VIII.
A.
TUBE RESEARCH AND DEVELOPMENT
MAGNETRON DEVELOPMENT
Dr. S. T.
Martin
A. G. Barrett
1.
Testing and Design of High-Power 10. 7-Cm Magnetrons
MF-8B magnetron was baked out at 600'C for 91 hours and attained a pressure of
-6
3.5 x 10
mm Hg at this temperature. After checking the direct current emission,
the tube was connected for pulsed operation with the Model 16 rotary gap modulator
and later with the 20-Mw modulator.
Performance in both cases was poor, with a maxi-
mum output of 0. 85 Mw for the following conditions:
Anode Voltage
24. 8 kv
Magnetic Field
1450 gauss
Current
128 amp
Pulsed Duration
1. 0 4sec
PRF
350 pps
Operating time for the tube while still undergoing tests on the pumps totaled 9 hours.
A leak at the window metal-ceramic seal occurred while pressurizing the output test
tank to reduce sparking and ended further tests.
Dissection of the window disclosed
that the metal-to-ceramic bond was structurally weak,
vacuum-tight.
although it had originally tested
Only 15 psi pressure was required to cause failure.
Cutting the tube open revealed
damage that was apparently due to
overheating.
In addition, the kovar
window cup was pushed into the copper output transition piece by the
window jig with enough force to make
Z
0
an undesired diffusion joint.
UJ
This
radical change in the internal output
UJ
C'
circuit geometry accounted for much
of the poor performance of the MF8B magnetron.
As a result of MF-8B experience,
tests were conducted to determine
Lile proper tiesI
PRESSURE psi GAUGE
HYDROSTATIC
UI WIIUUW
iau
m
WiLLI
AI-200 ceramic when subjected to
Fig. VIII-I
Deflection vs. static load, AI-200 ceramic
window.
-33-
static loads parallel to the window
axis. Deflections of the ceramic
(VIII.
TUBE RESEARCH AND DEVELOPMENT)
disk were measured as functions of applied hydrostatic loads for two window s using the
arrangement shown in Fig. VIII-I. At 110 psi gauge the difference in deflections for
one window was 2. 7 mils, and 3.5 mils for the other window.
window are given in Fig. VIII-I.
The curves for the latter
Two windows were loaded to the breaking point.
Fail-
ure occurred at 120 psi in one case and at 155 psi for the window of Fig. VIII- I .
Figure VIII - 2 is a photograph of
the window of Fig. VIII - 1 taken from
the vacuum side after rupture.
Ink has
been flowed onto the window to bring
out the cracks.
In this case the metal-
to-ceramic bond had adequate strength
and failure occurred in the ceramic
disk.
On the basis of these tests all windows are now load-tested and then
again vacuum-tested before acceptance
for assembly to a magnetron.
Each
window is subjected to a load of 110 psi
for one hour.
Fig. VIII-2
During magnetron oper-
ation pressure in the test tank will be
AI-200 ceramic output window after load test.
limited to 90 psi so as not to exceed
the total pressure of the load test.
The gold-wire diffusion method of attaching the window to the magnetron has produced vacuum -tight seals each time it has been used.
The seal is now made before
mounting the cathode and the end plates by maintaining the tube at 750°C for 90 minutes
in the hydrogen furnace.
The jig is removed after firing and the tube is ready for final
assembly.
MF- 9B magnetron was the first tube to be assembled with the latest techniques and
construction sequence.
This tube appeared to bake out rapidly on the pumps, but leaks
developed in the glass portion of the vacuum system and in the end plate structure of the
tube and prevented completion of the processing cycle.
This tube has now been set
aside for rebuilding.
Two attempts have been made to rebuild MF- 8B magnetron; the second is still in
process.
The first attempt, designated MF-8BR, failed when the ceramic disk in the
window cracked during assembly to the tube.
The second attempt, designated MF-8BS,
is in the window-assembly stage.
Parts are on hand for assembling MF-10B magnetron, the last tube of the oxide
'cathode series.
Revision of the drawings for these magnetrons has been completed.
-34-
(VIII.
B.
TUBE RESEARCH AND DEVELOPMENT)
MICROWAVE TUBES
L. D. Smullin
G. Guilbaud
C.
Prof. L. J.
Chu
H. Haus
L. Roberts
Prof. J.
Thomas
J.
Houston
H.
E. Rowe
Krusemeyer
L.
Stark
E.
A. W. Boekelheide
1.
a.
M.
H. J.
E. Muehe,
Jr.
Noise and Space Charge Waves
Small signal theory for one-dimensional flow
Technical Report No. 207 has been prepared for publication.
L. D.
b.
