Fast wave mode conversion in three-ion component plasmas

Summary form only given. Traditionally the ion cyclotron resonance frequency (ICRF) heating of two-ion species fusion plasmas is separated into minority heating and mode conversion regimes. The ICRF antenna launches the fast wave (FW) that propagates predominantly perpendicular to the magnetic field. When the FW power fraction absorbed by the minority ions at the fundamental cyclotron resonance is larger then the power fraction converted to the slow ion Bernstein wave (IBW) the regime is called as "minority heating". On the contrary, when the main source of the FW energy dissipation is the conversion to the IBW the regime is called as "mode conversion". The FW power converted to the IBW is effectively absorbed by electrons. The width of the absorption layer is equal to a few IBW wavelengths. Therefore the power deposition profile is enough narrow to study the transport with a well determined heat source. The local plasma heating can also be used for the impurity profile control in the discharges with the impurity seeding. Thus the optimal experimental conditions should be determined to get as much fraction of the mode converted power as possible. The new ICRF heating scenario with two cutoff-resonance pairs is studied. This scenario can be produced in a multi-component plasma consisted of three or more ion species with different charge-to-mass ratio. H(D,³He) plasma is considered as an example. The analytical expression for the mode conversion coefficient is derived. It is shown that the evanescent layer associated with D minority acts as the isolated R-cutoff in the theory of triplet configuration and the mode conversion can be enhanced and reach 100 %. The important feature of the considered scenario is a good mode conversion for small values of parallel wavenumber kN that is preferable for good antenna-plasma coupling. The second positive feature is the smooth dependence of the mode conversion coefficient on different parameters such as magnetic field - - value and antenna frequency compared to the triplet configuration case. The developed theory can help to analyze recent inverted JET ICRF experiments.