`Non-local' response of RTP ohmic plasmas to peripheral perturbations

Abstract

A `non-local' response of the plasma core triggered by peripheral plasma perturbations other than laser ablation is found in the RTP tokamak. Oblique pellet injection (OPI) has been used to induce fast cooling of the peripheral plasma. In response, an inward cold pulse (Te drop) and a slightly delayed core Te rise are observed. A somewhat similar `non-local' response is observed when the peripheral plasma is heated by modulated electron cyclotron heating or by fast current ramps, i.e. the core temperature drops in response to the peripheral heating. The plasma conditions for the occurrence of the `non-local' response have been investigated. The core Te rise following OPI is associated with the formation of a large temperature gradient in the region 1 < q < 2. The Te rise is largest at low electron density and for large pellet deposition radii. Above a critical density the Te rise disappears and only the (weaker) drop in core Te is observed. Time dependent transport simulations show that the propagation of the inward cold pulse is consistent with local transport, while the core Te rise is a slower phenomenon requiring a large transient drop of χe in the region 1 < q < 2.