Metal X-band waveguide cell for study of microwave propagation through magnetoplasm

Holt, E. H.

Noon, J. H.

Reynolds, J. F.

English

NASA Technical Reports Server

Rensselaer Polytechnic Institute 
Troy, New York 
AN X-BAND WAvEGulDE CELL FOR 
STUDY OF MICROWAVE PROpAGBTIOl4 
"ROUGH MAGNEPOFUSW 
by J . H .  Noon, E.H. Holt, J.F. Reynolds 
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PIASMA RESEARCH IABORAIMRY 
TECHNICAL REPORT NO. 16, November, 1964 
This work was partially supported by the National Aeronautics and 
Space Administration under grant no. NsG 48-60. . 
https://ntrs.nasa.gov/search.jsp?R=19650011715 2020-03-16T23:51:00+00:00Z
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Abstract 
A metal X-band waveguide ce l l  has been developed for studying micro- 
wave propagation through plasmas immersed i n  a longitudinal magnetic f ie ld .  
The c e l l  consists of a length of circular waveguide mounted between tu rns t i l e  
junctions. 
charge electrodes on the magnetic f i e ld  axis. 
discharge can be operated successfully a t  pressures i n  the range 0.25 t o  1 
Modification of these junctions permits the in s t a l l a t ion  of dis-  
No glass c e l l  i s  required. The 
Torr. Measurements of the changes in  the amplitudes of the circular  components 
of the wave polarization due t o  the presence of a magnetoplasm i n  the ce l l ,  
allow the electron density and electron col l is ion frequency t o  be calculated. 
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Introduction 
The use of glass  discharge vessels fo r  guided wave propagation of 
microwaves through a p lasm has several disadvantages: construction and 
assembly a re  more d i f f i c u l t  than with an all-metal system, and uncertainties 
In  the  appropriate boundary conditions are introduced by the presence of the 
glass.  
t o  have both discharge electrodes p r a l l e l  t o  the axis of the vessel  and t o  
Where a longitudinal magnetic f i e l d  is involved,i t  is  also advantageous 
the direct ion of microwave propagation. This axial electrode symmetry 
minimizes w a l l  e f fec ts ,  i n  contrast t o  discharge vessels used by other workers 19.2 
i n  longitudinal magnetic f ie lds .  F i g u r e  1 shows two types of glass discharge 
c e l l  i n  which one or both electrodes are perpendicular t o  the  axis of the ce l l .  
It has been found experimentally t h a t  with asymmetrical electrode construction 
the  plasma may be displaced i n  the  magnetic f i e l d  t o  the  extent of s t r ik ing  
one of the w a l l s  of the  glass  vessel, even f o r  magnetic f i e l d s  as low as 600 
gauss and discharge currents as low as two milliamps. This e f f ec t  does not 
occur with symmetrical electrode design f o r  f i e l d s  as high as 6000 gaus.s. 
This s i tua t ion  i s  represented diagrammatically i n  Figures 2a and 2b. Discharge 
vessels ranging i n  length from 15 t o  50 cm and i n  diameter from 1 t o  5 cm have 
all shown t h i s  e f fec t  fo r  pressures ranging from 2 Torr t o  0.2 Torr. 
Several other types of discharge vesse l  which could be incorporated 
i n  waveguide geometry were t r i e d  out i n  magnetic f i e lds .  The a l l -g lass  
design of Fig. *,with extended glass tubing,did not prevent w a l l  e f f ec t s  
near the  electrodes and i s  d i f f icu l t  t o  assemble. The hollow cathode d is -  
charge c e l l 3  of Fig. 3b is  more convenient t o  mount i n  conjunction with waveguide 
1. V. E. Golant, e t  al, Sov. Phys. Tech. Phys. 6, 
2. L. Goldstein, e t  al, p. 121, "Electromagnetics and Fluid Dynamics of Gaseous 
Phsma," Polytechnic Press (Interscience), (1962). 
3. K. C. Stotz,  NASA TN D-226, (1963); E. H. Holt, K. C .  Stotz,  Rev. Sci. Ins t .  
34, 1285, (1963). 
(1961). 
2 
Components but a l so  suffers from wall e f f ec t s  which extinguish the discharge 
i n  ,mgnetic fields. 
A sa t i s fac tory  design was achieved using symmetrical axial electrodes 
i n  a cy l indr ica l  metal c e l l  which was coated with an insulating layer of 
