A meniscus of plasma-beam boundary in H− ion sources largely affects the extracted H− ion beam optics. Although it is hypothesized that the shape of the meniscus is one of the main reasons for the beam halo observed in experiments, a physical mechanism of the beam halo formation is not yet fully understood. In this letter, it is first shown by the 2D particle in cell simulation that the H− ions extracted from the periphery of the meniscus cause a beam halo since the surface produced H− ions penetrate into the bulk plasma, and, thus, the resultant meniscus has a relatively large curvature.

Articles you may be interested inStudy of plasma meniscus and beam halo in negative ion sources using three dimension in real space and three dimension in velocity space particle in cell modela) Rev. Sci. Instrum. 85, 02A737 (2014); 10.1063/1.4854976 Effect of basic physical parameters to control plasma meniscus and beam halo formation in negative ion sources High-intensity ion sources for accelerators with emphasis on H − beam formation and transport (invited)a) Rev. Sci. Instrum. 81, 02B311 (2010); 10.1063/1.3272825 Simulating ion beam extraction from a single aperture triode acceleration column: A comparison of the beam transport codes IGUN and PBGUNS with test stand data Rev. Sci. Instrum. 79, 043304 (2008); 10.1063/1.2908170High intensity beams from electron cyclotron resonance ion sources: A study of efficient extraction and transport system (invited) Rev. Sci.

Our previous study shows that the curvature of the plasma meniscus causes the beam halo in the negative ion sources: the negative ions extracted from the periphery of the meniscus are over-focused in the extractor due to the electrostatic lens effect, and consequently become the beam halo. In this article, the detail physics of the plasma meniscus and beam halo formation is investigated with two-dimensional particle-in-cell simulation. It is shown that the basic physical parameters such as the H− extraction voltage and the effective electron confinement time significantly affect the formation of the plasma meniscus and the resultant beam halo since the penetration of electric field for negative ion extraction depends on these physical parameters. Especially, the electron confinement time depends on the characteristic time of electron escape along the magnetic field as well as the characteristic time of electron diffusion across the magnetic field. The plasma meniscus penetrates deeply into the source plasma region when the effective electron confinement time is short. In this case, the curvature of the plasma meniscus becomes large, and consequently the fraction of the beam halo increases.

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Our previous study by two dimension in real space and three dimension in velocity space-particle in cell model shows that the curvature of the plasma meniscus causes the beam halo in the negative ion sources. The negative ions extracted from the periphery of the meniscus are over-focused in the extractor due to the electrostatic lens effect, and consequently become the beam halo. The purpose of this study is to verify this mechanism with the full 3D model. It is shown that the above mechanism is essentially unchanged even in the 3D model, while the fraction of the beam halo is significantly reduced to 6%. This value reasonably agrees with the experimental result.