Fast ion orbit effects in high power ICRH modulation experiments in the JET tokamak

The discovery of 'Monster' sawteeth in JET has provided the opportunity to perform ICRH thermal wave measurements which are uncompromised by the locking of sawteeth to the RF modulation frequency. The aim of such studies is to measure the electron heat diffusivity, χe, and the electron heating profiles. Modulated ICRH experiments have been carried out in JET using on-axis heating with both 3He and hydrogen minority ions. The results with 5% 3He concentration gave 1.5<χe (m2/s)<3 and have been reported previously. Those data could be readily interpreted in terms of a simple heat diffusion model containing on-axis Gaussian heating profiles for both the collisional electron heating by the minority ions and the direct heating due to mode conversion, transit time magnetic pumping and electron Landau damping. Experiments with lower minority density produced modulated electron temperature profiles which were hollow and which cannot be simulated by such a simple model. In this paper we propose an interpretation of these data in terms of the response of the energetic minority ions to modulated RF. (author) 4 refs., 6 figs