A large-orbit model of fast ion slowing down during ICRH: Comparison with JET data

Abstract

Localized and intense ion cyclotron resonance heating (ICRH) of H and 3He minority ions in plasmas produced in the Joint European Torus (JET) tokamak create fast ion distributions having average energies in the multi-MeV range, similar to 3.5 MeV fusion alpha particles expected in a reactor. For typical values of the JET poloidal magnetic field, the radii of the particle orbits can significantly exceed the size of the RF deposition. Moreover, in the presence of a steep radial gradient in the fast ion slowing-down time, the large radial excursions of the heated ions take them into the outer plasma region where the frictional drag is larger than in the hot plasma core. In modelling the fast ion distribution with ICRH, therefore, it is important to include a correction for the finite orbital size since this gives rise to a significant reduction in the calculated energy content of the minority component. A large orbit width model for ICRH is described which has been used to predict the global fast ion energies of both H and 3He minority ions during ICRH in the JET tokamak. The model is capable of explaining certain discrepancies which have previously been observed on JET between the measured fast ion energy contents and those which have been calculated using a zero orbit width model. The largest corrections to the global energy content of the fast ions are found to be ≈56% for low current H minority discharges in which the ion-electron slowing-down time is long (≈ 1.6 s at the magnetic axis). Within experimental scatter, we can place an upper limit of Dfast < 0.18 m2·s−1 on the fast ion diffusion coefficient for any additional non-classical energy loss processes.