III
JIOD±a
A
"LECTROiIC TECMII
UhS APPLIED TO PHYSICS AND EIGINEERIG
STUDIVS IrADIING TO THE DESIGN OF A MICRO AVE
Staff
ACCELERATOR
Plofessor J C Slater
Dr J Halpern
Dr J H Bostick
M Labbitt
L 0 Maier
J Terrall
S J Mason
M E Van Valkenburg
Since the Final Report under Contract OEMsr-262,
of the Laboratory,
dated June 30, 1946, progress has been made in the various features of design, and
we feel that we have gone as far as nossible by using models involving the various
comnonents, as we have been doing during the past
believe tat
ear
There is good reason to
the accelerator will work, but the final testing of it must be done on
a real system, and for that reason, we are pronosing as a next step the corstruction
of a twenty foot accelerator,
operating with one magnetron per foot, fed by a beam
of two million volt electrons from a belt-driven electrostatic generator, and capable
of accelerating the electrons to a maximum energy of the order of magnitude of thirty
or forty million electron volts
We believe that the main problems which will be
eventually met in the design of a much larger machine, for the billion electron volt
region, will be met in the twenty foot model, so that it mijht well be possible to
p'rss from that model directly to a very large machine
It is proposed that appli-
cation be made for additional funds to construct such a twenty foot machine under
the auspices of the Research Laboratory of Electronics and additional space and
Since the program has
personrel will also be reauested to implement the program
reached a definite stage of advancement, we shall give here a discussion of its
various phases, and their current situation, together with the specific proposal
which we are making for future development
The main problems associated with the
accelerator are the accelerator tube, the electronics of the electron beam in the
tube, and the probable behavior of the emergent beam, the electron gun feeding the
accelerator,
the magnetrons feeding power into the tube, and the method of their
coupling, and the modulators feeding the magnetrons
These features will be taken
up in succession
The Accelerator Tube
The fundamental object of the accelerator tube is
to produce an alter-
nating field, at microwave frequencies, which can be analyzed into a number of
traveling waves, one of which travels with approximately the velocity of the
particles to be accelerated, so that as seen from the particle the wave exerts an
approximately constant force on it,
nroducing continuous acceleration
Only this
wave traveling with the correct velocity is effective in producing acceleration.
The field is fed by a nower source, in this case m;rnetrons
whose power is dissi-
pped in the resistive losses in the wall of the tabe, the
nnwer beinp fed into the
-24-
accelerated par'icles being negligible in comparison
For a given input power,
we naturally %rant to build up the maximum voltage in the traveling vwve which is
in resonance Yith the particles, which we may call the resonant rave
It can b
proved that this voltage E, in volts per meter is given by the formula
= C6
where a is a numerical constant,
.
depending on the geometry of the tube, and of the
order of magnitude of unity in important cases, P is the -ower
input into the tube,
in watts per meter, Q is the unloaded Q of the cavity, and X the wavelength in meters
The quantity a can be com-uted in simple cases, and measured in more complicated
cases
We note fiom this formula the obvious fact that the voltage per unit length
is proportional to tFe souare root of the power in)ut
the field
Furthermore we can see how
in volts per meter, depends on the wavelength
its dimensions, Q will be proportional to
wil] be pronortional to the
-1/4power,
but favoring the short wavelenLths
For a tube scaled in all
, so that for a constant value of P, E
showing small varl tion with wavelength,
Combining this fact iith
the large powers oer
unit length which can be realized with magnetrons, this shows the considerable advantage of the microuave region for linear accelerators, other things being ecual
Two geometrical structures of accelerator tube have been studied
are (1)
the iris tube, and (2)
the reentrant cavity tube
The first consists of a
cylinder, with uniformly spaced partitions a half wavelength apart,
having a circular hole in its center
Fig
1
These
each partition
Such a tube his a resonant mode as shown in
Breaking this standing 'uveup into waves traveling in opposite directions,
l-
---
-g---I,--
---.-- g.--I---g---.--..-
i
Figure 1
we find that by choosing suitable dimensions we can adjust, within wide limits, the
phase velocity of the wave, which of course must be chosen to equal the velocity
of the particles being accelerated
With no iris at all, the phase velocity is
faster than the velocity of light.
As the holes in the irises become smaller and
smaller, the phase velocity decreases, and c-n be made half or less of the velocity
-25-
of light, without serious difficulty, but to make the velocity much less than this,
the holes in the irises must be rather small. We have made experiments on tubes
of this nature, up to twenty half wavelengths long, and have determined the correct
dimensions for securing desired velocities of propagation
shunt resistance and
Q of
We have studied the
these cavities both experimentally and theoretically, and
have studied the mode structure of such tubes experimentally.
We find that for the
Thus a
neighborhood of the velocity of light, the dimensions are very convenient
tube 4" in diameter, with irises with holes 2" in diameter, spaced (10 7)/2 cm
a half wavelength) apart, will have a mode of resonant wavelength of 10 7
This tube forms a
cm, and will have a phase velocity equal to that of light
very satisfactory accelerator for electrons whose velocity is very nearly that of
(that is,
Tor the dimensions given, M has been found both theoretically and experiThus feeding a power of three megawatts per meter into
mentally, and is about 0 5
light
a tube whose
Q is 18,000 at 10 7 cm, will build up a value of 3 of about 7 million
These figures are of
volts per meter, or the order of 2 million volts per foot
the order of magnitude to be exoected in our proposed model, so that an estimate of
30 to 40 million volts for a 20 foot tube seems not unreasonable
Tube No 2, the reentrant cavity tube, is a cvlinder, containing smaller
The tube has
concentric cvlinders, spaced one to a wavelengti, supported by posts
The beam would travel through holes in
a mode similar to that shown in Figure 2
Figure 2
the hollow inner cylinders
Such a structure has convenient dimensions when the
velocity is quite small compared to the velocity of light, but as the velocity increases, the diameter of the inner cylinders decreases, so that near the velocity
of light the holes in the cylinders would be unsatisfactorily small to accommodate
Such a structure is probably the most convenient for nositlve ions, whose
a beam
velocities in the lower energy range would be small compared to that of light, and
We
it is the structure used by Alvarez at longer wavelen&ths for positive ions
are carrying out an extensive set of measurements so as to give camplete design
-26-
information on this tube, as on the iris tube, believing the results to be of
scientific imuortance, but are not contemplating use of the reentrant cavity
tube for the oresent purpose
The Electronics of the Beam
The electronics of a beam of ions or electrons in a traveling voltage
wave of the tye we are considering is not hard to work out
The problem is ouite
different depending on whether we deal with ions or witn electrons, tnough both
cases can be treated as limits of a general problem
The reason for the difference
is that the electrons travel with nearly the velocity of light throaghout their
With positive ions, we find
path, while the ions have much smaller velocities.
