Effect of Surface Produced H^-Ion on the Plasma Meniscus in Negative Hydrogen Ion Sources

Abstract

To extract intense ion beams with good beam optics from ion sources, controlling the distance d eff between the plasma meniscus (i.e., beam emission surface) and the beam extraction grid is important. This study conducts a novel investigation into the dependence of the effective distance d eff on the amount of surface Hproduction S H-. For this purpose, a 3D PIC (three dimensional Particle-in-Cell) simulation is conducted to obtain a model geometry of the extraction region for a Hion source with S H-as a parameter. Based on results, d eff significantly depends on S H-and the H --electron density ratio (α = n H -/n e ) in front of the extraction aperture for the same plasma density; as S H-increases, d eff decreases. The results suggest that S H-is critical for controlling d eff and the resultant beam optics extracted from the negative ion source. c

Numerical study of the plasma meniscus shape and beam optics in RF negative ion sources

“…surface H − production (𝑆 H − ) has been studied under the case with a large amount of surface H − production [12]. The results show that 𝑑 eff decreases as 𝑆 H − increases.…”

“…It is exactly the same model geometry as that in ref. [12] for the extraction region of the LINAC4 ion source [1,2]. The simulation domain is three dimensional, and the initial plasma is generated inside a cylindrical source on the left side of figure 1.…”

“…As will be briefly summarized below, the boundary condition is imposed on each boundary as in the same manner as in ref. [12]. As shown in figure 1, Dirichlet boundary conditions are used for the PG surface, as well as the inner wall of the plasma chamber, where the potential is set to zero.…”

“…In addition, the boundary conditions for the particles reaching the simulation boundaries are the same as those in ref. [12].…”

“…They are almost the same as those in ref. [12]. Reduced size scaling has been used with a scaling factor of (= 3.5 × 10 −2 ) in the same manner as in refs.…”