Abrupt sonic transition in the Hall thruster

Summary form only given. Plasma acceleration to supersonic velocities occurs both in space and laboratory plasmas. For a laboratory plasma accelerator, it is desirable that the plasma flow be stable, continuous and controllable. Yet, both experiments and numerical simulations of the Hall thruster exhibit in many cases an oscillatory behavior of the accelerated plasma. Steady flows rarely exist, a fact that is a serious restriction on the flexible use of the accelerator and an obstacle in the quest for an accelerator of variable thrust. Employing a simple model, we show that the acceleration of the plasma to supersonic velocities as a steady flow occurs only when certain relations between the flow parameters are satisfied. The desirable smooth steady acceleration in which the sonic transition occurs inside the accelerator is therefore an exception rather than the rule. We demonstrate that point by a comparison of the plasma acceleration to the acceleration of a gas to supersonic velocities at a Laval nozzle. We then demonstrate that by forcing an abrupt sonic transition, via the introduction of a discontinuity at the plasma flow (here by placing a floating electrode inside the channel), we enlarge the regime of parameters where a smooth sonic transition occurs. Moreover, the formation of a large electric field at the plane of the abrupt sonic transition provides a more efficient acceleration.