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Relationship between Surface-Produced H⁻ Density and the Potential Structure of an Ion-Ion Plasma near the Extraction Aperture in Negative Hydrogen Ion Sources

Katsuya HAYASHI¹⁾, Kazuo HOSHINO¹⁾, Kenji MIYAMOTO²⁾, Akiyoshi HATAYAMA¹⁾

1)

Keio University, Kanagawa 223-8852, Japan

2)

Naruto University of Education, Tokushima 772-8502, Japan

(Received 18 August 2025 / Accepted 8 January 2026 / Published 31 March 2026)

Abstract

The so-called “ion-ion” plasma, which consists primarily of negative and positive ions, has been observed near the extraction aperture in negative hydrogen ion (H⁻) sources with a substantial amount of surface-produced negative ions, according to several experiments. In this study, to elucidate the characteristics of ion-ion plasmas with substantial surface H⁻ production, the relationship between surface-produced negative ion density and electric potential is investigated using three-dimensional Particle-In-Cell simulations with the KEIO-BFX code. The results reveal that the plasma in a negative ion source can be classified into four distinct regions based on the relationship between H⁻ density and electric potential: Region (I), where H⁻ follows the Boltzmann relation; Region (II), where H⁺ follows the Boltzmann relation; Region (III), a transition region between the plasma region and the beam acceleration region; and Region (IV), the beam acceleration region. In negative ion sources with surface-produced H⁻ ions, plotting the H⁻ density against the electric potential offers a useful method for identifying the plasma meniscus, which is crucial for optimizing extracted beam quality and ensuring proper operation of beam extraction systems.

Copyright (c) 2026 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

negative ion source, particle in cell, surface-produced negative ion, ion-ion plasma

DOI: 10.1585/pfr.21.1401019

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