Study of Edge Transport Barrier Formation on CHS Plasma

Abstract

An edge transport barrier (ETB) similar to the tokamak H-mode has been observed for beam-heated plasma with two co-injected Neutral Beam Injectors (NBIs) in CHS. The Hα emission showed a clear spontaneous drop followed by an increase of line-averaged electron density at the L-H transition. Stored energy increased by ˜ 40% with H-factor improvement up to ˜ 30% compared to the international stellarator scaling (ISS04v03). A rapid density increase in the edge region to two-times level of the L-mode was observed to accompany a ˜ 15% increase in the density gradient. An ETB was formed when the plasma density exceeded the lower density limit by ˜ 1.5 × 10¹³ cm^-3 and the total NBI power exceeds the threshold level (P_deposit / n_e ˜ 200 × 10^-13 kW/cm^-3 at B_T = 0.95 T, R_ax = 92.1 cm, where P_deposit is the deposited NBI power, n_e is electron density, B_T is troidal magnetic field strength and R_ax is location of magnetic axis). The power threshold increased with the magnetic field strength, as in tokamak scaling, and depends on the magnetic field configuration of the helical plasma.