and parameters. As previously discussed, operation of an ignitron is
dependent upon the current pulse width because the cathode spots tend to move towards
the walls. Referring to Fig. 4, the top row of figures show one photograph and a
sequence of example tracings of the cathode spots moving in a circular ring away from
the ignitor [6]. The movement may be due to the diffusion outwards of partially ionized
vapor in the vapor jet associated with cathode spots. The diameter of this ring of spots
for a small, glass-walled ignitron (2" diameter) has been plotted as a function of time for
various voltages (voltage applied across a capacitor in parallel with the ignitron) [7].
There were voltages for which the ring of spots continued to expand even after peak
current. Here, the ring expansion seemed limited by its velocity rather than the need for
arc spots. For lower voltage tests, the diameter of the ring of arc spots reached its
maximum near peak current. In this case, the expansion of the ring seemed to be
governed by the need for arc spots. Experiments resulted in a formula for the velocity of
these spot rings which is given as
12
Increasing Time
nG.4
TRACINGS OF CATHODE SPOT MOTIONS [6]
13
'MUif-)
u = spot velocity (cm/sec)
Uo= 2.2 * 10 cm/sec
P = 0.70
in
•
Vc = capacitor voltage, kV
\^o= lo kV
fjj = ringing frequency, kHz
f1 = 200 kHz
In one investigation of groups of spots carrying a total of 75 A, each spot was found to
carry an average of 3.5 to 15 A (usually 5-9 A). Other work showed tiiat each spot was
actually a cluster of tiny emitting sites [7]. The spot earned up to 50 A and each emitting
site carried 1 to 2 A with a current density of 2-5*10^ A/cm^. These spots expanded as a
circular line of continuous emitting sites. Furtherresearchinto spot movement showed
that with a di/dt > 10*7 A/sec, new spots formed ahead of the old. This phenomenon
(called leap-frogging) increased the front velocity while die spot velocity remained the
same. Propagation in this mode appears to be possible by ionic charging of an insulating
film on the mercury cathode staning a new spot by either Maker effect emission or
dielectric breakdown. Malter effect emission is field emission from a conducting surface
by charge building up on insulating films or particles. This emission could trigger an
avalanche and lead to new arc emitting sites. Many spots also form spontaneously on
both the pool and other areas like the walls and baffles. Experiments have shown that at
high pulse currents, some wall spots originate near the lower end of the anode and move
up as though blown by the vapor blast from the pool. Crazing of the glass insulators has
been sometimes shown to be caused by the presence of plasma associated with spots
forming on the gradient grid. Arc spots are the cause of many of the limitations of the
performance of the tube and of many of the failure modes of the tube [7].
14
Pragtigal Aspggts
This section will cover the more practical aspects of ignitron size, firing
reqiurements, conditioning, and mounting. Basically, ignitrons come in five sizes : A,
B, C, D, and E. Some companies add to these types with custom sizes such as Jumbo C
or C+. At the moment, the diameter is the usual determining factor for the size of an
ignitron. Table 1 gives general geometrical and weight values for the major ignitron
sizes. Usually, the bigger the ignitron, the more power (peak current, coulombs, etc.) it
can handle. However, different ignitrons are made with specific purposes in mind and
thus certain characteristics are maximized depending on the use.
The ignitor is the element in the ignitron which has to be initially sparked in order
for the ignitron to conduct. Figure 5 shows the voltage and current associated with a
typical ignitor pulse. The initial ignitor current rise is approximately linear and is a
function of the ignitor-pool resistance until some time, T, when a cathode spot is formed.
At this time, the voltage assumes an arc-drop value of around 12 V. The maximum and
minimum voltage requirements vary slighdy from mbe to mbe and vary as a function of
the pulsewidth. Some ignitor firing circuits provide 500 V for 500 |is while shoner
pulsewidths require larger voltages (up to a few kV's) to initiate conduction. For
elementary ignitrons, the excitation circuit can be a single capacitor charged to several
thousand volts and switched by a solid state device, a krytron, or a thyratron. Repetitive
triggering or synchronous triggering of ignitrons requires more complicated or more
powerful trigger generators. The trigger generator utilized in this project will be detailed
later in this repon.
The ignitron's lifetime is directiy proponional to proper care of the tube before
and during use. There are a few precautions that should be taken in storage and handling
prior to use. The ignitron should stay in the original packing and under dry conditions.
This is because the boxes that the switches are supplied in are specifically designed for
15
TABLE 1
COMMERCL^L IGNITRON SIZES
SIZE
DL\METER
HEIGHT
WEIGHT
A
2"
8"
6 LBS
B
2.75"
12"
10 LBS
G
4"
10-14"
15 LBS
D
5.5"
12-20"
20-25 LBS
E
9"
22-42"
100-150 LBS
16
CURRENT
FIRING CURRENT
VOLTAGE
FIRING VOLTAGE
C DROP VOLTAGE
FIG. 5
VOLTAGE AND CURRENT OF TYPICAL
IGNITOR PULSE
17
these mbes. For example, the ignitron may be elevated with a circular section cut out so
that extensions from the bottom of the ignitron are not harmed. An ignitron is a very
powerful but delicate switch. There should be no shock to the tube or strain on the
terminals because there are sometimes glass insulators involved. Most of all, the ignitron
should never be inverted. This applies before, during, or after use. Mercury could
adhere to the surfaces of grids, insulators, anodes, and walls, especially if the areas are
cool.
In the case of a new ignitron or one that hasn't been used in a long time, a few
initial conditioning steps should be taken. The tube should initially be heat conditioned.
This step helps to drive mercury droplets away from the anode area of the mbe and
hopefully back down into the pool Heat conditioning involves heating the anode smd to
between 100-125° G while keeping the cathode near room temperamre for at least two
hours. The next step should involve some voltage conditioning. This step helps smooth
out any sharp areas on the anode or other structures which may cause early arcs or
predischarges. Apply a low dc voltage across the ignitron (ignitor not connected) with a
series combination of a 1 to 4 |jp capacitor and a 1 firesistorin parallel with the ignitron.
Slowly increase the voltage, do not be alarmed by early breakdown. Continue the
process until the ignitron settles down to its final breakdown value. An alternate method
of voltage conditioning involves connecting a variable ac voltage source across the
ignitron. Slowly increase the voltage, allowing a maximum of 30 mA to flow. During
mbe operation, the best performance results when the mercury pool is below 40° C and
the anode is few degrees hotter than the cathode (even during the cooldown after
operation). The water cooling pipe which encirles most commercial ignitrons is wound
with more turns at the cathode. This allows the cathode to be cooled to a greater degree
by the cooling water. Water used for cooling must be must be clean and free from
18
corrosive chemicals. The water mbes must not be blocked or the ignitron temperature
could rise during long or very high current conductions.
Basically, ignitrons come with two types of mounts: standard and coaxial. The
standard mount has a cathode smd protruding from the bottom of the mbe. This mount
can be adapted to just about any type of connections necessary. Some mbes come with
direct coaxial mounting ability. A coaxial mount can be made from a standard cathode
stud by bringing coimections upfromthe smd around the mbe in a squirrel cage fashion.
In most mbes, the outside envelope is acmally connected to the cathode and a band or
mbe can be used to encircle the mbe and allow connections. There are some general mles
about mounting. The mbe should be mounted within 3 degrees of vertical. This factor
involves trying to keep the mercury from wetting the ignitor or any other pan of the
ignitron and trying to keep the main arc centered and away from the walls. Make very
low resistance connections to the anode and cathode. The contact area should be clean
and the terminals securely bolted. This is in reference to the large amount of current that
the ignitron may carry and the related ohmic heating caused by faulty connections.
Rnally, a coaxial or squirrel cage mount is recommended if at all possible. This mount
creates a symmetric magnetic field inside the mbe.