glyptal .  
porating a t  each end a modified tu rns t i l e  
waveguide ports and a coaxial metal stub assembly. The inner insulated metal 
rod  of t he  coaxial stub functions both as part of the  microwave matching un i t  
Microwave propagation through the c e l l  w a s  made possible by incor- 
consisting of four X-band 
and as a discharge electrode. Pressure windows vacuum seal the waveguide. 
The size of the circular  guide r e s t r i c t s  propagation t o  the  
permits both senses of c i rcular  polarization. 
TEll 
The change i n  c i rcu lar  components 
mode but 
of the polarization due t o  the magnetoplasma allows electron density and 
co l l i s ion  frequency t o  be determined direct ly .  
therefore  possible with the  ce l l :  f o r  example, a study of diffusion decay of 
a plasma i n  a magnetic f i e l d  , orof microwave propagation i n  the direct ion of 
A var ie ty  of experiments are 
6 
the  magnetic f i e ld  through a magnetoplasma. 7 
Microwave Design 
I n  the  cell the  modified tu rns t i l e  junction i s  used as a six-port de- 
vice i n  which c i rcu lar  and rectangular ports  are matched simultaneously by 
the  adjustment of two coaxial cyl indrical  metal stubs A and B, shown i n  Fig. 4. 
When correct ly  matched the theoret ical  power d is t r ibu t ion  between the  various 
ports  f o r  different  input modes i s  shown i n  Tables 1 and 2. 
I n  the  ac tua l  ce l l ,  see Fig.  5, it i s  possible t o  launch any polarization 
4. R. S. Potter,  NRL Rep. 4670, (1955). 
5. 
6 .  
7.  D. W. Mahaffey, Phys. Rev. 129, 1481, (1963). 
P. J. Allen, R. D. Tompkins, Proc. IRE 41, 1231, (1959). 
V. E. Golant, A. P. Zhilinskii ,  Sov. Phys. Tech. Phys. 2, 699, (1961). 
~8 
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down the circulaz guide by appropriate input conditions. 
polarized wave i s  fed  i n  port 1 with matched terminations on the  other three 
ports,  a l i nea r ly  polarized wave w i l l  be propagated. 
c i r c u i t s  on arms 2 and 4, it i s  possible t o  generate c i rcular  or e l l i p t i c  
polarizations.  If the  t u r n s t i l e  junctions are correct ly  matched, there w i l l  
be no losses  of power by re f lec t ion  o r  through t h e  coaxial transmission l i n e  
of the matching stubs. 
If a l inear ly  
With adjustable short  
The approximate dimensions of the coaxial stub assembly t o  be used as 
t h e  matching u n i t  were eas i ly  estirnated. 4y 
involved a compromise between the microwave design, the requirements f o r  
However, the ac tua l  construction 
sui table  vacuum sealing, and for  discharge electrodes which would operate 
sa t i s fac tor i ly .  The waveguide window assemblies were brazed t o  a short  brass 
cylinder of 2" inner diameter and 1/4" w a l l  thickness. A 3" t r ans i t i on  taper 
section w a s  then brazed between the waveguide ports and the cy l indr ica l  wave- 
guide of inner diameter 2.06 cm. An overa l l  length of 46.5 cm between the 
waveguide ports at  opposite ends was chosen t o  suit the available magnetic 
f i e l d  coi ls .  
12", so that  assembly of the waveguide c e l l  presented no problem. 
preliminary t e s t s  were carr ied out i n  another solenoid magnet w i t h  an in t e rna l  
diameter of only 6". 
chosen f o r  the c e l l  ends with waveguide pressure windows attached. 
The air-core solenoid magnet used had an in te rna l  diameter of 
However, 
For t h i s  reason a m a x i m  overa l l  diameter of 5" w a s  
Vacuum and Discharge Considerations 
U l t e k  curvac flanges were used t o  vacuum s e a l  the  open end of the 2" 
diameter brass cylinder. The whole matching un i t  w a s  constructed as an 
in t eg ra l  part of one of the  vacuum flanges so that it could be eas i ly  removed, 
adjusted, or replaced. The electrode stubs were of tungsten and the insulat ion 
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used w a s  teflon. 
the end of the te f lon  where loca l  heating from the discharge would otherwise 
