Impact of ICRH on the measurement of fusion alphas by collective Thomson scattering in ITER

Collective Thomson scattering (CTS) has been proposed for measuring the phase space distributions of confined fast ion populations in ITER plasmas. This study determines the impact of fast ions accelerated by ion cyclotron resonance heating (ICRH) on the ability of CTS to diagnose fusion alphas in ITER.

Fast ions with large perpendicular velocities, such as the populations investigated here, can be detected with the 'enabled' part of the proposed ITER CTS diagnostic. The investigated ICRH scenarios include pure second harmonic tritium heating and He-3 minority heating at a frequency of 50 MHz, corresponding to an off-axis resonance.

The sensitivities of the results to the He-3 concentration (0.1-4%) and the heating power (20-40 MW) are considered. Fusion born alphas dominate the total CTS signal for large Doppler shifts of the scattered radiation. The tritons generate a negligible fraction of the total fast ion CTS signal in any of these heating scenarios.

The minority species He-3, however, contributes more than 10% of the fast ion CTS signal at locations close to the resonance layer for He-3 concentrations larger than similar to 1%. In this particular region in space for resolution of near perpendicular velocities, it may be difficult to draw conclusions about the physics of alpha particles alone by CTS.

With this exception, the CTS diagnostic can reveal the physics of the fusion alphas in ITER even under the presence of fast ions due to ICRH.