Operation of the ITER Electrostatic Residual Ion Dump with a Perturbed Field

Abstract

ITER will use a novel electrostatic method to remove the unwanted residual charged component from the neutral beam injectors in place of the usual magnet separation system. This technique has not been tested experimentally and is subject to the additional complication of plasma formation perturbing the electric field. Previous calculations have shown that whilst this is not significant for the 1 MeV heating beam systems, the lower energy diagnostic beam system will be susceptible. An analytical model of the electrostatic dump has been developed that includes the perturbation of the vacuum electrostatic field by both plasma and the separating positive and negatively charged residual beams. An approximate solution of Poisson’s equation is formulated that allows analysis of the space charge field when plasma density is insufficient to ensure zero electric field at the anode. The resulting modified electric field is then incorporated into a particle trajectory code to determine the deposition of the residual ions on the ERID panels. It is shown that the effect of plasma formation is to introduce an asymmetry into the deflecting field and the effect of the separating charges is to weaken the deflection of the residual beams. As a consequence the reference design for the ITER diagnostic beam will not collect all of the residual ions and it is recommended that the deflection voltage be increased by at least 50%.