Measurements of the behaviour of neutral atom density in a diffuse vacuum arc by laser-induced fluorescence (LIF)

The method of laser-induced fluorescence was used to study the behavior of the absolute neutral vapor density of a diffuse vacuum arc on FeCu contacts. The local and temporal resolutions were 1 mm/sup 3/ and 10 mu s, respectively. The arc current had a sinusoidal shape of 5.8-ms duration with peak values of 90 and 510 A. It was found that the maximum densities of the iron and copper atoms are 1.2*10/sup 17/ m/sup -3/ and 7.5*10/sup 17/ m/sup -3/, respectively. During the arc the density was correlated with the current. At current zero the measured densities decreased to 10/sup 16/ m/sup -3/. After current zero, an exponential density decay with a time constant of about 100 mu s was observed, indicating the recovery of dielectric strength after current zero. Measurements of the neutron iron vapor density at different spatial positions in the electrode gap reveal a nonisotropic distribution. From the measurements of the population distribution of the iron ground-state multiplet a/sup 5/D, the excitation temperature was derived. This temperature was low compared with the cathode spot temperature 2500-4000 K and decreased from 1600 K at the current maximum to 1000 K at current zero. The results indicate that the generation of neutrals is caused by flying evaporating metal droplets rather than by molten surface areas.>