STUDY OF LOW FREQUENCY OSCILLATIONS IN THE OPEN TRAP WITH MINIMUM B MAGNETIC FIELD

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STUDY OF LOW FREQUENCY OSCILLATIONS IN THE OPEN TRAP WITH MINIMUM B MAGNETIC

FIELD

V. Ziltsov, P. Kozarev, I. Makashin, D. Panov, A. Skovoroda, A.G. Scherbakov

To cite this version:

V. Ziltsov, P. Kozarev, I. Makashin, D. Panov, A. Skovoroda, et al.. STUDY OF LOW FREQUENCY OSCILLATIONS IN THE OPEN TRAP WITH MINIMUM B MAGNETIC FIELD. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-663-C7-664. �10.1051/jphyscol:19797322�. �jpa-00219314�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZdment au n07, Tome 40, ~ u i Z Z e t 1979, page C7- 663

STUDY OF LOW FREQUENCY OSCILLATIONS IN THE OPEN TRAP WITH MINIMUM B MAGNETIC FIELD

V.A. Ziltsov, P.M. Kozarev, I.N. Makashin, D.A. Panov, A.A. Skovoroda and A.G. Scherbakov.

Kumhatov I n s t i t u t e of Atomic Energy, Moscou, U.S. S. R.

I n the minimum ^bo p ~ n t r a p s a t the @.en- s i t y w OisWci ,when a f l u i t i n s t a b i l i t y i s damped,it has been experimentdy observed the presence of low frequency d r i - f t o s c i l l a t i o n s / l ,2,3/ ,The origins of the o s c i l l a t i o n s and there r o l e i n transport phenomena were not invest i c a t e d i n these references .In r e f ./4/ the existence of mechanism of ions trasport across magne- t i c f i e l d due .tp the action of d r i f t

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end i o n cyclotron (HF-)oscillations was experimentaly shown.In t h i s paper the properties of I;F o s c i l l a t i o n s and causes 'of there r i s e i n minimumB f i e l d were investigated.

The experiments were c a r r i e d out with the open t r a p "Ogra-3B" with minimum magnet set.Magnetic f i e l d with 17 kG strength i n the center w a s fabricated from baseba- l l - t i p e c o i l .*The distance between mirrors was 70 cm.Longitudinal and r a d i a l mirror r a t i o were2,ob and 2 ,respectively.The maximum plasma parameters i n t h e middle of the t r a p were:denaity ,bIOc,-3,

electron temperature 0,5keV,plasma l i f e - time 7 s e c ( i t was determined by charge exchange process) ,plasma radius 7 cm, plasma length(a1ong Z a%is)7cm

.

LF and HF o e c i l l a t i o n s were measured by the s e t of e l e c t r o s t a t i c antennae, disposed a t the plasma midplane and along mag- n e t i c force 1ines.U o s c i l l a t i o n s of

"cold ion" and eleckron flows along force l i n e s were measured too.Fluctuations of the electron density i n the center of the t r a p were measured by absorption of super high frequency(SH3') electromagnetic wave a t the electron cyclotron frequency /4/ .The me asurements of fluctuations of density and flows gave us an opportuning t o obtain some of the properties of oscil- l a t i o n s i n plasma i n t e r i o r .

Let us consider the conditions of LF wa- ves exitation.'Phe e x i t a t i o n can be "aoftn a t low deneity ( n 109cm-3) ,when cyc- l o t r o n fluctuations are absent (Flg.1)

.

The cyclotron o s c i l l a t i o n s , a r i s i n g a t the density

n

> 109c-3 (threshold depends on the magnetic f i e l d value),have a burst s t r u c t u r e .They l e a d %o %he hard e x i t a t i o n o f lowfrequency oscillations(Fig.2) with E f i e l d amplitude about some t e n s V/cm.

The b u r s t s of HF o s c i l l a t i o n s are accom- panied by the junrps of plasma p o t e n t i a l

with an amplitude about some hundreds V.

LF o s c i l l a t i o n s can be also exited a f t e r switching off the beam,when plasma density i s f a l l i n g down and cyclotron fluctuations are damped with help of electron heating (Fig.a).l'he I;F o s c i l l a t i o n s spectrum, measured by the e l e c t r o s t a t i c antenna set- t l e d a t the midplane (l),by an absorption of SHF wave (2) end by electron flow are similar(P'ig.4) .Some differance of the amplitudes of t h e separate constituents of spectrum i s explanea by the s p a t i a l l o c a l i z a t i o n of o s c i l l a t i o n s with diffe- r e n t r a d i a l structur.LF o s c i l l a t i o n s are r a t h e r regular between HI? burats.During the burst sharp rebuilding of I;F oscilla- t i o n s takes place(the t i m e l e s s than

IIF

o s c i l l a t i o n s period).Time i n t e r v a l between t h e RE' bursts depends on the plasma densi- t y and can be r a t h e r smal,l(some msec.).

