Harmonic launching of ion Bernstein waves via mode transformation

Abstract

Ion Bernstein wave excitation and propagation via finite ion-Larmor-radius mode transformation are investigated theoretically and experimentally. It is shown that in the ion cyclotron range of frequencies ω≲4Ωi, with modest ion temperatures (Ti≲10 eV), the finite-Larmor-radius effect removes the wave singularity at the lower-hybrid resonance layer, enabling an externally initiated electron plasma wave to transform continuously into an ion Bernstein wave. In an ACT-1 hydrogen plasma (Te≂2.5 eV, Ti≲2.0 eV), externally excited ion Bernstein waves have been observed for ω≲2Ωi as well as for ω≲3Ωi. The finite ion-Larmor-radius mode transformation process resulting in strong ion Bernstein wave excitation has been experimentally verified. Detailed measurements of the wave dispersion relation and of the wave-packet trajectory show excellent agreement with theory. The dependence of the excited ion Bernstein wave on the antenna phasing, on the plasma density, and on the neutral pressure (Ti) is also investigated. The present work provides information relevant to the physics of ion Bernstein wave propagation and waveguide coupling as well as to the rf–plasma interaction during ion-cyclotron-range-of-frequency heating in fusion plasmas.