ITER neutralizer modeling (abstract)

Abstract

In the neutralizer of the ITER Neutral Beam Injector, a 1MeV-D− beam passes through an structure filled with D2 gas, where negative ions are mainly converted to fast D0 atoms. Once that the beam is neutralized no further optical correction is possible, i.e., transport from the neutralizer to the confinement chamber is ballistic. Because of this, the transport through the neutralizer determines ultimately the geometrical properties of the neutral beams. The ionization of the buffer gas (D2) filling the neutralizer induced by the D beam creates a rarefied and low temperature plasma (ionization degree ≈10−3, electron temperature ≈20eV). This plasma can screen the electrostatic well of the D beam and, consequently, affect the properties of the extracted beam and the energy transport to the neutralizer walls. On the other hand, the plasma will eventually escape from the neutralizer and move back in the accelerator chain, toward the accelerating grids and the source. We present particle-in-cell simulations of the beam propagation and plasma formation through the neutralizer. Particle-particle and particle-wall collisions are treated using a Monte Carlo approach. Simulations show that the secondary plasma effectively screens the beam space charge preventing beam radial expansion due to Coulomb repulsion between beam ions. First results suggest that the current of plasma ions (D2+) into the accelerator would be of the order of I(D−)∕100, with I(D−) the negative ion current.