.-•
Report No.
GENERAL PROBLEMS OF BROADBAND AMPLIFICATION
IN THE MICROWAVE FREQUENCY RANGE
*
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CONTRACT NDNR TB34(oa)
Project No. NR-373-162
ELECTRICAL ENGINEERING RESEARCH LABORATORY
ENGINEERING EXPERIMENT STATION
UNIVERSITY OF ILLINOIS
URBANA, ILLINOIS
GENERAL PROBLEMS OF BROADBAND
AMPLIFICATION IN THE
MICROWAVE FREQUENCY RANGE
Progress Report No 1
Contract No Nonr 1834(08)
Project No NR373-162
30 April 1956
Period Covered:
1 January 1956
Prepared by:
M„ L„ Babcock
K R Brunn
Approved by:
to
31 March 1956
jV tuaAir~^
H„ Von Foerster
Profe/ssor
Electron Tube Section
Electrical Engineering Research Laboratory
Engineering Experiment Station
University of Illinois
Urbana, Illinois
Digitized by the Internet Archive
in 2011 with funding from
University of Illinois Urbana-Champaign
http://www.archive.org/details/generalproblemso01babc
CONTENTS
Part I General
Page
1 . 1 Preface 1
1.2 Personnel 1
Part II Experimental Work
1. Investigation of Barium Migration in the Hollow Cathode 2
1.1 Discussion 2
1.2 Plans for the Next Quarter 2
2. Triode with a Hollow Cathode 2
2. 1 Discussion 2
2.2 Plans for the Next Quarter 3
3. The High Voltage Hollow Cathode Investigation 4
3.1 Introduction 4
3.2 Hollow Cathode A 8 4
3.3 Hollow Cathode A- 9 4
3.4 Hollow Cathode A- 10 4
3.5 Hollow Cathode A-ll 5
3.6 Hollow Cathode A- 12 8
3.7 Hollow Cathode A-13 9
3.8 Hollow Cathode A 14 9
3.9 The Potential Field in a Hollow Sphere 12
3.10 Miscellany 16
3.11 Plans for the Next Quarter 17
PART I GENERAL
I I Preface
This is the first quarterly progress report for Contract No. Nonr
1834(08), which became effective 1 April 1956 as a replacement for
Contract No N6-ori 07156 The ONR Project Number., NR 373 162. remains
the same under the new contract
Research under this project deals with " General Problems of Broad
band Amplification in the Microwave Frequency Range The work is, in
general, a continuation of research initiated under the terms of
Contract No N6 on 071 Task XIX.
This report covers the period 1 January 1956 to 31 March 1956.
1.2 Personnel
The following staff members have been assigned to Contract No
Nonr 1834(08):
Percent Time
Supervisor :
H„ M. Von Foerster; Professor 17
Graduate Associates and Assistants:
Murray L. Babcock. Research Assistant 100
Kenneth R Brunn, Research Associate 100
Technicians and Assistants
Robert N. Waggener. Senior Glass Blower 33
Donald D Pritchard, Storekeeper 25
On 1 February 1956. Mr J F Lowe joined the staff of the subject
contract, quarter time, as an instrument maker.
-1
PART II - EXPERIMENTAL WORK
1. INVESTIGATION OF BARIUM MIGRATION IN THE HOLLOW CATHODE -
M L Babcock
I I Discuss ion
The newly designed cathode was assembled and seems to be satisfactory
However, the older lens system used in the previous beam analysis tube
was found to be inadequate for the present use since the position of the
cathode aperture with respect to the lens aperture could not be satis
factorily controlled for the measurements desired Therefore, a new lens
system was designed and the parts have been made In addition, a new
supporting and positioning mechanism has been built for use in the beam
analysis tube With this mechanism it is hoped that the emission from
the inside of the aperture can be separated from the emission from the
barium which has migrated to the outside of the aperture, and that an
indication of the size of the two emissions relative to one another can
be obtained
I 2 Plans for the Next Quarter
The tube will be assembled in its final form and tested.
2. TRIODE WITH A HOLLOW CATHODE M. L. Babcock
2 I Discussion
The triode with the modifications mentioned in the last report was
assembled and partially tested The various faults with the first triode
have been corrected in the modified triode with one major difficulty.
