Pulsed dye laser diagnostics of vacuum arc cathode spots

The ignition and arc phases of vacuum arcs were investigated using differential dye laser absorption photography with simultaneous high spatial (micrometer) and temporal (nanosecond) resolution. The discharge duration was 800 ns, the current 50-150 A, the electrode material copper, and the cathode-anode distance less than 50 μm. A 0.4 ns laser pulse (tunable, γ=480-530 nm) was used to obtain momentary absorption photographs of the cathode region. During ignition, an optically thick anode plasma expanded toward the cathode, decaying within 25 ns after bridging the electrode gap. In the arc phase, a fragmentary structure of the cathode spots was observed in situ for the first time. The microspots have a characteristic size of 5-10 μm. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)×10²⁶ m^-3