A. Shimizu

C. Suzuki

D.A. Spong

K. Matsuoka

M. Isobe

N. Nakajima

S. Nishimura

S. Okamura

T. Akiyama

National Institute for Fusion Science (NIFS-Repository)

§ 19. Orbit Topology and Confinement of Neutral-Beam Injected Beam Ions in Quasi-Axisymmetric System Isobe, M., Shimizu, A., Nakajima, N., Suzuki, c., Akiyama, T., Nishimura, S., Okamura, S., Matsuoka, K. (NIPS) Spong, DA (ORNL) The quasi-axisymmetric stellarator CHS-qa, which is one of the so-called advanced stellarators, has been designed to provide good neoclassical confinement as well as magneto-hydrodynamic stability while realizing a tokamak-like, toroidally-symmetric magnetic structure in Boozer coordinates [1,2]. In CHS-qa, the neoclassical transport is significantly improved compared with that in the existing CHS [3]. However, we must verify orbits and confinement property of energetic ions since residual non-axisymmetric magnetic field components exist in the edge region and these residual ripples can enhance the radial diffusion of toroidally trapped energetic ions. In this work, the orbit topology of collision less beam ions is numerically studied for CHS-qa configuration in both Cartesian and Boozer coordinates[4,5]. The global confinement property of neutral beam (NB)-injected energetic ions is also investigated by use of particle simulation code DEL TA5D following guiding center orbits in presence of slowing-down and pitch angle scattering processes on Boozer coordinates. Figures I a and I b show three-dimensional plots of typical passing and toroid ally trapped orbits of neutral beam(NB)-injected energetic ions in Cartesian coordinates by following full gyro motions in the vacuum magnetic field produced by 20 modular coils for B, of 1.5T[6]. It can be seen that particle orbits in CHS-qa are quite similar to those in tokamaks because of the quasi-axisymmetry of the system. [n an evaluation of NB ion confinement, it is important to consider the slowing down and pitch angle scattering processes. In order to evaluate the global beam ion confinement in the presence of collisions, the particle simulation code DEL TA5D was employed. The guiding center beam ion orbits are tracked in the equilibrium magnetic field obtained from the VMEC code which are then transformed to Boozer coordinates. Figure 2a and 2b show the time evolution of the percentage of energy lost «E> lost) from the beam averaged over the ensemble of I ,000 ions in volume-averaged magnetic field strength <8> of 1.5 T with R,ru, =1.85 m, corresponding to the parallel injection and the. pitch angle distribution of escaping ions, respectively. The plasma parameters used here are n,(O)=6.0xlO19 m·3 with a profile of n=n(Ojo(l-(rla)2), and T,(O)=T,(O)=1.5keV with a profile of T = T(Ojo(l-(r/al'l'. n, (hydrogen) is given as 0.91-n, and one impurity component is considered. The percentage of <E> lost is determined when it reaches a saturated plateau in its time evolution; this plateau is associated with the average beam ion decelerating to the thermal energy level of 312-T,. It appears that tangentially co-injected beam ions are well 318 confined even in the presence of collisions, and <E> lost is evaluated to be about 9.5% in this case. The pitch angle distribution is very peaked around the deeply trapped range, indicating that some of banana particles are lost due to residual non-axisymmetric magnetic field ripples. [n regard to this study, detailed description is available in Ref. 4 and 5. Z(m) 0.5 0 -0.5 ·1 a) x Z(m) 0.5 0 -0.5 ·1 b) Fig. 1. Collisionless beam ion orbits in Cartesian coordinates for CHS-qa. Three-dimensional plots of orbits of (a) energetic passing W ions (v/v-I) and (b) toroidally trapped ions (v/v...{J.26) with an energy of 38keV in B, of 1.5T. 50 25 10) <B>=1.ST • • Ib) 40 Eq:38keV ~20 Rtan=1.8Sm m ~30 ~ ~15 " .~ 0 'x ~ 20 ~10 0 v • 10 .0 5 / E , '\ 0 z 0 0 0.01 0.02 0.03 -0.1 -0.05 0 0.05 0.1 Time (s) VIIN Fig. 2. (a) Time evolution of <E> lost (%), (b) Number of exiting beam ions as a function of pitch angle v,/v References I) Matsuoka, K. ef aI., Plasma Phys. Rep. 23, (1997)542. 2) Okamura, S. ef al., Nuclear Fusion 41, (2001)1865. 3) Nemov, V.V. ef al. Plasma Phys. Control. Fusion 45 (2003)1829. 4) Isobe, M. ef al., 30th EPS Conf. on Contr. Fusion and Plasma Phys. St. Petersburg, Russia,7-11 July, 2003 ECA Vol. 27A, P-3.25. 5) Isobe,M. ef aI., accepted for publication in J. Plasma Fusion Res .. 6) Shimizu, A. ef aI., Fusion Engineering and Design 65 (2003)109. 

Orbit Topology and Confinement of Neutral-Beam Injected Beam Ions in Quasi-Axisymmetric System

Orbit Topology and Confinement of Neutral-Beam Injected Beam Ions in Quasi-Axisymmetric System

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