Ignitron Uses
There are five basic uses for ignitrons in normal operations: ac-control service,
power rectifier service, crowbar service, pulse modulator service, and capacitor
discharge service. Some ignitrons are designed for use in a specific category and are
given ratings geared to the service intended. Ignitrons for ac-control service are usually
used in welders and have non-simultaneous maximum ratings for average anode current
and demand kVA. In general,rectifierignitrons can supply large dc loads from a single
or multi phase ac main. These ignitrons have peak and average anode current ratings
19
given for two values of peak anode voltage. Crowbar ignitrons are used as safety
grounds for many large systems (generators, capacitor banks, etc.) and in conjunction
with other services such as capacitor discharges. High and/or large current carrying
capability are useful in this service. Pulse modulator service involves ignitrons with fast
recovery (depending on therepetitionrate). Ignitrons for capacitor discharge service are
usually designed for lower average current but much higher peak current and peak
holdoff voltage than the previous cases. In this case, the maximum current-per-coulomb
transfer capability of the ignitron is basically constant Ignitrons for this purpose may be
required to reach a fully conductive state very quickly and at an accurately controlled
point in time. To do this, the ignitor is usually pulsed very hard (high voltage) and very
quickly. Depending upon the circuit values, the ignitron may or may not be in an
oscillatory mode. This factor could be important depending upon the ignitron. In
general, the current carrying capability of each ignitron is a function of the energy
dissipated per pulse. This is the product of the average arc voltage drop, the pulse
length, and the current, and is measured in joules (J) [2], [3].
It is the usefulness of ignitrons as a closing switch in capacitor discharge service
which warrants its further development for the pulsed power industry. Envisioned
applications in the pulsed power industry include the use of ignitrons as closing switches
for capacitively driven railguns, crowbar switches in energy recovery circuits, and
closing switches for high-power laser flash lamps. These applications have caused a
renewed interest in improving these switches and in general research on the interactive
effects of materials, geometry, and environment on the switches.
Simultaneous
achievement of higher currents (1 MA) and higher coulombs (1000 C) without drastic
reduction in lifetime (1000 shots) is a major goal of all parries involved in the project
described here.
20
Theoretical Cnnf^i derations of Magnetic Field Effects
It was the anticipation that an axial magnetic field could prolong the life of an
ignitron and/or improve certain mbe characteristics that prompted tiiis smdy. Past work
on the effects of axial magnetic fields on ignitrons is documented by Knight [6]. He
found that axial fields tend to confine the spontaneous cathode spot formation to the area
of the mbe wall below the face of the anode. That work also reported that an axial field
(even as low as 0.01 T) reduced the holdoff voltage that the mbe could withstand by a
factor of 7. Furthermore, the application of an axial field of 0.1 T caused the differential
mbe voltage trace to become significantiy smoother than without a field. In comparison,
much work has been done on the effects of magnetic fields on vacuum arcs. Vacuum
interrupters are the major commercial devices utilizing vacuum arcs. The effects of
magnetic fields on their current interruption ability and arc characteristics have been
smdied extensively. A major goal of some of this research was to minimize the energy
dissipated within the switch [8]. This energy causes sputtering of metallic vaporfromthe
electrodes which contaminates the interelectrode space and leads toreignitionafter current
interruption. Yanabu et al., reported that with an axial field, the arc demonstrated a
tendency to spread over the entire electrode surface and was confined and stabilized by
the field. As a result, fewer hot spots were formed and less sputtering from the anode
occurred. It was also found that the arc voltage had a minimum at a certain value of the
magnetic field.
Any plasma arc is characterized by cathode spots, a diffuse interelectrode region,
and a diffuse electron collection at the anode. A theory by Kimblin states that the applied
magnetic field creates radial electric fields within the arc plasma due to the very different
gyration radii of electrons and metal ions [9]. The result is a radial space charge
distribution with negative charge concentrated in the center. Kimblin states that this leads
to a redistnburion of positive ions in the bulk of the plasma discharge with a resulting
21
reduction of the negative space charge in front of the anode. Gonsequentiy, the anode
voltage drop (and thus the arc voltage drop) is reduced.
CHAPTER n
EXPERIMENTAL APPARATUS
Test Ignitrons
This report discusses test results from three very different types of ignitrons: a
commercial NL-2909 ignitron, a glass-walled ignitron, and a custom designed
demountable ignitron. Figures 6, 7, and 8 show dimensional drawings for all three
mbes. The 2909 was developed from an NL-5555 mbe used for welding applications by
removing the splash baffle and changing the cathode connection for high pulsed currents.
It contains two ignitors and no additional baffles or grids of any kind. The glass-walled
mbe (of which two identical mbes were tested) is the largest one of its kind ever tested at
the levels used in this program. The demountable ignitron (DIG) was designed at this
laboratory and manufactured by National Electronics/Division of Richardson Electronics.
This mbe resembles a conventional steel-walled mbe but has a total of four viewports
permitting visual access to the ignitors and discharge in the cathode and anode region.
All components of the DIG (i.e., ignitors, anode, mercury filling, and any viewport) are
easilyremoved,replaced, altered or otherwise modified forresearchpurposes.
The DIG was designed with a small exhaust mbe on the top flange through which
the ignitron could be evacuated. In terms of pressure, the ignitron must be pumped down
to the vapor pressure of mercury at room temperature, which is 1 micron (1 milli-Torr).
This value falls on the extreme low side of most mechanical roughing pumps. Therefore
a pumping station was designed which utilized both a mechanical pump and an oil
diffusion pump. The mechanical pump used was a Franklin Electric Roughing Pump
while the diffusion pump was a CVC Type VMF-11 Oil Diffusion Pump. The reason
this diffusion pump was used was that it was one of the few pumps which did not
22
23
All dimensions in inches
FIG. 6
NL-2909 IGNITRON
24
All dimensions In Inches
6.25
no. 7
GLASS-WALLED IGNTTRGN
25
All dimensions in inches
no. 8
DEMOUNTABLE IGNITRON (DIG)
26
contain an alimiinum tower or wall (it used nickel plated steel internally). Pressure
moiutoring for the entire pumping station was done with a Stokes McLeod Gage. The
McLeod gage is based on the principle of a mercury filled U-mbe manometer with a
magnified readout of small pressure values achieved by a system of small capillaries. An
example drawing of a McLeod gauge can be seen in Fig. 9. Operation of the gauge is
dependent on conformance to Boyle's law, in that the pressure-volume product before
compression is equal to that after compression. The method of operation is to raise the
mercury column through the sample volume and up to a fixed mark on the enclosed
capillary. The height in the open capillary will then have a linear relationship with
pressure [10]. The rest of the components necessary for the pumping station were: red
mbber vacuum hose, stainless-steel nozzles and connections, a stainless-steel vacuum
valve, and a special mercury pump filter at the exhaust of dieroughingpump. Care was
always taken in terms of safety and proper cleanup when mercury was concerned. The
exhaust of the roughing pump was filtered and then directiy vented. Mercury spill
procedures were posted and mercury absorbent was labeled in clear view. Furthermore,
monitoring of the area was done with a mercury vapor detector. A picture of the
pumping station is shown in Fig. 10.
M^^jnr TeMX Stand Components
The major components rcquired to test the ignitrons in a critically damped
configuration included: the coils to provide the axial magnetic field, the capacitor banks,
proper system damping resistors based upon die inductance of the test stands, and the
power supplies to charge the banks and provide current for Uie electromagnets. The axial
magnetic field for all tests was provided by multiple (2 or 4) Magnion water-cooled, high
power electromagnets. A sketch of one magnet is shown in Fig. 11. The inside of the
electromagnet consisted of 90 spiral mms of a conductor with an edge cooling system.
27
To Vacuum System^ ^
Capillary G2
Low Vacuum Readings
Capillary Gi
High Vacuum Readings '
From Mercury Reservoir
no. 9
McLEOD GAUGE
28
DIFFUSION PUMP
ROUGHING PUMP
McLEOD GAUGE
FIG. 10
PICTURE OF PUMPING STATION
29
water connectio
:ions>^
coil windings
23.5"
21.25"
current and water connections
* coils are 2.75" thick
no. 11 MAGNION ELECTROMAGNET
30
The totalresistanceof each coil was 0.02 i l The water cooling system provided flowing
water of about 13^0 which was filtered and tiien flowed through the magnets. A
pressure gauge moiutored the water pressure and a separate flow gauge monitored die
water flow tiirough die catiiode cooling system. DC current tiirough tiie coils (which
were always connected in series) was calculated by measuring the voltage drop across an
800 A, 50 mV current shunt The electromagnets were stacked and spaced to provide a
relatively uniform axial field throughout the length of die ignitron along with providing
maximum viewing capability in the cases of the glass-walled and demountable ignitrons.
