Experimental investigation of particle production by arcing on tungsten for assessment of its influence on plasma operation

Arcs are one of the main erosion mechanisms in magnetic confinement fusion devices with metallic walls. They are also a source of dust particles which can limit their operation and introduce impurities in the plasma. The capacity of a given particle to introduce impurities into the core plasma within the closed magnetic flux surfaces is dependent on the particle velocity, size, and angle of emission. Due to the relevance of tungsten as a plasma facing material, it is of interest to characterize the parameter distributions of the W particles produced by arcing so that predictions about their influence on the operation of fusion devices can be made. Multiple experimental methods are used in this work, including measure- ment of individual particles by scattered light and high-speed recording of the particles produced by vacuum arcs in laboratory conditions. Supporting mi- croscopy studies provide an additional measurement of the size distribution by analysis of deposited particles, and analysis of the damage on the cathode sur- face. A central part of this work is the development of an experimental method based on the recording of the produced particles with a high-speed video cam- era, which allows direct measurement of a particle velocity and emission angle. Modeling of the cooling mechanism the free-flying particles experience pro- vides a prediction of their temperature evolution, which can be compared with the thermal radiation curves measured with the high-speed camera to obtain size and temperature measurements. This new method provides a two order of magnitude increase in the number of measured W particles compared with what is achievable with the light-scattering method, improving the statisti- cal significance of the parameter distributions. The obtained distributions are presented and the dependencies found between parameters are discussed. For the first time, temperatures of particles at the moment of emission are measured, revealing that a significant fraction is emitted at temperatures below the melting point of W. The emission of particles in solid state is confirmed by observation of rotation of non-spherical particles and by microscopic analysis of deposited particles. This finding contradicts the current understanding of W particle production by arcs, which assumes molten metal droplets are produced exclusively. High-speed recording also reveals a new phenomenon consisting in the si- multaneous emission of large numbers of particles from the same location. These ”burst” emission events account for one third of the detected parti- cles and represent a new mechanism of particle emission. The study of these events suggest that surface damage plays a role in their emission mechanism, and a physical hypothesis is presented based on the explosive release of weakly attached layers on the cathode surface. Lastly, the relevance for fusion devices of the results is discussed, and a simple estimation is done to showcase how the measured distributions of par- ticle parameters can be used to make predictions of the amount of impurities introduced in the core plasma of fusion devices.     «

Arcs are one of the main erosion mechanisms in magnetic confinement fusion devices with metallic walls. They are also a source of dust particles which can limit their operation and introduce impurities in the plasma. The capacity of a given particle to introduce impurities into the core plasma within the closed magnetic flux surfaces is dependent on the particle velocity, size, and angle of emission. Due to the relevance of tungsten as a plasma facing material, it is of interest to charact...     »