INFLUENCE OF STATIC, RADIAL ELECTRIC FIELDS O N TRAPPED PARTICLE INSTABILITIES IN TOROIDAL SYSTEMS

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1 IC/66/100 INTERNATIONAL ATOMIC ENERGY AGENCY INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS INFLUENCE OF STATIC, RADIAL ELECTRIC FIELDS O N TRAPPED PARTICLE INSTABILITIES IN TOROIDAL SYSTEMS A. A. GALEEV R. Z. SAGDEEV AND H. V. WONG 1966 PIAZZA OBERDAN TRIESTE

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3 IC/66/100 INTERNATIONAL ATOMIC ENERGY AGENCY INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS INFLUENCE OF STATIC, RADIAL ELECTRIC FIELDS ON TRAPPED PARTICLE INSTABILITIES IN TOROIDAL SYSTEMS A.A. GALEEV* R.Z. SAGDEEV* and H.V. WONG TRIESTE August 1966 "*To be submitted to "Physics of Fluids" * Permanent address: Institute for Nuclear Physics, Novosibirsk, USSR. 7

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5 ABSTRACT The drift motion of trapped particl&s in toroidal systems leads to a new branch of the gravitational instability. This is stabilized by radial eleotric fields. However, a new unstable mode then appearb, which is similar to that due to ion impurity density gradients

6 INFLUENCE OF STATIC, RADIAL ELECTRIC FIELDS ON TRAPPED PARTICLE INSTABILITIES IN TOROIDAL SYSTEMS I. INTRODUCTION 1) Recently B.B. KADOMTSEV and 0, POGUTSE JmJ derived a new branch of the gravitational instability which is immune to Bhear stabilization. The instability arises in toroidal confining systems and is due essentially to the drift motion of the "trapped" particles. In this paper we show that this instability can be stabilized by 2) radial electric fields arising in the plasma» in other words by plasma rotation. The effect of the electrio field is to push the "trapped" ions to the tail of the Ion distribution function so that the contribution of the trapped ions to the dispersion relation becomes small. A new unstable mode then becomes possible. This mode is analogous to that due to ion impurity density gradients, the trapped electrons taking the role of the impurity ions. II. PARTICLE ORBITS Fig. 1. We consider a symmetrio toroidal system with dimensions as shown in Fig. 1 Toroidal co-ordinates - 2 -

7 R is the major radius, a is the minor radius, and a/r <&l* We choose co-ordinates (r,^^) to describe the geometry of the system. The (r, f ) plane contains the major axis, and the position of this plane is defined by the angle. The main magnetic field is due to a current flowing along the major axis and is given by Bn = B Q (l - (r/r Q ) oosv). A rotational transform i about the minor axis is produced by the field By, = -i(r/r ) B «, B. «T o o o The equations of motion of the partiole-guiding oentre in zero order approximation' aret and we have We denote by v ti and v t the components of particle velocity parallel and perpendicular to the minor axis. We assume the presence of a static. radial electric field - H. The particles experience drifts due to curvature and gradients in the magnetic field. It is convenient to introduce a new quantity <p t which desoribes the variation from the zero-order motion of the p coordinate. For particles with charge e. and mass m., the equations of motion of the guiding oentre, including drifts, can be written dlt -flu;, -r i - ^. c^<r {&) where v, = (v^ + v.,,) /il. H the magnitude of the curvature and gradient drifts, v~, =s (e,/#.&.) (335^/dr) the drift due to a static, radial & j J J / 3 - T

8 electric field and q 1 = dq/dr = (d/dr) (l/i). - ( O ^ /dt) t Q. = (e,b /m.o) the cyclotron frequency, The particle trajectories corresponding to Eqs. (4) - (6) have "been discussed by BERK et al. ' Two types of orbits exist: the untrapped particles which circulate around the minor axis, and the trapped par- 2) tides which do not. The particles are trapped when ' Thus the trapped particles have velocity v,, = v -n/& an( * *" or! ar S e V E or small 0 they could lie in the tail of the distribution function. In the absence of electric fields, v,, < /r/r V and they are in the main part of the distribution function. We can estimate the period of oscillation of the trapped particles by using the zero order equation 1 +" + \T. The term Q0B <P is of order S ^ «- smaller than Q\T j "f is the Larmor radius. We have! where r = r o when tf>= 0, ^ = %, the magnetic moment a. ~ \Jj*- Zx»-. and we have neglected terms containing Z and ^ lt The trapped particles are reflected at *f= J (lc) given by cos ^f f = 1 = 2% 2) p We require )C < 1. The period of osoiallation Tie where K is a complete elliptic function of the first kind, and ol is 2 ' given by oosoc 1-21C. -4-

9 III. DISPERSION RELATION We shall now derive the dispersion relation. We assume that 0- (8xnT) / B <<; 1, where n is the density and T the temperature of the plasma, and we consider only electrostatic perturbations. The equilibrium distribution function f f (r, v t j,/x) is taken to be a local Maxwellian, where vf, is the velocity along the line of force. We neglect the dependence on V 5» since it does not make an essential contribution to the dispersion relation. The linearized equation for the perturbed distribution function in the drift approximation is T, is the velocity of the guiding centre. is the potential of the perturbed electric field -loot where i is an integer. In the usual way we obtain for f.' where n* JL II j L ^^fo) and we have replaced 0 by Y QQP~ X-j from Eq. (3) The time integration must be taken over the guiding centre trajeotory. We have two groups of particles which we must consider separately: 12 The trapped particles have a periodic oscillatory motion in restricted regions of (r,*p)-spaoe. We can expand $(r',f) in a sura over the harmonics of this periodic motion. The integration over time becomes trivial. We obtain a contribution to the dispersion, relation which is similar to that obtained when we consider the effect of Larmor orbiting in the magnetic field. The frequency of the periodio motion plays the same role as the Larmor frequency, except that here it would be a function of the velocity of the particles. We shall be interested only in the zero harmonic. It is sufficient therefore to average the periodio variation of the integrand over one period of oscillation. v