that there is a tendency to bunch in the part of the wave in which the accelerating
field is increasing with time.
The reason is that in that case an ion which
happens to be ahead of the bunch will find itself in a smaller field, will not be
accelerated so fast, and will lag behind, rejoining the center of the bunch, while
an opposite situation arises for ions behind the bunch
Thus if ions are started
out in all phases, bunches will automatically be formed in this increasing part of
the wave
At the same time, these ions will be violently defocussed
is that as an ion enters a gap,
it
is
The reason
focussed, and as it leaves the gap it
is
defocussed, as a result of the shape of the lines of force in the gap, as seen in
Fig
2
Since tne field is increasing, the defocussing effect will be greater than
the focussing, and the net effect is mathematically equivalent to a force pushing
the ion away from the axis, proportional to the distance from the axis
This will
result in an exponential sprerding of the beam, unless means are taken to counteract it, and calculation shows that it is so great that the beam will spread out
completely in a very few wavelengths
spreading by means of foil focussing,
the gap is eliminated
Alvarez has prooosed to counteract this
in which the defocussing part of the field in
He feels that this is not practicable at microwave dimensions,
and we are inclined to agree with him, the beam would spread too far, even with
foils, to travel in the small hole in the inner cylinders of Fig
2
Alvarez and
Serber have concluded that at best the beam can be confined to a diameter of something like 2k", which indicates a wavelength of the order of what they are using
An alternative method of focussing which might be considered would be a longitudinal
constant magnetic field, but calculations Indicete that it
value of many thousand gauss to be effective,
difficult
It
would have to have a
so that this method as well seems
is for these reasons associated with the defocussing of the beam that
we are not contemnlating a microwave accelerator for positive ions at present
The electron beam, provided it
forms a much simpler problem
starts wit
If the velocity is
nearly the velocity of light,
substantially that of light from
the beginning, the phase of an electron will not change during its
motion
An
electron which happens to start in the phase given by the peak of the acceleration
will travel down the tube in that same phase, and will pick up an energy of 3 per
meter, where this is the same value mentioned earlier
Electrons starting in other
phases will pick up less, while tnose starting in such a phase as to be decelerated
-27-
will soon slow down, get out of phase on account of tneir differing velocity, and
be lost to the beam
Not only does tne phase of an electron traveling with the
velocity of light remain fixed, but there is no defocussing effect
Even if the
electron is in the phase in which the field is increasing as it goes through the
gap, it turns out that there is a magnetic force acting on it on account of the
magnetic component of r-f field, which exactly cancels the electric defocussing
force
Thus a beam started with the velocity of light will travel down the tube
with continuously increasing energy, and hence electron mass, without focussing
or bunching.
An initial electron velocity of five million electron volts would be
enough to secure the situation described in the preceding naragraph
On tne other
hand, wita an initial velocity of two million electron volts, such as we propose
to use, the situation is not seriously altered
The velocity corresponding to this
energy is approximately 0 98 of the velocity of light
Witn a field of the order
of magnitude of tnat considered, tne acceleration to practically the velocity of
light will occur within about two feet
Exact calculation of the orbits shows
that in these first two feet there will be slight buncning, which has the fortunate
effect of concentrating r-tner more electrons than otnerwise in the phase giving
maximum acceleration, and trere will be slight defocussing, resulting in the most
Since
unfavorable phase in sometning like a doubling of the diameter of the beam
an incident beam of about 1/4
diameter seems feasible, and since the holes in the
irises will be 2", a final diameter of -" seems entirely satisfactory
After the
first two feet, there should be practically no further spreading of the beam, so
that the focussing problems met in a maciine of any length should be faced in a
twenty foot model.
These calculations are without any auxiliary focussing; an
entirely practicable amount of magnetic focussing in the first
concentrate the beam very greatly compared to these figures
electrostatic belt generators
two feet should
Thus in standard
witaout the linear accelerator feature
it is
practicable to concentrate a beam of 1/4" diameter to the order of 0 01",
there
seems to be no compelling reason why the same sort of focussing could not be
carried out in the injection part of the linear accelerator, but it does not seem
necessary for a first design of tube
The Electron Gun
A very suitable electron gun for the accelerator is a belt-driven
electrostatlc generator.
Trump has a well tested design of such a generator, com-
plete with an electron accelerating tube, producing two million volt electrons
The beam produced by such a generator should be about 1/4" in diameter, witnout
focussing, and by means of magnetic focussing a much narrower beam should be
obtainable.
