Optical emission spectroscopy of electrical focii discharges

Abstract

The optical emission from a dense magnetized plasma column (the Z pinch) was studied in experiments with a 1 kJ plasma focus using an optical multichannel analyser (OMA III). The plasma column (about 10 mm length and about 1 mm diameter) generated at the end of the co-axial discharge had a density of about , a duration of about 200 ns and a maximum circulating current of about 170 kA. All the most intense lines from the carrier gases (neutral atoms and ions) were found in the spectra in the optical range 340 - 700 nm, showing that the line intensity was independent of the pinch intensity. Also, an intense emission from other elements such as copper (belonging to the brass electrodes) and silicon (from the glass insulator) was observed with a strong dependence of their line emission intensities on the pinch intensity. By partially covering the electrode surfaces with aluminium sheets, and studying variation of the aluminium and copper line emission intensities with the pinch intensity, it was shown that the source of contamination could be attributed in part to the power energy density deposition of the current sheath during the breakdown and run down stages of the discharge, before the formation of the dense plasma column. This more energetic electrode (and glass insulator) erosion can be associated with a thinner current sheath, a condition that is also conducive to more efficient pinch formation. The possibility of covering the last centimetre of the electrode surfaces with materials with higher evaporation points offers an opportunity to reduce the contamination effect when the plasma focus is used as a surface-treatment apparatus.