10 The co-ordinate C makes small fluctuations about an average value which increases monotonically in time. For times longer than the period of one oscillation, the fluctuations are small compared with its average value and can be neglected. We can therefore approximate t (t') by set-) < : ^ where < > imply averaging over one period of oscillation. The contribution of the trapped partioles to the integral in Eq. (11) "beoomes 2) The untrapped particles circulate completely around the minor axis and are characterized by larg«angular velooities r(df/dt). Neglecting the drift contributions, we can approximate ^(t') by (11) The contribution of the untrapped partioles to the integral in Eq. Assuming quasi-neutrality, we have for the dispersion relation -6-

11 This is an integral-differential equation which we must solve to obtain the eigenfrequencies. This is a difficult problem, and we shall oontent ourselves here "by estimating the simplest eigenvalue of the dispersion equation. He assume that * 2? e where m is an integer. We shall neglect the radial motion of the wave packet (for further discussion of this point see ref. 3)). We obtain the dispersion relation similar to that first obtained by KADOMTSEV and POGUTSE 't -i/ «"» where * '; T>; / ~Q)f, i * ' (17) IV. STABILITY ANALYSIS l) Zero static electric field So far the value of (m Xq) has not been specified. We consider the oase where -7-

12 1* V We assume the ion and electron temperatures to be equal T = T., and static eleotrio fields to be zero* He neglect the contri'bution of the untrapped particles. The dispersion relation becomes (19) where e (20) and -r L E is a complete elliptic function of the second kind with argument sinflf/2. -8-

13 The trapped electrons and ions are in the main part of the distribution function, and we may assume their densities to be equal. The large terms containing- coo* in Eq. (19) cancel- and we obtain have approximated since v L «* v for the trapped particles. Now we can write Eq. (22) in the form where N 2rr to r A 6O* (24) ^\^ = p 0 - function. ^ Sq. (23) must now be solved to obtain a value for A, KADOMTSEV and POGFUTSB ' derived this equation and solved it numerically. They in fact considered the limit where (m - Jtq.) << 1» and also neglected the contribution of the untrapped particles.they found an aperiodic instability with growth rate Y- Im co given by ^) *) Kadomtsev has shown, using numerical analyses, that P^^, decreases rapidly with I m-m' I. Therefore it is reasonable to derive the approximate dispersion relation taking simply A, =^mm, >Ie ^ri- 11 use tlle Kadomtsevtype approximation later on. -9-

14 y yc T (2Y) 2) Won-zero static electric field e wish to point out, however, that Eq. (22) was obtained on the assumption of equal densities of trapped ions and electrons. If radial electric fields develop in the plasma (see ref. 2)), the plasma rotates aoout the minor axis of the toroid. The effect of this is not only to produce a frequency shift. At the same time the trapped particles are pushed out to the tail of the distribution function, since their parallel velocity -v u ~v^/& (see Eq. (?)) For equal ion and electron temperatures, the density of trapped ions decrease more rapidly than that of the electrons, and exact cancellation of the large terms in Eq. (19) do not occur. The instability will therefore be eventually quenched*)and we shall be left with a purely periodic mode with frequency to ^ e- 6J^ -X (28) *) The stability condition can be written in a form On the other hand, when the density of trapped ions has become small, a new instability arises. The untrapped ions now play a significant role. Assuming 1 that Eq. (IS) is satisfied, we neglect in Eq. (16) the untrapped electrons, and retain the imaginary contribution of the untrapped ions coming from the half-residue in the velocity integration. The dispersion relation is -10-

15 where we again & consider T. = T. i e If we neglect the effect of the radial electric fields on the trapped electrons, we can -write Eq." (2ft) in the form where F 777 (29) and F We must solve Eq. (29) to obtain a value for relation represented by Eq. (30) can "be written E J (31) The dispersion (32) This equation is similar to that derived for the ion impurity density gradient instability ', the trapped electrons playing the part of the ion impurities. The worst case results when the second term in the denominator of Eq. (3 ) is of order unity, and we have _.. ^j To satisfy Eq. (l8) we require ^ tl (34) ACKNOWLEDGMENTS The authors are grateful to Professor Abdiis Salam and the IAEA for their hospitality at the International Centre for Theoretical Physics, Trieste. H.V. Wong was supported by a CIBA fellowship. f- -11-

16 REFERENCES 1. 3.B. KADOP-iTSEV and 0. POGUTSE, International Sympositun on Experimental and Theoretical Aspects of Toroidal Confinement, Princeton, ILL. BERK and A.A. GALEEV, ICTP, Trieste, preprint IC/66/ B. COPPI, H.P. PUHTH, M.JT. ROS2Jn3LUTH and R.Z. SAGDEEV, Phys. Rev. Letters 17., 377 (1966) ") 9

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18 Available from the Office of the Scientific Information and Documentation Officer, International Centre for Theoretical Physics, Piazza Oberdan 6, TRIESTE, Italy