Experimentally-based ExB drifts in the DIII-D divertor and SOL calculated from integration of Ohm's law using Thomson scattering measurements of Te and ne

Abstract

The 2D spatial distributions of cross field drift velocities are calculated from 2D Thomson scattering measurements of Te and ne in the divertor and SOL of DIII-D. In contrast with the method that has been used on DIII-D where the 2D distribution of plasma potential Vplasma is obtained from measurements of the probe floating potential of reciprocating probes, the present method does not require insertion of a probe into the plasma and can therefore be used in high power discharges. The 2D spatial distribution of Vplasma is calculated from Ohm's Law for the parallel electric field E|| along each flux tube, E∥(s∥)=−1.71dTe/ds∥−(Te/ne)dne/ds∥, where the Thomson scattering values of Te and ne are used. To within a constant of integration, Vplasma is obtained by integrating E|| along the flux-tubes (field lines); the constant is obtained for each flux tube using the sheath drop at the target calculated from the characteristic of Langmuir probes built into the divertor tiles. The 2D distributions of Eradial=−dVp/dsradial, Epoloidal=−dVp/dspoloidal, vExBpoloidal=Eradial/B and vExBradial=Epoloidal/B are then calculated as well as the particle drift flux densities ΓExBpoloidal=nvExBpoloidal and ΓExBradial=nvExBradial for electrons, fuel ions and impurity ions, using the appropriate values of particle density, n. Keywords: DIII-D, divertor, drifts, Thomson scattering