Drift and non-uniformity mitigation in H⁻/D⁻ sources with Plasma Ion Funnel extraction

The H - multiaperture ion sources requested by Neutral Beam Injectors (NBI) for fusion applications need fair plasma uniformity on the beamlet extraction apertures placed in the Plasma Grid (PG), both to facilitate perveance matching of all beamlets and to balance erosion of cesium layer in long pulses. Particle drifts due to both the extraction electric field and the magnetic filter B , needed in the extraction region to reduce electron density temperature, form a pileup resulting as top/bottom plasma asymmetry. The plasma density can be controlled by funnel electrodes and bias plate (BP) with proper polarization. Assuming that filter current flows vertically, as in SPIDER, and in the designs for MITICA and DTT (Divertor Test Tokamak), we have B horizontally directed and drift velocity v d vertically directed. Several remedies were proposed, based on modification of the E × B direction, to reduce plasma flow accumulation at specific points (source bottom or top): in the funnel concept, the BP is supplemented by many electrodes inside the extraction region. Voltages among PG, BP, funnel and wider plasma chamber walls, as well B , are key parameters. Due to the large computation size of the full problem, two approximate simulation methods are proposed. 3D simulations with no plasma space charge have shown good ion extraction conditions for preventing direct electron co-extraction. An empirical model for plasma sheath and space charge is also solved in 2D (using nonlinear multiphysics solvers) and a discussion on drift trajectories that mostly confirm similar 3D results is introduced. Beam divergence, current improvements and asymmetry of current between top and bottom beamlets are also reported; in the 2D model, this asymmetry may be canceled with some tuning of funnel voltages.