Unipolar arc plasmas on nanostructured tungsten surfaces under perpendicular magnetic field

• Published in: Plasma sources science & technology 2020-12, Vol.29 (12), p.125015
• Main Authors: Hwangbo, Dogyun, Nishijima, Daisuke, Kajita, Shin, Doerner, Russell P, Ohno, Noriyasu
• Format: Article
• Language: English
• Subjects: fiberform nanostructure; helium plasma; Physics; tungsten; unipolar arc
• ISSN: 0963-0252; 1361-6595
• DOI: 10.1088/1361-6595/abc817

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, dfuzz, 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 dfuzz increases. Interestingly, arc spots make a circle while moving outward. With increasing dfuzz, 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.