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Submitted on 1 Jan 1979
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RADIOFREQUENCY DISCHARGE TESTS WITH HELIUM FLOW
M. Talaat
To cite this version:
M. Talaat. RADIOFREQUENCY DISCHARGE TESTS WITH HELIUM FLOW. Journal de
Physique Colloques, 1979, 40 (C7), pp.C7-165-C7-166. �10.1051/jphyscol:1979781�. �jpa-00219487�
JOURNAL DE PHYSIQUE Colloque C7, suppl6ment a u n07, Tome 40, J u i l l e t 1979, page C7- 165
RADIWREQUENCY DISCHARGE TESTS W I T H HELlUM FLOW
M.E. Talaat.
U n i v e r s i t y o f Maryland, Department o f Mechanical Engr. C o l l e g e Park, Maryland 20742, U.S. A.
Steady s t a t e eleven magacycl e radiofrequency discharge c a s e ) , the RF discharge, e.g. a t 0.682 discharge t e s t runs were conducted with helium ampsre and ( E / P ) = 1.89 V/cm-torr, tended t o con-
0
flowing inside a quartz tube, having an ID of 2 . 2 s t r i c t near the center and t o extend, almost sym- cms, a t reduced pressures, po=pgas (273.1 5/Tgas), metrically, beyond the electrodes a t both ends.
i n the range of 210 t o 360 t o r r s , flow veloci t i c s , (b) A t a helium flow velocity of about 480 me- u , in the range of 440 t o 485 m/s and e l e c t r i c ters/second, with the external high voltage e l e c t - f i e l d t o reduced pressure r a t i o s , ( ~ / p , ) , in the rode being upstream and the grounded electrode range of 1.7 t o 3.7 V/cm-Torr. External cyl inclrical being downstream, the RF discharge (e.g. a t 0.495 electrodes which were wrapped around the 2.5 cm OD ampere and
€ 1 ~ ~
of 3.574 vots/cm-torr) extended of t h e quartz tube provided the terminals f o r ap- about 2.2 cms upstream beyond the high voltage plying the raiofrequency discharge voltage. Eacl? ring electrode opposite t o the direction of the gas electrode had an axial length of about 2.4 cms andthe two electrodes were separated by an axial i n t e r - electrode distance of 2.2 cms, with one electrode being connected t o the ground of an 800 watt, 11 megacycle RF power supply. The t e s t section was f i t t e d i n t o the closed 1 oop Magnetopl asmadynami c (MPD) F a c i l i t y of the University of Maryland, and the system was operated w i t h high purity heliuo under steady s t a t e conditions a t the desired mass flow r a t e s , pressure and temperature l e v e l s . The radiofrequency ionization of the high velocity helium was maintained by establishing a 0.1 t o 0.5 ampere steady s t a t e 11
MHz
discharge p a r a l l e l t o the flow axis of the quartz tube t e s t section and the RF discharge current versus the applied RF voltage acro.ss t h e external ring electrodes were measured f o r each s e t of steady s t a t e dynamic oper- a t i n g conditions.Several important observations were made, both i n regard t o the shape of t h e RF discharge and i t s q u a n t i t a t i v e character. In regard t o t h e shape of the PF discharge i n helium with flow:
flow, b u t the discharge was not constricted.
( c ) A t a helium flow velocity of about 465.4 meters per second, with the external high voltage electrode being downstream and t h e grounded e l e c t - rode being upstream, t h e RF discharge (e.g. a t 0.455 ampere and €/pO of 3.212 v o l t s cm-torr) extended by about 2.2 cms downstream beyond t h e high voltage ring electrode in the direction of the He gas flow.
Y'
HELIUM I I MHzH V E L DS
( a ) When the flow velocity was on t h e order of one o
1.5 210 2.5 (E/P,{ V/CM-TORR
meter per second o r l e s s ( i .e., almost the s t a t i c
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979781
Again t h e discharge was n o t c o n s t r i c t e d .
From a q u a n t i t a t i v e viewpoint, Figure 1 gives p l o t s of t h e RF discharge c u r r e n t , I , versus ( ~ / p , ) , f o r f o u r d i f f e r e n t helium flow v e l o c i t i e s v i z . u <
1 meter/second ( i :e. ? nearly t h e s t a t i c discharge c a s e ) , u=451.8 m/s, u=465.3 m/s and ~ ~ 4 8 5 . 1 m/s.
These curves correspond t o d a t a taken with t h e voltage ring e l e c t r o d e being l o c a t e d downstream and t h e grounded e l e c t r o d e l o c a t e d upstream. They i n d i - c a t e c l e a r l y t h e s e n s i t i v e dependence of t h e RF d i s - charge c u r r e n t f o r a given value of ( E / P ~ ) on t h e flow v e l o c i t y u. Thus, f o r example, a t an ( ~ / p , ) value of 2.42 volts/cm-torr, t h e observed RF d i s - charge c u r r e n t decreases s u b s t a n t i a l l y from a value of 0.936 ampere a t a flow v e l o c i t y u < 1 m/s t o 0.194 ampere a t u=451.8 m/s, t o 0.155 ampere a t u=465.3 m/s t o 0.1 16 ampere a t u=485.1 m/s. Or s t a t e d d i f f e r e n t l y , we f i n d t n a i , f o r exaniple, f o r an RF discharge c u r r e n t o f 0.325 ampere, t h e r e q u i r - ed ( E / P ) value t o maintain t h a t c u r r e n t i n c r e a s e s
0
from 1.39 V/cm-torr, f o r t h e flow v e l o c i t y of l e s s than one meter per second t o 2.74 V/cm-torr f o r a flow v e l o c i t y of 451.8 m/s t o 2.92 V/cm-torr f o r a flow v e l o c i t y of 465.3 m/s t o 3.39 V/cm-torr f o r a flow v e l o c i t y of 485.1 m/s.
Figure 2 g i v e s p l o t s of t h e q u a n t i t y I / ( E / P ~ ) versus ( E / P 0 ), f o r t h e same s e t of f o u r helium flow v e l o c i t i e s of Figure 1 . The q u a n t i t y I / ( E / P ~ ) gives us a measure of t h e e l e c t r o n d e n s i t y generated by t h e discharge. For example, i f we c o n s i d e r , t h e e l a s t i c c o l l i s i o n frequency of t h e e l e c t r o n s t o be a c o n s t a n t with energy of t h e e l e c t r o n s and equal f o r he1 i u m t o (2.55)lO po second-' (F. 9 H. Reder and
S. C. Brown, Phys. Rev., Vol. 95, Nov. 4 , 1954, p.
885), then t h e e l e c t r o n d e n s i t y nee a t t h e c e n t e r of t h e discharge may be obtained by multiplying t h e q u a n t i t y I/(E/P,) i n ampere-cm-torr/vol t by
(9.0508)lO /yn (Ai sq cm) 18
,
where Ai i s t h eHELIUM I I MHZ L . 2 . 2 C M S r,,= 1.1 C M H V. E.L. D S
FIG.
2
discharge c r o s s s e c t i o n a l a r e a i n s q . cm., and yn i s an averaging f a c t o r which depends on t h e r a d i a l s p a c i a l d i s t r i b u t i o n of t h e e l e c t r o n d e n s i t y ( e . g . yn = 0.4323 f o r a zero o r d e r Bessel function d i s t r i b u t i o n ) .
We can s e e from Fi-~ure 2 t h a t i n o r d e r t o maintain a c e r t a i n value of l / ( € / p o ) ( o r t h e corresponding value of
