Three-ion ICRH scenarios: new and efficient schemes for plasma heating and fast particle generation in current and future fusion devices*

Three-ion ICRH scenarios [1] offer new exciting possibilities for the application of ICRF heating in fusion devices. In a plasma consisting of two main ions, ICRF power absorption is strongly enhanced in the vicinity of the ion-ion hybrid (IIH) layer because the RF electric field E+ is rotating nearly completely in the direction of the plasma ions. Candidate absorber ions are those for which the resonance layer is located close to the IIH layer and thus can be a third ion species with Z/A in between that of the two main ions or fast main ions, with a Doppler shift that displaces their resonance layer in between that of the two main ions. These ions may be typical H or He isotopes or impurities intrinsic or extrinsic to the plasma such as 9Be, 22Ne, Ar, etc.

Both plasma heating and fast particle acceleration are possible with this technique. 
We summarize recent and past experiments with various three-ion ICRH scenarios on
TFTR [2], C-Mod [1], JET [1] and AUG [3], using third ions and fast accelerated beam
particles [4] as resonant absorbers and with on- and off- axis heating. We will illustrate the promising potential of three-ion ICRH scenarios for future JET D-T, ITER and DEMO operations and for testing the fast particle confinement properties of the optimized stellarator W7-X at the high densities expected in this device in the coming years.

Finally, applications of three-ion scenarios are not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3He-rich solar flares.

[1] Ye.O.Kazakov, J.Ongena, J.Wright, S.Wukitch et al., Nat.Phys. 13,973–978(2017) 
[2] J.R.Wilson et al., Phys. Plasmas 5,1721(1998) 
[3] A.Kappatou et al., 45th EPS Plamsa Phys.Conf (Prague 2018) Paper O2.105 (2018) 
[4] J.Ongena, et al.,EPJ Web of Conferences, 157,02006(2017)