This difficulty, electrical leakage between cathode and grid, is still
present in the modified triode, but to a much lesser degree The leakage
now present seems to result from a deposit on the Alsimag ceramic spacers
since it is not present at the start but becomes greater as the length of
operation of the tube becomes greater To eliminate this leakage may re-
quire only a change in the ceramic spacer material or it may require a
completely new design However, it is hoped at present that the latter
will not be necessary
2
Even with the leakage present, the results of the tests appear prom
ising For example, the grid shows some control over the electron flow,
although not as much as would be expected This lack of control may be
due to inaccuracies due to leakage or to the present grid structure.
This grid has a mesh spacing of 0.010 inch for 0.002 inch diameter wires,
Thus only one or two grid wires intercept the electron flow. New grids
with a mesh spacing of 0.001 inch for 0.0003 inch diameter wires have
been obtained and will be used in the triode as soon as the grid-to
cathode leakage is eliminated This grid should exhibit much greater
control since the spacing is such as to place about 20 wires in the elec-
tron flow path instead of the present one or two wires.
Another result of the tests to date is the variation of current
with the variation of cathode temperature- This variation appears to be
much greater than the similar variation in the diode tube. However^ due
to the leakage present in the triode, this may be a false indication, and
so the results must still be considered only qualitatively.
2.2 Plans for the Next Quarter
A new triode has been constructed and will be tested soon This
triode has a mica spacer separating the grid from the cathode If this
fails to correct the leakage, then another tube with completely separate
cathode and grid-and plate assemblies will be constructed. Most of the
parts for this latter tube are available at present.
3. THE HIGH VOLTAGE HOLLOW CATHODE INVESTIGATION -
K. B, Brunn
3 I Introduction
The current series of hollow cathode experiments was concluded
during this period and the data on Models A-8 to A-14 is reported
below. Electrolytic tank measurements of the potential field in-
side the hollow spherical cathode were made and the results are
given. As a result of the experience obtained on the series of
oxide emitter cathodes it was .concluded that a pure emitter would
have many advantages in the high valta,ge regions of operation and
such a cathode has been designed and is under construction.
3 2 Hollow Cathode A-8
It was intended to mask off an area about the aperture having
a diameter of 0.125 inch when hollow cathode A-8 was sprayed with
the Ba-Sr oxide emitting material. However, an autopsy revealed
that the masking was very ineffective and a non-uniform coating
was actually present on this area ranging from no coating to nor-
mal coating. Consequently the, tests on this cathode appear to
have little value.
3=3 Hollow Cathode A-9
The cathode consisted of a bare; grade A, nickel sphere, and
was intended to verify experimentally that no direct emission from
the nickel surfaces of either the cathode sphere or heater assembly
is present in the current aeries of tests. Heater difficulties
doomed this experiment.
3-M- Hollow Cathode A- Ifi
This test was a repeat of the previous experiment with a new
heater assembly and bare nickel cathode. No measurable currents
were obtained. The anode- to- cathode spacing was 0.036 inch and a
maximum anode voltage of 9000 volts was applied at a maximum tem-
perature of 915°C. Thus there can be no doubt that the experimen-
tally observed currents of the hollow cathode must originate from
the oxide coating alone.
3 5 Hollow Cathode A- 1 I
Hollow cathode A- 11 was assembled according to the same specifica-
tions as Model A-4,* that is, an annular region about the aperture
having a diameter of 0.25 inch was masked off when the cathode was
sprayed. The cathode was converted with a maximum temperature of 943°C.
The activation was normal, i.e., the initial current was essentially
zero and slowly built up to a stable value in several hours. This
cathode yielded higher currents than Model A-4, but the characteristics
were similar.
