Mode conversion of fast Alfvén waves at the ion–ion hybrid resonance

Abstract

Substantial radio-frequency power in the ion-cyclotron range of frequencies can be effectively coupled to a tokamak plasma from poloidal current strap antennas at the plasma edge. If there exists an ion–ion hybrid resonance inside the plasma, then some of the power from the antenna, delivered into the plasma by fast Alfvén waves, can be mode converted to ion-Bernstein waves. In tokamak confinement fields the mode-converted ion-Bernstein waves can damp effectively and locally on electrons [A. K. Ram and A. Bers, Phys. Fluids B 3, 1059 (1991)]. The usual mode-conversion analysis that studies the propagation of fast Alfvén waves in the immediate vicinity of the ion–ion hybrid resonance is extended to include the propagation and reflection of the fast Alfvén waves on the high magnetic-field side of the ion–ion hybrid resonance. It is shown that there exist plasma conditions for which the entire fast Alfvén wave power incident on the ion–ion hybrid resonance can be converted to ion-Bernstein waves. In this extended analysis of the mode conversion process, the fast Alfvén waves can be envisioned as being coupled to an internal plasma resonator. This resonator extends from the low magnetic-field cutoff near the ion–ion hybrid resonance to the high magnetic-field cutoff. The condition for 100% mode conversion corresponds to a critical coupling of the fast Alfvén waves to this internal resonator. As an example, the appropriate plasma conditions for 100% mode conversion are determined for the Tokamak Fusion Test Reactor (TFTR) [R. Majeski et al., Proceedings of the 11th Topical Conference on RF Power in Plasmas, Palm Springs (American Institute of Physics, New York, 1995), Vol. 355, p. 63] experimental parameters.

On electron Bernstein waves in spherical tori

“…In this approximation the X mode resonance is no longer at K ⊥ = 0 but at K ⊥ − n 2 = 0 which, for small n , is a small correction. From an analogous analysis carried out in [2], we find that the fraction of the power on the X mode that is converted to EBWs is given by…”

Electron cyclotron heating in NSTX

“…We show that for a given plasma density and magnetic field profile, an appropriate choice of the frequency allows one to form a triplet (cutoff-resonance-cutoff) X to EBW mode-conversion scenario. This is similar to the low-frequency, ion-ion hybrid resonance scenario for which 100% mode conversion was shown to be possible [2]. The mode-conversion efficiency for DIII-D and NSTX is calculated.…”

“…This wave equation can be solved approximately as we have shown before. [2] We find that for an incident X-mode from the low B-field side, the fraction of power resonantly absorbed at the UHR (hence, mode converted to the EBW) is given by…”

“…This is the so-called "triplet" mode-conversion scenario, which has been encountered in the ion cyclotron range of frequencies (ICRF), and for which we have provided a complete analytical description. [2] We use a similar analysis for the mode conversion of the X-mode to EBW. In the following we summarize our analysis and illustrate the mode conversion and EBW damping for tokamak parameters of DIII-D and NSTX.…”

Tunneling and mode conversion of fast magnetosonic waves in the magnetospheres of Earth and Mercury

“…If the transmitted FW undergoes additional wave reflection, once a right-hand polarized R-cutoff (ǫ R = n 2 ) or a supplementary MC layer is present in a plasma, the resulting reflection and mode conversion coefficients depend on the constructive/destructive interference between the partially reflected fast waves (Majeski et al 1994;Fuchs et al 1995;Ram et al 1996;Kazakov et al 2010;Kim et al 2011). For the cutoff-resonancecutoff triplet (see Fig.…”