Mitigating Hall effect and controlling plasma potential in negative ion source via magnetic field redesign

The negative ion source concept currently under development for the ITER neutral beam injector exhibits several design flaws, notably: (i) plasma asymmetry along the direction parallel to the ion extraction grid, caused by the transverse magnetic configuration inducing an electron drift (Hall current) that is bounded by one of the lateral walls (Hall effect), [1, 2] and (ii) an excessively high plasma potential, which—according to current hypotheses—adversely affects beam divergence in the accelerator. The objective of this study is to propose a novel ion source concept that addresses these fundamental issues. The proposed approach is primarily based on modifying the magnetic field configuration to both eliminate the Hall effect (by closing the electron drift path and thereby removing plasma asymmetry) and control the plasma potential amplitude (and its gradients) using electrodes positioned at the magnetic mirrors. Experimental results obtained with a custom-built magnetized ICP source featuring a radial magnetic field configuration are presented, supported by three-dimensional (3D) particle-in-cell (PIC) simulations. Our observations show that the plasma parameters – density, electron temperature, and plasma potential – exhibit axisymmetric and relatively flat profiles in the expansion chamber. Moreover, the plasma potential amplitude can be effectively controlled by biasing electrodes placed at specific locations along the wall in the ionization chamber.

[1] F. Gaboriau et al., Appl. Phys. Lett. 104, 214107 (2014)

[2] G. Fubiani et al., New Journal of Physics 19, 015002 (2017)