by Johannes Baggemann
Within the HBS project an innovative target concept has been developed to fulfill the special requirements of HBS, namely high brilliance and high proton current at moderate proton energy. The result is a compact all-solid target made of tantalum with integrated microchannel cooling (see the left picture), designed for a 70 MeV proton beam with an average thermal power of 100 kW (1 kW/cm²b). The reliable heat dissipation in the design state was already successfully demonstrated experimentally in 2021 at the electron beam test facility JUDITH-2 at IEK-4 at Forschungszentrum Jülich. This first experiment has also shown that the target has significant power reserves, as only about 1/3 of the microchannel surface is involved in heat dissipation due to the near-surface heating of the electron beam. The actual power limit of the cooling structure and thus the safety reserve during operation has been determined experimentally in September 2023. For this purpose, the heat flow in the JUDITH-2 system was continuously increased until the critical heat flux (CHF) was reached and thus until the target failed. A hole, shown in the right picture, was melted into the target at a power density of 1.7 kw/cm² due to a locally formed isolating gas layer between the tantalum target and the cooling water. The resulting safety factor during operation is determined to be higher than 2 for the design goal of HBS.