Effect of impurities on the transition between minority ion and mode conversion ICRH heating in (3He)–H tokamak plasmas
Journal article, 2013
Hydrogen majority plasmas will be used in the initial non-activated phase of ITER operation. Optimizing ion cyclotron resonance heating (ICRH) in such scenarios will help in achieving H-mode in these plasmas. Past JET experiments with the carbon wall revealed a significant impact of intrinsic impurities on the ICRH performance in (3He)–H plasmas relevant for the full-field initial ITER phase. High plasma contamination with carbon impurities resulted in the appearance of a supplementary mode conversion layer and significant reduction in the transition concentration of 3He minority ions, defined as the concentration at which the change from minority heating to the mode conversion regime occurs. In view of the installation of the new ITER-like wall at JET, it is important to evaluate the effect of Be and W impurities on ICRH scenarios in (3He)–H plasmas. In this paper, an approximate analytical expression for the transition concentration of 3He minority ions is derived as a function of plasma and ICRH parameters, and accounting for typical impurity species at JET. The accompanying 1D wave modelling supports the analytical results and suggests a potential experimental method to reduce the 3He level needed to achieve a specific heating regime by puffing a small amount of 4He ions additionally to (3He)–H plasma.