Transient transport barriers in ohmic and electron cyclotron heated RTP plasmas: Relation with rational magnetic surfaces

Oblique pellet injection experiments have been performed in RTP ohmic and electron cyclotron (EC) heated plasmas. Peripheral cooling induces a transient rise of the electron temperature (ir,) in the plasma core, with the formation of a region of a high T-e gradient (transient transport barrier). The position of the barrier appears to be linked with the location of low-order rational magnetic surfaces. For Ohmic and EC heated plasmas with resonance moderately off-axis, the T-e rise is peaked in the plasma centre and the footpoint of the barrier is located close to the q = 2 surface. For EC plasmas with far off-axis resonance, i.e. in plasmas exhibiting hollow T-e profiles, the T-e rise starts off-axis, near the innermost low order rational surface (typically q = 3). It then propagates to the centre in a seemingly diffusive way.