cause deterioration of the insulating material. 
However, as shown i n  Fig. 6, a ceramic t i p  was added t o  
A more sat isfactory vacuum vessel could be obtained by using s ta inless  
s t e e l  throughout, ra ther  than brass, and by the use of ceramic insulation 
which could be soldered d i r ec t ly  t o  the  metal. Another improvement would 
be the use of te f lon  insulation on the in t e r io r  w a l l s  of the discharge vessel 
instead of gly-ptal. However, t e s t s  have shown that the e l e c t r i c a l  insulation 
of glyptal  withstands baking at temperatures of 600%, so that a low temper- 
a ture  bakeout of the vacuum system is  possible. 
A prototype metal c e l l  with 1/4" holes along the guide, sealed by waxed- 
i n  glass  windows, has been used t o  make visual  observations of the discharge. 
A glass  c e l l  with ident ica l  electrode structure and similar geometry w a s  a l so  
used. 
Torr and currents of 5 milliamps. The discharge i s  quite s table  and f i l l s  
the whole vessel  uniformly. However, i f  one electrode and the metal w a l l s  
a re  both at  ground potent ia l  the  discharge becomes unstable and the p l a sm 
near the electrodes tends t o  be displaced towards the adjacent w a l l s  i n  sp i t e  
of the layer of gly-ptal insulation. 
pulsed operation, an isolat ing transformer, the discharge i s  quite st'able. 
When a magnetic f i e l d  i s  applied the discharge constr ic ts  and for  a 
f i e l d  of 1000 gauss i s  r e s t r i c t ed  to  the cent ra l  region w i t h  a radius of approxi- 
mately 1 cm. 
A discharge can be successfully operated a t  pressures from 0.25 t o  1 
With a "floating" DC supply or, for  
Matching Considerations 
Adjustment of the matching units at  both ends w a s  made t o  give a constant 
frequency response over as broad a range as possible. Th i s  i s  necessary because 
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t he  change i n  d i e l ec t r i c  constant of the plasma i n  a magnetic f i e l d  alters 
the  e f fec t ive  wavelength of the propagated microwave signal. An X-band sweep 
osc i l l a to r  w a s  used t o  deliver power t o  t h e  c e l l  and t o  generate the osci l lo-  
scope sweep display of the response of the c rys t a l  detectors mounted on the  
output ports. The dilnensions of the components of the  matching units when 
correct ly  adjusted are shown i n  Fig. 6 and the  frequency response of the c e l l  
i n  Fig. 7. 
s t an t  t o  within 0.5 db, but drops o f f  outside t h i s  range. 
Within the  range 9.4 t o  9.8 G c / s  the  transmitted s ignal  i s  con- 
The re f lec ted  power l e v e l  is  more than 30 db below the  incident p o w e r  
level. 
l eve l  of (25  db. 
would be required t o  achieve th i s .  
The amount of cross-coupling between adjacent arms represents a power 
This should be zero but perfect symmetry of the junctions 
Analysis of Microwave Output Signals 
The signals from the  f o u r  waveguide output ports  provide information 
about the change i n  polarization of t he  propagated I24 wave due t o  the magneto- 
plasma. The polar izat ion parameters are defined i n  the  following way: 
L e t  the  orthogonal components of the  e l e c t r i c  f i e l d  be represented by 
E = A1 C O S ( - )  1 
A1 and A are the  amplitudes of the components, 2 where 
w = t he  angular frequency 
$ = the  phase difference 
The amplitude r a t i o  A2/A1 may be wri t ten as 
A /A = t an  012, O <  0 <n, 2 1  
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and the  right and l e f t  c i rcular  components can be expressedsyg as 
C+ = k o s  0/2 + s i n  ( 8 / 2 )  s i n  @ - i s i n  (9/2) cos @ ] / 2  
The response of crystals  mounted on ports 1 and 2 a t  the output end can be 
used t o  obtain 0. The outputs of ports 3 and 4 are combined a t  a hybrid 
junction and the difference i n  e l ec t r i ca l  length between port 3 and the 
hybrid junction and port 4 and the hybrid junction i s  adjusted by means of 
