Tomographic reconstruction of the beam emissivity profile in the negative ion source NIO1

A versatile negative hydrogen ion source named NIO1 of a moderate size (130 mA total extracted H- current, 9 apertures, 60 kV total acceleration) has been developed and installed at Consorzio RFX. It will allow a large experimental flexibility, very beneficial for studying several important issues related to the beam extraction, optics and performance optimization, in view of SPIDER and MITICA, the two full scale experiments for the ITER neutral beam injector (NBI) under construction at RFX. The main target of emission tomography applied to an ion beam is the reconstruction of the emissivity profile, from which the ion density distribution can be obtained. The measurement of the beam density profile and of its uniformity throughout the pulse duration with a non-invasive diagnostic as tomography, would represent an effective method for monitoring the ion source operation and for malfunction detection. The application of this diagnostic to the NIO1 beam will represent the experimental verification of the possibility to reconstruct a multi-beamlet profile, in the interest of the next tomography systems for SPIDER and MITICA. In this paper, a feasibility study of the tomographic diagnostic for NIO1 is presented. A tomography code based on algebraic reconstruction techniques has been developed for this purpose and the transport of the 9 H- beamlets is simulated with a Monte-Carlo particle tracking code from the ion source to the tomography plane, where the beam emissivity profile to be reconstructed is calculated. The reference emissivity profile is reconstructed by the tomography code considering different possible layouts of the detection system, in order to find the best compromise between the quality of reconstructions and the complexity of the diagnostic. Results show that a tomography system based on 6 linear CCD cameras should be capable of reconstructing the NIO1 emissivity profile with an rms error lower than 10 %. How the instrumental noise in the integrated signals affects the reconstructed beam emissivity profiles is also studied. A simple low-pass filter is found effective if the noise level is less than 10 %, otherwise a more sophisticated filtering technique must be considered.