On the plasma density of metal vapour arcs

The plasma density of metal vapour arcs in vacuum is discussed on the basis of ion currents measured at cylindrical walls around the electrodes. It is suggested the density varies inversely with the square of the distance to the cathode spot, reaching values up to 10 26 m -3 within the spot. From the distribution of the plasma ions it is concluded that the ion current to the cathode area within the spot reaches several per cent of the arc current. Up to a pressure of at least 2 atm, the background pressure does not change the internal spot processes that lead to retrograde motion.