Solutions to Mitigate Heat Loads due to Electrons on Sensitive Components of ITER HNB Beamlines

The operation of neutral beam injectors provides challenges in the thermal management of beamline components. Sensitive components such as the cryogenic pumps at beamline periphery shall be protected from the heat flux due to stray electrons. These are emitted by the negative ion accelerator or generated along the beamline by interaction with background gas and surfaces, and subject to backscattering. In this article the case of the ITER Heating Neutral Beam (HNB) is discussed, for which the beam parameters and the required pulse length of one hour is a major leap forward with respect to the present expertise with neutral beam systems. The engineering solutions adopted for effective cryopump protection against the heat load from electrons are described. The use of threedimensional numerical simulations of particle trajectories in the complex geometry of the HNB beamline was needed for the quantitative estimations of the heat loads. The presented solutions were optimized to minimize the impact on gas pumping and on the functionality of other components.