Ion behaviour in the HBTX1A/B reversed field pinch

Abstract

A study of the ion behaviour in the HBTX1A/B Reversed Field Pinches (RFP's) is presented. A comparison of measured and calculated energy spectra of the neutral deuterium atoms emitted from the plasma enabled the deuterium ion temperature to be measured and its radial profile estimated. The measured ion temperatures of 80.0eV to 600.OeV cannot be explained by collisional heating of the ions by the electrons even indirectly through typical (2.0% CV, 2.0% OVI, 0.1% FeXII) or extreme (9.0% FeXII) concentrations of impurity ions. Measurements of the Doppler broadening of carbon ion line emission, CV(2271.oA), enabled estimates to be made of the CV ion temperature of 150.OeV to 450.OeV and fluid velocity of 3.0x103ms1 to 1.4x10 ms1 toroidally. The measured ion temperatures are shown to be consistent with the heating power per particle being proportional to the particle mass. An ion power balance model is described in which it is assumed thatthe excess power input into the plasma above the Ohmic heating power into the electrons, typically 6.OMWnf3 on average, heats the ions locally through the damping of fluctuations. The resistivity estimated from a helicity balance model can be made to agree with the Spitzer value only if the excess toroidal loop voltage due to helicity loss at the plasma boundary is included. Ion energy losses are calculated from the relative neutral deuterium density profile assuming equal ion and electron energy and particle diffusion coefficients of typically 50.0m s on axis. Using this model the calculated ion temperature profile and ratio of ion to electron temperature, Ti/Te, are consistent with those measured over a wide range of conditions. Ti/Te is predicted to increase with increased helicity loss at the plasma edge. This was demonstrated by insertion of a tile into the plasma edge which increased Ti/Te from 1.0 with limiters to 2.0 and by removal of the limiters which decreased Ti/Te to 0.5 to 0.3 as predicted. A review of ion heating mechanisms indicated that ion heating by viscous damping of fluctuations is the most likely candidate. Estimates of the order of the viscous heating powerusing measured fluctuation levels are of the required magnitude. It isconcluded that the ion temperatures observed can be explained if the power input to the fluctuations associated with the RFP 'dynamo' heats the ions through the viscous damping of these fluctuations.