The generator as designed by Trump can be operated horizontally
It seems likely that it will be necessary to Incorporate in the electron source
in the electron gun an arrangement to pulse the grad in such a way that a beam will
be emitted only during the pulses during which the accelerator is activated
-28-
Other-
wise during almost all of the cycle unaccelerated electrons will nass tarough the
tube, and tney will form a background which will swamp the effect of the high
energy electrons coming during the pulse
Preliminary experiments passing a beam
of electrons from a 300 kv generator through a single high Q cavity have shown
this effect
In addition to the electron gun work is underway on the design of
a magnetic analyzer for the outcoming electrons, so as to make measurements of
their velocity distribution
The M~netrons and their Oouplin
The magnetrons which have been used for the work so far have been the
tunable BE7T 1 s produced by Raytheon
These magnetrons have a maximum output of
tne order of one megawatt, at a pulse length of the order of two microseconds, to
obtain tnls output, a relatively mild duty cycle must be employed
Since high
power is very desirable, and since a long pulse length is required by tne time
necessary to build up the oscillation in the h-gh Q cavity, and then to pass a beam
of electrons down tae tube (two microseconds being the Irreduciole minimum for
this, and four or five microseconds being desirable), an improvement of existing
magnetrons would be highly advantageous
It seems likely that the insertion of
the Bartol thoria cathodes in the HT would make a great improvement in that tube
for tne present purposes, and it would be greatly to the advantage of the project
if Raytheon had a service contract for the development of HK7T's with tnoria
cathodes, which unfortunately they do not have at present
We have explored the problems associated with feeding the magnetrons
into the load in great detail
In the first place, tnere is a difficulty in feeding
a magnetron into a single high Q load
circuits tned to the same frequency,
Both the magnetron and the load are resonant
and coupled together, and by well-known
princioles this results in the existence of two modes close togetner, so that tuere
is the likelihood that the magnetron will o-erate in the wrong mode, or that tnere
will be fluctuation between the right and wrong modes
in unstable operation, spar ing, bad spectrum,
etc
This shows itself in practice
By insertion of resistance
effectively in series with the load, Halpern has shown how to eliminate the possibility of operation in the undesired mode
This resistance absorbs some of the
power, so that not all of the power of the magnetron can be fed into the load, we
have succeeded In feeding about 70% of the power into the load, and it does not
seem possible to carry the percentage much higher than this in a reliable manner
The next difficulty comes when several magnetrons have to be fed into
the same load, with the requirement that they be locked together in phase
be shown theoretically that if
It
can
the conductances of two or more magnetrons are
effectively in parallel with each other, which can be secured by suitable adjustment of the lengths and transformer ratios of the outputs from the magnetrons to
the loads, the magnetrons should operate like a single magnetron whose voltage is
that of each magnetron, but whose current is the sum of the currents of the various
magnetrons,
the frequency being determined by the reactive comnonent of the load,
as well as by the average of the resonant frequencies of the various magnetron
By careful study
cavities, with the magnetrons all operating in the same phase
of the output characteristics of the HKM9, Everhart,
Labbitt, and others have
been able to design couplings between the magnetrons and various types of loads,
Two magnetrons have been operated in synchronism
so as to secure this result
into at least three types of loads, quite different physically, but equivalent
Three
electrically to each other, and to the load which we expect to use
magnetrons have been operated smoothly Into a load similar to that which will be
used. After considerable experience, circuits have been designed which are not
very critical, and which are essentially preplumbed, so that not much final adIt is inJustment is required after a magnetron is inserted into the circuit
terepting to observe the behavior of several magnetrons operating in parallel on
the spectrum analyzer. If one of the magnetrons is initially out of tune, it will
be seen to have a separate spectrum from the others
incidence with the others,
As it
is
tuned toward co-
its spectrum will move toward the others, until at a
There is
certain point it will suddenly jump into coalejcence with the others
every evidence that under these circumstances the magnetrons are actually operating
The best evidence comes from an experiment in which two magnetrons were
When such a cavity is fed with high
operated into a single cylindrical cavity
in phase.
power, producing fields of the order of a million volts per inch, which we have
secured, there is enough residual gas and ionization in the cavity so that electrons
are liberated, are accelerated, and produce penetrating x-rays.
The penetrating
power of these x-rays shows that the voltages present in the cavity are of the
order of magnitude which we find from the knowledge of the r-f fields
The in-
teresting observation is now made that when we go from one to two magnetrons
feeding such a cavity, the penetrating power of the x-rays increases, indicating
a real increase in peak voltage
the magnetrons are not locked together, turning
If
on the second magnetron merely increases the intensity of the x-rays
without in-
creasing their penetrating power, indicating the adding of intensities of two noncoherent oscillations.
The sudden jump in penetrating power can be used as an in-
dication of the locking in of the magnetrons,
and it
agrees with the evidence of
the spectrum analyzed
In the final design of linear accelerator, we wish to have magnetrons
spaced uniformly down the tube, approximately one per foot
The spacing adopted is
determined by geometrical considerations; we do not want to have them so close that
there will not be physical space for the magnetrons and their magnets, and the various
modulator components
The spacing of one per foot does not seem to be a limit, pro-
bably by clever design several times as many could be accommodated, but for a first
design we do not wish to push things too far
magnetrons could be fed into such a tube.
There are several ways in which the
In the first place, they could be fed
directly into the tube at equivalent points along its length, such as occur at
intervals of a wavelength (so that we should use a spacing of three wavelengths,
or about 12 6")
Por a short tube,
this is
-30-
certainly possible.
There is a possibility
however, that for a long tube it
may involve difficulties
A tube of the type we
are considering has not only the mode we desire, but as many other modes as there
are segments, distributed through a range of frequencies which is independent of
Thus as the number of segments is increased, the modes
the number of segments
get closer and closer together
The mode next to the one we desire is one in
which one half the length of the tube is operating as we wish, the other half in
the opposite phase, with a transition region between; obviously in such a mode the
electrons would be accelereted through half the tube, decelerated through the other
half, with no net result
correct mode.
Thus it is very important to avoid operation in an in-
If the incorrect mode is far enough in frequency from the desired
one, and the magnetrons are tuned to the desired freauency, there will be no danger
of operation in the incorrect mode, but If the frequencies are very close this
One of the Important results to be obtained from the twenty
danger may be real.
foot model proposed is to test practically whether this difficulty is real or not.
