Technologies for the realization of large size RF sources for negative neutral beam systems for ITER. Challenges, experience and the path ahead

Technologies for manufacturing small and medium size ion sources (up to four RE drivers) for positive and negative neutral beam systems have evolved over many decades. However, for large negative ion sources to be used at ITER for diagnostic and heating purposes, several existing manufacturing technologies have to be upgraded and re-evaluated to adapt these sources for ITER, such as an operation environment which considers the highest vacuum quality class in a nuclear environment. The diagnostic neutral beam source is the first of such a series of three sources being manufactured according to the ITER built-to-print specification. The experience gained with the progress of manufacturing has shed light on many unforeseen challenges which need due consideration to ensure successful manufacturing of the source as per ITER specifications. These challenges are related to use of the materials with a controlled percentage of the constituents adaptable to a radiative environment, special requirements of the weld joint configuration to enable full penetration with 100% volumetric inspectability, dissimilar material welding technologies, machining process development to meet stringent dimensional accuracies (in the range of 10-50 microns) of individual 'angled' grid segments to achieve overall aperture to aperture alignment of +/- 0.2 mm, electro-deposition of copper with thickness >3 nun over the angled surfaces with control over distortion, and development of post insulators with threaded connection between metal and alumina, with a load carrying capacity of 10 kN and electrical isolation of 90 kV in vacuum. The paper highlights the experience generated in the development of the above-mentioned manufacturing technologies, the methodologies adopted for mitigating the practical limitations, prototyping to establish and qualify the manufacturing procedure, and evaluation of the non-conformities and assessment of deviation proposals, in compliance with ITER requirements.