In Chapter 1 we report the final results of our extensive experimental and theoretical study on current sheet canting. The phenomenon of current sheet canting in pulsed electromagnetic accelerators is the departure of the plasma sheet that carries the current from a plane that is perpendicular to the electrodes to one that is skewed, or tipped. Current sheet canting is a ubiquitous problem in electromagnetic PPTs and results in a degradation of their propulsive e ciency. An experimental study in which photographic, magnetic, and laser-interferometric measurements of the canting angle of the current sheet in an experimental accelerator were made. The goal was to identify the mechanism(s) underlying the effect. The results of the experiments using advanced diagnostics and eight different propellants (hydrogen, deuterium, helium, neon, argon, krypton, xenon, and methane) were combined with a theoretical model and led to a detailed explanation of current sheet canting. In the resulting picture, canting is due to a depletion of plasma near the anode, which results from axial density gradient induced diamagnetic drift. Rapid penetration of the magnetic eld through this region ensues, due to Hall effect, leading to a canted current front ahead of the initial current conduction channel. The acquired fundamental understanding allowed the development of design prescriptions to reduce canting and abate its detrimental effects. These are discussed in the final chapter.
In Chapter 2 we report on the recent results from our exploration of current sheet initiation in PPTs. In particular we explore the physics of photo-induced and surface-assisted discharge initiation which holds the promise for providing azimuthally unuform and erosion-free discharge initiation in PPTs.
Specifically we demonstrate that a discharge can be initiated in a PPT at an undervoltage by shining an IR laser pulse on the thruster s backplate. The technique has the potential for achieving uniform and erosion-free discharge initiation. Three candidate mechanisms are investigated: thermionic emission, cathode vaporization, and gas desorption. Mass spectroscopic measurements and theoretical calculations implicate water desorption from the backplate as the likely explanation for the observed effects. It is then shown that while thermionic emission was not operative in the experiments, it can be used as the basis of the design of a discharge initiator.
We finally conclude in Chapter 3 with a discussion of how the insight gained from our fundamental study can be used to design better thrusters.