Computer design technique of electron gun for use in spacecraft transmitter

Knight, R. I.

Roberts, L. A.

English

NASA Technical Reports Server

DEVELOPMENT O F  A 25 - 50 WATT HIGH 
EFFICIENCY, X-B!iND, TRRVELING-WAVE TUBE 
Les ter  A.  Roberts 
Richard J. Knight 
and 
File No, JV-J 70-4085R8 
Contract No. 952931. 
This  work was performed f o r  the Jet Propulsion Laboratory,  California Instil Lit:> ~f 
Technology, a s  sponsored by the National Aeronautics and Space Administration 
under Contract NAN-100 .  
QUARTERLY REPORT NO. 2 
NOVEMBER 1970 - JANUARY 1971 
'C*Jnt!rins-Johnson Company 
3333 Hillview Avenue 
Palo Al to  , California. 
https://ntrs.nasa.gov/search.jsp?R=19710016455 2020-03-11T20:09:11+00:00Z
ABSTRACT 
This is the second Quarter ly  P r o g r e s s  Report on the development of a 25 - 50 watt, 
d m 1  mode, high efficiency, X-band space-type traveling-wave tube. During this 
quar te r ,  the electron gun design was  completed, the gun was  built and beam trajcc- 
tory measur.emeizts were  made, impedance matching design tests w e r e  completed, 
subassembly construction wzis ca r r i ed  out, PPM magnets were  received, the two- 
stage collector was designed and p a r t s  machined, and work was  being done on t h e  
attenuator design at the  end of the period. 
point where assembly of first tube should take place ear ly  in the next quar te r ,  
Technical p rog res s  had proceeded Go the 
SUMMARY 
The purpose of this project  is to develop a dual mode, high efficiency, X-ban 
traveling-wave tube suitable f o r  use as the power amplif ier  in a spacecraft t rans-  
mi t t e r  f o r  deep space communications systems. 
ments are 42 to 45 percent  overal l  efficiency, 25 to 50 wat ts  power outpul 
operating frequency range of 8.4 to 8 .5  GHz. Cathode design f o r  a lifetim 
ing 100 ,000  hours i s  required. 
The major  performance r 
The tube design is based upon l a rge  overvoltage techniques coupled with two stag 
depressed col lector  operation to achieve the efl'iciency performance. 
design is based upon rare-earth-cobalt magnet niaterials to achieve high perfor 
ance  P P M  electron beam focusing. 
Cathode technology. 
Magnet 
Long life design is based upon Coated Particle 
During this quar te r ,  the electron gun design was  ca r r i ed  out using the computee~ 
design technique, The gun was then constructed and tested f o r  beam trajeclory a n  
peiveance in  the beam testing system, Impedance match design and testing was 
completed f o r  the coax to helix transitions. Subassembly construction was carried 
out to the point where a tube could b e  assembled, The PPM focusing structure was 
built and tested with platinum-cobalt magnets. At the end of the  period %m::rium- 
Cobalt magnets were received and preliminany tests were  made. The two stage 
col lector  was designed and analyzed under saturated RF beam condition using 
computer techniques. Parts w e r e  made in  preparat ion f o r  fabrication. 
of the  per iod,  work on the final internal attenuator coniiguration was undca-vl,ag, 
The first tube should be  assembled ear ly  in  the next quar te r .  
At the en 
- 
1. INTRODUCTION 
Scope and Purpose of the Project  
The purpose of this project  is to develop a dual-mode, high efficiency X-band 
traveling-wave tube which can be used a s  a spacecraft  t ransmit ter  f o r  deep--:<p~~=c: 
communications systems. It is required to operate  at the efficiency level of 42 l o  
45 percent  in the power level range of 25 to 50 watts over  the frcqirciicy range of 
8.4 to 8.5 GHz. The tube is to be  designed to meet all requi remmls  of spac:~r,i:1 
operation and launching environment. Life requirements demand cathode desi  g,m 
having a n  ultimate possible life capability of 100,000 hours. Detailed perloriii,] " c ~ c  
requirements  are given in Specification No. CS505093 and the enviroiiinental 
requirements  are given by Specification No. TS 504550 (Mariner X'iars 1971),  
Easis of Tube Design 
The high efficiency tube design is based upon the technology devctopccl at M7;11ki13s- 
Johnson Company of l a rge  over-voltage operation of a traveling-wave tube, ti:;? 
t h i s  technique, a high value of basic beam efficiency can be achieved, The ovci- 
efficieiicy of the tube is fur ther  increased through the use of depresxd  coll~:'::~~ 
operation. 1mpro:iernent in normal dcpressecf collector efficiency mill. be acce~ 
