Spontaneous transition of core radial electric field driven by magnetic islands in the H-1NF heliac

This paper reports an experimental observation of spontaneous transition of the core radial electric field to a large positive value (E_r ∼ 5 kV m^-1), with a strong electric-field shear (∼700 kV m ^-2) in a low temperature (T_e ∼ 10 eV) radio frequency generated argon plasma in the H-1NF heliac stellarator. The transition, which seems to be driven by a spontaneous excitation of m = 2 magnetic islands near the core, is associated with a localized increase in the plasma density and excitation of coherent low frequency (∼3 kHz) oscillations possibly due to unstable E_r shear driven modes. Evidence suggests development of the core electron-root scenario, which previously has been observed only at high temperature electron cyclotron heated plasmas.