Prior to any testing, die magnetic field for all test stands was measured and profiled using
a Bell Gaussmeter Model 640 with a Hall sensor probe. Four cabinets of ten capacitors
each comprised the 2.56 mF, 10 kV capacitor bank. Each bank contained two carbon
disk resistors (0.2 Q, each) in parallel for an alterable damping resistance. A 10 kV,
1 A power supply was used to charge the banks. For the holdoff tests, a 1.89 |iF,
60 kV high energy capacitor was utilized along with a series stack of four 0.2 CI carbon
disk resistors. A 120 kV, 10 mA supply was used to slowly charge this system until
the test specimen conducted. Power for all electromagnets was provided by two high
current power supplies. For cmrents of up to 640 A, a Hobart arc-welding generator
was used. Precise tuning of the Hobart supply at low curtent levels (lO's of amps),
presented a problem. Therefore, a Perkin Model 3671 power supply was used to
provide precisely mnable currents of up to 150 A.
Two test stands were utilized in the testing of the diree mbes. The first stand,
shown in Fig. 12, was used for testing the commercial mbe and the glass-walled tube.
Figure 12 also shows the plot of the central axial field versus applied cmrent for this
stand. The stand consisted of four Magnion electromagnets with an inner diameter of
7 inches and a double-sided, symmetrical current return made from 1/4" by 4'
aluminum. The close proximity of the test ignitron to the inner magnet wall required a
31
In from Cspadtxx Banks
Return to
Capacito^
Banks
^
a
CAMERA VIEW
w^mmmj
f
Return to
Capacitor
Banks
'
fl'
|»iliM
!!^^^!:!:^^!•!•!•;;!;W^|.^|.^!.|.'|.^'!.|^
Current direction
Current Return
100
n o . 12
T ' r
200 300 400
Current [A]
500
600
700
FOUR-MAGNET STAND AND MAGNETIC
FIELD RELATIONSHIP
32
currentreturndesign which went around the magnets. The electromagnets are spaced 2"
between the two center magnets to allow for full viewing of the glass-walled tube
volume. The other magnet spacing is 1" to stretch die field uruformity over die entire
length of the commercial ignitron. Rgure 13 shows the field uruformity profile for this
test stand. The second stand is shown in Fig. 14 along with its central axial field versus
current plot and was used for testing the DIG. The stand consisted of two Magnion
electromagnets with an irmer diameter of 12 inches and a four-sided, symmetrical current
return made from two 1/4" aluminum plates and eight 1/2" aluminum rods. The larger
diameter of the demountable ignitron (10") forced the use of alternate electromagnets
which had a larger inner diameter (12"). The spacing of the two magnets is 4" to
facilitate a full view of the inner structure of the DIG through all viewports. Figure 15
shows the field uniformity profile for this test stand.
Triggering and Non-Optical Diagnostics
A block diagram of the diagnostic and triggering systems for the critically damped
conduction tests is shown in Fig. 16. A Tektronix 7834 Analog Storage oscilloscope and
a Nicolet 4094A Digital Storage oscilloscope were housed in a screenroom near the
experiment and were connected to the experiment via shielded diagnostic lines. The
entire firing sequence was initiated widi a manual trigger to die Cordin Model 437 Trigger
Delay Generator. The trigger signal for die oscilloscopes (which were set in single shot
modes) was provided by die non-delayed output of die trigger generator. The delayed
output of die trigger generator then sent a signal to die transmitter section of the fiber
optics. The fiber optics transmitted a light pulse out of die screenroom and to the
experiment. The fiber optic receiver at die experiment dien triggered the TRW camera.
and die ignitor pulser which fired die ignitron. Schematics and a general description of
die fiber optic receiver / transmitter and die ignitron pulser are given in Appendix .A.
J2
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Distance from top plate [inches]
15
34
magnet coil stack
TOP RETURN PLATE P
TOP VIEW
Wooden structure
Return connections
gnet Coils
Current Return Plates
SIDE VIEW
'
100
I
200
' — I — • — I — • — r — • — I — r
300
400
500
600
700
Current [A]
no.
14
DEMOUNTABLE STAND AND MAGNETIC
FIELD RELATIONSHIP
35
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Two Tektronix P-6015 HV probes were used to record die differential voltage across the
ignitron. These probes fed into a shielded differential amplifier / line driver which
outputs a single signal through die diagnostic lines, into die screenroom, and into one
channel of the Nicolet oscilloscope. The schematic and a general working description for
the differential amplifier is given in Appendix A. For all critically damped conduction
tests, one or two ignitron cmrent monitoring devices were utilized: a 50 kA Pearson Coil
(Model 301X) or a Rogowski Coil. The signal(s) from the current monitor(s) was(were)
fed into the screenroom through the diagnostic lines and registered on one or both
oscilloscopes. In the case of the test stand shown in Fig. 12, the current monitors
registered the full discharge current In the case of the test stand shown in Fig. 14, the
current monitors were placed around one of the eight return rods. To ensure that the
stand had a symmetrical current distribution between the rods, the current monitors were
either placed on opposite legs and on legs atrightangles to each other for various shots.
Optical Diagnostics
Two cameras were utilized in the smdy of the effect of an axial magnetic field on
the discharge plasma of the glass-walled ignitron. The first camera to be discussed is die
TRW Model ID Image Converter Camera. In die TRW camera, light energy radiated
from the event under smdy is focused on the image converter tube photocathode which
converts the photon image to an electron image. This electron image is accelerated and
focused on the photoanode of the image converter tube with a brightness gain.
Depending upon which plug-in unit is being utilized, frames can be taken in rapid
sequence or streak pictures can measure die time rate of change of information along one
axis of die event The signal from die fiber optics receiver is received by die Model 46A
Trigger Delay Generator of die camera which further delays the triggering of die camem
(if necessary). The major plug-in unit utilized in this experiment was the Model 9B
38
Microsecond Framing Unit This unit provides three frames in a vertical array widi
exposure durations of 0.5, 1,2, 5, or 10 \is. The time between exposures can be varied
from 10 to 500 ^is at preset levels.
The second camera utilized for this project was a Dynafax High Speed
Continuous Writing Framing Camera. The camera used 35 mm film which is rotated
around the inside of the camera drum. A separaterotatingmirrorflashedalternating rows
of exposures onto the rotating film. The combinedrotating-drum,rotating-mirrordesign
of this camera allowed framing rates of up to 26,(X)0 picmres per second. The top
rotational speed combined with the 3 p,s exposure stops (called the diamond stops)
allowed around 40 |is between frames. This camera was run in an open shutter mode
for the duration of each shot, then the shutter was closed. One problem associated with
this camera was to relate each exposed frame from the camera to a section of die current
trace to within a few \is. The shuttering of the individual frames is provided by the
rotating mirror in conjunction with the diamond stops.
A method for time
synchronization was devised which utilized therotatingmirror, a photomultiplier, and the
varying intensity lightfix)mthe ignitron during its conduction period. The basic diagram
for the system is seen in Fig. 17. Figure 18 shows an example output from the
photomultiplier.
Since light emitted from the conduction period was used, the
photomultiplier gave a signal only during conduction. In essense, the hght from the
experiment was transmitted through one optical fiber, reflected off the rotating mirror,
and received by another optical fiber which goes to a photomiultiplier. The optical fibers
were arranged so maximum reflection occured when the mirtor surface was flat with
respect to the optical fibers. Since frame generations occur when the mirtor surface is at
45 degrees with respect to die optical fibers, each photomultiplier pulse was precisely
between two frames. There was then a photomultiplier null (i.e., zero voltage level) as
die mirrorrotatedto its next surface and the frame-pulse-frame sequence continued. The
Fiber Optic to pick
up light from experiment
Inside Camera
Rotating
filmdnun
Fiber optic
to photomultiple
Photomultiplier
J
Electrical signal
to oscilloscope
n o . 17
DYNAFAX OPTICAL TIMING DIAGRAM
V
I
Baseline
2X->|
0.5X
* = point where exposure is taken
1= photomultiplier signal
X = time between exposures
n o . 18
DYNAFAX PHOTOMULTIPLIER SIGNAL
39
40
jitter for this method was a function of the photomultiplier mbe response and the fiber
optic-mirror alignment
CHAPTER m
TESTING AND RESULTS
Voltage Holdoff Te5;ts
The effect of an axial magnetic field on the holdoff voltage of each of die diree
ignitons was determined using a 1.89 ^iF, 60 kV high energy capacitor. Additionally,
the DIG was tested before and after the insertion of a dielectric cylinder. The actual
testing circiut diagram is shown in Fig. 19. This type of capacitive testing provided an
ample current pulse when complete tube breakdown occured and ruled out the
inaccuracies caused by partial mbe breakdown. The Second Workshop on High Power
Ignitron Development held at LLNL in April, 1988 determined this to be die most
accurate method of testing for ignitron holdoff voltages [11], [12]. Measurements of the
breakdown voltage were done with a Fluke Model 8024B Multimeter and a Fluke
Model 80k-40 HV probe.