a phase sh i f t e r  t o  be a multiple of n/2. 
. 
8 It can be shown that the magnitudes 
of the signals from crystals  mounted on the H and E arms of the hybrid 
junction are  the same as the mgnitudes of the r igh t  and l e f t  c i rcular  com- 
ponents of the wave. Thus the second polarization parameter @ can be de- 
termined. 
If the amplitudes of the circular wave components entering and leaving 
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the magnetoplasma are  C+ and C - and C+ and C - respectively, then 
where k+ = the propagation constant of the appropriate c i rcular  component - 
= B+ + a+ 
where f3+ = the phase s h i f t  per unit  length - 
a+ = the attenuation per unit  length 
- 
L = the length of the magnetoplasm i n  the direction of propagation. 
It i s  possible t o  evaluate the phase s h i f t  and attenuation parameters separately. 
8 
9. 
R. E. Haskell, Rensselaer Polytechnic Ins t i tu te ,  Plasma Resemch Laboratory, 
TR 8, (1963)- 
K. C. Westfold, J. Opt. SOC. Am., Q, 717, (1959). 
7 
Values of the r e l a t ive  output signals a t  9.5 Gc/s in arms 1 and 2 f o r  
a discharge current of 4 ailliamps are shown i n  Fig. 8 f o r  various values of 
magnetic f i e ld .  These show the sens i t iv i ty  of the technique since qui te  
large s ignals  are obtained. 
Relation t o  Plasma J?arameters 
The ro ta t ion  of the  plane of polar izat ion per unit length is  given by 
When the electron collision frequency Y 
angular frequency u), 
s i t y  ne direct ly .  It is necessary t o  know the radius of the region contain- 
ing the  plasma re l a t ive  t o  the radius of the guide, so  that v isua l  inspection 
of the plasma column i n  the  magnetic f i e l d  i s  required. 
When u) 
i s  very much less than the microwave 
can be used t o  determine the average electron den- 
- 
i s  close t o  the cyclotron frequency, )U w i l l  be dependent 
on both ne and )r . The col l is ion frequency )f may be calculated from 
the  attenuation factor. 
I '  . 
Port 
Power k v e l  
TABU3 I 
1 2 3 4 5 6 
P P/4 0 P/4 P/2 0 
Power d is t r ibu t ioh  between the  various ports  when microwave power 
enters  through one of the rectangular arms. 
I. 
Port 
Power Level 
TABU3 11 Power d is t r ibu t ion  
enters  through the 
1 2 3 4 5 6 
~ / 2  o ~ / 2  0 P 0 
between the various ports when microwave 
circular guide. 
power 
TAB= III End Matching Conditions f o r  Turnst i le  Junctions 
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Captions fo r  Figures 
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Figure 1 
Figure 2 
Figure 3 
Figure 4 
Figure 5 
Figure 6 
Figure 7 
Figure 8 
G l a s s  discharge vessels with off-axis electrodes A and C. 
(a) 
(b) 
Plasma between off-axis electrodes A and C displaced i n  an 
external magnetic f i e l d  towards vessel  walls. 
Plasma between axially symmetric electrodes A and C is  not 
i n  contact with vessel w a l l s  i n  an external magnetic f ie ld .  
(a) 
(b) 
G l a s s  c e l l  w i t h  extended glass tubing t o  r e s t r i c t  the 
plasm t o  the central  region of the vessel. 
X-band waveguide hollow-cathode discharge ce l l .  
Turnsti le junction comprising four rectangular waveguide ports, 
one circular  port, and one coaxial port. 
Schematic diagram of the waveguide c e l l  showing the modified 
t u r n s t i l e  junctions at each end of a length of c i rcular  guide. 
Schematic diagram of the coaxial metal stub assembly used fo r  
microwave matching and i n  which the inner rod ac t s  as  a discharge 
electrode. For dimensions see Table 3. 
Transmission characterist ics of the c e l l  as a function of frequency. 
Signal levels  of the two orthogonal output arms  for  a l inear ly  
polarized 9.5 Gc/s input s ignal  passing through the magnetoplasma, 
as a function of magnetic f ie ld .  
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An x-band waveguide cell for study of microwave propagation through magnetoplasma technical report no. 16

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