A comparison between hypervapotron and multitube high heat flux beam stopping elements

Structures capable of accepting high heat fluxes (⩾10 MW/m²) in steady state are an important aspect of fusion engineering. The hypervapotron principle has been used successfully in the JET (Joint European Torus) neutral beam injectors at power densities up to 10 MW/m². Tests have been carried out in the JET Neutral Injection Test Bed to assess the safe operating limits of these elements, which form the basic subunit from which larger components (beam dumps, scrapers, etc.) are assembled. In addition, a multichannel water-cooled element (multitube) has been developed and tested as a possible alternative to the hypervapotron. The results from these tests are reported and a comparison is made between the data obtained from the two types of elements. In particular, the dependence of the peak power density limit as a function of total incident power, coolant flow, and pressure are presented. Measured and computed temperature profiles in the elements are used to assess the internal stresses and their subsequent implications for the fatigue life. Both the hypervapotron and the multitube type of element are shown to be suitable for the 140-kV beam injectors for JET