Electrostatic ion-cyclotron instability driven by a slow electron drift

Abstract

The electrostatic ion-cyclotron instability can be excited in a single-ended Q-machine by a positively biased collector. According to infinite plasma theory the instability is triggered by the drifting electrons. Recently this explanation has been questioned since experiments by another group have shown that the excitation takes place solely near the collector. The excitation has been ascribed to the collector sheath which has strong axial and radial gradients. The authors try to prove that drifting electrons are able to excite the ion-cyclotron instability. They produce an electron drift without the formation of a localized two-dimensional sheath. To that end the plasma electrons are delivered by a source which is spatially separated from the ion source. If the central section of these electrons is slightly accelerated by a grid they observe the instability.