The status of negative ions R&D for Fusion Applications

In several research fields, ranging from neutron generation to synchrotron accelerators, sources of negative ions are used to obtain powerful beams with high brightness. Particularly, in present and future nuclear fusion devices, neutral beam injection systems are expected to play a key role in plasma heating, current drive and configuration control. Aimed to address the most outstanding open issues, experiments and simulations are presently devoted, in a huge worldwide effort, to the investigation of the physics of negative ion beams, including formation, extraction and acceleration. At the Test facility for the ITER NBI under advanced realization in Padova the project and realization of the injector is carried out by taking into account the outcome of this international R&D. In this contribution an overview of the present status and perspectives of the negative ion physics and its impact on the ITER NBI project is described. The atomic and molecular phenomena occurring in the source and their influence on the plasma properties are discussed; the complex dynamics of molecules, negative and positive ions and electrons is described; particularly the generation of negative ions is presented, by illustrating the main production mechanisms and the beneficial effects of cesium addition, and highlighting the relevance of detailed studies of cesium dynamics. The effect of the magnetic field in filtering the most energetic electrons and in increasing the amount of negative ions in the extraction region is presented. Two-dimensional and threedimensional numerical codes are applied to the investigation of extraction and acceleration physics to interpret the experimental beam features. Space charge compensation of the accelerated beam and the effect of the neutral gas density profile as well as the production of secondary and back-streaming particles are also discussed. The latest advances in physics understanding and in modeling have been extensively benchmarked to experimental results in existing fusion facilities and applied to the design of the high power beams for ITER, where the physical requirements have to meet the engineering constraints and additional issues regarding the obtainment of a high degree of uniformity, in terms of particle density and beam divergence, over the cross-section of large-area beams.