Fast ion distributions driven by polychromatic ICRF waves on JET

Experiments have been carried out on the JET tokamak to investigate fast ³He and hydrogen minority ion populations accelerated by ion cyclotron range of frequencies (ICRF) waves launched with multiple frequencies (i.e. up to four frequencies separated by up to ≈15%). This 'polychromatic' heating is compared with single-frequency, 'monochromatic', ICRF heating of reference discharges with similar power levels. Information on the fast ion populations is provided by two-dimensional gamma-ray emission tomography and the measurements are compared with numerical modelling. Polychromatic heating with resonances in the plasma centre (R_res ≈ R₀) and on the low magnetic-field side (LFS) (R_res > R₀) is found to produce predominantly high-energy standard trapped ions, while resonances on the high magnetic-field side (R_res < R₀) increase the fraction of high-energy passing ions. Monochromatic heating with a central resonance produces stronger gamma-ray emission with the maximum emission in the midplane close to, and on the LFS of, the resonance, in agreement with the calculated radial distribution of fast ion orbits. Both the fast ion tail temperature and energy content are found to be lower with polychromatic waves. Polychromatic ICRF heating has the advantage of producing smaller-amplitude and shorter-period sawteeth, consistent with a lower fast ion pressure inside the q ≤ 1 surface, and higher ion to electron temperature ratios.