DIELECTRIC RECOVERY OF VACUUM ARCS AFTER STRONG ANODE SPOT ACTIVITY.

Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs have been measured for flat OFHC (oxygen-free high-conductivity) Cu contacts (D equals 25 mm, d equals 7. 5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5. 5-ms duration with peak values up to 11 kA. In comparison with diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Postarc currents have been simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The amount and decay of the residual plasma have been evaluated from the measurements of postarc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.