Beam duct for the 1 MW neutral beam injector on TCV

The 1MW power of the neutral beam injector (NBI) enters the plasma of the Tokamak a Configuration Variable (TCV) through a duct. The original beam propagation model predicted a maximal heat flux on the internal faces of the duct below 350 kW/m(2), leading to an acceptable temperature rise for the 2 s nominal pulse duration. During commissioning, the NBI showed unacceptable overheating in the duct, the beam divergence being higher than expected. Several ion source grids have been tested to mitigate the beam divergence however the overheating of the duct is still problematic. Since then, the NBI operates with reduced power and duration to avoid damaging the beam duct. This paper describes the design and thermal analysis of a new beam duct capable of withstanding NBI shots at nominal regime for a range of modeled neutral beam divergences. The design drivers are the high heat flux protection and the accumulated heat dissipation. The concept features a thermal shield composed of a smooth elliptical castellated layer surrounded by an actively cooled structure.