Impact of bumpiness control on edge plasma in a helical-axis heliotron device

In the helical-axis heliotron configuration, bumpiness of the confinement field εb is introduced to control the plasma transport. The plasma performance were experimentally investigated in Heliotron J for three configurations with εb = 0.01, 0.06 and 0.15 at ρ = 2/3. The obtained volume-averaged stored energy depends on the configuration. To understand the observed difference in global energy confinement, the εb-control effects on the edge plasma is discussed. For εb = 0.01, the plasma density and temperature in the peripheral region is low compared to other cases. This poor plasma edge relates to the observed low stored energy or poor energy confinement for εb = 0.01.