Study of plasma confinement in the L-2M stellarator during the formation of an edge transport barrier

A plasma confinement mode characterized by the formation of an edge transport barrier (ETB) was discovered in the L-2M stellarator after boronization of the vacuum vessel wall. The transition into this mode is accompanied by a jump in the electron temperature by 100–200 eV at the plasma edge and a sharp increase in the gradient of the electron temperature Te in this region. The threshold power for the transition into the ETB confinement mode with an increased electron temperature gradient is Pthr▿Te = (60 ± 15)ne [1019 m−3] kW. The formation of the ETB manifests itself also in a substantial change in the electron density profile. A density peak with a steep gradient at the outer side forms at the plasma edge. The threshold power for the transition into the ETB confinement mode corresponding to a substantial increase in the plasma density gradient near r = a is Pthr▿Te = (67 ± 9)ne [1019 m−3] kW, which agrees to within experimental error with the threshold power for the transition into the ETB confinement mode determined from the sharp increase in the gradient of the electron temperature Te. The value of Pthr for the L-2M stellarator agrees to within 25% with that obtained from the tokamak scaling. In the ETB confinement mode, the plasma energy W and the energy confinement time τE determined from diamagnetic measurements increase by 20–30% as compared to those obtained from the stellarator scaling for the confinement mode without an ETB. When the heating power increases by a factor of 2–3 above the threshold value, the effects related to improved energy confinement disappear.