Change of plasma boundaries due to beta in heliotron plasma with helical divertor configuration

Abstract

We analyse the change of plasma boundaries due to changes in beta in large helical device plasmas based on electron temperature and density profile measurements, and compare the change with the prediction by a MHD equilibrium code without a priori assumptions of the nested magnetic surfaces, the HINT code. In the open magnetic field line region, which is predicted to exist at high beta, a small but non-zero electron pressure and its gradient are observed. The shift of the geometric centre of the peripheral magnetic surface due to the beta value is systematically consistent with the prediction, which suggests that the HINT code is quite a useful method for identifying the shape and location of the plasma boundary in heliotron plasmas with a divertor configuration. However, in the open field line region, a discrepancy arises because the electron mean free path is much shorter than the connection length of the magnetic field line to a wall, which suggests that the additional effects such as the inertial effect and/or the viscosity in the equilibrium force equation should be taken into account.