Ion Cyclotron Instability in the Magnetic Well of DECA II

A deuterium plasma with a density of 109-1010 particles/cm3 is confined in the magnetic well of DECA II. Study of the fast neutral atoms emitted by the plasma shows that the principal cause of plasma losses is charge exchange with the neutral gas desorbed by the walls during injection. From this study the authors deduce the perpendicular energy distribution function of the hot plasma, which has a mean energy Ti = 1.8 keV. The plasma is the seat of an instability at the ion cyclotron frequency and its harmonics; the instability is of low amplitude and does not contribute measurably to the plasma losses. The spectrum of the unstable frequencies varies with the density of the plasma. For densities of the order of 109 particles/cm3, instability occurs at the centre of the well at the cyclotron frequency (Ωi) and at its harmonics (2Ωi, 3Ωi, 4Ωi). For higher densities (∼ 109-1010 particles/cm3) the instability appears only at the first and third harmonics (2Ωi, 4Ω)i. The statistical width of each of the observed lines is remarkably narrow, being less than the resolution of the instruments (of the order of 3%). The azimuthal mode of the ion cyclotron frequency is m = 2; that of the first harmonic is m = 4. These modes become deformed as the plasma moves along the flux tube: the phase is maintained on a magnetic field line; the parallel wave length is at least twice the length of the plasma (λ‖≥ 14 cm). The measured growth rates are of the order of 5% of the cyclotron frequency. The main source of energy for feeding the instability appears to be the non-monotonic nature of the perpendicular energy distribution (distribution of the loss cone). Numerical study of the dispersion equation of a two-component plasma (a hot plasma having the experimental distribution as its distribution function, and a cold Maxwellian plasma corresponding to the plasma which may be produced by charge exchanges) shows that the latter reduces the density threshold above which the instability may be triggered sufficiently to explain the experimental results. This study also shows that the frequency ω = Ωi is stabilized when the proportion of cold plasma exceeds 10%, the most unstable frequency then being ω = 2Ωi. (author)