Confinement, Additional Heating (NBI, LHH, ECH), and Current Drive of Stellarator Plasma in JIPP T-II

The confinement of ohmically heated stellarator plasma is studied experimentally. The energy confinement time τ
^E
increases with the total rotational transform angle τΕ in the case of ɩΣ/
₂
π < 0.25, that is, qΣ(a) > 4, while τ E decreases with ɩΣ/
₂
π > 0.3, that is, qΣ(a) < 3-4, which is consistent with the Alcator scaling. When the rotational transform angle due to helical field ɩh/
₂
π is larger than 0.14, no major disruptions are observed even for qΣ(a) as low as 1.7 (ɩh/
₂
π= 0.14, Ip ≳ 100 kA at Bt = 2.2 T), although strong MHD oscillations with low frequency (~1 kHz) appear when qΣ(a) < 2. Tangential injections of neutral beams are carried out for additional heating of stellarator and tokamak plasmas. With co-injection, the increase in ion temperature ΔTi at the center per injected power PN into the torus is 1.6 eV/kW in the stellarator and is 1.9 eV/kW in the tokamak configuration. In the case of counter-injection, ΔTi/PNis 1.6 eV/kW in both configurations. Lower hybrid heating (LHH) with the frequency of 0.8 GHz is applied. In both cases of stellarator and tokamak, ion heating of 0.8 eV/kW is observed with input power up to 160-180 kW. Electron cyclotron heating (ECH) by use of klystrons with frequency of 35.5 GHz is carried out. Preliminary results show heating of bulk electrons with the heating efficiency of 1.6 eV/kW. Current drive in steliarator plasma by lower hybrid wave is observed in the case of low density plasma (n¯e ~5 × 1012 cm-3).