Progress of long pulse discharges by ECH in LHD.

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. Fusion55 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  ×  10¹⁹ 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  ×  10¹⁹ m⁻³ and T_e0 of 1.7 keV, and the 30 min discharge with n_{e_ave} of 0.7  ×  10¹⁹ m⁻³ and T_e0 of 1.7 keV. [ABSTRACT FROM AUTHOR]