The higher emission permitted the use of the oscillographic pres-
entation of the characteristics, and tracings of photo-oscillograms
taken with the cathode temperature as the parameter are reproduced in
Fig. 1. The characteristic shape is typically that of the normal hollow
I
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Hollow Cathode A-ll
12 Jan 1956
954°C
Spacing 0.034"
931°
f S^
912°
904°
Dot.
i
0
Figure
4 .6 .8 to
Anode Voltage in kv
1.2
1.4
Hollow Cathode Characteristics
with an Uncoated Area of 0 25 inch
Diameter About the Aperture
See Page 19, Progress Repo Nj 2 of he p . N- N6- on-O/ i56 October 30,
1955
10
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o
.06
.04
02
.01
Hollow Cathode A- II
II Jan 1956
920°C .016"
6 8 10
20 30 40 60 80 100
Anode Voltage
200
400 600 800 I0C0
Figure 2 Logarithmic Plot of the Characteristic with an Uncoated
Area of 0 25 inch Diameter About the Aperture
cathode except that of the region near the origin, i.e.. below approx-
imately 100 volts. In order to observe this region more closely, a
typical set of measurements have been plotted on a logarithmic scale
in Fig. 2 For anode voltages less than about 50 volts the charac-
teristic follows a three-halves power voltage law. This is the
classical space-charge limited relationship except that one important
difference must be pointed out namely, that the current at these
anode voltages is not independent of temperature as can be observed
in Fig. 1.
The temperature dependence of the emission of this cathode was
found to differ from the fully coated cathode in that it did not
vary as an exponential function of the reciprocal temperature. The
observed temperature dependence is plotted in .Fig. 3^
(A
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ex
E
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c
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3
.6
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^ 4.6 volts
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^2.2 volts
Hollow Cathode
A-ll
12 Jan 1956
.034" 1.3 kv
X^
T
°c
900°
8.0
Figure 3
82 4 8.4 86 8.8
10 /T (T in degrees Kelvin)
Hollow Cathode Current versus Reciprocal
Temperature for a Cathode with an Uncoated
Area of 0 25 inch Diameter About tne
Aperture
The cathode yielded reproducible results, and, after about 19 hours
of actual operation, was removed and inspected. The cathode coating
looked very good and no visible evidence of cathode material could be
observed on the masked off area or on the edges of the aperture.
3 6 Cathode A- 1 2
Cathode A- 12 was not a hollow cathode, but a normal cathode
approximating a parallel plane diode. It was assembled just as the
previous hollow cathode models except that instead of coating the
internal surface of the cathode sphere, the exterior surface was
coated on an annular region about the aperture having a diameter of
0.125 inch. Thus the arrangement is an ordinary diode utilizing the
same geometry as the hollow cathode experiments.
The results were as one would predict, namely, true space-charge-
limited operation. By operating this cathode at relatively low tem-
peratures so that it "saturated" at low anode voltages, a direct
comparison of its characteristics and the hollow cathode character-
istics was obtained. A typical comparison is shown in Fig, 4» The
'Internal coating'" curve is that of Model A~6 at a temperature of
100
300
400
200
Anode Voltage
Figure u. Comparison of the Normal and Hollow Cathode Characteristics
917°C and an anode- to- cathode spacing of 0.042 inch. The "external
coating" curve of cathode A-12 was taken at a spacing of 0.050 inch
and about 700°C. It should be observed that the "saturation" region
does not show a true saturation, but continues to increase as the
anode voltage is raised. This cannot be due solely to the Schottky
effect for two reasons. First, the curvature of the characteristic
is contrary to that of the Schottky effect, which requires that the
slope increase as the anode voltage is increased, and second, the
field strengths are too small to aocount for the magnitude of the
increase in current. The main factors which influence this portion
of the characteristic are probably the roughness of the coating
surface and the variation in work function of different areas of the
coating which cause different portions of the cathode surface to
saturate at different anode potentials. Consequently, the transition
region between complete space-charge-limited operation and completely
saturated operation is extended over a rather large range of anode
voltage. This situation complicates the calculation of temperature-
limited currents from oxide coated cathodes encountered in practice.
A comparison of the two curves of iFig. 4 shows that even under
practical conditions a rather distinct knee in the curve of the
ordinary cathode is clearly observable, while this is not the case
in the internal or hollow cathode case.
3=7 Cathode A-13
Cathode A- 13 was a repetition of the externally coated cathode
of model A- 12 and yielded the same results. True space-charge-
limited operation was obser/ed5 and when the temperature was re-
duced the same "saturation" characteristics as shown in Fig. 4 were
obtained.
