Physikalische und emissionsspektroskopische Untersuchungen am Lichtbogen unter Helium-Drücken bis zu 50 at

This study deals with the application of the d.c. arc as a spectrochemical source of light at high helium pressures, viz. power reactor conditions, for identifying spherical graphitic fuel elements which had been deliberately contaminated with defined impurities. Tipped tungsten tacks and graphite electrodes of cylindrical or spherical shape were used: the tacks served as cathodes, the graphite electrodes as anodes. The He-gas pressure during discharge was varied between 1 atm and 50 atm. The runs at high He-pressure show that the arc changes at pressures > 30 atm into a form of discharge governed by convection. At 10 atm however the arc was still found stabilised by the electrodes. At pressures > 30 atm we got distinct evidence for vaporisation. The change in characteristics of the gas type arc into a vapour arc is marked by an increase in operating voltage at currents greater than 20 amps. Not unexpectedly the operating voltage increases rapidly with increasing gas pressure during discharge. The lines of the spectrum are broadening proportionally to the gas pressure. A homogeneous magnetic field, simultaneously applied along the axis of the arc, centralises and stabilises the arc discharge and reduces the material transport from the anode. With increasing magnetical induction the line broadening reduces. The good thermal conductivity of the He and the increasing heat convection with rising pressure both result in rapid increase of power and radiation density with the consequence of higher temperature. Temperature measurements with two pairs of Mg II lines using a 20 amps arc discharge at He pressure of 40 atm yielded about 13000 K. In an example for a pair of ion lines in the Co/Ni element combination a satisfactory spectral-analytical calibration curve was obtained at helium pressure. It is possible to increase the reproducibility and narrow down the lines of the spectrum with the consequence of greater accuracy of the resulting analytical curve if a homogeneous magnetic field is applied.