On radio frequency current drive in the ion cyclotron range of frequencies in DEMO and large ignited plasmas

Abstract

Abstract. To explore the possibility of efficient Fast Wave current drive in an ignited plasma in the Ion Cyclotron (IC) range of frequency in spite of competition from absorption by ions, we have added to the full-wave toroidal code TORIC a set of subroutines which evaluate absorption by these particles at Ion Cyclotron harmonic resonances, using a realistic "slowing-down" distribution function, and taking into account that their Larmor radius is comparable or even larger than the Fast Wave wavelength. The thermalized population of α-particles is not a serious competitor for power absorption as long as their number density is compatible with maintenance of ignition. By contrast, the energetic slowing down fraction, in spite of its even greater dilution, can absorb from the waves a substantial amount of power at the cyclotron resonance and its harmonics. An extensive exploration both in frequency and in toroidal wavenumbers using the parameters of one of the European versions of DEMO shows that three frequency windows exist in which damping is nevertheless predominantly on the electrons. Designing an antenna capable of shaping the launched spectrum to optimize current drive, however, will not be straightforward. Only in a narrow range when the first IC harmonic of Tritium is deep inside the plasma on the highfield side of the magnetic axis, and that of Deuterium and Helium is still outside on the low-field side, it appears possible to achieve a satisfactory current drive efficiency with a conventional multi-strap antenna, preferentially located in the upper part of the vessel. Exploiting the other two windows at quite low and quite high frequencies is either impossible on first principles, or will demand novel ideas in antenna design.PACS numbers: 52.25 Fi, 52.25 Ya, 52.55 Fa

A new ion cyclotron range of frequency scenario for bulk ion heating in deuterium-tritium plasmas: How to utilize intrinsic impurities in our favour

“…Direct absorption of ICRF power by energetic alpha particles is a potential showstopper for the (Be)-D-T heating scenario at higher T i . Using the recently upgraded version of the TORIC code [42], we have estimated that this absorption is reasonably small up to T 0 ≃ 10 − 15 keV: at T 0 = 10/15 keV the fraction of alpha-particles in the plasma core is computed to be about 0.2%/0.5% and they absorb p α ≈ 7%/19% (p Be = 72%/52%) of the incoming RF power, respectively. However, channeling of fusion alpha-particle power using minority ion catalysis [43] could potentially facilitate the application of Be impurity heating in D-T plasmas at higher T 0 .…”

Toroidicity effects on the absorption of IC waves by electrons

“…In ITER frequency windows for hf current drive in the IC range (ICCD) are difficult to be found [17,18]. In the initial non-activated phase, ICCD might be marginally possible at the upper end of the planned frequency range.…”

Survey of heating and current drive for K-DEMO

“…Current drive is optimized by minimizing damping by any ion species, so the frequency should be just below 2Ω cD at the outboard edge of the plasma (with the 2Ω cT resonance at R < R axis ) and the parallel wavenumber should be chosen to minimize damping by alphas [13,14]. Accordingly, we have considered only a midplane launcher, because alpha damping cannot be strictly minimized with off-midplane launchers: it is more difficult to launch a pure k spectrum [15], and the opposing requirements on k to both enhance electron absorption and minimize fast alpha absorption produce higher alpha absorption than for a midplane launcher [16]. Although the studies of ICRF current drive cited above were limited to a few EU DEMO designs, their qualitative guidance for midplane launchers has been validated by both the (advanced) ACT1 [17] and (conservative) ACT2 [18] design studies.…”