Diagnostic method for radial electric fields in Tokamaks by the observation of ripple-trapped ions

Abstract

Weakly collisional ions trapped in the toroidal field ripples at the outer plasma edge of a Tokamak can be prevented from escaping the plasma due to grad B drift by a counteracting radial electric field. This leads to an increase in the density of ripple-trapped ions, which can be monitored by the analysis of charge exchange neutrals. The minimum radial electric field Er necessary to confine ions with energy E and charge q (q=−1: charge of the electron) is Er=−E/(q×R), where R is the major radius at the measuring point. Slowing-down ions from neutral injection are usually in the right energy range to be sufficiently collisionless in the plasma edge and show confinement by radial electric fields in the range of tens of kV/m. The density of banana ions is almost unaffected by the radial electric field indicating that the change in the fluxes of ripple-trapped ions is, indeed, caused by the radial electric field and not by direct changes in plasma parameters. Single particle calculations for real ASDEX Upgrade geometry and magnetic fields but with an assumed electric field profile give a rule of thumb estimate of the electric field strength. They also make plausible the reason why the charge exchange flux may react in less than 100 μs to an abrupt onset of a radial electric field with a halfwidth of at least the Larmor diameter of the particles in question and thus make this measurement an interesting tool for the investigation of the edge radial electric field.