New synergy effects of the lower hybrid wave and the high harmonic fast wave current drive

Abstract

The combined drive current of the lower hybrid wave (LHW) and the high harmonic fast wave (HHFW) is studied for the first time, based on the use of low and higher β_e operational parameters in EAST. Broad and significant synergistic effects are found in the simulation, the local synergy factor reaching a factor of 4.3 in the off-axis region. The current drive (CD) efficiency is greatly improved, and the current profile is modified as a result of the synergy between the two types of waves . the LHW interacted with the resonant electrons in low parallel velocity region and pushes them into the adjacent resonance region of the high phase velocity wave (HHFW), the number of fast electrons resonant with the HHFW is increased dramatically, and the driven current is enhanced. Therefore the synergy effect strongly depends on the positional relational between the velocity resonance regions of the two waves. Moreover, the effects of the parallel refractive index and the wave power on the synergy effect are examined. Some problems well known in the single LHW CD or the HHFW CD may be overcome by the combined CD.

Helicon and lower hybrid current drive comparisons in tokamak geometry

“…The last form is sometimes referred to as an oblique whistler wave. Moreover, helicon waves can propagate in the high harmonic fast wave (HHFW) range of frequencies (Ono 1995;Lashmore-Davies et al 1998) and are sometimes referred to as HHFWs (Yin et al 2022).…”

“…As a result, the efficiency is the ratio J H+LH || /P H+LH cd , which for a single frequency and wavenumber recovers the same expression as given at the end of § 6. Moreover, the cross term may allow more current to be driven and more power to be absorbed, perhaps as observed in the EAST numerical simulations of Yin et al (2022). Synergy is possible, but not assured, since helicon and lower hybrid waves independently act on perpendicular and parallel motion, respectively, to make the electrons less collisional.…”

“…2021; Li, Li & Liu 2021; Yin et al. 2022). Both HCD and LHCD rely on a Landau resonance and the preferential heating of electrons, although HCD uses a perpendicular component of the applied electric field rather than the parallel component used for LHCD.…”

“…Investigating helicon (or whistler) waves as a means to drive a parallel current in a tokamak is a less studied area of current drive (Pinsker 2015) than the more common mechanism of lower hybrid waves (Fisch 1978;Karney & Fisch 1979, 1985Fisch & Boozer 1980;Fisch & Karney 1981;Cordey, Edlington & Start 1982;Taguchi 1983;Cohen 1987;Giruzzi 1987;Chiu et al 1989;Ehst & Karney 1991) which is extensively reviewed by Bonoli (2014). Nonetheless, helicon current drive (HCD) remains of considerable interest (Vdovin 2013;Prater et al 2014;Pinsker 2015;Wang et al 2017;Lau et al 2018Lau et al , 2019Pinsker et al 2018) because of its many similarities to lower hybrid current drive (LHCD) and its ability to efficiently access higher density regions (Li et al 2020a,b;Lau et al 2021;Li, Li & Liu 2021;Yin et al 2022). Both HCD and LHCD rely on a Landau resonance and the preferential heating of electrons, although HCD uses a perpendicular component of the applied electric field rather than the parallel component used for LHCD.…”

“…For LHCD the applied rf fields are tailored to make the first or v || J 0 term dominate. For HCD the rf fields must be applied in a manner that makes the second or v ⊥ J 1 term dominate (Prater et al 2014;Pinsker 2015;Pinsker et al 2018;Yin et al 2022), so J 1 will not be expanded by assuming its argument is small. Of course, both HCD and LHCD rely on there being a Landau resonance.…”