ELECTRON-DENSITY MEASUREMENTS IN VACUUM ARCS AT ANODE SPOT FORMATION THRESHOLD

CO2 laser interferometry was used to determine electron density profiles in copper vacuum arcs just before and after the transition from the low-voltage quiescent mode to the high-voltage mode associated with anode-spot formation. Just before transition, the electron-density profiles are peaked on axis, with maxima of 9×1014 cm-3 and 6×10 14 cm-3 in the cathode and anode regions, respectively. At about 0.1 ms after transition, the electron-density profile in the cathode region remains unchanged, while the central electron density near the anode drops be a factor of about 2. No constriction in the density profile was observed at any axial position. Previously reported anode surface temperature measurements are used to attribute the sudden drop in electron density near the anode just after transition to a local increase in the electron-ion recombination rate caused by a sudden influx of relatively cold neutral atoms from the anode surface. The implications of these observations with regard to a mechanism for transition are discussed.