Conceptual design and integration of a diagnostic neutral beam in ITER

In ITER, one of the key issues to achieve 400 s driven-burn operation at Q about 10 (Technical Basis for the ITER-FEAT Outline Design—Section I-3.2.2.IAEA) is helium ash accumulation. As a result, the real-time measurement of the thermalised helium density profile in the confinement region is of fundamental importance. This paper outlines the design of a modulated, 100 keV, hydrogen, diagnostic neutral beam (DNB), together with preliminary calculations of the performance for the measurement of helium ash by charge exchange recombination spectroscopy (CXRS). The DNB uses a negative ion beam as the primary beam to achieve a required performance with acceptable system efficiency (a higher neutralisation efficiency and smaller beam divergence). This uses the same negative ion source as the ITER H&CD injectors, coupled to a single stage accelerator. The design concept and hence the geometry of the beamline components, i.e. the neutraliser and the residual ion dump and the calorimeter, are similar to those utilised for the H&CD injectors with some minor differences due to the lower beam energy and power. An overview of the design is presented giving the basic information on beam transmission and neutralisation, gas flow and pumping. Selected aspects of the design of the high heat flux components are included. Specific layout and safety requirements of the ITER plant require the installation of the DNB within the common secondary confinement of the NB H&CD system (the NB cell); as a consequence, the DNB has to share the vacuum vessel access through a single port with one of the H&CD injectors.