3 8 Hollow Cathode A-!4
Hollow cathode A- 14 was assembled with the oxide coating present
only on an annular region having a diameter of 0.148 inch about the
aperture. That is, the region in the vicinity of the aperture is
capable of emission and the areas distant from the aperture are not
capable of emission. The cathode was converted at a maximum tem-
perature of 1050°C and activated normally.
The current-voltage characteristic of this cathode was found to be
the same as that of the typical fully coated hollow cathode over the
range of voltages measured, i.e., below 1000 volts. A tracing of a
typical photooscillogram of the characteristic is reproduced in Fig.
5. Such a result substantiates the hypothesis that the area in the
1.0
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V.
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Hollow Cathode A-14
26 Jan 1956
978 °C .050"
,
>^
100
400
500
200 300
Anode Voltage
Figure 5= Characteristic of a Hollow Cathode Coated in
the Neighborhood of the Aperture Only,
600
vicinity of the aperture contributes the major portion of the current
over the range of voltages investigated. Typical data has also been
plotted on a logarithmic scale in Fig. 6 which essentially duplicates
similar plots for the fully coated hollow cathode. *
* See Fig. 4, page 10, Progre:s Report No. 2 of this contract
10
1000
800
600
400
200
100
80
M
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Hollow Cathode A- 14
25 Jan 1956
922°C .050"
^Ti
1
8 10 20 40
Anode Voltage
60 80 100
200
400 600 800 1000
Figure 6
Characteristic of the Hollow Cathode Coated in the
Neighborhood of the Aperture Only
11
The cathode was removed after about 98 hours of operation and
inspected. The coating looked good and no visible signs of migration
through the aperture could be observed,
3 9 The Potential Field in a Hollow Sphere
In any analysis of the operation of the hollow spherical cathode,
a knowledge of the field distribution inside the cathode in the ab-
sence of space charge would be a valuable bit of information. An
exact theoretical calculation of the potential field is quite diff-
icult, but an approximate solution in terms of Legendre Polynomials
is readily available,6 However, from a practical computational view-
point this solution has limited value as it is in a rather cumbersome
form involving a double infinite series containing integral coefficients,
Anticipating graphical and numerical methods of solutions, the more
direct approach of the plotting tank method seemed to be in order.
While the general method of the electrolytic tank potential
measurement is well known, a few considerations pertinent to the
particular problem are worthy of note, The geometry of the model
is shown in .Fig, 7. A wedge angle of five degrees and a signal
Axis of Symmetry
R
d
a
t
Tank Model , inches
!6,3
!=74
°87
= 2)75
Actual Cathode, inches
= 375
= 040
,020
,005
Aperture Radius
Norma! ized to Uni ty
>8 = 7
2
i
= 25
Figure 7= Geometry o* the Hollow Spherical Cathode Tank Model
Morse, P.M., and Feshbach, H* 'Methods (jf Tn^sreticai Ph,. rsics, " Pa:: ; II
McGraw-Hill, New Ysrk, 1953, P. 1283
12-
frequency of 60 cps were used with the usual bridge circuit and an
oscilloscope null detector. The size was scaled as large as possible,
principally to make the model aperture size large enough to insure
that variations in the electrolyte meniscus in the vicinity of the
aperture would have a negligible effect on the potential levels with-
in the sphere.
Due to the fact that the potential values within the sphere are
so very small, it was not practical to measure the fields everywhere
within the sphere using the single anode and cathode electrodes
sketched in Fig. 7. Such a procedure would be subject to large errors
due to the fact that these potentials are in the region of the lowest
sensitivity of the bridge, because of the. very small unbalance signal
magnitudes available with reasonable anode voltages, and the difficul-
ties of sufficiently shielding the detector from stray pick-up voltages
of comparable magnitude to the bridge signal voltages near the null
position. Consequently, the fields were measured in the region near
the aperture and a new electrode was then made to conform to the
measured equipotential line corresponding to one percent of the anode
potential. This new electrode was then used with the cathode electrode
to measure the fields farther into the sphere- Then a third electrode
was made to conform to the equipotential line corresponding to one
percent of the second electrode, or 10° times the original anode po-
tential. Using this third electrode and the cathode electrode, the
remaining potential distribution could be measured. The resulting
potential distribution of the entire sphere has been reproduced in
Fig. 8. The values of the plotted equipotential lines have been
normalized to unity anode voltage.
