Ion Distribution Functions in Collisionless Surface Ionized Plasmas

For a low-density surface-ionized plasma (e.g., in a Q machine) where Coulomb or binary collisions are negligible, a simple steady-state model of the plasma is obtained through the Vlasov and Poisson equations. Among the main predictions of this model, the electron-rich regime involves a truncated Maxwellian distribution function for the ion velocities parallel to the magnetic field. Experiments have been carried out in a short single ended Q machine for a cesium plasma. Curves of density versus neutral flux are plotted. With the help of a heated probe and an electrostatic analyzer, whose work functions are carefully taken into account, the ion distribution functions are deduced. In our observations they correspond to beams of 1 km/sec to 1.5 km/sec mean velocity and velocity spread which can be characterized by a parallel temperature of 150-300° K. These experiments and the theoretical model are in good agreement.