Characterization of SPIDER beam optics in volume operations with visible cameras

SPIDER (Source for Production of Ion of Deuterium Extracted from RF plasma) is the full-size prototype of the negative ion source for future ITER neutral beam injectors. With a maximum acceleration voltage of 100 kV, a negative ion beam is extracted and accelerated, and different diagnostics are developed to characterize the performance, shape, and deflection of the beam as well as the width of the beamlets. In particular, the optics of the accelerated beam is studied with a set of 15 cameras installed all around the beam, and they measure the visible emission; the spatial profile of the emission gives an estimate of the beamlets that compose the beam and also information about its optics. In this paper, experiments performed in volume operation (without cesium) are analyzed, studying the role of the extraction and acceleration voltages, polarization of plasma grid and bias plate, and magnetic filter field. It is shown how the width of the beam is not spatially uniform. Two types of non-uniformity are pointed out: a top/bottom asymmetry at large spatial scale and a smaller one at the single driver scale.