A possible excitation mechanism for observed superthermal ion cyclotron emission from tokamak plasmas

Abstract

Intense superthermal ion cyclotron emission (ICE) has been observed from tokamak plasmas. The power spectrum displays narrow peaks at multiple harmonics of the background ion cyclotron frequency [Cottrell and Dendy, Phys. Rev. Lett. 60, 33 (1988)] in Ohmic deuterium plasmas, and the radiation appears to be driven by the fusion ion population in the edge plasma. Theoretical investigation of this phenomenon may be rewarding, in terms of the information about the behavior of energetic ions in tokamaks that can be extracted from ICE observations. The interpretation presented here is based on the resonant excitation of fast Alfvén waves with ion Bernstein waves supported by an energetic ion species (α), in the presence of a more numerous thermal ion species (i). Because the ion cyclotron frequencies may be commensurate (lΩα=sΩi for some low integers l,s), and observations indicate that ω is comparable in magnitude to Ωi, the standard theory [Belikov and Kolesnichenko, Sov. Phys. Tech. Phys. 20, 1146 (1976)] which assumes ω≫Ωi is not immediately applicable, and is accordingly extended here to the low-frequency regime. The results show that excitation of the fast Alfvén wave at proton cyclotron harmonics can occur for fusion proton concentrations nα/ni as low as 10−7, and that multiple cyclotron harmonics can be simultaneously unstable. Furthermore, while fusion protons born at 3.0 MeV are above the energy threshold required to drive the instability, the other primary fusion products in deuterium—1.0 MeV tritons and 0.82 MeV helium-3 nuclei—fall below it, consistent with the observation that radiation at cyclotron harmonics of the latter is not detected. However, the mechanism considered here cannot easily explain the generation of radiation which is observed at deuterium cyclotron harmonics that are degenerate with proton half-harmonics.