High-heat-flux testing of helium-cooled heat exchangers for fusion applications

High-heat-flux experiments on three types of helium-cooled divertor mock-ups were performed on the 30-kW electron beam test system and its associated helium flow loop at Sandia National Laboratories. A dispersion-strengthened copper alloy (DSCu) was used In the manufacture of all the mock-ups. The first heat exchanger was manufactured by Creare, Inc., and makes use of microfin technology and helium flow normal to the heated surface. This technology provides for enhanced heat transfer at relatively low flow rates and much reduced pumping requirements. The Creare sample was tested to a maximum absorbed heat flux of 5.8 MW/m(2). The second helium-cooled divertor mock-up was designed and manufactured by General Atomics (GA). If consisted of millimetre-sized axial fins and flow channels machined in DSCu. This design used low pressure drops and high mass flow rates to achieve good heat removal. The GA specimen was tested to a maximum absorbed heat flux of 9 MW/m(2) while maintaining a surface temperature below 400 degrees C. This temperature was selected to be compatible with a 2-mm-thick beryllium the at the plasma interface, where the maximum surface temperature must be below 700 degrees C. A second experiment resulted in a maximum absorbed heat flux of 34 MW/m(2) and surface temperatures near 533 degrees C. The third specimen was a DSCu, axial flow, helium-cooled divertor mock-up filled with a porous metal wick designed and manufactured by Thermacore, Inc. The internal porous metal wick effectively increases the available heat transfer area. Low mass flow and high pressure drop operation at 4.0 MPa were characteristic of this divertor module. It survived a maximum absorbed heat flux of 16 MW/m(2) and reached a surface temperature of 740 degrees C. Thermacore also manufactured a follow-on, dual channel porous metal-type heat exchanger. This ''unit cell'' divertor module was an improvement over the previous design because it utilized short flow lengths to minimize pressure drops and pumping requirements. It survived a maximum absorbed heat flux of 14 MW/m(2) and reached a maximum surface temperature of 690 degrees C.