Beamline optimization for 100keV diagnostic neutral beam (DNB) injector for ITER

The 100 kV negative hydrogen ion source based Diagnostic Neutral Beam (DNB) injector, which forms a part of the IN Procurement Package for ITER, targets a delivery of ~18-20A of neutral hydrogen atom beam current into the ITER torus for charge exchange resonance spectroscopy (CXRS) diagnostics. Considering stripping losses, ~70A negative ion current is required to be extracted from the ion source, which leads to a production of 60 A of accelerated ion beam. Subsequent process of neutralization, electrostatic ion separation and transport to the duct leads to a large separation between the points of generation of ion beam to the point of delivery of the neutral beam into the Torus (~23 m). This forms one of the most important constraints for the transport of neutral beams to ITER. The requirements are not only for a stringent control over ion optics, the transport to electrostatic separator, minimum loss of beam due to intercepting elements, low reionization loss, focusing to control interception losses, adequate compensation of residual magnetic fields to overcome magnetic field induced deflections also forms important design issues for a reasonable transmission efficiency. Due to the multi parameter dependence, it becomes necessary to assess different scenarios using numerical codes. In the present case the assessment has been carried out for the DNB using the beam transport codes PDP, BTR and the MCGF codes which are developed by the Russian Federation. An optimized configuration of the beam line has been arrived at on the basis of these codes enabled studies. These parameters are: listing of the vertical and horizontal focal lengths as 20.6 m, a spacing between ground grid and neutralizer of 1 m, positioning of RID at a distance of 0.75 m from the neutraliser exit. Further, optimizing the gas feed to the source and neutralizer lead to a final transmission of ~35% of the extracted beam power to the torus. The paper shall present the methodology, the issues concerned and the final configuration which forms the basis for the present engineering.