The effect of the axial magnetic field on the holdoff voltage of each mbe is plotted
in Figs. 20, 21, and 22. Each set of points can be fitted to the same basic curve. In
summary, each ignitron holds off its fiill voltage until a certain minimum axial magnetic
field is reached. At this point, there is a steep dropoff in holdoff voltage until a minimum
voltage is reached; thereafter, the holdoff voltage slowly rises. The minimum field
required to lower the holdoff voltage and the minimum holdoff voltage varies from test
specimen to test specimen. The NL-2909 displayed a drop in holdoff ability going from
over 55 kV to 800 V at an axial field value of around 0.005 Tesla. Data points of full
holdoff potential (around 55 kV) are not shown to the extreme left of Fig. 20 in order to
provide a higherresolutiongraph of the other data points. The minimum holdoff voltage
was 340 volts which occurred at 0.013 Tesla. At the maximum magnetic field value
41
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800-k
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600
=
400
I
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I
0
0.1
I
0.2
0.3
Axial Magnetic Field [Tesla]
FIG. 20
NL-2909 HOLDOFF VOLTAGE GRAPH
Axial Magnetic Field [Tesla]
n o . 21
GLASS-WALLED IGNTTRGN HOLDOFF
VOLTAGE GRAPH
44
1400
Axial Magnetic Field [Tesla]
n o . 22
DEMOUNTABLE IGNFTRON HOLDOFF
VOLTAGE GRAPH
45
attempted in die experiment, 0.35 Tesla, die holdoff voltage rose to only 1.23 kV. The
glass-walled mbe displayed its drop in holdoff voltage at an axial field value of
0.022 Tesla. The holdoff voltage initially dropped from an average of 45 kV down to
13 kV. The minimum holdoff voltage for the glass-walled ignitron was 9.41 kV which
occurred at an axial field value of 0.034 Tesla. The glass-walled ignitron regained a
holdoff value of 38 kV with the application of an axial magnetic field of around
0.25 Tesla. In the case of the other glass-walled mbe, the full holdoff voltage of 32 kV
reoccured at around 0.2 Tesla. Testing in this case was not done with a capacitive circuit
but with a HV power supply and a current limiting resistor. The demountable ignitron
held off an average of 23 kV up to an axial field value of 0.014 Tesla at which point it
dropped to 750 V. Again, the plot of the data points for this mbe, shown in Fig. 22,
does not include the full holdoff points in order to improve the resolution of the graph.
The holdoff voltage of the demountable ignitron showed a minimum of 330 V at around
0.03 Tesla. The amount of holdoff voltage recovered at the highest field value
(0.162 Tesla) was 680 V.
Comparisons of the results from the three ignitrons reveal the drastic effect that
axial magnetic fields have on ignitrons. The NL-2909 and the demountable ignitron
displayed large reductions in their respective holdoff voltages, dropping to less than 4%
of their original value atrelativelylow axial magnetic fields. Furthermore, both of these
mbes showed a very slow recovery toward full holdoff potential as die axial magnetic
field was increased. However, the recovery of the NL-2909 was almost twice as fast as
that of the demountable ignitron. On the other hand, widi an applied field, the holdoff
voltage of the glass-walled mbe dropped to around 30% of its original value. The
magnetic field value at which this reduction took place was almost twice diercquucdfield
by die DIG and over four times die field required by the NL-2909. The glass-walled
ignitron also recovered to full holdoff potential more than ten times faster than the two
46
metal-walled mbes. Tests widi reversed polarity of die magnetic field showed diat die
predescribed effects were independent of die polarity of die magnetic field within die
resolution of the diagnostics.
It was hoped that the insertion of the dielectric cylinder into the DIG would cause
die DIG to behave qualitatively more like die glass-walled ignitron rather dian die 2909.
A picture of the cylinder is shown in Fig. 23. When die dielectric cylinder was initially
inserted into die demountable ignitron, itreachedto within 0.5" of the top flange of die
mbe. Holdoff tests without magnetic field revealed continually low values ranging up to
1.5 kV. Upon inspection of die inside of the mbe, an arc spot was plainly visible on the
anode, radially adjacent to a surface discoloration of the dielectric cylinder and wall. It
was theorized that the increased field intensity caused by the change in dielectric constant,
Er = 4, introduced by the cylinder residted in a surface flashover from the wall across the
top of the cylinder and to the anode. The dielectric cylinder was then raised until it was
flush with the inner side of the top flange of the demountable ignitron. Further testing
again revealed an extremely low holdoff voltage even without any axial magnetic field.
The extremely low holdoff voltage displayed by the DIG when die dielectric cylinder was
inserted made further experimenting with the cylinder impossible. Modifications of the
geometry are planned for future experiments. These events are theoretically explained in
Chapter IV of this report
Conduction Tests
Critically damped conduction tests were performed mostly on the glass-walled
ignitron and die DIG and to a lesser degree on die NL-2909 ignitron. Basically, die 2909
was used for test runs on the four magnet stand with just a few shots run at vanous
curtents to get an idea of the arc resistance.
In all cases, the arc voltage (and
correspondingly arc resistance) was measured at peak curtent to mle out any inductive
•.,
FIG. 2?
PICTURE OF DIELECTRIC CYLINDER
48
(i.e., di/dt) effects in the differential ignitron voltage. Arcresistancewas calculated by
dividing the measured arc voltage by die measured value of die peak current for diat shot
One problem encountered was diat the two test stands had different inductances and
resistances. Therefore, the current pulse in each ignitron had a different risetime, peak
current, and general shape. When designing the stands, availability took precedence over
inductance. A general figure of 1 \ili was desired and some calculations were done to
try to get close to this figure. The stands were test fired and the oscilloscope traces used
to adjust the bank resistors to critically damp the system. The oscilloscope traces were
then used to obtain all the circuit parameters. Figure 24 shows the basic test circuit for
the critically damped conduction test Figure 25 is a plot which closely cortesponds to
an actual oscilloscope trace for the four magnet stand. The numbers given at die top of the
graph are the capacitance of the banks, the inductance of the particular stand, and the
system resistance (including the stand, the banks, and an approximate ignitron arc
resistance). This shows an inductance for the four-magnet test stand of 1.9 |J.H. The
same method was utilized with the two-magnet stand and the results are shown in
Fig. 26. This stand had an inductance of around 0.56 }iH. The risetime of the current
pulse for each stand was, therefore, different. The four magnet stand had arisetimeof
around 72 jis while the risetime for the two magnet stand was around 42 pis.
The plots of the data points taken for the NL-2909 are shown in Fig. 27. Very
few points were taken and the points at the higher current levels are in question because
of extremely noisy disharges. Needless to say, due to the drop in holdoff voltage caused
by the magnetic field on any metal-walled tube, neither the 2909 or the DIG were tested
widi the magnetic field on. In general, the average value of the arc resistance of the
NL-2909 varied from 8 mCl at 13 kA to 11.5 mii at 79.5 kA. Too few data points
were taken to give strong confidence in these results.
49
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1200
1000-
«
600-
60
100
Peak Current [kA]
0
FIG. 27
20
40
60
Peak Current [kA]
y'arc
^ and R arc OF NL-2909 IGNTTRON
53
Figure 28 shows the effect of peak current on the arc voltage and the
corresponding arcresistanceof the glass-walled ignitron. This graph definitely seems to
show a decrease (as opposed to an increase in the case of die 2909) in die arc resistance
as a function of peak current Beginiung with a value of 10 mii at 12 kA, the arc
resistance for the glass mbe dropped to a single test shot value of 5 mii at 100 kA. More
data points on the effect of an axial magnetic field on the arcresistancewere taken with
dus mbe for two values of peak current A summary of the results is given in Fig. 29.
