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.

Articles you may be interested inLinear and nonlinear physics of the magnetoacoustic cyclotron instability of fusion-born ions in relation to ion cyclotron emissionThe magnetoacoustic cyclotron instability is a mechanism by which waves on the perpendicular fast Alfvkn-ion Bernstein branch can be excited through cyclotron resonance with an energetic ion population. It is a candidate emission mechanism for the superthermal ion cyclotron radiation, apparently associated with the products of fusion reactions, that has been observed from tokamak plasmas. Iri the present paper, an extended shell model is adopted for the energetic ion distribution function, f,(v) -II, exp[ -(u-vc) 2/u$]. An analytical formulation of the dispersion relation is obtained, whose numerical solution yields quantitative information on the role of VT in stabilizing wave growth at ion cyclotron harmonics. The results show that, for typical plasma parameters of interest, the degree of instability is significantly depressed, relative to its level for v,=Oj once uT/vO~O. 1. Gaps appear in typical multiple cyclotron harmonic excitation patterns for 0.1 0.25. Thus the energetic ion shell-driven magnetoacoustic cyclotron instability typically occurs only when the shell is relatively narrow in velocity space.

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Electron emission spectra of metallic Ca, Sc, Ti and V excited by rare-gas-ion bombardment are presented. The spectra display distinctive atomic characteristics as well as solid-state structure due to M23VV Auger emissions. By energy considerations, we show that their main spectral features can be explained by atomic autoionizations of the excited metal atoms in 3p53dn+14s2 states with excitation energies less than the solid-state 3p ionization energy. The relative importance of the autoionization channels M23N1N1, M23M45N1 and M23M45M45 is investigated by comparing theoretically calculated spectra with the experimental spectra. Our work suggests that M23M45N1 transitions rather than super-Coster-Kronig M23M45M45 transitions dominate the decay channels.