Radial profile and q dependence of electron heat diffusion measured with ECH modulation in RTP

Abstract

Perturbative measurements of the electron thermal diffusivity (χpert) in the RTP tokamak are presented. Electron temperature perturbations are induced by on- and off-axis modulated electron cyclotron heating (MECH) and the sawtooth instability. The radial profile of χpert is deduced from the phase and amplitude profiles at several harmonics of the modulation frequency. The effect of the edge safety factor (qa) on the profile is investigated in a qa scan with 3.2 < qa < 5. It is found that χpert increases approximately parabolically between the sawtooth inversion radius (rinv) and the edge. Outside rinv no dependence of the χpert profile on qa is observed. In this region χpert exceeds the power balance diffusivity (χpb) by a factor of 2 to 3 at all qa values. Different behaviour with the ECH resonance just in or outside rinv provides evidence for a transport barrier in this region. All data, i.e. for all qa and with MECH at different radii, can be fitted using a single χpert profile, which features a transport barrier near rinv. Placing these results in the context of earlier studies at both tokamaks and stellarators, it is found that no single, local model can account for the combined phenomenology