If
it is not, the direct feed of the magnetrons into the tube will almost certainly
If the difficulty comes up, one possible solution will be
be the simplest method
to introduce resistive material into the tube, at such points as to damp out the
This probably will be possible, we have tried it with success with
undesired mode
the reentrant cavity type of tube, as has Alvarez,
but have not yet experimented
with mode elimination with the iris type of tube
If
the mode problem persists,
another more complicated type of circuit
which we have investigated would almost certainly remove it
This is a circuit
in which the magnetrons are fed directly into a long waveguide, and the waveguide
in turn has side arms at suitable intervals, between successive magnetrons, leading
into the accelerator tube
We have investigated the behavior of the waveguide,
three magnetrons, and arms leading into water loads
This circuit locks the
magnetrons satisfactorily into parallel operation, and feeds each of the side arms
in the same phase
If the accelerator tube were fed from this cirulit, since the
side arms would be located every three wavelengths,
it
would feed only those modes
which have the same phase at intervals of three wavelengths,
and these are the
same as the modes of a tube only three wavelengthb long, and which are therefore
very widely separated
In other words, it
seems almost impossible that this scheme
would get Into difficulty with modes, and it will be used if
it is necessary
It
is not actually much more complicated than the direct feed into the tube, and will
very probably be tried out with the twenty foot section in any case
It
seems to
us, therefore, that the chances of meeting serious obstacles in the problem of
feeding even a long accelerator tube with many magnetrons are small
The Modulators
The problem of feeding many magnetrons obviously involves a wholesale
modulator problem
Bostick has been analyzing this problem, and suggesting solutions
The least satisfactory solution would be to use separate complete modulators, triggered
from the same circuit
This is what has been done so far, feeding three magnetrons,
but it is obviously foolish to duplicate in large numbers any parts of the modulator
-31-
which could be common to many magnetrons
The other two schemes suggested by
Bostick involve combined circuits for the power sources, with separate pulse
One of his proposed schemes is
transformers feeding the separate magnetrons.
to use a d-c high voltage meter generator set for the power supply for a number
There
These could operate with a circuit as shown in Fig 3
of magnetrons
AN
d-c
8kv
^1 1----
Modulator Operating 10 Magnetrons from one 20 kw, d-c Generator
Figure 3
is a possibility of securing at a low price five motor generators suitable for
this purpose, each capable of feeding 10 or more magnetrons, so that two of the
five would operate the proposed twenty foot model, and the others could be kept
The remaining proposed scheme is to
as spares, or used for future expansion
use a transformer and rectifier, operating with the circuit of Fig
4
This
scheme must operate at a recurrence frequency of 120 pps, but that value seems
suitable for our purposes, and it has been verified that the 27T will operate
satisfactorily at this recurrence rate
One such transformer, capable of operating
20 magnetrons, exists in the vault in Building 22, but it presumably would be unwise to dismantle that equipment, since it probably will be eventually used as a
The necessary transformers can be built, but Westinghouse auotes a
For this reason it is likely that the motor generators,
delivery date of a year
which could be procured immediately, are the method to use at first, but it is also
modulator
likely that the transformer rectifier method would in the long run be better,
-32-
If7
,
115
60D
Modulator
Employing Power Supply and Rectifiers to Operate at 120 pps
Pigure 4
on account of easier maintenance,
and the fact that in the long run the transformers
would be easier to procure than motor generators (aside from the particularlmotor
generators which happen to be available at the moment)
be wise to carry out parallel developments,
It- therefore will probably
using the motor generators for im-
mediate results, but following up with the transformer rectifier development, so
that it will be familiar when larger machines sbould be designed
III.
B
ULTRASONICS RESEARCH PROGRAM
Staff
Dr
J
J
R
W
0
Kittel
K Galt
R Pellam
A. Rapuano
Roth
Description of Pro.iect
In this program advantage is taken of the radar pulse techniques and
microwave power sources now available to study ultrasonic properties of solids
The principal activity at this time is tne measurement of
liquids and gases
the velocity and absorption of sound waves in different media in the ultrasonic
frequency range
The techniques employed are di~cussed in the June 30th Report
Status
The princiual activities of this group consist of
(a) Measurements of velocity and absorption of sound in liquified
gases including liuid
helium at 15 me/sec and 45 me/sec
(b) Absorption measurements at 200 me/sec
-33-
(c) Microwave transducer deslgn and general transducer theory
(d) Absorption in solids 1 to 100 me/sec
Paramagnetic relaxation experiments using ultrasonic waves at
(e)
15 mc/sec.
(f) Non-linear wave generation in Rochelle Salt
The contents of Technical Reports 4 and 5 on "Ultrasonic Propagation
in Liquids" are scheduled to appear in the October issue of the Jornal of Chemical
Ph3 sics
Low Temperature Measurements on Argon
The eauipment develooed for low temperature work has been used to
(A)
measure the absorution and velocity of sound in liquid argon.
85770 cm/sec at 84 20K, and decreases as temuerature increases
P = Po
a (in
e - xc
)
is
a = 0 015 +50%
The velocity is
The absorption
This checks approximately with the value
calculated from viscosity and heat conductivity
Since the accuracy of the ab-
sorption measurement is better at higher absorptions,
equipment has been set up
to generate and receive r-f pulses at 45 me, so that a more accurate value for
a may be obtained
Since
M/1/2 is constant as frequency varies, the value ob-
tained at the higher frequency should be quite satisfactory
Ligcud Helium
Preparations are now complete for measuring the velocity and absorption
of souna in liouid helium at 15
mec/sec
by the pulse-method
For Hel tnose
measurements should furnish another example of a classical liquid (I
e , monatomic
like Hg and liquid Ar). ,It is expected that the measurements will be carried down
below the X - point (2 190K) where the unusual properties of Hell (the second, or
low temperature phase of liquid helium) can be studied
The low v.scosity and
high heat conductivity of Hell, as compared to the low heat conductivity and
normal viscosity of HeI,
should provide comuacrtive data.
