Electron heating by mode-converted ion-Bernstein waves in ICRF heating of tokamak plasmas

Abstract

In a tokamak plasma, ion‐Bernstein waves (IBW) can be excited by mode‐conversion of the externally launched fast wave for ICRF heating. This conversion process is known to be efficient for low k∥’s which carry substantial power from a single loop antenna. A detailed numerical analysis of the propagation of the IBW shows that the initial small k∥ are significantly enhanced along the rays due to toroidal effects. The upshift can occur for short radial distances of propagation and is large enough so that the IBW can Landau damp onto the electrons. This could help explain the observed strong electron heating by ICRF waves in tokamak plasmas. The numerical ray trajectory analysis is done in toroidal geometry for a hot Maxwellian plasma with gradients in temperature, density, toroidal and poloidal magnetic fields included in a WKB sense. A simple analytical model is derived which explains the upshift in k∥ and gives results very close to the numerically obtained values. Approximate analytical conditions for app...