Electrostatic Acceleration of Neutral Plasmas - Momentum Transfer through Magnetic Fields

Electrostatic plasma accelerators which avoid the space charge limitations of conventional ion rockets are described. Additional advantages for these devices include moderate requirements on magnetic field strength, and on power level. In cylindrical and annular geometries, neutral plasmas can exhibit axial acceleration under the influence of externally applied axial electrostatic fields in the presence of radial magnetic fields. Ions will accelerate freely in the presence of such ah axial electric field if both the ion gyro radius and the ion mean free path are large relative to the apparatus size. The axial drift of electrons, however, will be strongly inhibited by the radial magnetic field if the electron gyro radius is small relative to both the apparatus size and to the electron mean free path. The resulting circumferential drift of electrons constitutes an electric current which in turn produces a magnetic reaction force on the external field coils. The electric field is impressed upon the plasma hy means of an upstream anode and a downstream cathode. This cathode serves as a source of electrons for space charge neutralization of the exit plasma beam. A critical engineering question concerns the effectiveness of the radial magnetic field in inhibiting the upstream diffusion of electrons. This diffusion is an energy loss mechanism. Both classical and anomalous (Böhm type) electron diffusion models are considered. Experiments are described which substantiate the existence of the here described mechanism for momentum transfer to neutral plasmas. The experiments are in approximate agreement with the anomalous (Böhm type) diffusion model and are in clear disagreement with the classical diffusion model. The engineering significance of this result and possible approaches for dealing with it are considered. This work was sponsored by the Air Force Office of Scientific Research of the Office of Aerospace Research, United States Air Force under Contract No. AF 49(638)-659.