by using a two stage collector system wherein electrons of diffei*cnt velociiy c l i  
can be sorted out and collected at optimum potentials. 
X-band operation imposes cer ta in  rest r ic t ions on the TWT design due to tlzc 14 
beam cur ren t  density necessa iy  €or  th i s  type OS high efficiency operation_. 
general ,  this fo rces  the use of low7er values  of beam pem'eance i n  t h e  helix L'C 
than can be achieved at lower frequencies. This  present  design utilizes the hi7acr 
coercive fo rce  of new magnetic mater ia ls  such as s a m a r i u ~ ~ - c o b a ~ t  to providc ti;e 
high magnetic flux density in thc electron beam necessary to focus the high v a l u r s  
of cu r ren t  density. Use of the new rare-earth-cobalt malcr ia ls  allows the  use of :i 
high value of plasma wavelength to magnetic period ratio in the pci-iodic pcrti-i? 
magnet focusing structure.  Th i s  a s s u r e s  good focusing under large- signal, de 
collector conditions. 
In 
Long life operation of the cathode of the tube will be  accomplished by using Conlccl 
Particle Cathode (CPC)  technology. Th i s  is a special form of the oxide cathode' 
D. W. h l au re r  and C. M. Pleass, The CPC: A Medium Current Dmsily High 
Reliability Cathode, BSTJ Vol. XLVI, No. 1 0 ,  Dec. 1967. 
1 
- 1 -  
which overcomes most of the major life limiting factors of the simple oxide catb 
This is accomplished by coating each particle of the basic triple-carbonate e-ith 
coating material with a microscopically thin shell of high purity nickel material. 
These particles a r e  then coated onto a nickel substrate which provides reducing 
agents at  a controlled rate f o r  converting the barium oxide material to free barium. 
The C P C  cathode combines a high current density capability with low teinper;lture 
operation and long-life performance. 
- 2 -  
11. WORK ACCOMPLISHED DURING THIS QUARTER 
Additional Phase  Velocity Measurements 
Additional phase velocity measurements  were  made on the helix structure.  These 
were made on the helix of a design corresponding to the output section of t h e  
32 watt design described in  Table I of Quarterly Report No. 1. This  Iiad 100  TPI 
v e r s u s  the 75 T P I  of the experimental  helices measured be-Core. The normalized 
phase velocity data reduced f rom these  more  recent  in ea surements  l ie within oilre 
percent  of the data previously shown as  Helix No. 2 in Figtire 1 of Quarter ly  
Report No. 1. These most  recent  data were  then used as the normalized mode 
f o r  all dcsign and analysis  calculations based upon this helix configura 'Lion with 
four  Be0 supporting rods. 
Electron Gun  Desivn 
Following the initial calculations on the WJ-251 gun design, modification coinpuII:i 
tions were  begun using the electron gun computer progrxin. 
i nc rease  the distance f rom the anode electrode io the point where the beam m a e l ~ c t l  
its minimum diameter.  
computations were made i n  a serics of steijs start ing from the WJ-251 gun desi;,:? 
and moving s a r i o u s  surs'aces of the gun elemeiits to i m p ~ o v e  the  bcniii t ra j  eelory 
and laminnrity. A s e r i e s  of 21 sequential calculations were made 'Lo a r r i v e  at Inc? 
final gun element configuratjon. Some shortcuts  were  used on the c a l c ~ ~ l a t i o n s  to 
save computer t ime by s tor ing space-charge matr ix  iniorinatiqn and using it Lo 
reduce the number of relaxations required €or  the calculations to coi>-\rezge. T h e  
final beam trajectory and electrode configuration is shown in Figure 1.. This p 
does not include the effect of thermal  electrons. 
It was dcsived io 
It was a l so  desirable  to improve beam lsminarity. Thesc, 
The calculations showed that the new design would have severa l  inip'l'ovements. 
First the gun perveance increased i rom the value of 0.177 micrope-ms for the 
WJ-251 gun to a value of 0.285 micropervs.  This will resu l t  in a lower OperatllJg 
anode voltage, Second, the anode to beam minimum distance increased f rom 115 
inch to . 4 1 0  inch which allows a much bet ter  gun polepieee confignration. Thi 
the cur ren t  density a c r o s s  the suxface of the cathode became v e r y  unis'orni, X 
iollowing Table gives the  pertinent pa rame te r s  for  the  32 watt  design. 
- 3 -  
~ 2 3 4 6 9  - 4 -  
Pew eanc e .285 micropervs 
Cathode Current  Density , 3 4 0  amps/cm 
Beam Current  49 niA 
Cathode Diameter ,172 inches 
Beam Diameter 
