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Hydrogen majority plasmas will be used in the initial non-activated phase ofITER operation. Optimizing ion cyclotron resonance heating (ICRH) in suchscenarios will help in achieving H-mode in these plasmas. Past JET experimentswith the carbon wall revealed a significant impact of intrinsic impurities onthe ICRH performance in (3He)-H plasmas relevant for the full-field initialITER phase. High plasma contamination with carbon impurities resulted in theappearance of a supplementary mode conversion layer and significant reductionin the transition concentration of 3He minority ions, defined as theconcentration at which the change from minority heating to mode conversionregime occurs. In view of the installation of the new ITER-like wall at JET, itis 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 thetransition concentration of 3He minority ions is derived as a function ofplasma and ICRH parameters, and accounting for typical impurity species at JET.The accompanying 1D wave modeling supports the analytical results and suggestsa potential experimental method to reduce 3He level needed to achieve aspecific heating regime by puffing a small amount of 4He ions additionally to(3He)-H plasma.

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