Progress of long pulse discharges by ECH in LHD

摘要

Using ion cyclotron heating and electron cyclotron heating (ECH), or solo ECH, trials of steady state plasma sustainment have been conducted in the superconducting helical/ stellarator, large helical device (LHD) (Ida K et al 2015 Nucl. Fusion 55 104018). In recent years, the ECH system has been upgraded by applying newly developed 77 and 154 GHz gyrotrons. A new gas fueling system applied to the steady state operations in the LHD realized precise feedback control of the line average electron density even when the wall condition varied during long pulse discharges. Owing to these improvements in the ECH and the gas fueling systems, a stable 39 min discharge with a line average electron density n_(e_ave) of 1.1 × 10~(19) m~(-3), a central electron temperature T_(e0) of over 2.5 keV, and a central ion temperature T_(i0) of 1.0 keV was successfully performed with ～350 kW EC-waves. The parameters are much improved from the previous 65 min discharge with n_(e_ave) of 0.15 × 10~(19) m~(-3) and T_(e0) of 1.7 keV, and the 30 min discharge with n_(e_ave) of 0.7 × 10~(19) m~(-3) and T_(e0) of 1.7 keV.