Parametric Limitations on Discharge Performance in the Maryland Centrifugal Experiment

Abstract

Inherent in any experiment is a set of bounding limitations. In the Maryland Centrifugal eXperiment (MCX) these limitations are manifest as restrictions upon discharge performance as defined by program goals. We therefore examine the multidimensional parameter space in which MCX operates so as to ascertain the performance boundaries along each of the variable axes. In as much as is possible, the variables are expressly decoupled in order to determine their unique effect upon discharge performance. This study has led to the following conclusions. First, the nascent cause of transitions within the MCX discharge from a higher voltage mode (H-mode) into a lower voltage mode (O-mode) appears to be consistent with the formation of a secondary breakdown within the vacuum vessel. These transitions have been shown to be mitigated by auspicious choice of insulator position and geometry. Second, the effect of variations to the radial extent of the discharge has been studied and a set of parametric dependencies determined. These show the MCX discharge to be diffusive in nature, and highly dependant upon radial extent. Thus leading to the conclusion that a minor increase to the midplane vessel extent should result in substantial performance enhancement. Finally, a global limitation to the MCX discharge has been discovered. It is manifest as an abrupt maximum attainable voltage across the plasma. This has been interpreted as a velocity limitation due to the direct relationship between plasma voltage and rotation velocity (v(phi) = Vp/aB). An examination of a discharge struck in a background helium environment, as opposed to the standard hydrogen backfill, has shown that this limit is consistent with Alfven's theory of a critical ionization velocity. While this limitation has been observed on the historic predecessors to MCX, there have also been cases where supercritical velocities have been achieved. Suggestions for facilitating access to this regime as well as projections for an enhanced iteration of the MCX device are listed.