K. "Saito

T." Watari

"Saito,  K.

Watari,  T."

NIFS-Repository (National Institute for Fusion Science)

§23. Rabbit-Ear Distribution Function of High Energy Ion Produced by ICRF Electric Field Saito, K., Watari, T. The dependence of the count number of high-energy particles on pitch-angle was studied on LHD with the time-of-flight neutral particle analyzer (TOF-NPA). The angle of the line of sight was scanned shot by shot using five successive discharges sustained by ICRF heating. As shown in Fig. 1, a "rabbit-ear" structure, a large population at a certain pitch-angle, was observed [1,2]. Ions are accelerated in the perpendicular direction at an ion cyclotron resonance layer by ICRF heating. Therefore a pitch-angle increases until the turning points of banana orbit reaches the cyclotron resonance layer. It is known in tokamaks that the distribution function of ICRF heated plasma have thus a "rabbit-ear" structure. The distribution function fo is calculated with a bounce-averaged Fokker-Planck equation [3]. - 1 f dl 1 8 /i 8 C(~)=- -C(~)=-2 mv(-+--) Z"b V II V 8E E8/i where, 1 8 q () 1 ( 2) L res =-(-+--), a= LlVI/ +- (Llv.L ) , m 8E mw 2v where (..1 VII )' (Ll V// )2) , and (Ll V.L)2) are the Coulomb diffusion coefficients, and m and q are the mass and charge of the resonant particle, respectively. OJ and n are the applied frequency and an ion cyclotron frequency of the resonant particle, respectively. E and J.l are the energy and the magnetic moment, respectively, and E1eft is the left hand component ofRF electric field. In the helical magnetic configuration, there are many helical ripples along the magnetic line of force. The coefficients Z" b' C, and q must be calculated at each helical ripple. The particle trapped in one helical ripple is transferred to the other ripples via drift motion. Therefore, the calculation of drift orbits is required in order to deduce the number of bounces in i-th ripple, nj • The coefficients Z"b' C, and q were weighted by nj: Z"b = r.njZ"bj I r.nj j i c=r.njc/ r. nj j j q =r.niiJr.nj i i The calculation successfully simulates the "rabbit-ear" structure as shown in Fig. 2. 10 8 1. 6 -l > 2 700 0.'"'"2 -'&"0 -"""2-.... 4-.... '--8----1~0-....!12 v,/vlttO Fig. 1 Contour of count number of neutral particles detected with the TOF-NPA. 20 15 5 ~ 10 >-1 S 0 -20 -10 0 10 20 vJlVth Fig. 2 Contour of the calculated distribution function by solving the bounce-averaged Fokker-Planck equation. References [1] Saito, K., et al., Plasma Phys. Control. Fusion 44 (2002) 103 . [2] Saito, K., et aI., to be published in Journal of Plasma and Fusion Research. [3] Stix, T.H., Waves in Plasmas, AIP, (1992), pS21. 27 

Rabbit-Ear Distribution Function of High Energy Ion Produced by ICRF Electric Field

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