Anode to Beam Min. Dist. .410 inches 
2 
.016 (without thermal  electrons) 
The  design w a s  then constructed and placed under t e s t  in the beam tester belt jar 
system. 
ion-focusing effects and the cu r ren t  density i n  the beam cross-section can be 
scanned with a pin-hole collector system. 
axially away f r o m  the gun anode as well as a c r o s s  the beam which then gives 
cu r ren t  density profile measurements  as a function of distance f r o m  the gun. 
t h i s  way, the position of the minimum beam diameter  can be determined ns we 
as the diameter  of the electron beam as  a function cf distance. 
In this system, the gun i s  tested under pulsed conditions to eliminate 
The pin-hole collector can bc  moved 
In 
Various perturbations of the design positions of the cathode with req3ect l o  the 
focus electrode and the anode with respect  to the cathode w e r e  tried. It was forand 
that the position of the beam minimum occurred slightly ahead of the design location 
and that the measured minimum beam diameter ,  which includes therinal eiectrons, 
was s o m e w h ~ t  l a r g e r  than predicted. 
of the electrode positions w a s  determined which gave the smallest  beam mini imum, 
Th i s  produced a beam diameter  of 0.023 inches as compared with the desired vzi 
of 0.017 inches. Beam perveance measured to be {he value predicted by the coni-- 
pu te r  calculations. Figure 2 shows the measured beam proflle f o r  the optimum 
This w a s  expected. The optimtzm cornhimtioi: 
I__ - 
design. - 
It is planned to u s e  th i s  gun design in  the f i r s t  tube to evaluate its pcrformanee. The 
additional reduction in beam diameter  can easily be accomplished by magnetic 
convergence in the first few entrance cells of the P P M  stack. 
Impedance Matching Tests 
Helix impedance measurements  w e r e  made using an H P  Network Analyze3.. With 
great care taken to minimize connecting l ine lengths and spurious discontinuili e s  of 
connectors, smooth data w a s  measured ove r  the X-band frequency range. Helix 
impedance va r i ed  f r o m  175 ohms to 200 ohms over  the 8.2 to 12.4 GHz range, 
- 5 -  
060 
.040 
* 020 
0 
- 0 .zoo .400 600 
Distance from Anode - Inches 
Fig. 2 - Plot of measured beam diameter vs distance from the anode of the 
aa 23645 electron gun. - 5 -  
The technique used to match the helix impedance to the 50 ohm coaxial fine imp 
cons is t s  of a quarter-wavelength t ransformer  in  the coaxjal line section betwce 
vacuum window and the beginning of the helix. This  technique can  be used at  
because the  short  wavelength allows it to physically f i t  into this space. Since o 
relatively narrow band matches are required f o r  the limited frequency range 
requirement ,  a single quarter-wave t ransformer  is used. F igure  3 is an exyan 
Smith Chart  plot of this match a c r o s s  the 8.0 to 9 . 0  GHz range. Over the r e  
frequency range of 8.4 to 8.5 GKz the VSWR is less than 1.07:l. This  ploi inelu 
the effects of the OSM coaxial connector, the vacuum window, the t ransformer ,  a 
the helix t e rmi  nation, 
Subassembly Fabrication 
Parts and j igs  were  received f r o m  the machine shop during the qua r t e r  and t h e  various 
assembl j e s  w e r e  fabricated and checked f o r  assembly problems and vacumi  ii7tegritJT- 
An exploded view of the tube assembl ies  is shown in Figure 4. 
assembled as  they mill be in a completed tube is shown i n  Figure 5. 
assignment number is WJ-3703. 
collector. 
A view of the parts 
The Lubc type 
This view shows the tube  with the single-stage 
The major  difficulty encountered in  the assembly hrzs been in drawing thc hcjix b a r r e l  
tubing to precisely the right inside diameter.  The h d j x  supportcd by f o u r  ro6s insid:: 
a deformed b a r r e l  requi res  more  cr i t ical  dimensional control over defori11 
a conventional th ree  rod helix. 
Our plans w e r e  to quantitatively measure  the  ba r re l  ID before and during d 
tion with a precis ion air gauge which can measure  dimensional changes aczuii*alely 
to 0.0001 inch. Delivery on this gauge has  been delayed until ear ly  in the nest 
ter so alternative ways of controlling deformation have had to b e  worked out, It has 
been necessary  to draw sets of tubing with the  inside d iameter  increased in  steps of 
0.0005 inch. 
based upon t r i a l  and e r ror  tests on finished helix assemblies .  
