A Comparison of Time Dependent Simulations with Experiments in Ion Cyclotron Heated Plasmas

Comparisons between time dependent simulations and experiments in ion cyclotron heated plasmas in the JET tokamak have been done. The heating is simulated with a time dependent code, PION-T. The scenario which has been analyzed is minority heating of hydrogen at the fundamental cyclotron resonance in a deuterium majority plasma. For the analyzed scenario both second harmonic deuterium heating and fundamental hydrogen cyclotron heating take place. For large power, anisotropic high energy tails will be formed on the velocity distribution of the resonating ion species. The formation of such a tail on the deuterium is assessed by monitoring the 2.4MeV neutron flux from (D,D) reactions. By taking the difference between two different plasma energy measurements, which put different weights on the parallel and perpendicular energy, the anisotropic energy content can be measured. This is compared with that calculated for hydrogen and deuterium. For low power levels or at the beginning of the heating pulse good agreement between calculations and measurements is found supporting that the power deposition is correctly described and that the fast ions, responsible for the anisotropic energy, slow down by classical collisions. For large amount of coupled power or later in the heating phase a clear difference between calculations and measurements is found. This discrepancy is identified, in the analyzed discharges, to be caused by finite orbit effects. By including the width of the drift orbits, good agreement between simulations and experiments is obtained also for the high power cases. Fast ions have earlier been seen to be expelled from the centre of the plasma in connection with sawtooth crashes. By studying the rapid changes in fast energy content in connection with sawtooth instabilities we find the reduction to be consistent with that about 40% of the fast ions are expelled from the central region to the outside of the q=1 surface and that prompt losses are negligible.
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JETP93001 1.59 Mb