Shaping the Aperture of the Absolute Collimator in DIII-D Neutral Beamline

The absolute collimator currently in service at the DIII-D NB injection system has experienced localized melting and damage. As part of the DIII-D 210-deg beamline co-counter conversion, a new absolute collimator was needed, and the opportunity to resolve melting was found on the off-axis beamline configuration. The pulsed high heat flux and uneven distribution of the heat loads required the aperture surface to be axially extended to spread out and reduce the surface heat flux. Geometric sculpting of the absolute collimator aperture based on the baseline dimension was performed using ANSYS CFX software. The reshaped absolute collimator aperture surface reduces the impinged heat flux to below ~4 MW/m². Two interchangeable inserts are designed to occupy the high heat flux region for mitigating the thermal-induced stresses. The design achieves the objective of 6-s pulse lengths with 10-min repetition rates using the original peripheral conduit cooling system in the new collimator with minor changes.