RADIAL BEAM VELOCITY MODULATED MICROWAVE TUBE

56 Pages·02.487 MB·English

by  LOB, CHESTER GODFREY

COPYRIGHTED
by
CHESTER GODFREY LOB
1951
RADIAL BEAM VELOCITY MODULATED
MICROWAVE TUBE
BY
CHESTER GODFREY LOB
B.E., Tulane University, 1946
M.S., University of Illinois, 1949
THESIS
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING
IN THE GRADUATE COLLEGE OF THE
UNIVERSITY OF ILLINOIS. 1951
URBANA, ILLINOIS
UNIVERSITY OF ILLINOIS
THE GRADUATE COLLEGE
May 1U, 1951
I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY
SUPERVISION BY. CHES1ER GODFREY LOB
p-\TTTXT.F.r> RADIAL BEAM VELOCITY MODUIA TED MICROWAVE ITJBE
BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF_ DOCTOR OF PHILOSOPHY
WtuA<h l/taJ L4erU-L
Recommendation concurred inf
(P r/ ^yJaXT^vai^wv
Committee
on
Final Examination!
t Required for/fbetoris degree but not for master's,
M440
i ii
TABLE OF CONTENTS
Page
ACKNOWLEDGMENT iv
I. INTRODUCTION 1
II. VARIOUS DETAILS OF THE TUBE . U
A. THE CAVITY U
B. THE CYLINDRICAL GUN k
C. GENERAL CONSTRUCTIONAL DETAILS 5
III. INFORMATION OBTAINED FROM THE OSCILLATING TUBE 11
(HOT-TEST MEASUREMENTS)
IV. DETERMINATION OF D.C. FIELDS IN THE DRIFT REGION ll*
A. ANALYTICAL DETERMINATION OF POTENTIAL DISTRIBUTION lU
B. EXPERIMENT TO CHECK THE ANALYSIS 2$
(D.C. BEAM PROBE MEASUREMENT)
V. CONSIDERATIONS CONCERNING THE BEHAVIOR OF TIME VARYING FIELDS IN
THE DRIFT REGION 33
A. ANALYTICAL DETERMINATION OF TRANSIT TIME FOR A PARTICULAR
CASE 33
B. CALCULATION OF CUERENT REQUIRED TO GIVE CONDITION ASSUMED
FOR THE TRANSIT TIME CALCULATION OF SECTION A 36
C. R.F, BEAM PROBE MEASUREMENT 38
a. CALCULATION OF EXPECTED AXIAL BEAM DEFLECTION 38
b. EXPERIMENTAL RESULTS * 39
c. AN INDEPENDENT EXPERIMENT CLOSELY ALLIED TO THE BEAM
PROBE MEASUREMENT. THE DUAL BEAM TUBE UO
CONCLUSION 1#
iv
ACKNOWLEDGMENT
The work contained in this thesis was done under the sponsorship of the
United States Air Force through Contract Number "W33-038-ac-lU7U2.
As with many acknowledgments, for a relatively long term project such as
this, the entire staff of the Electron Tube Research Laboratory is to be thanked
for their helpful advice and assistance. The author is particularly indebted
to D. F. Holshouser, H. M. von Foerster and N. Wax for their sincere interest
and helpful criticisms*
CHAPTER I
INTRODUCTION
The radial beam klystron was first suggested by D. F. Holshouser as a possi
ble means for obtaining relatively high power oscillations in an r.f» geometry
which lends itself to ease of tuning. This tube, the basic configuration of
which is discussed shortly, incorporates the simplicity, ease of construction and
adjustment of a reflex klystron and yet has many of the advantages of a two-gap
klystron. Other possible advantages and a number of interesting features of this
device are discussed after a brief description of the geometry.
Basically, the tube appears as is shown schematically in Fig. 1« It con
sists of a coaxial resonator, one-half wavelength long, cut through the center
perpendicular to its axis of symmetry. Surrounding the resonator is a gun
structure consisting of a ring cathode and two focusing electrodes. The resonator
is actually two coaxial cavities, each operating in the quarter wavelength mode.
Qualitatively, the operation can be described as follows. Consider a portion
of the beam which leaves the cathode at a particular azirauthal angle. This
portion becomes velocity modulated while crossing the interaction region, is
bunched when traversing the drift region, and finally gives up its r.f. energy to
the interaction gap at a point diametrically opposite to the point at which it was
previously bunched. The action is obviously that of a two-gap klystron, but
occurs around the entire periphery of the tube.
The tube may also act as a reflex klystron, i.e., the portion of the beam
previously discussed may be velocity modulated and turned around before reaching
the center of the drift region, thus giving up its r.f• energy at exactly the
same point at which it was previously velocity modulated. This reflex action can
be effected by the existence of a cylindrical virtual cathode in the drift region.
2
As is shown later, the establishment of such a virtual cathode is readily
possible.
This type of geometry has several interesting features some of which are now
evident. First, the available cathode area, and hence input current, is not a
limitation since this device, of necessity, uses an outside cylindrical cathode
of considerable current capacity. Second, the tube is easily designed to dissi
pate considerable heat and hence the power input may be relatively large.
Finally, the r.f. geometry is merely a shorted coaxial transmission line and may
be tuned symmetrically by varying the position of the short (length of cavity).
With such tuning the fields would remain symmetrical and hence one of the major
drawbacks of many tuning arrangements would be overcome.
Aside from these purely practical aspects, the investigations on the K3 have
