Compensation of Beamlet Deflections and Focusing Methods in the Electrostatic Accelerator of MITICA Neutral Beam Injector

Abstract

Neutral beam injectors are the main source of plasma heating in advanced fusion devices. To support the realization and optimize the performances of the injectors of ITER tokamak, a dedicated test bed is under construction at Consorzio RFX, Padova. This facility will include a full scale prototype of the injector, named megavolt ITER injector concept advancement. Its aim is to demonstrate and optimize a 16-MW neutral beam power, obtained by neutralization of a 56-A deuterium negative ion beam <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">$D^{-}$ </tex-math></inline-formula> accelerated to an energy of 1 MeV. Since the expected current density available at the source is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">${\sim}{\rm 300}~{\rm A}/{\rm m}^{2}$ </tex-math></inline-formula> , a large extraction area is needed in order to obtain the required current, so that the beam is formed by the superposition of 1280 beamlets extracted from the source by means of a set of grids biased at increasing potentials. This paper presents the studies carried out to optimize the beam optics, in order to satisfy the ITER requirements on beam aiming and focusing over long distances. This includes the compensation of the electrostatic interaction among beamlets and the effect of supporting structures. The most promising methods to steer the beamlets are studied and a comparison among different design configurations is made.