Long-pulse Plasma Discharges by Upgraded ECH System in the LHD

Abstract

Until 2009, three high-power, over 1MW each, 77GHz gyrotrons have been installed and applied to the LHD experiment. In addition, a 154GHz gyrotron of 1MW was installed in 2012. The 77GHz gyrotrons suffer gradual increases of internal pressure during long-pulse operation delivering power to LHD. To mitigate the problem, quasi-steady operation by combination of on-off operations of the 77GHz gyrotrons was performed. Applying two 77GHz gyrotrons alternately with intervals of two minutes and an 84GHz gyrotron continuously, a 30 min. long-pulse discharge with the line average electron density n e_ave of 0.7×1019 m−3 and the central electron temperature T e0 of 1.7keV was achieved by the time average injection power P inj of 260kW in 2012, showing significant progress in sustained density from the former 65 min. discharge with n e_ave of 0.15×1019 m−3 and T e0 of 1.7keV by P inj of 110kW of 84GHz wave. In 2013, one of the 77GHz gyrotron was improved to furnish a sub-window to remove stray radiation inside the tube. And the new 154GHz gyrotron was applied to the long-pulse discharge experiment. Using three gyrotrons: 154, the improved and an existing 77GHz with P inj of 340kW in total, higher temperature plasma having steep temperature gradient typical for internal transport barrier, with n e_ave of 1.1×1019 m−3 and T e0 of 3.5keV was quite stably sustained for 325s.