On resonant ICRF absorption in three-ion component plasmas: a new promising tool for fast ion generation

We report on a very efficient ion-cyclotron-resonance-frequency (ICRF) absorption scheme (Z)-Y-X, which hinges on the presence of three ion species residing in the plasma. A mode conversion (cutoff-resonance) layer is well known to appear in two-ion species plasmas. If the location of the L-cutoff in Y-X plasmas, which can be controlled by varying the Y:X density ratio, almost coincides with the fundamental cyclotron resonance of the third ion species Z (resonant absorber), the latter-albeit present only in trace quantities-is shown to absorb almost all the incoming RF power. A quantitative criterion for the resonant Y : X plasma composition is derived and a few numerical examples are given. Since the absorbed power per resonant particle is much larger than for any other ICRF scheme, the here discussed scenarios are particularly promising for fast particle generation. Their possible application as a source of high-energy ions for the stellarator W7-X and to mimic alpha particles during the non-activated phase of ITER tokamak is briefly discussed.