The actively cooled version of the JET diverter plates use a finned structure as a heat exchange surface between the copper alloy base plate and the cooling water (hypervapotron). The turbulence created by the fin structure gives a remarkably good heat transfer even before the onset of boiling. The boiling heat transfer is stabilized by the colder fin structure. Finite element calculations confirm that the heat transfer can be explained by turbulent boiling heat transfer. Power densities of up to 25 MW/m$+2$/ can be removed with a pressure drop of 4 bar per meter. Beryllium tiles brazed to the CuCrZr base plate can withstand a pulsed power loading of up to 16 MW/m$+2$/. Limited in strength is the intermetallic layer between the braze and the beryllium tile. The test sections, mounted rigidly against a strong back, withstood the stress caused by thermal expansion.
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