Studies of end loss from a theta pinch using a Twyman--Green interferometer. Scientific report 77-4

Abstract

The transient rate of flow of plasma from the ends of a short, high density, low temperature linear theta pinch was studied experimentally and analytically. Diagnostic devices were used to study the discharge and plasma properties; these included a Rogowski coil, a magnetic field probe and a Twyman-Green double pass interferometer which was illuminated by a 7 mW He-Ne laser. The interferometer was used to monitor the passage of fringes with respect to time at two radial positions simultaneously by the use of a photodetector consisting of two fast silicon photodiodes with supporting circuitry. One complete fringe represented a change in number density of 2.942xl0 16 cm" 3 .Compatible interpretation of the above-mentioned diagnostics allowed evaluation of the loss rate of plasma. The major uncertainty in obtaining 1/e particle end loss times from the Twyman-Green interferometric data was associated with lack of data on the density profile. Two different techniques were developed to provide estimates of the profile. These led to loss times estimates of 1.82 usee and 2.85 usee for a 50 millitorr fill pressure of H_, and 2.39 usee and 2.81 psec for a 65 millitorr fill pressure. These loss times were compared with: results obtained from magnetic field probe data on plasma column oscillation which indicated the loss rate; loss times analytically determined by scaling experimental results from a larger theta pinch; and loss time predicted by a zero dimensional computer code. It was determined that, for the 65 millitorr fill pressure, agreement between loss time predicted by these four techniques was good; however, both experimental uncertainties and loss time uncertainties were greater for the 50 millitorr fill pressure.IV

Study of flow and loss processes at the ends of a linear theta pinch. Progress report, June 1, 1977--May 31, 1978

“…T /TT~G From Equations (6), (7), (9), (10), (11) and (15), i , K » a -q//ro , (16) and n = n /l -6(1 + G) , (19) where m. is the ion mass, n is the fill density, r is the internal discharge l o The measured plasma outer radius quoted is the r ff value arrived at through a determination of the radius in which 90% of the plasma is contained.…”