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

Here, we propose the heavy ion-beam probing (HIBP) project for the mega amp ere 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 phy sics (the nature of anomalous transport). The calculations of the probing b eam trajectories made for typical MAST parameters: B-0=0.6 T, I-p1=1 MA, an d Cs+ probing ions show the applicability of HIBP for the spherical torus M AST with energy of about a few hundred keV. The detector line connecting th e 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=11 0 keV allows us to get a series of profiles in the internal part of the pla sma 0<rho<0.4 during a single shot by a fast scan of the injection angle. T he effect of splitting of the sample volume due to the intersection of the secondary fan in a rapidly decreasing toroidal magnetic field of the spheri cal tokamak was observed: two sample volumes can appear instead of the conv entional one. It changes the principles of HIBP: the notion of a detector g rid became more general and a new problem appears: how to separate the sign als coming from different sample volumes. Using this effect we can spread t he detector grid and observe the outer part of the plasma, 0.4<rho<1. It wa s shown that these two parts can be united and the total plasma profile can be observed in one shot. (C) 1999 American Institute of Physics. [S0034-67 48(99)75101-4].