Measurement of MTF target plasma temperature using filtered photodiodes

Abstract

Magnetized Target Fusion (MTF) requires the fast compression of hot, dense plasmas by a conducting liner. We have used two-dimensional MHD calculations to study the electromagnetic implosion of metallic liners driven by realistic current waveforms. Parametric studies have indicated that the liner should reach velocities of 3-20 k d s , depending on the magnetic field configuration, and reach convergence ratios (initial radius divided by final radius) of at least 10. These parameters are accessible with large capacitor bank power supplies such as SHWA or ATLAS, or with magnetic flux compression generators. One issue with the high currents that are required to implode the liner is that Ohmic heating will melt or vaporize the outer part of the liner. Calculations have shown that this is a realistic concern. We are currently addressing questions of liner instability and flux diffusion under MTF conditions. Another issue is that the magnetic fields needed to inhibit thermal losses to the walls will also heat, melt, or vaporize the inner wall surfaces. For initial fields between 5-50 Tesla, the wall heating is significant but does not result in rapid melting. As the implosion evolves, flux compression leads to fields in excess of 100 Tesla. Calculations which include flux diffusion, Ohmic heating, and realistic material properties show that a significant fraction of the inner surface of an aluminum liner will have melted and vaporized in the final microsecond of implosion. It is not clear at this time that such material mixes will the hot plasma. We are conducting studies to determine the extent of wall-plasma interaction under these conditions. Magnetized Target Fusion (MTF) is an approach to fusionwhere a preheated and magnetized plasma is adiabatically compressed to fusion conditions. Successful MTF requires a suitable initial target plasma with a magnetic field of a1 least 5 T in a closed-field-line topology, a density of roughly 10l8 ~m -~, a temperature of at least 50 eV but preferably closer to 300 eV, and must be free of impurities which would raise radiation losses. The goal of these experiments is to demonstrate plasma conditions meeting the requirements for an MTF initial target plasma. The plasma is produced by driving a z-directed current of 1-2 MA through either a static gas fill or a 38 pm diameter polyethylene fiber. The data obtained from an array of filtered photodiodes is used to estimate the plasma temperature. The filter material and thickness for each diode is chosen such that the lowest absorption edge for each is at a successively higher energy, covering the range from a few eV to 5 keV. The analysis assumes a fully stripped optically thin plasma which radiates as either a blackbody, a bremsstrahlung emitter, or a group of emission lines (gaussian-like). 288

Analysis of data from z-pinch MTF target plasma experiments

“…1 in Ref. 3). The radial profile of the magnetic field is consistent with a diffuse zdirected current.…”

“…5 show that the polyethylene fiber discharges produce higher maximum open diode signal compared to the gas fill discharges. This is due to the strong m=0 "sausage" instability events which occur in the fiber discharges but not in the gas fill discharges [3,4,5]. The data also shows that, particularly for the hydrogen fill discharges, the maximum open diode signal does not depend on capacitor bank output voltage.…”

“…The energy content of a 5 T, 10 18 cm -3 , 150 eV plasma filling the containment region is ≈ 2 kJ. The estimated radiation energy for hydrogen fill discharges is 0.6 kJ [3], which also assumes uniform plasma conditions. This would imply only 30% of the plasma energy is lost to radiation.…”

“…4) is then used as a qualitative measure of the plasma temperature; i.e., when the ratio is large (several hundred), the plasma temperature is high (of order 200 eV) as shown in Ref. 3, and when the ratio tends to one, the plasma has become very cold (< 10 eV). A hot plasma lifetime is defined as the time it takes for the open to visible ratio to drop from the maximum value to 5% of the maximum value.…”

“…1 of a companion paper in this proceedings [3], and the parameters of the Colt capacitor bank are summarized in Ref. 3.…”