Experimental studies on NBI and ICRF heated plasmas in the large helical device

In the Large Helical Device (LHD), upgrading of the key hardware system since the last EPS conference has led to (1) higher T_e (T_e(0) = 4.4 keV at 〈n_e〉 = 5.3×10¹⁸ m^-3 and P_abs = 1.8 MW); (2) higher confinement (τ_E = 0.3 s, T_e(0) = 1.1 keV at 〈n_e〉 = 6.5×10¹⁹ m^-3 and P_abs = 2.0 MW); (3) higher stored energy W_p^dia = 880 kJ; and (4) the highest β value in helical devices (2.4% at B = 1.3 T). The energy confinement was systematically higher than that predicted by the International Stellarator Scaling 95 up to a factor of 1.6. Ion cyclotron range of frequencies (ICRF) power up to 1.3 MW was reliably injected into the plasma without significant impurity contamination and a plasma with stored energy of 200 kJ was sustained for 5 s by ICRF alone. Long pulse discharges greater than 1 min in duration have been successfully achieved with ICRF power alone and with neutral beam injection (NBI) power alone. With NBI heating an 80 s discharge was achieved with a heating power of 0.5 MW at 2.75 T. The electron density was maintained at around 1.6×10¹⁹ m^-3 by controlled gas puffing. The central electron and ion temperatures were kept around 1.5 keV. With ICRF heating, a similar long pulse discharge was achieved for 68 s with a heating power of 0.85 MW. The sustained plasma parameters are: W_p approximately 110 kJ, T_e(0) approximately T_i(0) = 2.0 keV and 〈n_e〉 = 1.0×10¹⁹ m^-3. During these discharges, no increase in radiation power has been observed.