On the vertical uniformity of an ITER-like large beam

Abstract

Abstract A beam uniformity higher than 90% is a crucial requirement for the ITER neutral beam injection (NBI) system, together with very low beam core divergence ( The ELISE test facility, equipped with an ITER-like multi-aperture 3-grid system, produces large ITER-like beams, with half the size in vertical direction. Due to the distance of the beam diagnostics from the grids, the overlapping of the beamlets is large, and information on divergence and uniformity can be retrieved in terms of beamlet groups or beam segments. In general, good symmetry in accelerated current density between beam segments is achieved. The vertical beam uniformity is determined by caesium distribution and by plasma parameters at the plasma grid, which are inhomogeneous due to the interplay of filter field and bias potential. In this work, the role of filter field and bias potential on the vertical beam uniformity (in intensity and divergence) between the two beam segments is identified: the filter field mainly affects the extracted current density and its distribution within and between the segments; the bias potential can be used to tune the beam divergence, differently between the beam segments.