For peak currents in the range of 12 kA, the minimum measured arc voltage was 84 V
(corresponding to 6.8 mfi) which occurred at a magnetic field value of 0.05 T. For
peak currents of around 50 kA, the minimimi arc voltage was 267 Volts (corresponding
to 5 mii) which mostiy occurred at 0.1 T. In general, the arc resistance for almost all
the data points fell within a limited range of 5 to 12.5 mi^, with most of them falling in
the 7.5 to 10 mli range. In terms of die magnetic field's effect on the appearance of die
voltage. Figs. 30, 31, 32 and 33 show unretouched Nicolet plots of the voltage and
current traces for four increasing values of magnetic field. The smoothing of the voltage
measurement was an anticipatedrestdtof die field application.
Some of the Dynafax photo sequences which were taken of this mbe are shown in
Appendix B. The frames are numbered in order of their recording. If camera timing
signals were obtained, the time from cmrent start for each frame is listed below the
picture. Figures 34 and 35 show oscilloscope pictures of the camera timing signals for
bodi cameras. Figure 34 shows die TRW setup in a 3 frame framing mode and the
Dynafax signal along with the current signal for the shot Altemately, Fig. 35 shows the
TRW camera semp in a 200 pis streak mode and the cmrent signal. Photographs for
each current level were always taken with die same neutral density filter on the cameras.
For example, Dynafax photos taken at the 40 kA level were taken through a neutral
density filter with an N.D. = 1.6 (equivalent to 2.5% transmission). Appendix B.
54
B
E
400-
^
>
300B
B
B
B
>•
200-
100-
0-
1
0
20
1
40
—1
60
1
80
100
Peak Current [kA]
0
n o . 28
20
40
60
Peak Current [kA]
100
V„and R 3rc OF GLASS-WALLED
IGNFTRON
55
600•
•
500«
•
400-
•
#
«
>
b
CI
300 •"
•
>
11
200100 4!
B
B B
1
u T
B
1
1
0.10
0.00
1
B
B
0.20
0.30
1
Axial Magnetic Field [Tesla]
a
•
Ip» 12 kA
Ip=50kA
25
20-
15 O
CS
IO-
si •
I
0.00
I
I
0.10
• ' — 1 —
0.20
0.30
Axial Magnetic Field [Tesla]
n o . 29
Va^ and Rare vs. B FOR GLASS-WALLED
IGNITRON
56
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60
TRW: Framing mode = 3 positive square pidses
DYNAFAX: Framing operation = 4 inverted pulses
GURRENT SIGNAL = large critically damped pulse
n o . 34
GAMERA TIMING OSGILLOSGOPE TRACES:
TRW, DYNAFAX, CURRENT
TRW: Streak mode - bottom square pulse
CURRENT SIGNAL = top inverted pulse
HG. 35
CAMERA TIMING OSCILLOSCOPE TRACES:
TRW, CURRENT
61
Fig. B-1 shows a typical set of photographs for a shot widi no appUed magnetic field.
The plasma after peak current was very diffuse and filled die entire mbe volume. Early in
dus shot, sUghdy irregular patterns of plasma were prevalent There was also more
luminosity near die cadiode and walls early on. Further low field shots (as seen in
Figs. B-2, and B-3) show patterns similar to the no field shots. Again, there were
plasnia patterns and some vertical striations in die volume. As die field approached
0.05 T (which is die general area where die holdoff voltage effect was discovered), die
Dynafax photos began to loose much of dieir luminosity. Figures B-4, B-5, and B-6
show a very different picture of die same value of conduction current. The plasma is
very whispy and does not fill the volume as in lower field shots. The voltage traces at
diis level were also much smoother. Furdiermore, die plasma was significantiy more
confined to the interelectrode region in the later states. In smdying some of die
exposures, the plasma column seemed to sway from side to side within the mbe. One of
die Dynafax series. Fig. B-8, is a shot which self fired instead of being triggered. It is
obvious, that the primary discharge channel formed between the potential shield at the
cathode and the lower, outer edge of the anode. Some theories involving these locations
will be brought up later. The decrease in luminosity and volume of the plasma channel is
confirmed by some of the Polaroid pictures taken with the TRW camera in the framing
mode. Figure 36 shows two test shots of the same current but different values of axial
magnetic field. Streak photographs displayed the same change in luminosity.
The lower inductance of the two-magnet test stand used with the DIG allowed
much higher currents to be obtained for conduction experiments. Currents of up to
225 kA gave the data displayed in Fig. 37. This mbe by far showed the smallest change
in average arc resistance over the tested current range. This tube also showed the lowest
average arc resistance of around 5.5 mii. Even at the 225 kA level, which gave an arc
voltage of 1.2 kV, the arc resistance was only 5.3 mCl. The 225 kA level was obtained
62
TOPPOLOROID: NO FIELD
10 p.s exposures
50 fis between exposures 1 and 2
50 \xs between exposures 2 and 3
BOTTOM POLOROID: 0.035 TESLA
10 jis exposures
20 }is between exposures 1 and 2
50 )is between exposures 2 and 3
FIG. 36
TRW FRAMING MODE PHOTOGRAPHS
63
1500
1000
•
+
+
+
** 4**
500-
*i*
0
• * /
• ' I •
I
• ' I '
50
100
150
>
•
200
I I I
250
Peak Current [kA]
8
76-
O
4-1
1:
3-4
s
+
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•
41- + - H .
+
*
•»•
•
+
10
I
50
n o . 37
I
100
150
Peak Current [kA]
200
250
V_and R arc OF DEMOUNTABLE
IGNITRON
64
at 9 kV on the capacitor banks. Therefore, further testing at higher currents will require
a new stand design. It is hoped that die further smdy of the arc through die ports of diis
mbe will reveal some of the factors affecting arc voltage.
Risetime Tests
There was much concern over the values of arc voltage being measured in this
program. Measurements at LLNL gave much lower values for arc resistance at much
higher currents [13], [14]. The largest difference between their and our testing mediods
was, besides using different igiutrons, that their risetime was much slower. Therefore,
we tested the effects of piUse risetime on the arc voltage of the DIG. Two of die four
capacitor banks were removed and the damping resistors changed to bring the system as
near to critically damped as possible. The same method as described earlier was utilized
to obtain approximate circuit values for this system. A computer simulation of the pulse
for this system, very close to an acmal pulse, is shown in Fig. 38. The risetime for this
system changed from about 42.3 p,s with four banks to about 35.5 |is with two banks.
The same procedure that was done previously was again followed. The maximum
current with this two-bank system was 125 kA at 9 kV. The plots shown in Fig. 37 are
shown again in Fig. 39 with the new points for the two-bank system added to show the
differences. A definite effect can be seen, as most of the measured arcresistancesfor the
two-bank system arc higher than for the four-bank system, regardless of die current. The
higher arc resistance averages out to be 7.7 mn. The arc voltage is accordingly higher
for die same amount of current. It stands to reason diat if a faster rising current pulse
caused an increase in die arc voltage, dien a slower rising pulse would give a lower arc
voltage at the same current level. This could possibly explain some of die reasons behind
the high arc voltage values measured in the program as compared to those obtained at
LLNL (e.g., 100 V at 850 kA for an arc resistance of 0.11 mQ).
65
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1500
1000hm
500-
100
200
300
Peak Current [kA]
+ = shots with 4 banks
• = shots with 2 banks
Peak Current [kA]
n o . 39
Varcand R arc OF DEMOUNTABLE
IGNITRON WITH 2 AND 4 BANKS
CHAPTER IV
INTERPRETATION OF RESULTS / CONCLUSIONS
Voltage Holdoff Tests
The lowering of the holdoff voltage of an ignitron by an axial magnetic field
represented an explanation for some problems encountered in industry. There were
reported problems of ignitrons breaking down when a large current was triggered near
the mbe. Obviously, the magnetic field caused by the current was enough to lower die
breakdown voltage of the tube. In general, igiutrons which are supposed to behave
normally and hold off their full voltage continually should not be placed near large
currents. If it is not possible to separate the mbe from the current path, then either
magnetic shielding or a symmetrical current path inrelationto the mbe should be used.
The reduction of the breakdown voltage in the presence of an axial magnetic field
is caused by the deflection of primary electrons of suitable energy into a cycloidal motion
aroimd the anode, as depicted in Fig. 40. For the ignitrons with a metal wall (the 2909
and the DIG), the conditions for this effect are ideal in the anode-wall gap, where
electrons accelerated by a radial electric field are exposed to a perpendicularly oriented
axial magnetic field. Figures 41, 42, and 43 show the equipotential lines of the
potential distribution for the 2909 mbe, the demountable mbe, and the glass-walled tube.
