Design and Characterization of a Theta Pinch Discharge for Atomic Emission Spectroscopy

362 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.The high temperature and density plasma generated by magnetic constriction of a preexisting plasma was studied as an emission source for the elemental analysis of solid materials. This high energy plasma has the ability to directly sample solid materials. A theta pinch discharge was designed and constructed to create this magnetically compressed plasma. In the theta pinch geometry, magnetic compression is achieved by a pulsed, high current discharge through a coil. The coil encloses an insulated discharge vessel containing an ionized plasma at low pressure. The field generated by the high current discharge through the coil causes magnetic compression of the plasma, increasing both the plasma temperature and density. The main discharge source which generates the high current pulse has been operated at peak voltages of -31 kV, and has been shown to be capable of generating peak discharge currents of 60 kA. Two coil geometries, cylindrical and helical, have been examined with this source. Improved coupling between the high current discharge and the plasma was observed with the higher inductance, helical coil. Along with the effect of coil geometry, the effect of fill gas on coupling efficiency was also studied. The theta pinch was operated with both argo and helium fill gases. Coupling between the main discharge and the plasma was more efficient for discharges in argon than for discharges in helium, most likely due to the higher ionization potential of helium.This discharge was applied to aluminum, stainless steel, and tungsten samples. Evidence of sampling by the theta pinch plasma has been observed for each sample. From studies with aluminum samples, the efficiency of sampling by the theta pinch plasma was greater at the edges of the discharge plasma than at the center. Emission measurements from the tungsten and stainless steel samples showed that the sampling efficiency of this transient discharge was dependent on the surface area of the material. This agrees with the expected ablative sampling process.Studies of the time-resolved emission from the argon fill gas and an aluminum sample showed that the continuum and ionic emission follow the main discharge current waveform. Emission from neutral species also showed a dependence on the discharge current, but neutral emission was also observed after cessation of the discharge current. This continuation of neutral emission after cessation of continuum and ionic emission leads to improved signal-to-background ratios for neutral emission monitored after the cessation of discharge current. Not only lower background emission levels could be achieved after the discharge, but also spectral interferences associated with emission from ionic states would be eliminated.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD