(FT/P2-31) Beam-plasma Interaction in the ITER NBI

T.M. Minea1), A. Lifschitz1), F. Dure1), G. Maynard1), J. Bretagne1), A. Simonin2)
1) Laboratoire de Physique des Gaz et Plasmas (LPGP), Université Paris Sud XI, Bat. 210, 91405 Orsay Cedex, France
2) DRFC, CEA Cadarache, 13108 Saint-Paul lez Durance, France

Abstract.  In the neutraliser of the ITER Neutral Beam Injector, a beam of negative ions (D-) of about 1 MeV passes through a structure filled with deuterium gas, where negative ions are mainly converted into fast atoms. The ionisation of the deuterium buffer gas filling the neutraliser induced by the D- beam creates a rarefied and low temperature plasma which screens the electrostatic well of the D- beam and affects the properties of the extracted beam and the energy transport to the neutraliser walls. Moreover, the plasma can eventually escape from the neutraliser and move back in the accelerator, toward the accelerating grids and the negative ion source. The OBI-2 (Orsay Beam Injector 2 dimensional) code was developed to simulate the beam propagation and plasma formation. Particle-particle and particle-wall collisions are treated using the Monte Carlo collision approach and the plasma is treated via Particle-in-Cell. The neutraliser geometry has been chosen as cylindrical with volume to surface aspect ratio representative of real technical devices designed for ITER neutral beam heating. Simulations show that the secondary plasma effectively screens the beam space charge preventing beam radial expansion induced by the Coulomb repulsion between beam ions. Plasma ions created in the neutraliser form an upstream current impinging the accelerator grid. On the other hand, the presence of these plasma ions between the accelerator grids and the neutraliser entrance enhances the beam focusing.

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