Effect of the Nanostructured Layer Thickness on the Dynamics of Cathode Spots on Tungsten

Experimental data on the effect of the thickness of a nanostructured layer present on the surface of a tungsten cathode on its arcing properties have been analyzed. It has been shown that if the nanostructured layer is not completely destroyed as a result of the operation of a cathode spot cell, the spot “sinks” in the layer, the spot cells group together, and the trajectory of the spot motion becomes “entangled.” To interpret the experimental results, the grouping of arc spot cells was simulated using a random walk model based on the Monte Carlo method into which an attractive force was introduced and assuming that the cell plasma density, i.e., the cell initiation probability, is proportional to 1/r
²
, where r is the distance between two spot cells. It has been demonstrated that spot cells are entangled, forming a group and moving together when the attractive force between them is strong enough. When this force decreases and the directionality of the cells increases as the cathode spot shifts to a lower thickness region of the nanostructured layer, the velocity of motion of the spot increases and its trace left on the surface becomes narrower.