HAL Id: jpa-00219480
https://hal.archives-ouvertes.fr/jpa-00219480
Submitted on 1 Jan 1979
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
SOME CHARACTERISTICS OF AN ELECTRIC DOUBLE LAYER IN A MAGNETIZED PLASMA
L. Lindberg, S. Torven
To cite this version:
L. Lindberg, S. Torven. SOME CHARACTERISTICS OF AN ELECTRIC DOUBLE LAYER IN A MAGNETIZED PLASMA. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-151-C7-152.
�10.1051/jphyscol:1979774�. �jpa-00219480�
JOUIWAL DE PHYSIQUE CoZZoque C7, s u p p l 6 m e n t a u n 0 7 , Tome 40, J u i Z Z e t 1979, page
C7-
151SOME CHARACTEFUSTICS
ff
AN ELECTRIC DOUBLE LAYER IN A MAGNETIZED PLASMAL. Lindberg and S. Torven.
Department o f PZasma P h y s i c s , Royal I n s t i t u t e o f TeehnoZogy, S-100 44 S t o c k h o l m 7 0 .
E l e c t r i c double l a y e r s have r e c e n t l y a t t r a c t e d much i n t e r e s t and s e v e r a l new experimental r e s u l t s have been r e p o r t e d Ll] [2] [3] [LJ]
[5].
Recent reviews o f t h e phenomenon i n c l u d e a g e n e r a l s u r v e y [6] a s w e l l a s s e p a r a t e reviews o f e x p e r i - ments
IT]
and t h e o r yL8] .
S e v e r a l p o s s i b l e a p p l i -c a t i o n s t o space and a s t r o p h y s i c a l plasmas have been d i s c u s s e d
[S]
[lo] [ll].Most experiments s o f a r have been performed i n non- magnetized plasmas. S i n c e t h e e x i s t e n c e of double
l a y e r s i n magnetized plasmas has now been proved, much work remains t o s y s t e m a t i c a l l y i n v e s t i g a t e t h e p r o p e r t i e s of such double l a y e r s . The p r e s e n t paper r e p o r s s some i n i t i a l r e s u i t s .
The double l a y e r s t u d i e d i s one w i t h a monotonic p o t e n t i a l v a r i a t i o n b u t w i t h a r a p i d l y v a r y i n g spa- t i a l l o c a t i o n . I t i s a l s o accompanied by a compli- c a t e d spectrum o f waves with f r e q u e n c i e s from t h e i o n a c o u s t i c regime ( 5 kHz) up t o f r e q u e n c i e s l a r - g e r t h a n t h e plasma frequency (400 M H Z ) .
The double l a y e r i s produced i n a plasma column, r a d i a l l y c o n f i n e d by an a x i a l magnetic f i e l d (0.005- 0.15 T ) i n t h e vacuum chamber between E2 and A ( ~ i ~ . 1 ) . The plasma i s maintained by a source g i v i n g i n -
flow of plasma t h r o - ~ g h an a p e r t u r e i n E2, which de- f i n e s t h e diameter of t h e plasma column ( 1 . 5 cm).
The plasma source i s a dc a r c d i s c h a r g e a t about 1 mTorr between t h e mercury p o o l cathode C and t h e hollow e l e c t r o d e El. The mercury vapour p r e s s u r e i n t h e vacuum chamber i s k e p t t y p i c a l l y an o r d e r of magnitude s m a l l e r t h a n i n t h e source.. Double l a y e r s have been observed t o form f o r e l e c t r o n gyro f r e - quencies fce i n t h e i n t e r v a l f p / 3 5 f c e 5 5 f p . Here f i s t h e plasma frequency. The r e s u l t s repor-
P
t e d below r e f e r t o a weak magnetic f i e l d of 5 mT 1 8 -3 and a mercury atom number d e n s i t y of 3.10 m
.
As has been d e s c r i b e d p r e v i o u s l y [5], t h e double l a y e r evolves from an e l e c t r o n r i c h anode s h e a t h , when t h e axode p o t e n t i a l i s i n c r e a s e d t o some t e n s of v o l t s (depending on t h e background p r e s s u r e of
Helrnholz coils
F i g . 1. Schematic p i c t u r e of t h e d i s c h a r g e d e v i c e . A plasma column i s o b t a i n e d between E2 and A by i n - flow of plasma a l o n g t h e magnetic f i e l d through t h e a p e r t u r e i n E2. The plasma s o u r c e i s a dc a r c b e t - ween C and El.
n e u t r a l g a s ) above t h e plasma p o t e n t i a l . Then most of t h e o r i g i n a l p o t e n t i a l drop over t h e anode s h e a t h appears over t h e double l a y e r , and between t h e l a y - e r and t h e anode a new plasma ( t h e anode plasma) i s formed. The anode plasma i s m a i n t a i n e d by i o n i z a t i o n due t o e l q c t r o n s a c c e l e r a t e d i n t h e l a y e r , whereas i o n i z a t i o n p r o c e s s e s a r e n e g l i g i b l e i n t h e cathode plasma owing t o t h e low e l e c t r o n t e m p e r a t u r e t h e r e ( 2 eV). The diameter o f t h e anode plasma column i s a l s o l a r g e r t h a n t h a t of t h e cathode plasma.
