Collective Thomson Scattering Diagnostic of Confined Alpha Particle Distributions in ITER

Abstract

The presence of diffused, strongly non-Maxwellian and possibly anisotropically-distributed heating sources, the fusion-produced alpha particles, makes the real difference between the heating phase of an ignited plasma and present-day fusion plasmas. This stresses the importance of alpha particle diagnostics for ITER. The information required will be manifold. It is unanimously recognized, however, that knowing the space and velocity distribution of the confined alphas during slow down, up to their birth velocity v∝0 = 1.3 x 107 m/s (E∝0 = 3.5MeV), will be essential. Collective Thomson scattering (CTS) has the potential to provide this piece of information. Radiation from a high-power coherent source is launched into the plasma and the Doppler-shifted radiation scattered at a given angle is collected and analyzed. Under suitable conditions, the scattering process is primarily due to the shielding electrons moving along with the ions and therefore the spectrum is strictly related to the ion velocity distributions, including the alpha’s. The schematic of a CTS diagnostic, shown in Fig. 1 with reference to a proposed experiment based on the free electron laser (FEL)1, is conceptually similar at all frequencies.