led to several interesting and more general results.
The d.c* space-charge distribution (neglecting the effect of residual gas)
was determined analytically and measurements were made to check the results. The
measurements showed that there was enough residual gas present to completely
neutralize the space charge. As is well known, electronic space charge is
appreciable if one of the two following conditions is fulfilled:
a. The amount of residual gas is very small in the vicinity
of the electrons
b» The electronic space charge is time varying and such that
a period of this time variation is small when compared
with the time required for ions to move into the region
of high space-charge density.
Although the first condition is not realized with the K3, the second is. On the
axis of the K3 drift region there is present a very high-density (theoretically
3
infinite), time-varying space charge. An effort was made (both analytically and
experimentally) to determine the effects of this space charge on the action of
the K3» The results are reported herein.
An interesting side issue to the main investigation is a discussion on how
the effects of this time-varying space charge were utilized in the"Dual-Beam
Tube", an experiment which was discussed by D. F. Holshouser at the I.R.E. Tube
Conference in 1?5>0 and which will be briefly discussed in this thesis.
It should be pointed out that although the tube has many possibilities, the
primary aim of this work has not been to develop an efficient device, but rather
to study both analytically and experimentally the very interesting mechanisms
associated with the device. Logically speaking, however, this type of study is a
necessity for the development of an efficient tube.
CHAPTER II
VARIOUS DETAILS OF THE TUBE
A. THE CAVITY.
In general, the work was carried out in steps, the first being the design of
an r.f. cavity for use in the tube. The original cavity which was used for cold-
test purposes is shown in Fig. 2. This design, with only slight modification,
has been used throughout the course of this study. As can be seen from the
figure, two coupling loops were used for cold testing in order to determine the
coupling from one quarter wave section to the other. A tuner was also incorpo
rated. The particular cavity shown had a resonant wavelength of 3.75 cm, and a Q
of 900. This particular cavity is ideally suited for a wide tuning range. Tuning
is accomplished by a movable coaxial short at each end. Ths movement of the two
ends are linked so as to obtain symmetrical tuning. With such a system, the r.f.
field configuration is not disturbed during tuning.
B. THE CYLINDRICAL GUN.
The first step in designing an efficient gun structure was the design of a
relatively stable outside cathode. Primary attempts were unsuccessful because of
thermal expansion and resulting distortion. The final cathode design is rather
unique. Various views of this structure are shown in Fig. 3» The actual cathode
is a nickel ring inserted into a groove machined in the lava. Behind ths cathode
are three turns of coiled tungsten which serve as a heater, and behind the heater
is a nickel backing strip to reduce thsrmal losses. The nickel cathode is not a
complete ring; rather, a small segment is cut out to allow for thermal expansion.
The gun structure is shown on the assembly drawing of Fig. U. The focusing
electrodes are held to the lava cathode washers with molybdenum clips. The gun
design went through several stages. At first, it was decided to use a Pierce-
type structure with the focusing electrodes operating at zero potential,. The nor
mal plotting tank method was used and various focusing electrode shapes were tried
in order to get an anode-to-cathode potential distribution which was intermediate
between that of concentric cylinders and concentric spheres. After this, several
gun tests were made using the actual cathode and focusing electrode structure.
From then on, the process was one of cut and try until a fairly efficient
structure was achieved. The gun finally used in the tube was about 80$ efficient,
the focusing electrode potential being variable from 0 to approximately -100 v.
This design, though not optimum, was sufficient for the purposes of these studies.
The structure just described was used throughout the course of study and no
further gun work was carried out.
C. GENERAL CONSTRUCTIONAL DETAILS.
A sketch of the tube incorporating the cavity design and the gun structure
just described is shown in Fig. U. The tube incorporates two very important
features. First, for a relatively small structure, there is an available cathode
area of about one square centimeter. Second, the thermal properties are good
since there is a heavy copper wall to conduct heat along a relatively short path.
Figures 5> and 6 are top and side view photographs of an assembled tube with
out the glass enclosure shown in Fig. km Many structural details show up more
clearly in these photographs.