As tnese will be the
sound absorption measurements in Hell they siould contribute information re-
first
garding such controversial subjects as the mass transort phenomenon
It
is
an exrerimental fact that sound propagates with two velocities in
one mode of propagation behaving lime ordinary sound and the other, the so-
HeII,
called "second sound", benaving sonewhat like a "heat wave"
Consideration is
being given to the nossibllities of applying the pulse technique to second sound
in Hell
(B)
200 mosec Proram
A program of resetrch to explore the supersonic range between 200 me and
1000 me has been started
Some equipment nas been built and some measurements of
400 me r-f energy absorption in crystals have been made, but futher wore has been
postnoned pending some measurements of sonic absorptions in various lioulds at 200
me
A superhet receiver and a self pulsed oscillator delivering about 500 watts
at tris
frequency h-ve been bu-lt and are in tie process of being cleaned up.
-34-
(0)
Microwave Transducer
An unsuccessful attemnt was made to excite oulsed sound in a quartz
rod at microwave frequencies (S-band)
The ends of the quartz rod were flat
and Parallel to within an optical wavelength and the faces of tne quartz c-ystal
soldered to the rod were parallel to within 2 seconds of are
In a %raveguide by means of microwave energy, to transmit a
excite the cryst-1l
sound pulse down tne quartz rod, reflect it
microwave energy within the guide
signal pulse
The object was to
from the end and in turn produce
This energy should then apnear as a received
It is believed that the sensitivity of the crystal-rod unit has
been lowered eitner by sound absorption in the solder at tsese frequencies or
due to the baking process invo±ved in the soldering
Further work in tnis region
will be postponed until more experience has been gained at lower frequencies
Transducer Theory
It will include an
A paper on transducer taeory is being orepared
analysis of the niezo-electric relationships for tne transducer, the effect of
beam-shape on sound absorption measurements,
and the limitations ulaced upon
transducer design as tne result of using pulse metnods
(D)
Absorotion in Solids
The use of ultrasonics as a means for investigating the internal structure
of metals has been growing in imoortance in recent years
(rlO
Relatively low frequencies
me max ) were used to inspect castings for flaws, etc
Studies are now under
way to determine the maximum resolving power of such devices in terms of freauency,
material of specimen, dimensions,
temperature,
etc , as xwell as the effect of magnetization,
grain size, etc
Since absorotion is known to var
this variation are of extreme importance
with frequencj, tne factors leading to
In particular,
Investigation of tne
variations of ultrasonics absorption as a function of grain size of the metal
samples and frenuency of the incident sound wave was started July 1
After oreliminary studies were made with existing 10 and 30 me/sec
equipment, design of a continuously variable frequency source to cover the range
from 1 to 100 me was begun
The breadboard model has been completed and work on
an experimental model is under way
A urogram to obte.n sets of metal samples caliorated as to grain size
is progressing very well with some samples being supplied by several outside metal
companies.
The M
I
T
Department of Metallurgy is cooperating in this phase of
the worc
(E)
Relaxation in Paramagnetic Salts
The soin systemn in a paramagnetic substance in a magnetic field has
degrees of freedom which make a contribution to the specific heat
Consequently,
as a sound %wave iropagates through tne substance, the soin system absorbs and
-35-
If the period of the sound wave approximates the spin-atom
releases energy
relaxation time, the application of a magnetic field should change the sound
This relaxation period is
absorption
has been measured from the absorption of energy in an electromagnetic
It
salts
of the order 15 me/sec for some fM and Or
field by Gorter, et al
at the University of Leiden
A very preliminary calcula-
tion indicates that the acoustical effect is of the order of 0 1 db over a 5 inch
Equipment is being set up to try to observe this effecto
path
Non-linear Effects in Rochelle Salt
(7)
If a y-cut quartz plate is mounted on a Rochelle salt crystal in such
a way that the x axes of the quartz and Rochelle salt are parallel, and the x
axis of the quartz is perpendicular to the z axis of the Rochelle salt, the propagation of a pulse of sound from the quartz through the Rochelle salt should be
non-linear.
This effect should cause the production of a distorted pulse of sound
at frequencies harmonic with that of the exciting pulse
from the theory of Rochelle salt.
This can be predicted
An attempt has been made to observe the effect
experimentally at 15 me by observing the second harmonic signal produced
A
positive indication was obtained, but more experimental and theoretical work is
This work is being done in collaboration
planned in order to get a definite result
with Dr
III.
0
Huntington, who is now at Rensselaer Polytechnic Institute.
HIGH SPIED OSCILLOSCOPE AND HIGH VOIAGE PULSE MIEASURING TECONIQLUES
Staff:
0
T Fundingsland
L
D
A
F
Harris
Winter
Description of project
This project, which was initiated by the Radiation Laboratory and which
is
being continued in the Research Laboratory of Electronics is concerned with the
development of high speed oscilloscopes for the study of extremely short duration
recurrent transient electrical phenomena.
It is hoped that the employment of this
equipment will give new information regarding the nature of the build up of
oscillations in a pulsed magnetron and preparations are being made to use the
oscilloscopes for studying these effects
Status
High Seed Oscilloscope
At the beginning of this period there were two
main difficulties hindering satisfactory operation of the high speed oscilloscope
These were
(1)
Continuous conduction of the sweep thyratron on the slowest sweep;
(2) Time jitter of the order of 3 x 10
8
sec between the output trigger pulse and
the sweep, making accurate observation difficult.