The calculations were done on a Macintosh II computer using the "MacPoisson" code
[15]. Both steel-walled mbes have the maximum field strength in the radial direction
between the anode and the wall. Due to the smaller anode-wall spacing of the DIG, the
density of equipotential lines in the gap is about 2 times higher than for the 2909 tube.
Test calculations have shown diat the field distribution in the demountable tube is not
significantiy affected by die openings for the viewports. In contrast to die tubes widi
67
68
Steel wall (Cathode)
(1)
7(2)
(1) Electron path for B-rield = 0
(2) Electron path for B-field for min. voltage
(3) Electron path for high B-field values
n o . 40
SKETCH OF ELECTRON TRAJECTORIES IN
GROSSED ELECTRIC AND MAGNETIC FIELDS
69
FIG. 41
EQUIPOTENTIAL LEVELS OF THE
FIELD DISTRIBUTION IN THE 2909
TUBE
70
^y^y^y^yK
wall
n o 42
EQUIPOTENHAL LEVELS OF THE
FIELD DISTRIBUTION IN THE
DEMOUNTABLE TUBE
71
POTENTIAL SHIELD
FIG. 43
EQUIPOTENTIAL LEVELS OF THE FIELD
DISTRIBUTION IN THE GLASS-WALL TUBE
72
metal walls, die vector of die electric field (normal to die equipotential levels) in die
glass-walled mbe is predominandy oriented in die axial direction widi die highest radial
component near die lower potential shield. The radius of gyration of electrons moving
widi die speed, v, perpendicular to a magnetic field widi die flux density, B, is given by
R=-?^
.
(3)
It is interesting to note that the proper gyroradius for a cycloidal motion, as depicted in
Fig. 40, is only possible for electron energies in the range of some eV's to some ten's of
eV's, which is in the range of the ioiuzation energy of Hg molecules. For an assumed
mean electron energy of 8 eV and a magnetic flux density of 0.005 T, die obtained
gyroradius is 1.9 mm, which is about 15-30% of the gap-widdi of the metal-walled
mbes. Hg-ions are not contributing to this effect at all since their gyroradius is more than
6 orders of magnitude larger, because of their higher mass. The electron path is
determined by the superposition of the rotational motion in the magnetic field and the
radial acceleration in the electric field. The electron's kinetic energy is increased due to
radial movement towards the anode and subsequendy decreased by elastic and inelastic
collisions with Hg molecules. The Hg vapor pressure in the insulating mode at room
temperature is about 10"^ Torr. Therefore ignitrons operate in the holdoff mode on the
left side of the Paschen curve. The cycloidal motion of the electrons in the magnetic field
of proper magnimde therefore leads to a effective increase of the crossed gap distance and
a corresponding increase of the (pd) product determining the breakdown voltage. There
are references which discuss the effect of a crossed magnetic field on the Paschen
characteristics of certain gases or on the arc voltage between electrodes in vacuum [16],
[17]. One reference gives an analytical expression for die effective (pd) increase due to
die magnetic field [16]. The increase of die holdoff voltage above die observed minimum
level for higher magnetic fields can be explained if die electron padi ^3 in Fig. 40 is
73
considered. In this case the high magnetic field effectively hinders the electrons from
crossing the gap, providing magnetic insulation. This principal explanation of die
reduction of the holdoff voltage can also be applied to the glass-walled mbe. However the
field calculation (Fig. 43) shows that only a small radial field component exists at the
lower potential shield The onset of the predischarge development is therefore likely to
take place in this region. In fact, stable glow discharges could be observed at this place
before the main discharge with the magnetic field either on or off. Additionally, when
self breakdown of the mbe occurred, it was between the anode and the top of die potential
shield. The small radial component of the electtic field and the larger overall gap distance
explains the lower sensitivity of the glass-walled mbe to magnetic fields. The obtained
results show that the breakdown is dominated by the collisional ionization processes of a
Townsend mechanism and not by the mechanisms of a high vacuum discharge. In the
latter case the electron energy would be so high that the gyroradius would be larger dian
the anode-wall gap.
The insertion of the dielectric (nylon) cylinder into the demountable ignitron
(DIG) dropped the holdoff voltage of the mbe down to an extremely low level. It was
reasoned that the increase in field intensity caused by the dielectric mismatching of the
cylinder resulted in the lower holdoff. Figures 44,45, and 46 show a closeup panial
cross-section of the top comer of the DIG with a plot of the equipotential lines at 10%
intervals. Note that the height of the dielectric cylinder caused very litde difference in the
equipotential lines. This corresponds to ourresultsdiat die height of die insert in the tube
did not affect the lowered holdoff voltage. In smdying die change in electric field caused
by the cylinder, the geometry of the scene is shown in Fig. 47. In the absence of any
surface charge, the normal components of the flux density, D, are continuous at the
boundary between die vacuum of die ignitron and the dielectric cylinder. This implies
diat the normal components of the elecoic field intensity are discontinuous and related as
74
ANODESTUD
WALL
FIG. 44
CLOSE VIEW OF SECTION OF
DEMOUNTABLE TUBE
75
ANODESTUD
WALL
n o . 45
CLOSE VIEW OF SECTION OF
DEMOUNTABLE TUBE WITH SPACED
DIELECTRIC CYLINDER
76
ANODESTUD
WALL
RG. 46
CLOSE VIEW OF SECTION OF
DEMOUNTABLE TUBE WITH FLUSH
DIELECTRIC CYLINDER
77
ANODE STUD
GLASS INSULATOR
Dimensions in inches
REGION 1
Cr = 1
n o . 47
'
REGION 2
er = 4
REGION 3
e- = 1
DEMOUNTABLE - CYLINDER GEOMETRY
78
Di=D2
I^
e„Ei = e ^ E 2
.
(4)
Thus, die electric field intensity in die vacuum region is four times die intensity diat is in
die dielectric cylinder. Using diis diought process and summing die voltage drops across
die duree regions shown in Fig. 47, die DIG should stiU have held off 14 kV. Therefore,
diere must also have been odier processes at work. There is a possibility diat die surface
of die dielectric cylinder was being charged, in which case the problem becomes more
complicated to analyze, especially since die amount of charging cannot be determined.
Furthermore, secondary electron emnrission from die dielectric could cause gas to evolve
from the surface which could possibly cause Townsend breakdown to occur.
Conduction Tests
The conduction tests showed the effea of peak current and axial magnetic field on
die arc voltage of the test ignitrons. In the case of the glass-walled ignitron, an increase
in the peak current conducted generated a decrease in the arc resistance. In the case of the
two metal-walled ignitrons, an increase in peak current brought about a small increase in
arc resistance. The magnimde of theresistanceof any plasma depends on the frequency
of electron collisions. It is also known that the resistivity of a plasma decreases with
increasing plasma temperature because the time between collisions increases widi
temperature. Therefore, any increase in plasma resistivity implies a cortesponding
decrease in plasma temperature [18]. Furtherresearchon plasma parameters could check
diis argument The axial magnetic field did seem to cause a small drop in the arc voltage
in the glass-walled ignitron. Thisreductionin arc voltage (and dius arc resistance) was
more pronounced at higher peak curtent levels. Figure 48 displays the superimposed
waveforms for two shots at the same bank voltage (5.34 kV) but at different values of
axial magnetic field. As can be seen, the test shot with 0.05 T applied, (B), has a
smoother voltage waveform and a higher peak current waveform. The curtent waveform
79
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80
for shot (B) also looks slighdy more damped dian die no field wavefortn. This implies a
decrease in circuit resistance (i.e., arc resistance). Calculated arc resistances show diat
die arc resistance was 6.89 mii widiout any applied field and 5.41 mn with 0.05 T
applied.
The optical investigations of the glass-walled igiutron showed two major areas of
change. In general, with an axial magnetic field, the conduction plasma displayed a more
centered, uniform nature (much like a column). Additionally, the application of around
0.05 T or greater magnetic field made the luminosity of the plasma decrease as compared
to a no field shot Oie possible explanation of this is in the way a magnetic field interacts
with electrons or actually how it affects the electron velocity distribution. The magnetic
field decreases the number of high-velocity electrons in the distribution normal to the
field. As a result, the resistivity is higher in the direction perpendicular to the field lines
dian it is in the parallel direction where the magnetic field has no effect. This change in
resistivity would seem to effect the motion of the conduction electrons of the
plasma [18]. Additionally, since the velocity of the electrons is being influenced by the
magnetic field, it isreasonableto say diat their energy is being affected also. The change
in the energy of the conduction elecODns caused by the magnetic field probably affects the
number of ion-electron collisions which radiate, thus affecting the luminosity of the
plasma.
