  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 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 modeling supports the analytical results and suggests
a potential experimental method to reduce 3He level needed to achieve a
specific heating regime by puffing a small amount of 4He ions additionally to
(3He)-H plasma.