The cause of the first of these troubles was located in the sweep charging circuit within which the sweep condensers are resonantly charged from a d-c
If the recharge is too rapid the thyratron conducts
supply through an inductance
continuously,
since it has not deionized from the previous sweep
corrected by increasing the inductance of the charging reactor
This was
The second trouble, that of jitter between the output trigger and sweep,
was not so easily remedied
The phasing circuit originally used employed two lumped
constant delay lines to which a single pulse was applied the output of each passing
through a separate amplifier and then triggering a blocking oscillator
The two
blocking oscillator pulses were phased with respect to each other by making one of the
delay lines variable
The fixed-delay pulse was used to initiate the sweep, while the
variable pulse served as output trigger
In order to keep the physical size of these
lines small, they were designed with a high delay per section, as a result the lines
could pass only slow rising pulses
To reduce the jitter
below its value of 3 x 10
sec faster rising pulses were needed
Lines designed for the same circuit, to give
adequate delay, and to pass faster rising pulses
were found to be excessively large
After trying several electronic phasing circuits discussed below
the following scheme
was adopted
The final circuit used takes a half microsecond pulse from a 1500 volt
blocking oscillator
line
and applies this to a variable tap along a five microsecond delay
One end of thiq line is connected to the sweep thyratron grid
while the other
end is connected to the trigger output Jack, as shown in Figure 1
The line is made
The main part is a twenty section lumped constant line having a 1000
ohm characteristic impedance and five microseconds delay, with taps every quarter
in two parts
microsecond
The remaining part is a distributed winding over a grounded foil
A tap
slides along the length of the coil giving continuous phasing between the intervals
determined by the lumped line
The pulse to the thyratron grid is fed from the line
through a voltage doubling transformer, through a condenser, and onto the grid
II
Figure 1
This method of phasing gives smooth control over a range of
about
2 x 10
sec jitter
+
5 psec with
Using d-c on the thyratron heater eliminates the last
visible trace of jitter on the 60 in/gsec sweep
In seeking a solution to the problem, several schemes were tried before
-37-
These will be mentioned to point out their short-
using the method described above.
comings.
Both a modified Eccles-Jordan trigger circuit and a biased blocking oscil-
lator circuit were used.
Bias control of the triggering points was used to obtain
An overloaded amplifier pulser with subsequent
phasing in both of these circuits.
Phasing was accomplished in this case by controlling
voltage amplifier was al ,o tried.
In each case the ouput
the rise time of the pulse with variable capacity loading.
pulse was used to trigger a 1500 volt biased blocking oscillator.
These circuits were
found unsuitable, as short pulse blocking oscillators are difficult to trigger consistently. Therefore it was decided to eliminate all electronic circuits between the
initial and final pulses.
At recurrence frequencies greater than 2000 cps the trace begins to defocus.
No solution for this trouble has been found as yet.
However a repetition rate of 2000
cps is probably high enough for most applications.
Work on the two oscilloscopes under construction is complete except for the
sweep chassis.
installed.
One sweep chassis is operating but the parts are not permanently
The other is about one fifth wired.
Barring unforeseen troubles, both
'scopes should be assembled and ready for final testing about the middle of October.
A hard tube pulser has been built which is capable
Short Pulse Generator.
of delivering a
0.01
pLsec pulse (15kv) to a 1000 ohm load.
to 90% amplitude in less than 0.005
amplitude.
(See photograph).
sec.
The pulse rises from 10%
The pulse duration is measured at 95%
The pulser has provision to use maxim'rm of eight 5D21's
O
in parallel for the output switch.
0.05
0
srEc.
The basic circuit is shown in schematic in
Fig. 2.
The driver circuit consists of a network (shown in the dotted
box) which
is discharged through a hydrogen thyratron (3045) into a resistance
load. The
resistance is capacity coupled to the grids of the output
pulse tubes. The positive
driver pulse rises to full amplitude in about 0.02
-38-
Psec.
This time is limited by the
I10
LOAD
1000 ^L,
Figure 2
thyratron ionization time as well as the input capacitance of the 5D21 grid circuit
Leads in the driver c.rcuit are made as short as possible to kee' the inductance to a
The duration of the pulse applied to the 5D21 grids is determined by the
minimum
pulse forming network and length of the shorted cable connected across the thyratron
load resistor
The control grids of the 5D21 tabes are connected through individual small
resistors to a common "drive" ring
The screens are likewise connected to a common
ring and a mica condenser bypasses each screen to cathode as close to the tube as
possible
The plates are all connected to a common conductor which serves to support
the output high voltage condenser
This same pulser can be used for longer pulses by changing the length of
the shorted cable which clips the tail
of the pulse and modifying the nulse forming
networ'c in the driver, or by replacing the network with a piece of cable of the desired
length
It
is necessary to add a capacity in parallel with the output of this cable
to obtain a fast rise
The reason for this is that the distributed caacity of the
cathode circuit is more quickly charged from a condenser than from the constant impedance cable
The choice of the optimum number of outout switch tubes depends upon several
factors
(1)
For these saort times, the instantaneous current needed to charge the distributed
capacity of the load is more than one tube can supply,
(2)
Adding more tubes to obtain the current necessarily adds capacity in the grid
circuit which must be charged from the driver
(3)
The
exact nature of the ep-ip curves for these short pulses is not known and one
can only extrapolate from data obtained with longer pulses
(4)
The variation of thyratron impedance as a function of time is not known accurately
and most reasonable assumptions add a non-linear element to the circuit problem
It is difficult to establish criteria for determining experimentally the optimum number
For example a very rapidly rising pulse can be obtained with two or
of switch tubes
three tubes when the grid drive is excessive
However, the leading corner of the pulse