Risetime tests
The effect of die risetime (i.e., time to peak cmrent) on die arc voltage may
explain the wide range of arc voltages reported in various journal and conference
proceedings. By increasing our risetime by 16%, the average arc resistance was
increased by 30 to 40%. This effect is not taken to be a linear one. but it is safe to
assume diat a decrease in the risetime of the curtent will cause a decrease in the arc
81
resistance and correspondingly a decrease in the arc voltage. Therisetimefactor could be
related to the fonnation time of die conduction plasma.
Glass-Walled Ignitmn Fflilim>
After approximately 150 shots at varying peak currents of up to 100 kA, the
glass-walled ignitron failed to hold off voltage consistendy. Testing revealed diat die
mbe would hold off a few kV after some time. This behavior indicated that the mbe had
become gassy. It is known that if die curtent through a gassy mbe is allowed to flow for
some period, the gas can be removed and the mbe performance will improve temporarily
[2]. This is an example of the mercury vapor acting as a gettering material in die vacuum.
One paper has stated previous work with glass-walled mbes [7]. In this paper, oxygen
and nitrogen were foimd to be the major constitutents in most gassy glass-walled mbes.
The glass walls are a major source of oxygen and high current discharges tend to release
this oxygen. This mbe also experienced a wetted ignitor. The ignitor-cathode resistance
dropped to 0.3 CI but the mbe was still being triggered by the ignitor trigger generator.
Closing Remarks
This report has described new research into a wide variety of ignitron
characteristics. Furthermore, concentrated research into the effects of an axial magnetic
field on the holdoff voltage, arc voltage, and visible plasma characteristics has been
reported. The change in luminosity caused by the axial field could indicate a lesser
degree of harmful plasma near the wall of the ignitron and thus a new method to improve
die lifetime. The field also smoodied out die differential voltage across the tube which
indicates a more uniformly conducting plasma. However, die decrease in the holdoff
voltage of die metal-walled mbes when confn)nted widi an axial magnetic field makes an
externally applied field unrealistic. Applying the magnetic field simultaneous with the
82
conduction cmrent coidd prove to be very beneficial. This method has been used widi
great success for vacuum interrupters [8], [19]. The axial field did reduce die arc
resistance in the higher current shots. Inserting a dielectric cylinder into a metal-walled
ignitron lowered die holdoff voltage to well below the expected levels. A combination of
increased electric field intensity in the vacuum and possible surface charging of the
cylinder may be the reasons for this. If these factors could be overcome, the dielectric
insert could prove useful in shielding the wall from the discharge plasma, thus alleviating
one source of outgassing (i.e., the wall) but possibly providing another source of
outgassing (i.e., the insert) and also stopping the arc transfer to the wall. Therisetimeof
the cmrent (time to cmrent peak) does affect the arc voltage at cmrent peak. A faster
risetime allows less time for a fully conducting plasma formation which means a higher
resistance and thus a higher arc drop.
REFERENCES
1.
J. Slepian and L.R. Ludwig, "A New Mediod for Initiating die Cathode of an
Arc," Transactions of the A.IH.F.. vol. 52, pp. 693-700, June 1933.
2.
National Electronics, Industrial Tubes bv National: Operation and Maintenance
Hints for Ignitrons. Thvran-ons. and Rectifiers, pp. 1-14.
3.
General Electric Tube Products Department, Ignitrons: Capacitor Discharge and
Crowbar Service, publication PT 57B, October 1974.
4.
T.R. Burkes, M.O. Hagler, M. Kristiansen, J.P. Craig, W.M. Pormoy, and
E.E. Kunhardt, "A Critical Analysis and Assessment of High Power Switches,"
submitted to Naval Surface Weapons Center, pp. 49-67, September 1978.
5.
T.F. Turner and H.S. Buder, "Performance of Ignitrons in Pulse Service,"
Proceedings of the Seventh Svmposium of Hvdrogen Thvratrons and
Modulators, pp. 328-347, May 1962.
6.
H. de B. Knight, L. Herbert, and R.C. Maddison, "The Ignitron as a Switch in
High-Voltage Heavy-Cmrent Pulsing Circuits," Proc. of lEE. vol. 106, pt. A,
supplement n2, pp. 131-137, April 1959.
7.
D.B. Cummings, "Ignitron Discharge Growth During High-Curtent Pulses,"
IEEE Transactions of Communications and Electronics, vol. 82, pp. 514-523,
September 1963.
8.
S. Yanabu, S. Souma, T. Tamagawa, S. Yamashita, and T. Tsutsumi, "Vacuum
Arc Under An Axial Magnetic Field and Its Interrupting Ability," Proc. of TEE.
vol. 126, no. 4, pp. 313-320, April 1979.
9.
CW. Kimblin and R.E. Voshall, "Interruption Ability of Vacuum Inten^pters
Subjected to Axial Magnetic Fields," Proc. of lEE. vol. 119, no. 12,
pp. 1754-1758, December 1972.
10.
H.A. Steinherz, Handbook of High Vacuum Engineering. Reinhold Publishing
Corp., New York, 1963.
11.
Proceedings of die fu-st "Workshop on High-Power, High-Coulomb Ignitrons,"
Texas Tech University, Lubbock, Texas, April 21, 1987.
12.
Proceedings of the second "Workshop on High-Power, High-Coulomb
Ignitrons," Lawrence Livermore National Laboratory, Livermore, California,
April 21, 1988.
13.
R. Kihara, "Evaluation of Commercially Available Ignitrons as High-Current,
High-Coulomb Transfer Switches," Proceedings of the IEEE 6th Pulsed Power
Conference. June 1987.
83
84
14.
D.B. Cummings, R. Kihara, and K.S. Leighton, "High Curtent Ignitron
P^^^^oPJjen^" Proceedings of the 18th Power Modulator Svmposium.
June 1988.
15.
J.R. Cooke, D.C. Davis, and E.T. Sobel, MacPoisson: Finite Element Analvsis
and Poisson's Egnarinn with the Macintosh. Cooke Publications, Idiaca, NY,
April 1987.
16.
A.E.D. Heylen, "Paschen Characteristics of Gases in a Grossed Magnetic Field,"
Gaseous Dielectrics IT. edited by L.G. Christophorou, Pergamon Press,
New York, pp. 160-167, 1980.
17.
J.G. Gorman, G.W. Kimblin, R.E. Voshall, R.E. Wien, and P.O. Slade, "The
Interaction of Vacuum Arcs with Magnetic Fields and Applications," IEEE
Transactions on PAS, vol. 102, no. 2, pp. 257-266, February 1983.
18.
S. Glasstone and R.H. Lovberg, Controlled Thermonuclear Reactions. D. Van
Nostrand Company, Inc., New York, 1960.
19.
S. Yanabu, T. Tsutsumi, K. Yokokura, and E. Kaneko, "Recent Technical
Developments of High-Voltage and High Power Vacuum Circuit Breakers,"
Xnith International Svmposium of Discharge and Electrical Insulation in
Vacuum. Paris, vol. 1, pp. 131-137, June 1988.
APPENDICES
A. SCHEMATICS
B. DYNAFAX PHOTOS
C. RELATED INFORMATION
85
APPENDDC A
SCHEMATICS
Ignitron Trigger Generator
The schematic of the trigger generator is seen in Fig. A-1. It produces a high
voltage pulse adjustable from 600 V to 5 kV of approximately 1 \is duration. The pulser
is triggerable by a T.T.L. compatible input (2-5 V) widi a minimum pulsewiddi of 20 ns.
The delay of the generator is approximately 150 ns from input to output. The output is
isolated from ground by a 1:1 turns ratio pulse transformer. The output pulse is
generated by switching a charged, 0.6 jiF, capacitor into the pulse transformer. The
switch used is a cold cathode switch mbe called a KRYTRON. This provides a fast
risetime at the output (0 - 5 kV in 30 ns). A very fast SCR is used, via a small pulse
transformer, to trigger the Krytron. A type GB301A SCR which has a switching time of
10 ns, was used for this. This generator provides a low jitter reliable trigger for the
ignitron.
