Present status of numerical modeling of hydrogen negative ion source plasmas and its comparison with experiments: Japanese activities and their collaboration with experimental groups

Abstract

The present status of kinetic modeling of particle dynamics in hydrogen negative ion (H − ) source plasmas and their comparisons with experiments are reviewed and discussed with some new results. The main focus is placed on the following topics, which are important for the research and development of H − sources for intense and high-quality H − ion beams: (i) effects of non-equilibrium features of electron energy distribution function on volume and surface H − production, (ii) the origin of the spatial non-uniformity in giant multi-cusp arc-discharge H − sources, (iii) capacitive to inductive (E to H) mode transition in radio frequency-inductively coupled plasma H − sources and (iv) extraction physics of H − ions and beam optics, especially the present understanding of the meniscus formation in strongly electronegative plasmas (so-called ion-ion plasmas) and its effect on beam optics. For these topics, mainly Japanese modeling activities, and their domestic and international collaborations with experimental studies, are introduced with some examples showing how models have been improved and to what extent the modeling studies can presently contribute to improving the source performance. Close collaboration between experimental and modeling activities is indispensable for the validation/improvement of the modeling and its contribution to the source design/development. chamber wall (anode) and filaments (cathode). In the latter sources, the RF-electromagnetic field is used to generate and heat plasmas.In this review, we mainly focus on the modeling studies, especially the modeling study of H − source plasmas using kinetic approaches, such as the test particle Monte-Carlo model [14] and particle in cell (PIC) model [14,15], which has been reviewed in [16]. Here, we extend it with some new results. Close collaboration between the experimental and modeling study is very important as it can improve our basic understanding of source plasmas. Various examples of the model validation will be shown through comparisons with experiments and also how modeling studies contribute to the improvement of source performances.As for the multi-cusp arc-discharge source, we will first show a systematic study to help us understand the role of the EEDF on the H − VP in section 2.2. The study has been carried out for a compact multi-cusp arcdischarge source (SHI H − source: Sumitomo Heavy Industry H − source [17,18]) for medical application such as boron neutron capture therapy and the radioisotope production for molecular imaging technology.The SHI H − source is a typical tandem type H − source [19]. In such a tandem type volume sources, the plasma source volume is divided into two regions by the transverse magnetic field (the so-called magnetic filter filed: MF-field) to control the EEDFs and to promote the two step H − VP reactions explained above. Namely, the source volume consists of two regions: (1) the 'driver region' where the electron energy is high and the EV process is promoted, and (2) the 'extraction region' where the electr...

Characterisation of negative ion beam focusing based on phase space structure

“…Although the meniscus formation is a key to the beam focusing, it is extremely difficult to observe the meniscus experimentally. In numerical simulations, it is pointed out that the meniscus shape is important for the beam halo generation and the beam divergence [25][26][27]. A similar result was also reported by the inverse beam calculation to evaluate the meniscus shape [28].…”

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…Many other authors [63][64][65][66][67] have developed similar approaches for a better understanding of the formation of negative H − and D − in different sources.…”

“…Contrary to surface production sources, volume production sources operate with pure H 2 or D 2 , having the practical advantage of inherently caesium-free operation, which makes them an attractive alternative. While different processes might be responsible for the formation of negative ions [67,73,74,103], the volume production mechanism is an efficient path and mainly refers to the process of dissociative attachment (DA) of low-energy (cold) electrons to highly vibrationally excited molecules [19,66,73,103]:…”

Focus on sources of negatively charged ions

“…The combination of physics modelling and simulation with experimental methods is the key to improved understanding of NBI's hydrogen and deuterium plasma; Hatayama presents a thorough review of the low temperature plasma modelling [21] and Tsumori presents the most sophisticated measurement techniques to investigate plasma population's properties including those specific to large sources for fusion reactors' NBI heating [22]. As an example, Briefi applied a collisional radiative modelling to analyse high-density low temperature hydrogen plasma; he presents the simulation of inductively coupled plasma (ICP) heating of CERN's Linac4 ion source and its experimental validation via state-of-the-art optical emission spectroscopy measurements of the Balmer lines and Fulcher band [23].…”