Combination effects of co-, counter- and perpendicular neutral beam injections in CHS

CHS is a low-aspect-ratio heliotron/torsatron with a toroidal period number 8 (R=1.0 m, a=0.2 m, Bt ≤ 2 T). In low-aspect-ratio torsatrons, because the loss cone region of the ripple-trapped particles is considerably large, the confinement of high energy particles which have nearly perpendicular pitch angles to the magnetic field is one of very important research issues, both in neutral beam heating experiments and ICRF heating experiments. On the other hand, loss of high energy ions contribute in the formation process of the electric field which is an important part in confinement scenario for stellarators and tokamaks. Two neutral beams are installed (E0 ≤ 40 keV, Pinj ≤ 1 MW, for each beam) in CHS. One is in tangential direction to the torus (NBI-2) while another is movable (NBI-1). It is possible to make co- and counter-balanced injection or tangential and perpendicular combination injection experiments. In addition to the drift orbit losses of trapped particles, high energy beams have other loss processes produced by the shift of beam orbits from the magnetic surfaces due to the toroidal drift which degrades the passing particle confinement. Because the magnetic configuration of CHS which gives optimum confinement is inward shifted one (in major radius direction), there is no room between the inboard side vacuum chamber wall and the plasma boundary. As the beam ions injected in counter-direction (tangential to the torus) have inward shifted orbits, the significant direct losses on the wall are expected. On the other hand, co-injected ions are subject to the charge exchange energy loss outside of plasma boundary (at outboard side of a torus). The experiments using three kinds of injection angle (co-, counter- and perpendicular) give information to understand these processes. (author) 5 refs., 3 figs