Differential Probe Amplifier
The differential amplifier, a schematic of which is seen in Fig. A-2, is basically of
a standard instrumentation amplifier design. The operational amplifiers used were type
LF357's, which have a slew rate of 50 V/|is. The input impedance for each channel of
die amplifier is 1 MCI in order to match properly widi die high voltage probes used. A
combination output stage (LF357 & LH0002) provides a signal capable of driving a
50 CI line. Both inputs and the output have compensating networks to preserve the pulse
characteristics of the input waveform. The inputs are balanced by an internal trimming
potentiometer. The overall response time for the amplifier is 100 ns.
86
87
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89
Fiber Ontic System
The schematics for the two components comprising this system are shown in
Hgs. A-3, and A-4. The system consists of areceivermodiUe and a transmitter module,
each of which has two identical channels. The transmitter module accepts a 2 to 5 V
trigger signal and outputs a fixed 100 ns light pulse via an infrared emitter. The
transmitter can only beretriggeredafter the completion of die previously triggered event.
Additionally, die transmitter is triggered on die leading edge. Therefore, it is insensitive
to pulsewidth variations at the trigger input The transnutter's optical output is then
coupled to the photodiodes at the receiver via 0.5 mm fiber optic cable. The receiver
module receives the optical signal from the transmitter and converts this information to a
2 to 5 V output trigger pulse. Again, the receiver is sensitive only to the turn-on phase
(leading edge) of the incoming signal. It then produces a pulsewidth at the output
independent of the incoming signal. This method insures against false triggering due to
low level optical signals.
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APPENDDC B
DYNAFAX PHOTOGRAPHS
The following pages display some of the test shots of the glass-walled ignitron
taken with the Dynafax camera. Each page is a single conduction test with multiple
exposures taken at intervals over the 300 |is conduction period. The approximate time to
the center of each 3 us exposure is displayed below the photograph if camera timing
signals were obtained. The time between the center of each frame for different shots
varied from 45 ^is to 55 us because of variations of the Variac voltage applied to the
camera (which varied the speed of the camera). All shots of like curtent were done at
identical opticalfilteringregardlessof applied magnetic field. Shots at different current
levels are usually at slighdy modified filtering. All shots were done with the two-magnet
test stand which had a time to peak curtent of around 72 |is.
92
93
TIME TO CENTER OF FRAME FROM CURRENT START
#1 = 5 us
#3 = 95 .us
#5 = 135 us
#7 = 275 us
#2 = 50 ^Is
#4 = 140 lis
#6 = 230 ^ls
#8 = 320 [is
HG B-1
DYNAFAX PHOTO SEQUENCE FOR Ip = 40 kA.
B = 0 TESLA SHOT
94
TIME TO CENTER OF FRAME FROM CURRENT START
#1 = 0 ^s
#3 = 90 )is
#5 = 180 ^is
#7 = 270 \is
#2 = 45^15
#4 =135 MS
#6 = 225 pis
#8 = 315)is
HG. B-2
DYNAFAX PHOTO SEQUENCE FOR L = 40 kA,
B = 0.01 TESLA SHOT
95
TIME TO CENTER OF FRAME FROM CURRENT START
#1 = 55 i)s
#3 = 155 .us
#5 = 260 MS
#7 = 365 \is
#2 =105 MS
FIG. B-
#4 = 205 MS
#6 = 310 MS
# 8 = 415M5
DYNAFAX PHOTO SEQUENCE FOR L = 40 kA,
B = 0.02 TESLA SHOT
06
TIME TO CENTER OF FRAME FROM CURRENT ST.\RT
#1 = early light #3 = 90 MS
#5 = 190 MS
#7 = 290 ^LS
#2 = 40 M^
#4 = 140 MS
#6 = 240 \is
#8 = 340 [is
RG. 3-4
DYNAFAX PHOTO SEQUENCE FOR L = 40 kA,
B = 0.035 TESLA SHOT
97
TIME TO CExNTER OF FRAME FROM CURRENT ST.\RT
#1 = 55 MS
#3 = 145 MS
#5 = 235 us
#7 = 325 MS
#2 =100 MS
#4 = 1 9 0 MS
#6 = 280 MS
#8 = 370 Ms
n o . B-5
DYNAFAX PHOTO SEQUENCE FOR L = 40 kA,
B = 0.05 TESLA SHOT
Qv!
-^y^-V:i
Anode
Anode
Cathode'
Cathode-
Anode
-- V Anode
©•••^
Cathode
Calnode.
Anode
Anode
.-r W
( •••» . i.#- .V- fc. i*4\
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...V .... Cathode
^/•>.
Anode
>'-vr
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'"Anode;
./.. -V ." • :
Cathjode... .-. -^..
SELF-BREAKDOWN CONDUCTION PICTURES
FIG. B-6
DYNAFAX PHOTO SEQUENCE FOR Ip
B = 0.06 TESLA SHOT
= 40kA,
or»
FIG. B-7
DYNAFAX PHOTO SEQUENCE FOR L = \C>0 IcA,
B = 0.04 TESLA SHOT
100
SELF-BREAKDOWN CONDUCTION PICTURES
HG B-8
DYNAFAX PHOTO SEQUENCE FOR Ip = 100 kA,
B = 0.06 TESLA SHOT
APPENDDC G
RELATED INFORMATION
The following pages give some empirical equations and some other figures
relating to ignitron use. All of the equations and figures come from Reference [11] and
represent one of thefirstattempts to collect information on the various models and curves
relating to ignitrons. Most of the information has its source listed with it.
101
SOME EMPIRICAL EQUATIONS AND MODELS
1.
^^^
Number of expected operations multipUer = e***^'^
Rated maximum peak anode voltage
Actual peak anode voltage
, _ Rated maximum peak anode current
Actual peak anode current
_ Rated maximum average current
~
Actual average cunent
or
Rated maximum energy switched
_ at acmal operating conditions
~
Acmal energy switched
Source:
2.
A. Shulski, Richardson Electronics
312/232-4300
Ignitron Model D (GE 3720 7 A) at 200 kA
(At 100 kA, arc drop - 100 V)
AAAr-^^^^^
Lj + Lconnection - ^^^ ^^
R = 1 m a Ls = 250 nH
Source:
3.
R. Cook, GEM, University of Texas at Austin
Varc=8V + 0 . 3 7 m n * I
(at I<50kA and size D ignittons)
Source:
J. Melton, LANL
505/667-5031
512/471 -4496
103
1,000,000
100,000
-
10,000
-
1000
.
>
I I I mm
100
0.01
0.1
I I I iiMii
1.0
I I I iiiiii
I I I iiiiii
10
100
pd (torr-cm)
A
Hackam
D
>
o
Seddon
Llewellyn-Jones and Galloway
Gusewa and Klarfeld
HG. C-1
PASCHEN CURVES FOR MERCURY
I I I Mill
1000
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107
TABLE 2
HIGH GURRENT, HIGH COULOMB TEST DATA FOR IGNITRONS
Dat:a
Point
Tube
Coul
kA
# Shots
Mcxiel
Source
1
400
100
15-20,000
5553
U-ITL
2
1100
84
9,300
5553
lANL
3
1700
95
13,000
5553
lANL
4
1150
100
42,000
5553
lANL
5
200
600
1
GL37207A
LLNL
6
168
525
5
GL8205
LLNL
7
168
475
2
NL496
LLNL
8
110
300
500
GL8205
LLNL
9
920
414
2
NL1053
Rirharri-son
10
600
54
127,000
5553
LANL
11
700
63
30,000
5553
LANL
12
55
95
37207
LL2^
13
1000
96
190,000+
5553
LANL
14
300
160
15f
5553
LANL
15
300
300
20-50+
37207A
CEM
16
75
330
5+
BK488A
EEV
17
350
40
600+
EK488A
EEV
18
225
233
EK496
EEV
19
100
120
50+
NL488
PI
20
8600
24
20+
21
168
475
6
22
110
300
435
23
800
375
47
24
160
500
25
300
26
1000
Hux^es
GL37207A
LLNL
NL496
LLNL
NL1058
Richardson
110
8205x100
LLNL
340
100
1058X100
Maxwell
300
300
97
1058x100
Maxwell
27
600
325
74
1058x100
Maxwell
28
709
151
1
8205
UY a t A u s t i n
29
2171
306
1
8205
UT a t A u s t i n
30
300
340
1058x102
Richardson
212+
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