is not square but has a spike of about 10-20% amplitude and several millimicroseconds
If a flat top pulse is desired, the grid drive must be reduced resulting
Depending on the particular shape desired the optimum number
in a slower rising pulse
duration
of tubes is somewhere between two and six
A 725A "X" band magnetron was substituted for the 1000
MaetrBon BAbayi
ohm resistance load when the circuit had been adjusted to give a flat-fopoed pulse of
The added capaci0 01 sec duration rising in about 0 005 Psec on the resistance load
tance of the magnetron and its filament transformer decreased the rate of voltage rise
That is no conduction
by almost a factor of two but the magnetron failed to start
current flowed and no r-f output radiation was detectable
behaved as a capacitance
The magnetron apparently
The only observable current was proportional to the rate of
rise of applied voltage and did not vary in accordance with the dynamic impedance
characteristic for normal conduction current during oscillation
Even with amureciably
higher voltage than is normally applied to the tube it was necessary to increase the
pulse duration to more than 0 02 psec before a true conduction current appeared and
r-f radiation was detectable
Since these are only cursory observations on one magnetron
no general conclusions can be drawn
Puluas
oeaur nTechuiuman
Conventional RC voltage attenuators have been
found unsatisfactory for viewing these short pulses
even with the best refinements we
could devise in shielding, balancing of time constants
and cable matching
The funda-
mental difficulty is that an attenuator with high enough imoedance to minimize disturbance of the pulser circuit cannot be satisfactorily matched to low impedance (100 ohm)
Any compromise in this resuect produces distortion of the signal
cable
However
apparently reliable viewing of voltage pulses was accomplished by placing the 'scope as
near as possible to the pulser load (within a few inches) and using a "field probe"
The probe consists of a small plate conn-cted directly to the CR-'lbe deflecting plates
by a very short lead
A capacity divider is thus formed by the capacitance of the probe
The voltage across a non-induc-
to the load and the capacitance of the OR-+ube plates
tive resistance load as observed by this method was in good agreement with the current
pulse observed with a cqaxial current viewing resistor and matched cable
III
D
LIQUID FILLED CHAIMBER TO BE USED IN THE PHOTOGRAJHY OF VERY HIGH ENERGY
IONIZING PARTICLES
Staff
Dr W H Bostick
M Labitt
Description of project
The purpose of this project is to develop an apparatus to be used in the
photography of very high energy ionizing particles
Because high pressure Wilson
cloud chambers are difficult and expensive to make, a process previously described
by W
H
Bostick1 is being investigated
In this apparatus it is hoped to provide
a condition whereby bubbles of gas or vapor can be created along the path of the
ionizing particle, tnus making the path visible
1
Bostick, W
H , Fizz Chamber, December 16, 1945
-40-
R
L
E
hectographed note
Status
A hexane filled chamber has been constructed and operated at room temperature in such a way that the gas pressure above the hexane was suddenly reduced from
one atmosphere to a value 2 where M is a pressure which can be adjusted to any value
between 0 05 atmosphere and one atmosphere
A clearing field in the chamber was produced by a potential of 10 kv placed across tuo electrodes in the chamber
As R was
varied in the operation of the chamber, all stages of cavitation from no bubbles to
violent boiling were obtained in the hexane, but in no instances did the bubbles
appear in any formation that might be called a track
The bubbles, 'hich were for
the most part, hexane vapor rather than dissolved gas, aere usually formed at the
surfaces of the rubber gaskets used inside the chamber
Any further attempts at this Laboratory to obtain bubbles along the
tracks of ionizing particles will probably take the form of constructing an allmetal chamber energized sonically and completely filled with hexane
III
E
REPORT 011 EXPRIrMETATIOJ AID PzOTSLECTRIC-PO'TOSPCTROiDLTRY AND 0(
C00NSTRUCTI0A AND USE OF PACKAGED
TUBES
Staff
TRE
IPLI IER UNITS EdPLOYL4G SUBLIZIATURE
Dr B Chance
J i Thurston
P L Richman
Description of project
The primary purpose of tas project was to develop a photoelectric
spectrometer for biophysical experimentetion
The problem being to make an
apparatus which would measure extremely small spectral shifts
In the course of
this development, work was begun on the use of subminiatare tabe assemblies as
amlifiers, voltage regulators, and so on
The advantage
of suca a technique
being that the circuits could be stabilized by means of iiverse feedback adjusted
for desired characteristics at assembly, and thereafter could be e'oected to hold
their calibration for long periods, their reulacement uoon failure being made by
changing subassenblies rather than replacing tubep
Because this work apneared
to have wide apolication in tne assembly of otner electronic eouloment, it has been
continued and expanded
Status
Item 1 A comparison of the electron-mult.plier phototube and the
ordinary uhototube for high-resolution spectroonotometry
A number of tests were condacted to determine tne comparative stability
and sensitivity of type 931A and 929 phototubes
Both of these tabes were tested
in a modified Coleman s-ectrophotometer in which the light intensity and tne supoly
nossible The drift of the outout of
voltages were stabilized as carefully as
either tube was then recorded as a function of time
Since it was desired to measure
an extremely small increment of light Intensity, (approximately 1 x 10 - 5 ) the output
of either tube was measured with a sensitive millivoltmeter using mechanical-switca
-41-
modulators described in previous renorts
In tests of the photomultiplier the
effects of fatigue already noted by Diekel caused not only a critical selection of
The
all available tubes but also in the end seriously limited their performance
best available Type 931-A (several Type 1P28 were tried without good results)
-4
per minute with a collector current of 10 mlcrodrifted at the rate of 1 x 10
amperes and annroximately 50 volts
ver stage
Type 929 a drift rate of between 1 x 10
-4
Without any special selection of
and 1 x 10 -
5
per minute was obtained
Thus for the light Intensities available (between 1 x 10
amneres) tne ordinar
9
and 1 x 10 -
7
micro-
pho-otube provided simpler and more stable operation in
measuring small increments of light intensity witnout the necessity for careful
selection of tubes
It should, however, be remembered that tne success for use