Smullin
Experimental study of noise on election beams
The apparatus described in the last report has been put into operation and preliminary results have been obtained on the noise- standing wave ratio in the electron beam.
A typical example of the general type of standing-wave pattern obtained by moving
the first cavity along the electron beam and measuring its output noise power is shown
in Fig. VIII-3.
Similar data have been taken for a range of beam voltages from 500 to
0
DISTANCE
IN
CM
Fig. VIII-3
Noise current standing wave on electron beam.
current < 20
pa, P = 10-6 mm, B = 325 gauss.
-35-
V
= 1500 v, I
o
= 3.25 ma, intercepted
(VIII.
TUBE RESEARCH AND DEVELOPMENT)
2000 volts, a range of beam currents from 1 to 5 ma, and a range of magnetic fields
from 200 to 800 gauss.
The plasma wavelengths obtained in this way have agreed very
closely with the calculated values.
One notable feature of this curve is the difference in the magnitude of the various
minima; from the theory it would be expected that all the minima would be the same
size, and, of course, that all the maxima would be the same size.
This difference in
the size of the various minima is believed to be caused by partition noise generated by
very small amounts of current striking the front of the measuring cavity; the intercepted current varies as the cavity moves along the beam, and, as would be expected
from the analysis carried out for partition noise at low frequencies, the partition noise
increases with the intercepted current.
It appears that the intercepted current must be reduced to a small fraction of 1 percent of the beam current before partition noise can be ignored.
In our preliminary
measurements the intercepted current was 1 to 1.5 percent and the SWR was approximately 7 db.
The data in Fig. VIII-3 were obtained with the intercepted current less
than 0.5 percent.
Present work is directed toward reducing interception to less than
0.1 percent of the beam current.
The vacuum in the tube becomes important when it
is required that less than 0. 1 percent of the electrons suffer any collisions, and pressure less than 10
-6
mm Hg must be maintained.
H. E. Rowe
c.
Traveling-wave tube design
A demountable,
wave tube designs.
continuously pumped system is being built for testing travelingA shielded electromagnet encloses a drift tube 2.25 inches in
diameter and 16 inches long, within which helices, velocity jump sections, and other
electrodes can be located and moved as desired.
A shielded housing is provided for
the electron gun.
2.
C. E.
Muehe,
L.
Roberts
Operation of Pulsed Magnetrons into a High-Q Load
The measurements described in earlier reports,
comparing the operation of the
magnetron with ballast loads of different impedance with the operation of the magnetron
using the switching device have been completed.
A technical report is being prepared.
H. J.
3.
a.
Krusemeyer
1-MEV Pulsed Electron Source
Tube
Mechanical construction difficulties have delayed the completion of the Pierce-gun
type, high-voltage diode but it is now in the final assembly stage.
culty was in spinning the large cup-shaped cathode electrode.
-36-
The principal diffi-
This was finally achieved
(VIII.
TUBE RESEARCH AND DEVELOPMENT)
with the use of sheet nickel, 0.037 inch thick before spinning.
In addition, it was found
that the kovar anode disk, pictured in the Quarterly Progress Report, January 15,
1951,
deflected enough to break the glass seal (8 5/8 inches in diameter) when the tube was
evacuated.
This condition was overcome by spinning the kovar into a spherical shape.
An oversize bake-out oven and vacuum system have been completed for processing
the tube.
Tests on a beryllium electron window,
0.003 inch thick, have shown this to be the
ideal material if such a thin sheet can be sealed with a vacuum-tight seal.
Its energy
absorption is low, its thermal conductivity is high, and it has sufficient tensile strength
to withstand the tube bake-out at 4500C with no noticeable creep.
However, all samples
obtainable to date have had too rough a surface for a vacuum-tight gasket seal with
either a copper or a gold gasket.
A strong braze joint was obtained between the sheet beryllium and nickel-plated
stainless steel, with the use of BT silver solder and a titanium hydride reducing agent.
However, oxide pockets in the beryllium were reduced in the process, thus forming
small vacuum leaks.
Because of these difficulties with beryllium the use of a stainless steel window
0. 001 inch thick is planned for the first tube.
of beryllium may be carried on later.
Further research on the possibilities
It is calculated that the stainless steel window
will reach a temperature of 6500C at the desired duty ratio of 1:2000.
A sample win-
dow was held at this temperature for several hours with no sign of creep or vacuum
failure.
Metallic zirconium is being used as a getter in this tube.
One piece will be attached
to the cathode heat shield, where it will operate at 300 C to 400 0 C, the range of maxi0
mum hydrogen absorption.
Another piece will be attached to the cathode cylinder itself
where it will absorb other gases at approximately 800°C.
A.
b.
