Properties of hotspots in plasma focus discharges operating in hydrogen-argon mixtures

We have investigated the properties of hotspots formed in low energy plasma focus (PF) discharges operating in hydrogen-argon mixtures, at 140 kA current level. A combination of filtered pinhole and slit-wire camera is used to measure the hotspot size and temperature. The results show that the best conditions for reproducible and localized hotspot formation are obtained by adjusting the base pressure in such a way that the mass load allows the time of first radial collapse to coincide with peak current. When the PF is operated with 20% argon content, rather uniform hotspots, of 115 mum characteristic size and 300 eV characteristic temperature, are produced with a better than 80% reproducibility in their axial localization. The electron density in the hotspots is estimated to be similar to10(20) cm(-3). Calculations performed with a CRE code indicate that a significant fraction of the radiation is emitted in the 3.2 to 3.88 keV region, corresponding to K-alpha emission from highly ionized argon.