Numerical simulation of the initial stage of unipolar arcing in fusion-relevant conditions

A model for the initial phase of unipolar arcing has been developed with account of an external energy source which triggers the arcing, the vaporization of the atoms from the heated surface, the ions and electrons produced by ionization of the vapor, the electron emission from the metal surface, and melt motion and surface deformation. Current transfer outside the arc attachment is taken into account and the potential difference between the plasma and the metal surface (the plate) is evaluated from the condition that the net current transferred to the plate is zero at each moment. The model is used for simulation of the interaction of an external energy load (laser beam) with a tungsten plate immersed in a helium background plasma. The results revealed the formation of a crater, but no jet formation or droplet detachment. If the plate is large (R = 100 mm), the peak temperature attained is 5200 K, and the plate potential remains below the plasma potential. If the plate is small (R = 10 mm), a peak temperature of 7500 K is reached, the potential of the plate surpasses the plasma potential, circulation of the melt at the pool periphery occurs, and the erosion (which is mainly due to the vaporization of the metal atoms in the spot) reaches the value of 37 mu g.