Reduction of Beam Losses and Heat Loads by Secondary Particle in MITICA Beam Source

MITICA is the prototype of the ITER Heating and Current Drive Neutral Beam Injectors, whose aim is to demonstrate the reliable and efficient production of a 17 MW neutral particle beam required for ITER plasma heating and current drive [1]. The beam source,housed in a high-vacuum chamber, will extract 1280 negative ion beamlets from a weakly ionized plasma inthe relatively high density plasma source, and then accelerate them to 1MV through a six-stage electrostatic accelerator producinga total negative ion current of 40 A. A considerable fraction of the extracted ions will be lost by various collision processes resulting in particle charge changing during the acceleration phase, through the extraction/acceleration grids of the beam source. These processes cause efficiency loss, beam non-uniformity, production of stray secondary particles whose trajectories causes heat of the mechanical components. The stripping loss fraction is directly proportional to the local density of Deuterium gas steadily injected in the plasma source and exiting through the grids' aperture. The gas pumping in the extraction and acceleration stages is therefore a key functionality of the Beam Source design. This aspect is being simulated with a 3D code named AVOCADO[2]. Different geometric solutions have been tested in numerical models, aimed at the reduction of the gas density while keeping as uniform as possible the distribution. The parameter space considered is limited by constraints given by optics, aiming, voltage holding and mechanical feasibility: hence, a best compromise among different needs is desired. In this paper, the guidelines of the optimization process are presented together with the proposed solutions and the results of numerical simulations. [1] L.R. Grisham, et al. "Recent improvements to the ITER neutral beam system design", Fusion Eng. Des. Vol. 87 issue 11 (2012) [2] E. Sartori and P. Veltri, "Avocado: A numerical code to calculate gas pressure distribution", Vacuum, vol. 90, pp. 80-88, April 2013