Ignition and erosion of materials by arcing in fusion‐relevant conditions

Abstract

This study investigated the characteristics of arcing initiated on nanostructured fuzzy tungsten (W) surfaces in helium (He) plasmas. Under the same He plasma conditions, the relationship between arc voltage and current was characterized as linear due to plasma resistance across the He plasma from the target surface to the plasma source. Properties of the erosion caused by arcing were investigated by focusing on two parameters: arc current and fuzzy layer thickness. The erosion per charge was significantly dependent on the thickness of the fuzzy layer but did not depend on the arc current. Sample surface analysis revealed that drastic changes in arc spot grouping features were accompanied by the fuzzy layer thickness variation. The ecton model indicated that the erosion as ions does not play a main role, but mechanical destruction by momentum transfer from the explosive electron emission centre to neighbouring nanowires seems to enhance the erosion greatly.

Field emission properties of nano-tendril bundles formed via helium plasma exposure with various additional impurity gases

“…Owing to the long distance from each other and high heightto-radius ratio, the field emission from the NTB is significantly higher than that from the fuzz [11,12]. The NTB structure can trigger arcing [13], which leads to many adverse effects on the divertor plate, such as erosion increment and W dust formation [14,15]. Therefore, the formation of the NTB is considered unfavorable for the divertor plate.…”

Motion analysis of arc spots on tungsten fuzz by means of self-avoiding random walk model

“…11) Thus far, a lot of research has been dedicated to understanding the characteristics of arc spot motion on fuzz surfaces when a transverse magnetic field (B) exists for simulating the inclined incidence of plasmas toward the wall surfaces. With much experimental evidence, [12][13][14][15][16][17][18] numerical simulations with a Monte Carlo (MC) random walk model 19) led to further understanding of (1) bifurcation of spots and self-avoiding effects, 19) (2) directionality to the retrograde ( −j × B, j refers to the arc current) direction depending on the strength of B, 18) and (3) entanglement of spots (arc spot "grouping") depending on the thickness of fuzz layer. 20,21) When it comes to arc spot motions under the presence of a magnetic field perpendicular to a sample surface ( −j × B = 0), frequent bifurcation of spots and clear zigzag random motions, 22,23) and the circular grouping of arc spots was found with the presence of an external axial magnetic field.…”

2D kinetic modeling of plasma jet in external magnetic field

“…In addition, the effect of AMF on the increase in the average ion charge in vacuum-arc ion sources is of interest [2]. Recently, interest has arisen in the behavior of cathode spots on the divertor surface in tokamaks, where the magnetic field is almost perpendicular to the surface [3]. Previously, various magnetohydrodynamic and hybrid quasi-neutral approximations were used to simulate a plasma jet with AMF [4][5][6][7].…”