Effect of the sample volume splitting in heavy ion-beam probing (HIBP) for spherical tokamaks (by the example of HIBP for mega ampere spherical tokamak)

Abstract

Here, we propose the heavy ion-beam probing (HIBP) project for the mega ampere spherical tokamak (MAST) to establish systematic research of the radial electric field as an idea to investigate both the reactor-related physics (H mode and advanced tokamak mode with reversed shear) and basic plasma physics (the nature of anomalous transport). The calculations of the probing beam trajectories made for typical MAST parameters: B0=0.6 T, Ipl=1 MA, and Cs+ probing ions show the applicability of HIBP for the spherical torus MAST with energy of about a few hundred keV. The detector line connecting the center and the edge of the plasma allows us to obtain a radial profile by variation of the energy shot by shot. A detector line of equal energy E=110 keV allows us to get a series of profiles in the internal part of the plasma 0<ρ<0.4 during a single shot by a fast scan of the injection angle. The effect of splitting of the sample volume due to the intersection of the secondary fan in a rapidly decreasing toroidal magnetic field of the spherical tokamak was observed: two sample volumes can appear instead of the conventional one. It changes the principles of HIBP: the notion of a detector grid became more general and a new problem appears: how to separate the signals coming from different sample volumes. Using this effect we can spread the detector grid and observe the outer part of the plasma, 0.4<ρ<1. It was shown that these two parts can be united and the total plasma profile can be observed in one shot.