of the phototube renuires the extremely stable mechanical switch modulator system
Complete circuit diagrams of tae equipment used for these tests
mentioned above
are available
A further test of the resolution possible in spectrophotometry was made
by employing a differential-photocell combination in
hich light at two slightly
The
different wavelengths fell upon a pair of phototubes in a balanced circuit
signal from these two phototubes was amplified by a similar modulation system
In
addition, a similar photoelectric circuit was used to control the light intensity
The results obtained from tans equinment v ere consiaerably better than those obtained
from the single photocell and drift rates approaching 1 x l0
- 6
parts per minute
Fere again comlete circuits are available
The differential spectrophotometer has been emoloyed in a study of the
kinetics of enzyme reaction and its performance has been compared with that of an
apnaratus built for the same purpose but constructed to use d-c amplifiers and less
sophisticated light control circults
It
appears that a factor of improvement of
roughly 100 has been obtained since equivalent signal-to-noise ratios are obtainable
with a 2 x 10 -
8
mole per liter
solution whereas a 1 x 10 -
6
mole per liter
solution
was previously reauled
Work on this project will presently be directed towards the stabilization
of the mercury are in order to extend these measurements to the ultraviolet and to
give increased intensity at the wavelengths desired in the visible region
Item 2
(J
N
Construction ana tests of various subminiature package units
Thurston and Peter Richman)
Corsiderable experience has been gained on the performance of the two
types of a-c amplifier which have been made
Unfortunately difficulty has arisen due
to premature deterioration of the cathode of some of the pentodes (Type 828A)
Con-
sultation with the manufacturer has indicated that these tubes were probably handmade
Production samples have been received recently but no data are yet available
to determine whether normal life (a few t}ousand hours) is obtainable with these
tubes
No difficulty is anticipated from this cause in the long run since numerous
life tests were made of type SD834 (614) in the Radiation Laboratory and successful
operation for several thousand hours at a few watts dissipation was readily obtainable
1
Dieke, G H , "A Study of Standard Methods for Spectrographic Analysis", Sport
W-193 Office of Production Research and Development
-42-
Present development work on these a-c amplifiers is directed toward imrovement in
stability, reduction of noise, and reduction of the tendency to oscillate Uhen
supplied from a high impedance source
A test circuit has been devised for measuring
the characteristics of these amplifiers and for selecting and testing the characteristics of individual tubes
The present design of direct-coupled amplifiers, although giving clite
satisfactory performance, is sensitive to variations of the negative supply voltage
and a more elaborate model has been constructed which consists of two casc-,ed
stages of differential amplifiers employing pentodes
The
use of a differential
amolifier for the second stage reduces considerably the sensitivity to supply
voltage changes
Furthermore the overall gain is just as great as that of the pre-
vious design, and as the accompanying circuit diagram indicates
put is obtainable
a push-pull out-
The cathode resistor of the first stage is split into two parts
in order to permit balancing of the tubes in order that zero differerce of potential
betweeen the first two gTids would give approximately zero output.
A few of these units have been built and performance data on them are
available
The majority of tubes tested have a grid current of less than 0 01
microampere and a noise level of between 10 and 15 microvolts over a bandwidth of
10 -c per sec
The common mode signal (common to both grids) uhich just exceeds
noise is about 1 millivolt
This figure may be increased to roughly 100 millivolts
by emoloying a screen battery and carefully balancing the tubes
One difficulty of
this circuit not completely solved is the sensitivity of the gain of a second stage
to screen voltage variation
If desired, some internal feedback may be used in
this amplifier to diminish this variation
At the present time Sylvania Electric Products Comnany is manufacturing
prototypes of the a-c amplifier using Type 828A pentodes
They propose next to work
on the direct-coupled amplifier and then branch out into the many applications of
this philosophy of circuit design
A preliminary design for a stable multivibrator
frecuency divider has already been com-leted and a unit for generating timing
markers, at 20, 10, 5, 1, and 1/2 cps has been built at the Laboratory
OtI*er im-
portant applications of these units are in selective ampliflers, sine wave oscillators,
square wave generators, etc
Item 3
WicLe-range voltmeter
The mechanical switch modulator-demodulator circuit has been used as a
basis of a wide-range voltmeter covering 500 microvolts to 1 kv
On the low ranges
(500 microvolts to 1 volt)
While on the high
the input resistance is 1/4 meg ohm
ranges (1 volt to 1000 volts) the input resistance is
250 meg ohms
The current
sensitivity of the instrument is roughly 4 x 10l2 amps but depends upon the noise
level and grid current of the first amplifier tube
device is
included since it
is thought that it
-43-
The circuit diagram of this
may have many uses in the Laboratory
TO
BE ADJUSTED IN FINAL TEST
Asw 1000
GROUND
AND
INPUT AND OUTPUT SALANCED
WIIT RsPEoT TO nOUIND
D C AMPLIFIER # 2
ON
TOP OF UNIT
ALTERNATE RESISTOR ARRANGEMENT
TI3R
ALL Ul1ST100SL 1% D WW
2
S.
r
*
RfV SWITCH
0
+ 150oV
x
J 1 Jw
_
r
_
_
w2
_
____
_
0WW
25M
025
BROWN CONVERTER
ili
WIDE RANGE VOLTMETER
ib
O
I
III
P
BIOPFYSICS PROJECT
Staff
Dr S Goldman
H N Bovoes
Description of nroiect
In order to facilitate the examination of the electrical potential distribution on the surface of the skull as an aid in the diagnosis of br-ir tumors,
attemnts are being made to utilize radar PPI presentation techniques
The problem
is extremely dif'icult because of the minute potentials available for presentation
(1 to 50 mlcrovolts)
This technioue when develooed is expected to have many
aonlicat.ons in the blot hysics field
Status
The preliminiry beam
Ticl-up tube mentioned in the renort of June 30,
1946 as being under construction, has now been built
as anticirated and rork is undervay to bring it
put level of 10 microvolts
The tube onerates about
down to an oerating signal in-
I When this is accomplished a scanning-beam pickun-
tube will be made for the nickup of actual skull potertall
-46-
mamping signals