Thermo-mechanical design of the neutralizer for CRAFT negative ion-based neutral beam injection system

The Comprehensive Research fAcility for Fusion Technology (CRAFT) has been launched in China and a negative ion-based neutral beam injection system (NNBI) is an important component of CRAFT. The primary target of the CRAFT NNBI is to realize a neutral hydrogen beam of >2 MW at the beam energy of 400 keV for the pulse duration of 100 s. In the CRAFT NNBI, the neutralization of the negative ion beam will be accomplished in a gas neutralizer mainly consisting of two vertical channels. Each channel has a rectangular cross-section of 0.145 m × 1.7 m and a length of 3 m, which is formed by several copper panels. A key issue of the neutralizer is to remove the heat load from the incident ion beam. This paper presents the thermo-mechanical design process of the neutralizer of the CRAFT NNBI. Because the heat load densities on the lateral walls of the panels are relatively low, their surface temperature can be well controlled under the safe values of copper by deep drilling of the cooling channels. The largest problem is the leading edge of the panels, where the heat load density is extremely high. Thus, the leading edge element (LEE) is carefully designed to avoid damage to these panels. The LEE is made of CuCrZr alloy and the twisted tape is inserted into the cooling channel to promote the turbulence and consequently the heat transfer capability. Finally, the thermo-mechanical properties of the heat load components of the neutralizer can be controlled under the required values in terms of the surface temperature, water temperature, stress, and deformation.