Study of edge transport barrier formation on CHS plasma

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 ∼1.5x1013 cm-3 and the total NBI power exceeds the threshold level (Pdeposit/ne-bar - 200 x 10-13 kW/cm-3 at BT=0.95 T, Rax=92.1 cm, where Pdeposit is the deposited NBI power, ne-bar is electron density, BT is toroidal magnetic field strength and Rax 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. (author)