caused some delay. 
The small  b a r r e l  size aceeniuates this  CiiCIiculty, 
The choice of the co r rec t  s i ze  of tubing to use has  had to be made 
This procedure bas 
PPM Focusing Structure  
A t  the design beam p e m m  nee value of 0.46 micropervs,  i t  is necessary  lo rase 
coercive force  permanent  magnets to simultaneously meet the requiremeills for 
axial magnetic flux density and shor t  magnetic period. A t  the desired .crztlr.c of the  
ra t io  of plasma wavelength to magnetic period, h /L, of 2. 8, sariiarium-,cob:irt 
magnets are required. At the lower value of hp /L = 2.2,  platinum-cobalt magnets 
1' 
- 7 -  
Fig. 3 - Expanded Smith Chart Plot of the coax l o  heli;.: transition including the 
vacuum window. 
aa 23647 - a -  
- 9 -  
- 10 - 
can  be  used. 
Cerium-cobalt magnets w e r e  also considered as  a substitute f o r  platinum-cobalt, 
but the bid p r i c e  was higher than platinum-cobalt. 
evaluated with both magnetic period values given above. The possibility of a hybrid 
stack will a l so  be investigated, 
Both platinum-colxilt and s:~iii;~rium-cobalt magnets w e r e  ordercd,  
Focusing performance will be 
W e  have recently received delivery of magiretizing and demagnetizing cquipmenl 
suitable f o r  u s e  with samarium-cobalt material ,  
Platinum-cobalt magnets w e r e  received in the middle of January. A complete 
magnet stack f o r  the first tube was built up and the field pattern 1 ~ ; ~ s  refined to 
give uniform peak fields by programming the individxal rxngnet cells. 
f o r  the first tube. 
It is ready 
The samarium-cobalt magnet inaterial was delivered just  at the end of the period, 
Prel iminary checks showed tiia t the  magnetizing equipment is capable of demagnct- 
izing the material  to one-half magnetizat!:ori with the autorriatic system and is 
capable of knocking thc magnetization all the way to zero with manual opera io::, 
The magnetization cannot be  reversed f rom the delivered polarity and be corny1 c.lclv 
saturated.  
to the saturated valuc. 
The magnets can, h o ~ ~ e v e r :  be remagnetized in the delivered po 
Attenuator 
The remaining work to be done before assembly of the first tube is to finalize-the 
attenuator configuration. 
supporting rods  of the helix. 
being workcd out with sprayed carbon coating. 
it will be duplicated with a pyrolytic carbon deposition. 
The loss  is being ilcpositecl o n  two of the €our ceramic 
Pat tern length, t a p e r s  and surface resist ivity a r c  
Wheii the desi red pat tern is aclii w e d ,  
The  attenuator work should be completed ear ly  in  the nest  period. 
will then be assembled. 
The f i r s t  tube 
Two Stage Collector 
The 'avo stage collector configuration was  analyzed using a computer program simi iiir 
to the electron gun program except that it a l l o ~ s  the electron beam to be injcetecl 
into the electrode region with an a rb i t r a ry  electron di stribution and an a r b j t m r y  
velocity on each electron. 
velocity f r o m  the helix in the l a rge  signal TWT calculations, 
was chosen a s  the most pessimist ic ,  that  i s ,  the fastest chlectrons on the axis pro- 
gressing to the slowest electrons on the outside surface of the electron beam. The 
trajectory plot is shown in Figure G. 
The velociiy distribution wL: s chosen a s  the exitiag 
The radial distribution 
- 11 - 
aa 23646 - 12 - 
An exploded view of the collcctor assemhly parts is shown in Figure 7.  
s tage  col lector  is planned f o r  u s e  on Tube S/N 2. 
not taken place a s  yet. 
T h e  tn.0- 
The f i r s t  assembly of p a r t s  has 
Program f o r  t h e  Next Quar te r  
Tube construction and testing will be the msjor activity f o r  the next quarter, 
planned to build and t e s t  tubes S/N 1, S/N 2 ,  arid S/N 3 .  
design will be made based upon the measured results. 
procedures  will take place as required during this  period. 
It is 
Modifications to the helix 
Refinements in  assembly 
- 13 - 
- 14 - 
This  r epor t  contains information prepared by Watkins- 
Johnson Company under J P L  subcontract. 
is not necessarily endorsed by the Jet P r o p d s i o n  Labora- 
tory, California Institute of Technology, o r  the N a l i o d  
Aeronautics and Space Administration. 
Its content 


Development of a 25 - 50 watt high efficiency, X-band, traveling wave tube Quarterly report, Nov. 1970 - Jan. 1971

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