Nonlinear excitation of ion-acoustic modes by rf waves

Abstract

We have detected a slow ion-acoustic wave propagating obliquely to the equilibrium magnetic field in a low-density, weakly ionized helium plasma produced by rf power in the linear mirror machine LISA. When an upper hybrid wave is launched perpendicular to the ambient magnetic field, we have observed that the upper hybrid wave decays parametrically into another upper hybrid wave and a slow ionacoustic wave.

Lower hybrid resonant layer excitation and mode conversion in the linear LISA machine

“…We observed that in the range 0 < 1 k , I r 6 2.5 (0 < r c 6.0cm), the E , profile was not modified by the Landau damping and 'I collisional effects near the hybrid layer. This agrees with the experimental data shown by [4], except close to r = 0. We can observe in Figs 2(b) and 2(c) that the respective electric fields, E, and E,, do not have the typical Bessel function dependence, which is obtained when k , is real.…”

“…Close to the resonance layer, the strong electric field E, induced by the antenna accelerated the electrons in the perpendicular direction, so that u,/u,, > 1. The lower hybrid resonance strongly modified the electric field distribution in the plasma, especially at the resonance layer, as shown by Rapozo et al [4].…”

“…The experiment was carried out on LISA, a linear mirror machine now operating at UFF [4]. For the experiment described here the d.c. current in the field coils was finely controlled so that the magnetic field varied from 200Gauss to 1200Gauss.…”

“…Figure 3 of Ref. [4] shows the profiles of the parallel temperature Tll as a function of the radius, for two different resonant volume sizes, namely large and small ones. At the resonant layer (Ar E l.Ocm), the parallel Landau damping on the electrons and the perpendicular Landau damping on the ion were responsible for the wave dissipation.…”

“…Figures 6(a) and 6(b) of Ref. [4] show the power deposition profile us. radius for a large resonant volume (dashed line) and small resonant volume (solid line) for the cases without and with plasma; these data permit us to obtain the quality factor Q of the LISA cavity.…”