Temperature structures in MST helical core plasmas*

The RFP configuration is dominated by several MHD tearing modes with poloidal periodicity $m=$\textit{1}. These modes are resonant at different radii within the plasma core, giving rise to stochastization of the magnetic field. Increasing the plasma current (and consequently the Lundquist number $S)$ tends to channel the perturbation energy to the innermost resonant tearing mode, leaving the other modes with negligible amplitude (QSH state). A further topological transition takes place when the main magnetic axis and island X point vanish leaving the O-point as the new helical magnetic axis (SHAx state). Previous work on RFXmod has shown the formation of a thermal structure that becomes wider during the transition between QSH to SHAx states. In MST we have performed experiments at similar $S $to RFXmod plasmas. Ion Doppler spectroscopy and Thomson scattering measurements reveal the presence of in-outboard asymmetries in the impurity ion temperature radial profiles and the formation of a hot electron region. The changes in both the temperature radial profiles are associated with the transitions to non-axisymmetric equilibria. Further analysis will be performed to characterize the thermal structures also through charge-exchange spectroscopy measurements of ion temperature.