W. Boekelheide
Modulator
The modulator has been completed and tested with a 1-p.sec pulse and repetition
rates varying from 80 pps to 400 pps.
primary of the pulse transformer,
Approximately 25 kv can be delivered to the
corresponding to 1. 25 Mv at the secondary.
At
higher voltages corona develops along the glass reservoir bulb of the thyrotron in the
vicinity of the anode lead flare which, of course, defines the upper limit.
Other com-
ponents in the modulator are capable of higher voltages but to use them to full capacity
would mean a series-parallel hookup of four thyrotrons.
It is not planned to include
these extra tubes at this stage.
The 75-kv, 0. l-pf capacitor had to be replaced by a pulse-forming network.
The
capacitor was found to break down at voltages in excess of 15 kv.
L.
-37-
Roberts
(VIII.
TUBE RESEARCH AND DEVELOPMENT)
4.
Spiral Beam Reflex Oscillator
Figure VIII-4 shows a cross section of a new type oscillator.
It consists of a TE
resonant cavity with an electron beam projected along the axis (perpendicular to the
E field).
An axial magnetic field is also provided, as shown.
It was observed that
with the beam collected on electrode R, the Q of the cavity was greatly reduced with
the magnetic field adjusted to cyclotron resonance at cavity frequency. (See L. P.
C.
35,
I. Shulman:
Smith,
Frequency Modulation and Control by Electron Beams, Proc. IRE,
644, July 1947.)
If electrode R was held at cathode potential, or lower, the system
oscillated at the cavity resonant frequency, with the same magnetic field as above.
With
a beam of about 10 ma at 700 volts, the power output was about 30 mw at 3000 Mc/sec.
Further experiments will be made to determine the effect of variations in the shape of
the reflector field, and beam impedance.
5.
J.
D. Smullin
Cut-off Frequencies of Single and Multi-Filar Helices
The cut-off frequencies predicted by S.
No.
E. Thomas, L.
Sensiper (forthcoming Technical Report
194) have been experimentally verified.
The results will be presented in Technical
Report No. 208.
6.
L.
Stark
A New Gaussmeter
A new type of gaussmeter is being developed.
magnet mounted on jewel bearings.
It consists of a small permanent
The natural period of the magnet is
in the static magnetic field being measured.
determined
A small coil of copper wire, driven by
an audio oscillator, is mounted so that it sets up an a-c magnetic field perpendicular
to the field to be measured.
of the magnet is reached.
One varies the audio frequency until the resonant frequency
The impedance of the coil changes rapidly near the resonant
frequency of the magnet, and this change is detected as an unbalance in a bridge circuit.
See Fig. VIII-5.
The resonant frequency of a magnet pivoted in a magnetic field is given by
f
1
BM
where B is the static magnetic field being measured,
M is the magnetic moment of the
magnet, I is the moment of inertia of the magnet, and o is the permeability of vacuum.
2
Thus, B is proportional to f2.
To get a high ratio of M/I, one must use a small magnet
of a strongly magnetic material.
Also, it is desirable to choose a magnetic material
of high coercive force to prevent demagnetization.
A rather crude test model has been constructed and tested.
The magnet consists
of a nearly cubical piece of Alnico V about 4 mm per side, mounted on pivots and
vee jewels.
The driving coil consists of about 1400 turns of No. 40 wire.
-38-
The device
ER
IN
Fig. VIII-4
Cross section of spiral beam reflex oscillator.
LMAGNET PIVOTS ABOUT
AXIS PERPENDICULAR
TO THE PAPER
DRIVING COIL
Fig. VIII-5
Bridge circuit for measuring resonant
frequency for probe magnet.
I.
2.
3.
4
5.
CYLINDER OF COBALT- PLATINUM MAGNETIC ALLOY
SHAFT OF POLISHED CARBALLOY
SAPPHIRE JEWELS
BRASS
DRIVING COILS
Fig. VIII-6 Cross section of oscillating magnet gaussmeter.
(VIII.
TUBE RESEARCH AND DEVELOPMENT)
has been tested over the range of 40 to 1200 gauss.
At 1200 gauss the resonant fre-
quency is 152 cps and the half-power bandwidth is 5 cycles.
The audio driving voltage
applied to the bridge is about 0.4 VRMS and the bridge unbalance at resonance is about
0.01 VRMS.
Both of these voltages seem rather independent of frequency over the
measured range of 20 to 152 cps.
A more refined model having as its magnet a cobalt-platinum cylinder 0.07 inch in
diameter and height is now under construction.
diameter.
It will fit into a probe 0.28 inch in
A cross section of the new design is shown in Fig. VIII-6.
J.M. Houston
-40-