Scaling mechanisms of vapour/plasma shielding from laser-produced plasmas to magnetic fusion regimes

Abstract

The plasma shielding effect is a well-known mechanism in laser-produced plasmas (LPPs) reducing laser photon transmission to the target and, as a result, significantly reducing target heating and erosion. The shielding effect is less pronounced at low laser intensities, when low evaporation rate together with vapour/plasma expansion processes prevent establishment of a dense plasma layer above the surface. Plasma shielding also loses its effectiveness at high laser intensities when the formed hot dense plasma plume causes extensive target erosion due to radiation fluxes back to the surface. The magnitude of emitted radiation fluxes from such a plasma is similar to or slightly higher than the laser photon flux in the low shielding regime. Thus, shielding efficiency in LPPs has a peak that depends on the laser beam parameters and the target material. A similar tendency is also expected in other plasma-operating devices such as tokamaks of magnetic fusion energy (MFE) reactors during transient plasma operation and disruptions on chamber walls when deposition of the high-energy transient plasma can cause severe erosion and damage to the plasma-facing and nearby components. A detailed analysis of these abnormal events and their consequences in future power reactors is limited in current tokamak reactors. Predictions for high-power future tokamaks are possible only through comprehensive, time-consuming and rigorous modelling. We developed scaling mechanisms, based on modelling of LPP devices with their typical temporal and spatial scales, to simulate tokamak abnormal operating regimes to study wall erosion, plasma shielding and radiation under MFE reactor conditions. We found an analogy in regimes and results of carbon and tungsten erosion of the divertor surface in ITER-like reactors with erosion due to laser irradiation. Such an approach will allow utilizing validated modelling combined with well-designed and well-diagnosed LPP experimental studies for predicting consequences of plasma instabilities in complex fusion environment, which are of serious concern for successful energy production.

Experimental simulation of materials degradation of plasma-facing components using lasers

“…However, we used 1064 nm laser excitation for this study and hence the majority of the laser energy will be coupled to the plasma rather than the target material after the plasma generation. Previous laser breakdown studies showed that nearly 90% of the energy will be absorbed by the plasma after its generation [15], which is consistent with the recent modelling results of plasma shielding during laser exposure [16]. Hence the energy density coupled to the target will considerably be less than the given value.…”

Self-Regulated Plasma Heat Flux Mitigation Due to Liquid Sn Vapor Shielding

“…Additional power handling capabilities such as evaporative cooling [6,7] and the vapor shielding effect [8,9] are inherently available for a liquid surface. The concept of vapor shielding encompasses several physical processes.…”

Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices

“…Additional heat transport by convective movement of the liquid, evaporative cooling 7 and a reduction of neutron issues 8 are other potentially beneficial properties of liquid PFCs. Finally and most importantly, when operating in the vapour shielding regime where a cloud of evaporated neutrals exists in front of the plasma-exposed surface 9 , 10 , any accidental exhaust power excursion leads to increased evaporation, which may mitigate the impact on the divertor armour by self-protection. Despite these advantages, liquid metals are still at a low technology readiness level and require further development.…”