Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

Abstract

This paper presents experimental studies of plasma-surface interactions during powerful plasma impacts of a quasi-stationary plasma accelerator (QSPA) on the Sn capillary porous systems (CPSs) in conditions simulating disruption and edge localized modes (ELM) like loads. Experiments were carried out using two QSPA devices. ELM-like plasma exposures were performed with QSPA-M test-bed facility. A large-scale QSPA Kh-50 device was used to simulate plasma disruptions and giant ELMs. Variation of the plasma stream energy density has been performed to study the onset of vapour shield. It is shown that during plasma exposures of a Sn-CPS target with the QSPA plasma load <1 MJ m −2 , single dust particles traces have been registered. A further increase in the heat load leads to the splashing of the eroded material. For ELM-like impacts, a rather weak melt motion was observed on the target surface. A post-mortem analysis has shown that the CPS structure was not destroyed in the course of many repetitive ELM-like pulses. Surface morphology has changed from a smooth surface to corrugation structures with the formation of some cavities in mesh cells due to the influence of the surface tension and capillary effects. Spectral lines of Sn I and Sn II have been identified by optical emission spectroscopy in the near-surface plasma. A plasma shield, that consists mostly of Sn neutrals appears at Q ∼ 0.1 MJ m −2 . An increase in the surface heat load resulted in the intensive emission of Sn II lines, which started to be observed at Q ∼ 0.3 MJ m −2 . The plasma electron density near the surface increases significantly at Q > 0.5 MJ m −2 , which corresponds to the strong vapour shielding of the exposed surface. A comparison between the obtained results on the vapour shielding of Sn CPS and available numerical simulation using the TOKES code has been performed.

The Effect of a Small Helium Addition on the Plasma-Surface Interaction in Qspa

“…The plasma stream was around 5 cm in the presence of a B-field and increased to 15 cm when the external magnetic field was switched off. The value of energy density in the axis region of the plasma stream varied in a range of 0.1 to 1 MJ/m 2 [6][7][8][9].…”

“…The plasma stream energy density and heat loads on the surface were measured with a set of movable calorimeters [7,8]. The plasma pressure was measured with piezoelectric detectors.…”

“…The exposure time ranged from 1 μs to 1 s; the spectral range was from 290 to 1100 nm. The velocities of the emitted particles and the instants of their ejection (from the target surface) are to be calculated using the information from the subsequent camera frames (with visible traces of the particles flying from the target surface after a plasma shot) [7][8][9].…”

“…Quasi-stationary plasma accelerators (QSPA) are capable of reproducing ELM impacts in terms of both heat load and particle flux to the surface and can provide plasma transportation in an external magnetic field, which mimics the divertor conditions [6][7][8][9]. The energy transfer from pure hydrogen plasma to material surfaces during plasmasurface interactions (PSIs) in a magnetic field has been investigated in the recently developed quasi-stationary plasma accelerator, the QSPA-M [7,8]. The study analyzed the features of plasma visible radiation and performed investigations of energy transfer to the material surface for varied plasma heat loads and external magnetic field values.…”

“…The experimental studies of vapour shielding of liquidmetal tin capillary porous structures (CPS) under plasma loads relevant to fusion reactor transient events have been also performed in complementary simulation experiments using QSPA-M and QSPA Kh-50 experimental facilities [8]. It has been shown that a pronounced vapour shield effect is dominating for exposed surfaces under the disruption heat loads.…”