Experimental study of vacuum arc with low cathode current density as a source of metal plasma

Summary form only given. Sources of metal plasma are applied in plenty of technological tasks, for example, in coatings deposition or electromagnetic separation of isotopes. Vacuum arc may be used as one of such source. Its main advantages are high productivity and ionization degree, but plasma of usual vacuum arc contains micro particles of cathode erosion products. External cathode heating decreases its current density and problem of micro particles disappears. In this work such diffusive vacuum arc was studied. Gadolinium and plumbum were used as cathode materials. For gadolinium 90% of arc current consist of thermal electron emission from cathode, and for plumbum this part is less than 0.01%.Arc was initiated in vacuum chamber with pressure of residual gases less than 10 mPa. The investigated substance which was the arc cathode was placed in molybdenum crucible with outer diameter 25 mm. Under the crucible there was an electron-beam heater with maximum power about 1 kW. Heater allowed changing the cathode temperature at fixed arc current. A stainless steel water-cooling disk was used as the anode. It had a hole with the diameter of 32 or 15 mm. The distance between electrodes was about 30 mm. Arc plasma was studied by spectrometric and probe methods. The discharge with plumbum cathode was studied in current range from 20 to 70 A and cathode temperatures from 1.2 kK to 1.4 kK. Typical discharge voltage was about 15 V. The arc with gadolinium cathode was studied in current range from 15 to 150 A, cathode temperatures from 1.9 to 2.1 kK and voltages form 3 to 50 V. Arc regime with singly high ionized (ionization degree is about 100%) gadolinium plasma was revealed. The average ion charge of plumbum plasma was estimated at the arc current 40 A. Average evaporation speed of gadolinium and plumbum was measured in arc conditions. It was almost two times less than without discharge at the same cathode temperature.