Collisional transfer and neutral copper vapour density during a diffuse vacuum arc

Measurements of the number density of neutral metal vapour during vacuum arcs by laser-induced fluorescence (LIF) are disturbed by electronic collisions which depopulate the upper fluorescence level. To overcome this difficulty, the sinusoidal arc current was rapidly forced to zero which resulted in a rapid reduction of the collisional interaction. This is shown by measurement of the collisional transfer from the optically pumped 4 2 P 3/2 level to the neighbouring 4 2 P 1/2 level of copper. Measurements of the laser-induced fluorescence were performed shortly before and after the artificial current zero. Due to the reduction of the collisional interaction, the LIF signal after current zero rises by an order of magnitude above its value before the artificial current zero, reaches a maximum two microseconds after current zero and decays as the vapour disperses. The vapour density inferred from the maximum LIF signal is considered as an approximate value for the density before the artificial current zero. By forcing current zeros at preselected times during a 50 Hz sinusoidal half-cycle with 200 A peak current, a lower limit for the vapour density during arcing was obtained. The copper vapour density rises to a maximum value of 3.6*10 18 m -3 near current crest and drops to 5*10 16 m -3 at the natural current zero of the half-cycle. Afterwards it decays by about two orders of magnitude within 1 ms.