SUMMARY

The divertor is a critical component in magnetic fusion reactors, inasmuch as it allows removal of impurities and fusion by-products from the plasma. The plasma-facing surface of the divertor is subjected to extremely high heat fluxes in the order of 10 MW/m^2. A variety of modular helium-cooled ‘finger-type’ tungsten divertor conceptual designs have been proposed for future commercial reactors; the use of helium cooling and tungsten alloy construction enhances safety and thermal performance without exceeding material temperature limits.

This study will experimentally evaluate the thermal performance of a modular helium-cooled finger-type tungsten divertor design similar to the helium-cooled multi-jet (HEMJ) divertor and the helium-cooled modular divertor concept with integrated pin array (HEMP), which uses jet impingement and/or cooling fins to cool the divertor surface. A series of experiments were conducted using air at Reynolds numbers spanning the prototypical conditions. A test section similar to the HEMP geometry was heated at heat fluxes up to 2 MW/m^2 by the flame of an oxy-acetylene torch. Nusselt numbers were determined from measurements of the heat flux and cooled surface temperature. These were used to predict the maximum heat flux that the design can endure when cooled by helium at prototypical conditions.

Data from preliminary experiments using helium at ambient temperature suggest that the air experiments are not dynamically similar to the helium-cooled divertor under prototypical conditions because of differences in the relative contributions of convection to the coolant and conduction through the structure. This proposal describes new experiments with geometries and material properties to assure prototypical test conditions. The experiments will use test sections fabricated from materials with a thermal conductivity that should ensure that the fraction of the heat removed by convection will be comparable to that for the actual divertor; the test section will be heated at heat fluxes as high as 10 MW/m^2. The new test results will be used to predict the maximum heat flux that can be accommodated by helium-cooled divertors at prototypical conditions (i.e. at temperatures >600°C and pressures >10 MPa). The results will also be used to develop generalized maximum heat flux correlations for these designs, which will be especially useful in system design codes for magnetic fusion reactors to predict overall performance.