Overview Of The Beam Physics Investigation At The ELISE Test Facility

Strict requirements are foreseen for the Neutral Beam Injection system (NBI) for ITER: a high extracted current density has to be achieved (33 mA/cm(2) for H- and 28.6 mA/cm(2) for D-) together with very small beam core divergence (< 7 mrad) and a beam uniformity of better than 90%, for a large beam extracted from 1280 apertures. The ion source tilling pressure has been set to <0.3 Pa, in order to keep the stripping losses in the accelerator to a tolerable level, and the ratio of co-extracted electrons to ions should be less than one. In the roadmap towards the development and design of the ITER NBI system, the ELISE test facility is an intermediate step, having half the size of the final ITER NM source. As well as important scientific and engineering results, ELISE provides highly valuable experience in the operation and performance of a large RE-driven negative hydrogen ion source. At the ELISE test facility it is possible to have an insight into the physics of the large beam by means of several diagnostics. The Beam Emission Spectroscopy (BES) diagnostic provides information on the beam uniformity as well as the divergence, along a vertical and a horizontal profile. Analysis of infra-red (IR) imaging of the beam striking a calorimeter provides a 2D map of the beam power density. Three main topics will be here reported: 1) Studies of the vertical beam homogeneity often show a vertical (top/bottom) difference in terms of beam intensity and, as a consequence, in terms of divergence (i.e. different beam optics tier different extracted beam currents). 2) The investigation of the possibility to measure the broad beam component (this being a small fraction of the beam with a significantly higher divergence than the majority) by means of the BES diagnostic, leads to different methods for a proper-fit of the Ha, main Doppler peak. Different fit methods correspond to different hypotheses on the origins of the broad component itself. 3) The investigation of the stripping losses inside the extraction system aims to provide a robust method for the BES data analysis, in combination with modeling of the gas density profile along the beamline, in order to give a proper estimation of the stripping losses to be compared with predictions as extrapolated from calculation for ITER (< 10% up to the extraction grid).