It was possible i n our experiments t o re- o e i m LF spectrum during the time l e s s t than 1 msec.At the same time the oscilla- t i o n s of the cold ions flow (2) and the o s c i l l a t i o n s , measured by the electrosta- t i c antenna, placed a t z=20cm, have i n p r i n c i p a l lower f~equency(Fig.5)~

For the simple model of plasma column with a parabolic r a d i a l d i s t r i b u t i o n of densi-t;y the dispersion r e l a t i o n f o r LF o s c i l l a t i o n s can be writen as i n Ref.6

2 2wOim 2

8 =

---

^{- (}^I

-

l / ( ~ + mu')

1

u c i a

where m

-

azimuthal mode number, a

-

^{p l a s}

ma radius, 6.1 *- frequency of ion d r i f t i n inhomogeneous magnetic field,X

-

^characte-

r i s t i c r a d i a l wave nuiuber,determined from t h e boundary conditions .This equation gives two different branches of s t a b l e oscil1ations:electron and ion.This branches f o r m=l are shown(the s o l i d l i n e s

-

experiment) i n Fig.6.The value of i n equation was taken t o normalizilr the lowese r a d i a l mode electron brunch theore- t i o a l dependence on the experimental poi- nts.Drift freguency o* w a s estima'r;ed from ion brunch measurements .Experimentaly the frequency values were received by help of e l e c t r o s t a t i c antenna t h a t were disposed a t the nidplane.!Phe plasma r o t a t i o n i n the ambipolar e l e c t r i c f i e l d introduces a con- t r i b u t i o n t o these frequencies.The value of: positive p o t e n t i a l grows propartionaly

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797322

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t o the density up t o

,

^,09cm-3 ^{. A t} ^{t h i s}

density t h e p o t e n t i a l i s about 1 kV.The frequency of plasma r o t a t i o n

w

z I O ~ P /

a 2 B

i s about 1

-

1 , 5 l o 5 sec-'.It r i s e s some- what the frequency of e l e c t r o n brunch and decreases t h a t f o r i o n brunch.Fig.7 shows a f a a t scan of t h e e l e c t r o n brunch o s c i l - l a t i o n s ( I ) a t t h e moment when the b u r s t of HF o s c i l l a t i o n s takes place and plasma p o t e n t i a l changes sharply (2).

Fig.6 shows t h a t t h e e l e c t r o n and i o n bru nches of L'F o s c i l l a t i o n s e x i s t i n the p l a sma.The dispersion of experimental p o i n t s i s connected with exi'i;ment of t h e diffe- r e n t r a a i a l modes.Note t h a t these brun- ches ere n e u t r a l y stable.Electron and ion o s c i l l a t i o n s have a p o s i t i v e energy.

?an the density n c

,

^109cm-3 t h e fluc- t u a t i o n s may be e x i t e d by noises of i n j e c t e d beam.The measured spectrum of beam f l u c t u a t i o n s i s p r a c t i c a l y white up t o t h e frequency about 200 kT3z.h amplitude of t h e p o l a r i z a t i o n e l e c t r i c f i e l d by i o n i z a t i o n is l a r g e too.LF o s c i l l a t i o n s a t

*.<,

109 cm-3 lead t o an appearence of anomalous p o t e n t i a l , discussed i n /5/

.

I f n r , 109c-3 HF o s c i l l a t i o n s a r i s e and appearence of LF o ~ c i l l a t i o n s i s cor- r e l a t e d with t h e beginning of HF o s c i l l a - tions.Let us note t h a t LP o s c i l l a t i o n s e x i s t s much more lonRer than HF b u r s t - (Fig.2).

Thus LF o s c i l l a t i o n s i n ^thefrequency range up t;o 500 kHz take place i n the open t r a p with minimum B .Ion and e l e c t r o n branches of o s c i l l a t i o n s are observed.

It i s shah t h a t they may be e x i t e d by i n j e c t e d beam noises and cyclotron oscil- 1 a t ions

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4Ot328(19bI

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F i g .1

i .

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