Thermal load calculations at TJ-II vacuum vessel under neutral beam injection

The neutral beam power transmitted into the helical axis stellarator TJ-II is calculated for the final duct design of TJ-II The transmitted beam is intercepted by the first toroidal field coil before reaching the plasma. The nontrapped fraction of the beam hits various vacuum vessel components. The adopted design of the graphite thermal shields used as vacuum vessel protection at TJ-II is presented. The design achieves a compromise between maximum power into the torus and minimum loads on sensitive parts. A three-dimensional version of the beam geometric code DENSE is set up to calculate the power loads due to shine-through neutrals on the shields under these circumstances. Power load maps are the input to the finite element code ANSYS for the calculation of temperature distributions. For the usual duty cycle at TJ-II (300-ms pulses every 300 s), the peak surface saturation temperatures at all surfaces remain under 650 degrees C, well below the tolerable limits for graphite.