Quenching of the current-driven ion-wave instability in the trapped-electron regime in a toroidal plasma

An analysis is presented of an experimental study of the scattering of microwaves by density fluctuations due to the current-driven ion acoustic wave instability in a toroidal plasma confinement device, the FM-1 spherator. It is found that the instability exists only in the collisional regime (where ν/sub eff/ is greater than ω/sub be/) and it is inhibited (i.e., quenched by two orders of magnitude) in the trapped-electron banana regime (where ν/sub eff/ is less than ω/sub be/). Further, it is found that for two different values of the ratio of the trapped to untrapped electron number density (i.e., for both 70 percent and 40 percent mirror trapping) this inhibition always starts to occur when ν/sub eff/ approximately equals ω/sub be/. Finally, it is shown that the nature of this inhibition is in general agreement with the recent theoretical notion that in the banana regime the entire Ohmic heating current is carried only by the freely circulating electrons