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 shielding 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 Emax. 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 paper presents three very different models of vapor shielding, demonstrating that Emax 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 Emax 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. Thus, Emax cannot be used for validation of shielding models and codes, aimed at the target material erosion calculations.

The work represents a next step in studies of longitudinal energy losses from linear magnetic plasma confinement devices. The paper describes the experimental technique that has been developed and used to measure the absolute values of longitudinal particle and energy density fluxes from an open magnetic mirror trap. Using this technique, it was possible to measure and account for the secondary electron emission coefficient from plasma-facing end plate, and perform a comparison with the existing theory of energy confinement in open traps. It is shown that the average energy transported by an ion-electron pair leaving the trap is close to theoretically predicted minimum value of 6-8 electron temperatures. As confirmed by the measured dependencies, this conclusion is valid in a broad range of parameters and the threshold value of the magnetic expansion ratio does not exceed its theoretically predicted minimum of √ m i /m e .

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