Unipolar arc plasmas on nanostructured tungsten surfaces under perpendicular magnetic field

Abstract

Unipolar arcing is an important phenomenon in terms of wall erosion and the subsequent plasma contamination in nuclear fusion devices. In this study, unipolar arc plasmas are experimentally explored under the presence of the external magnetic field perpendicular to a tungsten (W) sample on which a ‘fuzz’ nanostructure surface is formed. It is found that the fuzz layer thickness, d fuzz, plays a dominant role in determining the motion of arc spots, the arc ignition probability, and arc plasma parameters. High-speed imaging reveals that the motion of arc spots becomes more collective, as d fuzz increases. Interestingly, arc spots make a circle while moving outward. With increasing d fuzz, the amount of eroded W atoms increases, as indicated from an observed increase in the total visible emission intensity, while the arc ignition probability is found to drop. The physical mechanisms are discussed, in detail, based on spectroscopic measurements of the electron temperature, obtained from Boltzmann plots of both W I and W II, and the electron density, determined from Stark broadening of W I lines.

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

“…Note that the detail of the experiment is described elsewhere. 25) Figure 1 shows the representative observed motions of the arc spots. Expanding motion is distinguished, while circular movement is clearly seen on a relatively thicker fuzz layer, as seen in Fig.…”

“…From the experiments, it was found that (1) more than 80 arc spots existed simultaneously, (2) arc spots expanded outward from the origin, and (3) collective movement of the arc spots creating a circle while expanding depends on the fuzz thickness. 25) Based on these experimental findings, we shall develop a two-dimensional MC random walk simulation and compare between the experiment and the simulation.…”

“…If four relative probabilities are determined, the propagation probability to an adjacent site i ( Pi ) is then calculated using the following equation: More detailed images can be available in the previous study. 25) Fig. 2.…”

“…However, the number of spots increased up to ∼90 with the arc current up to ∼90 A in the present experiments. 25) In vacuum arc physics, it is believed that the arc current is observed as individual packets of emitting electrons, and every single arc spot has a minimum current. 27,28) Therefore, we newly define the probabilities of bifurcation and extinction based on the experimental observations 25) as follows:…”

“…24) In addition, a recent experiment that initiated arcing on "fuzz" surfaces under steady state He plasma exposure showed collective motions of numerous arc spots, expanding isotropically with increasing the fuzz thickness. 25) To understand the detailed properties of the collective motion of arc spots on the fuzz layer, we will utilize numerical simulation, which incorporates the experimental findings, such as bifurcation and extinction of spots, arc current for each spot, limitation on re-visiting previously damaged area, etc. In this study, firstly, we introduce the MC simulation to the experimental results at −j × B = 0, and compare the detailed properties between the experimental and numerical results.…”