Effect of impurities on the transition between minority ion and mode conversion ICRH heating in ( 3 He)–H tokamak plasmas

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 ( 3 He)–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 3 He 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 ( 3 He)–H plasmas. In this paper, an approximate analytical expression for the transition concentration of 3 He 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 3 He level needed to achieve a specific heating regime by puffing a small amount of 4 He ions additionally to ( 3 He)–H plasma.