3 Search Results

Three issues related to the physics of divertor detachment are analyzed in detail: the criteria for the onset of complete divertor detachment, the role of neutrals in “symmetryzation” of detachment in the inner and outer divertors, and the transition to divertor detachment. The results of comprehensive 2D numerical simulations with the SOLPS4.3 package are compared with some experimental data and predictions based on simplified analytical models. It is shown that it is the ratio of the upstream plasma pressure to the specific power flux entering the recycling region that controls the local onset of detachment on a specific flux tube.more » P_up/q_reclycl≥(P_up/q_reclycl)_crit remains the valid criterion also in the presence of seeded impurity, if the impurity radiation and hydrogen recycling regions are spatially separated. Detailed analysis indicates that the reverse plasma flow forming on the most heat loaded flux tubes in the outer divertor under the influence of the neutrals coming from the deeply detached inner divertor plays the key role in the detachment “symmetryzation” and allows the outer divertor to reach the detached regime. Finally, it is demonstrated that a gradual increase of the perpendicular heat transport in the edge plasma during transition to the detached regime can make this transition bifurcation-like.« less

The erosion of divertor targets caused by high heat fluxes during transients is a serious threat to ITER operation, as it is going to be the main factor determining the divertor lifetime. Under the influence of extreme heat fluxes, the surface temperature of plasma facing components can reach some certain threshold, leading to an onset of intense material evaporation. The latter results in formation of cold dense vapor and secondary plasma cloud. This layer effectively absorbs the energy of the incident plasma flow, turning it into its own kinetic and internal energy and radiating it. This so called vapor shieldingmore » is a phenomenon that may help mitigating the erosion during transient events. In particular, the vapor shielding results in saturation of energy (per unit surface area) accumulated by the target during single pulse of heat load at some level E_max. Matching this value is one of the possible tests to verify complicated numerical codes, developed to calculate the erosion rate during abnormal events in tokamaks. The study presents three very different models of vapor shielding, demonstrating that E_max depends strongly on the heat pulse duration, thermodynamic properties, and evaporation energy of the irradiated target material. While its dependence on the other shielding details such as radiation capabilities of material and dynamics of the vapor cloud is logarithmically weak. The reason for this is a strong (exponential) dependence of the target material evaporation rate, and therefore the “strength” of vapor shield on the target surface temperature. As a result, the influence of the vapor shielding phenomena details, such as radiation transport in the vapor cloud and evaporated material dynamics, on the E_max is virtually completely masked by the strong dependence of the evaporation rate on the target surface temperature. However, the very same details define the amount of evaporated particles, needed to provide an effective shielding to the target, and, therefore, strongly influence resulting erosion rate. Finally and thus, E_max cannot be used for validation of shielding models and codes, aimed at the target material erosion calculations.« less