Pulsed plasma propulsion

There are many different types of pulsed‐plasma engines, or accelerators, which may be classified by the geometry of the discharge, either a sheet or a line, and by the position of the electrodes, which are either directly in contact with the propellant, or inductively coupled with the propellant. It is the author’s belief that, of all the various types of pulsed‐plasma accelerators, only the sheet‐direct‐contact accelerator will ultimately become a working engine. The arguments in support of this belief are partly of a practical nature and may even be only intuitive. There are, however, two devices which fill even this narrow description: the so‐called “pinch engine” and the purely axial accelerator illustrated in Figs. 1 and 2. Note that both are characterized by coaxial geometries and require that the discharge be axi‐symmetric. In the “pinch” type or radial accelerator, the initial discharge formed by axial currents must be turned so that the currents become radial. No turning is apparently required in the axial accelerator. However, the magnetic‐field forces are inversely proportional to the radius, and some turning will take place. It would appear that both accelerators should maintain a current sheet normal to the walls of the electrodes. This may be more easily obtained in a curved channel than in a straight one. In addition to symmetry, it appears necessary to obtain a discharge which is relatively thin and impervious to neutral particles by virtue of the high degree of ionization in the discharge. The accelerating sheet, driven by Lorentz forces, should then drive out the propellant situated in the interelectrode region. In general, the thickness of the sheet and its degree of ionization is affected by the energy available to the discharge. Low energy per discharge coupled with a high pulse rate is desirable for low capacitor weight, but is detrimental to the formation of a good driving “magnetic piston”. Experimentally, lower energy per discharge appears to be required in the radial than in the axial accelerator in order to obtain symmetric and nonporous sheet discharges or “pistons”.