Particle transport with high power central ECH and ECCD in TCV

A coupled heat and particle transport phenomenon, leading to particle depletion from the plasma core is observed in a variety of plasma conditions with centrally deposited electron cyclotron heating and electron cyclotron current drive in TCV. This phenomenon, which causes inverted sawteeth of the central density in sawtoothing discharges and leads to stationary hollow profiles in the absence of sawteeth, has been linked to the presence of m/n=1/1 MHD modes. In particular, this phenomenon, known as 'density pumpout' can be suppressed by stabilizing the mode by means of operation at high triangularity. The correlation of pumpout with the loss of axisymmetry suggests that neoclassical transport processes involving locally trapped particles near the helically displaced magnetic axis, previously believed to be important only in stellarators, may account for the phenomenon in tokamaks as well. (author)