Once t h e l a y e r has formed, it can be moved t o any p o s i t i o n between E2 and A by an adjustment of t h e d i s c h a r g e c u r r e n t . The l a y e r p o s i t i o n i s not s t a - t i o n a r y , b u t t h e l a y e r moves i r r e g u l a r l y back and f o r t h w i t h amplitudes somewhat l a r g e r t h a n t h e l a y - e r t h i c k n e s s . The t y p i c a l v e l o c i t y i n t h i s motion (300 m / s ) i s much s m a l l e r t h a n t h e i o n a c o u s t i c speed [5]. A c o i n c i d e n c e t e c h n i q u e i s used t o r e - duce t h e e f f e c t o f t h e s e f l u c t u a t i o n s on t h e mea- surement s. A t a p o i n t , through which t h e double l a y e r o s c i l l a t e s , t h e p o t e n t i a l i s sensed by means
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979774
of an e m i t t i n g Langmuir probe and compared with a f i x e d value. A t coincidence a s h o r t t r i g g e r i n g pulse i s produced, which I s u t i l i z e d f o r sampling of t h e s i g n a l s from o t h e r probes. Such a sampling has a l s o been made d i r e c t l y from oscillograms. Diagrams ( a ) and ( b ) i n Fig. 2 show p o t e n t i a l and e l e c t r o n den- s i t y (expressed a s a l o c a l plasma frequency) mea- sured with movable Langmuir probes. Hot, e l e c t r o n e m i t t i n g probes were used t o measure t h e p o t e n t i a l , and t h e density was obtained from swept characte- r i s t i c s of a cold probe. The sampling method re- duces e f f e c t s of t h e l a y e r motion. However, f l u c - t u a t i o n s 5nn t h e asymptotic l a y e r v o l t a g e , i n t h e f r e - quency range 5 - 20 kHz, a r e s t i l l present a s i n d i - c a t e d ( 6 + ) i n diagram ( a ) . The p o t e n t i a l l e v e l of t h e whole anode plasma f l u c t u a t e s r e l a t i v e t o t h e quiescent cathode plasma. P o t e n t i a l f l u c t u a t i o n s measured simultaneously a t any two p o s i t i o n s within t h e anode plasma appear t o be i d e n t i c a l (within t h e bandwidth of t h e probes, 0
-
200 kHz] implying a propagation speed l a r g e r than 105 m/s.Diagram ( c ) i n Fig. 2 shows t h e s p a t i a l d i s t r i b u - t i o n of rf-power. This was picked up by a,n a x i a l l y
Potential
movable probe c o n s i s t i n g of an open coaxi'al cable of small dimensions, l e d i n t o t h e plasma through a c a p i l l a r y g l a s s tube. The power was measured by a spectrum analyzer a t a frequency of 350 MHz, and such a s p a t i a l v a r i a t i o n e x i s t e d i n a frequency band with a bandwidth of about 100 MHz. Electron d i s t r i b u t i o n functions a r e being measured t o corre- l a t e t h e observed s p a t i a l d i s t r i b u t i o n of t h e r f - power t o beam-plasma i n t e r a c t i o n . Double humped d i s t r i b u t i o n s have been observed on t h e anode s i d e of t h e l a y e r . Another rf-spectrum d t h an upper cut- o f f frequency a t about 150 MHz a l s o e x i s t s both i n t h e anode and t h e cathode plasma.
The existence of tlie marked minimum i n t h e e l e c t r o n d e n s i t y ( ~ i g . 2 ) , implying t h e p o s s i b i l i t y of t r a p - ping of Langmuir waves a t t h e double l a y e r C.21
,
has been confirmed f o r a wide range of parameter values
REFERENCES
[l] Lutsenko, E.I., Sereda, N . D . , and Kontsevoi, L.M., 1975, Zh. Tekh. F i z .
3,
789 (Sov. Phys.Tech.. phys
. 20,
498).[2] Qullbn, B.H. and Wong, A.Y., 1970, Phys. Rev. Let.- t e r s
3,
1393.Coakley, P., Hershkowitz, N . , Hubbard, R . , and Joyce, G . , 1978, Phys. Rev. L e t t e r s 40, 230.
Levine
,
J. S.,
Crawford, F .W.,
and 11=, D .B.,
1978, Phys.. L e t t .
m y
27.Torvdn, S. and Andersson, D .
,
1978, Report T R I T A - E P P - ~ ~ - ~ ~ , Royal I n s t . of Tech., Stock- B o b ( T O be published i n J. Phys. D.: Appl.pyurs. ).
,[6]
Block, L.P., 1978, Astrophys. Space S c i .55,
59.
[7] T o r d n , S.
,
1978, Report T R I T A - E P P - ~ ~ - ~ ~ , Royal f i s t . Teca.,
Stockholm (To be published i n Wave I n s t a b i l i t i e s i n Space Plasmas, Astrophysics and Space Science Book S e r i e s , Reidel Publ.camp..,
1979).[8] C a r l q v i s t
,
P.,
1978, Report TRITA-EPP-78-15, Royal I n s t . Tech.,
Stockholm (TO be published i n Wave I n s t a b i l i t i e s i n Space Plasmas, Astro- physics and Space Science Book S e r i e s , Reidel Publ. Comp., 1979).[g] Fllthammar, C.-G.
,
Akasofu, S.-I., and f l f v b n , H., 1978, Nature275,
185.[lo]
Shawhan, S.D., Fllthammar, C.-G., and Block, 'L.P., 1978, J. Geophys. Res. 83, 1049.E l l ] Alfvbn, H., 1978$ Astrophys. Space S c i . ,
54,
279.
[12] ~ a h l b e r g , C . , 1978, ~ e p o r t UFt'EC 7889R, I n s t . of Techn., Uppsala U n i v e r s i t y , Uppsala, Sweden.
ACKNOWLEDGEMENT
This work was supported by t h e Swedish Natural Science Research Council.
