Spectroscopic Study and Motion Analysis of Arc Spot Initiated on Nanostructured Tungsten

Abstract

Arcing on the nanostructured tungsten surface has been examined recently because it gives rise to the erosion of materials and impurity transport toward the core plasma in a nuclear fusion reactor. Arcing was initiated on a helium-exposed tungsten surface, on which nanostructured tungsten was formed, by irradiation with ruby laser pulses of 0.08 MJ·m-2. The motion of an arc spot was observed with a fast-framing camera. The magnetic field strength and arc current dependences of velocity were discussed on the basis of experimental observation. Arc trails were observed using a digital fine scope to determine the relationship between the grouping width of an arc trail and arc velocity; the changes in grouping width and condition of the specimen surface are discussed. Spectroscopic measurements were performed to determine the electron temperature by the Boltzmann plot method.

Unipolar arc plasmas on nanostructured tungsten surfaces under perpendicular magnetic field

“…Measured plasma parameters, T e and n e , of arc plasmas are affected greatly by d fuzz . Figure 11 Previous studies reported T e ∼ 0.5-0.9 eV from W I at d fuzz ∼ 1 μm [21,22], agreeing with the present values. It is found that T e tends to decrease with increasing d fuzz .…”

“…The discovery of fiberform 'fuzz' nanostructure growing on metal surfaces under fusionrelevant conditions [13] significantly eased unipolar arc ignition because the thermal [14] and electrical [15] properties drastically change on a fuzz surface. After the first laboratory experiment for the unipolar arc ignition on a tungsten (W) fuzz target [16], many studies have revealed a variety of characteristics: ignition conditions [17,18], motion of arc spots under the parallel magnetic field [19,20], arc plasma parameters [21,22], and so on.…”

“…To understand the possible effect of arcing on fusion devices, the characterization of arc plasmas is important, particularly in terms of the electron temperature, T e , the electron density, n e , and the ionization degree. Several recent studies attempted to estimate the electron temperature, T e , of arc plasmas on fuzz surfaces, which ranged at 0.5-0.9 eV [21,22]. However, for the electron density, n e , very limited literature is available, where the Saha-Boltzmann equation was used with an assumption that the fraction of doubly and highly ionized ions was negligibly small [22].…”

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

“…The increment of erosion rate with an increase in the arc current is obvious when erosion per charge is constant. Considering that arc spot velocity is proportional to arc current, [30,31] the area eroded per unit time was likely to be furthered as the arc current increased. Thus, the erosion rate, which is equivalent to eroded area per time, would be enhanced owing to increasing spot velocity.…”