Evaluation of beam properties using a checkerboard masking configuration at the NNBI test stand BATMAN Upgrade

Abstract

Negative ion based neutral beam injection (NBI) systems will be used for heating and current drive at the ITER tokamak and require stringent beam quality. Including a beamlet core divergence of less than 7 mrad and high uniformity across individual beamlets (better than 90%), at extracted current densities of 329 A/m2 for hydrogen and 286 A/m2 for deuterium. The BATMAN Upgrade (BUG) test facility, a 1/8 scale ITER source, is used to investigate and optimize these beam properties. Beamlet divergence and uniformity at BUG test facility are measured using a retractable Carbon Fiber Composite (CFC) calorimeter. However, under standard operation (with all 70 apertures open), strong beamlet overlap prevents the extraction of single-beamlet divergences from the CFC data. To overcome this limitation, a checkerboard masking configuration was implemented. This masking, combined with a multi-Gaussian model allows to obtain beamlet properties, including size, shape, and intensity along the whole extraction area. The experimental methodology and data analysis techniques are presented in detail. Their capabilities are showcased by presenting measurements performed at BUG under different configurations of the magnetic filter field (drift-up and drift-down conditions) and for both hydrogen and deuterium operation.