RADIAL PROFILE AND Q-DEPENDENCE OF ELECTRON HEAT DIFFUSION MEASURED WITH ECH MODULATION IN RTP

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