It is also of interest to know the value of the electric field
at the internal surface of the sphere. This is readily obtained from
the tank measurements and is plotted in Fig. 9° The fields can be
seen to fall off quite rapidly as one moves away from the edge of the
aperture. The measured field values in the immediate neighborhood of
the aperture are, of course, least accurate, but for values of 9 > 4°
the error should be small. The straight line portion of the curve
yields the relatively simple relationship (for 9 in degrees
_JL - 4so8 9"2<3?
Va/,B
13
Figure 8 Equ ipotent ial Plot of the Hollow Sphere as
Determined from an Electrolytic Tank Model
14
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10 20 30
0 in Degrees
50
70
HX»
180
Figure 9
Normalized Field Strength at the Internal Surface of the
Hoi low Sphere
15
which is valid for 6 £ 100°. Of direct concern to the high voltage
investigation is the result that with pulsed anode voltages of the
order of 200 kv one can obtain field strengths of the order of 10 v/cm
at the back of the cathode in the absence of space charge. Thus at
these high voltages one can feel assured that a considerable portion
of the cathode surface will actively contribute to the operation.
3o 10 Mi seel 1 any
As regards the high voltage investigations of the hollow spherical
cathode, the experience obtained with the series of cathodes using
oxide emitters has led to one principal conclusion, namely, that the
use of oxide coated cathodes is not particularly suited for the high
voltage experiments. This conclusion is based on several factors
including sparkingc reproducibility, and temperature-limited char-
acteristics of oxide cathodes. Sparking problems which were
troublesome but not insurmountable arose at anode voltages of
several kilovolts, but at voltages of several hundred kilovolts
there is little doubt that the results would be disastrous. As
greater portions of the cathode surface will be operating under
temperature limited conditions as the voltages are substantially
increased, the problems with oxide coated cathodes become in-
creasingly important. The fact that temperature-limited char-
acteristics of oxide cathodes are both nonpredietable and non-
reproducible within any reasonable limits makes even approximate
analysis invalid and interpretation of experimental data almost
impossible. In the light of these facts, the use of a pure metal
emitter would be an immeasurable improvement. The pure emitter
would be far more predictable and reproducible, would not be sub-
ject to variations due to sparking, and would completely eliminate
the conversion and activation irregularities inherent in the oxide
cathode, as well as poisoning effects. Another advantage which the
pure emitter offers is the elimination of the question of migration.
There are practical experimental disadvantages of the pure emitter
of course. Since higher work functions must be accepted it means
operation at rather elevated temperatures. For laboratory experiments
this is not a distinct disadvantage, and it is planned to heat the
16
pure emitter hollow cathode by means of RF induction heating. A problem
of greater concern is the reduction of the direct emission from the
external surface of the cathode sphere to a negligible value. The
basic technique will be to use a material having a work function suf-
ficiently greater than the internal emitting surface Since the sat-
uration current density varies exponentially with the work function,
such a technique becomes plausible. The following relationship is
easily derived from Richardson' s equation for the saturation current
density, where T is in degrees Kelvin and Aqp is the difference in
work function in volts
log10 ^ « "5Q5Q A<p .
Jo T
Thus, for a difference in work function of one volt at a temperature
of about 2(J00°KC one obtains a saturation current density ratio of
the order of 300. The direct emission can he further effectively
reduced by geometrical means such as measuring the current to the
anode directly opposite the aperture separately from the current
collected by the outer areas of the anode.
A tantalum emitter offers the advantage of being easily drawn
to shape and has a work function of 4« 13 volts. Platinum appears
to be a suitable metal for the external surface as it can be
plated or evaporated on the tantalum sphere,, has a high melting
point, and a work function of 5.36 volts. An experimental set-
up utilizing RF induction heating, reduction of direct emission
by geometrical means, and other details has been designed and is
under construction.
3-11 Plans for the Next Quarter
Having completed the series of tests on the low voltage hollow
spherical cathode, the results will be studied. The pure emitter
hollow cathode will be assembled and testing should be initiated
during the next quarter. Methods of utilizing the field plot of
the hollow spherical cathode obtained during this quarter for
analysis of high voltage operation will be investigated.
17
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