Haruhisa Nakano

Large plasma sources are employed in negative ion based heating neutral beam injectors for fusion applications. A review of the ITER beam source plasma properties is presented, in comparison with negative ion sources of comparable size, based on the latest experimental measurements from SPIDER and supporting numerical simulations. Spatial investigation of plasma parameters using several diagnostics, including the beam itself, retarding field energy analyzers, optical emission spectroscopy, and movable and fixed Langmuir probes, is key to understanding the source physics, to support the operation and improve the source performance. The influence of multiple RF-driver configuration on uniformity and the improvement flexibility it provides, as well as the implications of control parameters like filter field and plasma grid bias, are examined. Similarities and differences with other giant negative ion sources are discussed to highlight the key physical processes leading to optimal operation and viable paths for possible improvements to the ITER source design.

A two-dimensional fluid model of the RF ion source in NIFS test stand was established using COMSOL Multiphysics to examine the influence of the cusp magnets on plasma transport and RF power coupling. Particular attention was paid to the magnetic field intensity of the cusp magnets on driver backplane and its impact on plasma behaviour in both the driver and the extraction region. Simulation results reveal that strong magnetic confinement from the upper cusp magnets suppresses cross-field plasma transport, leading to reduced electron density near the plasma grid and less efficient RF power deposition. To explore potential optimization, a modified configuration with two smaller magnets of half the original size was investigated. This adjustment significantly enhances plasma density, increasing the peak density in the driver by nearly a factor of two. However, the enhanced plasma density is accompanied by increased spatial gradients in the extraction region, indicating that plasma uniformity is not improved under the modified magnetic configuration. These results reveal a trade-off between plasma density enhancement and spatial uniformity, arising from the interplay between magnetic confinement and plasma transport. This study provides insight into the role of cusp magnetic fields in regulating plasma generation and spatial gradients, and highlights the importance of balancing plasma density and uniformity in the optimization of RF negative ion sources.

Negative carbon (C−) and oxygen (O−) ions are expected to find wide applications for ion implantation processes. However, systematic experimental data required for their efficient generation and the elucidation of their underlying atomic processes are lacking. In this study, we systematically measured the angular and incident beam energy dependencies of the reflected carbon and oxygen ion properties from Mo, W, and low-work function material 12CaO⋅7Al2O3 electride targets, for low-energy (<1 keV) carbon (C+) and oxygen (O+) ion injections. For all targets, both C+ and C− ions were reflected for the C+ ion injection, whereas only the reflected O− ions were observed for O+ ion injection. The energies of C+, C−, and O− ions reflected from the electride and Mo decreased significantly with increasing reflection angle. The effect of the lower work function was an increase in the intensity of the O− ions reflected at larger-incident and smaller-reflection angles by comparing the reflected ions from Mo and electride targets. On the other hand, the effect of the work function on the production of C− ions was small. When 1 keV-O+ ions were incident on W, the fraction of the reflected O− ions was 10%–30%, and the rest were neutral oxygen particles. Good agreement between the measured and calculated incident angular dependency of reflected O neutral particles was observed.

The plasma-beam boundary is the main focus of the negative ion source study. The boundary behavior affects negative ion beam formation throughout the negative ion neutral beam injector. Simulation for optimizing the beam profile is under development and shows different results from experimental data. For this reason, experimental results from the region are valuable for constructing theory and as a guide for simulation development. This paper shows space potential and density measurement with a Langmuir probe scanning inside the extraction aperture. The density measurement shows the movement of plasma according to the arc power, magnetic field, and applied extraction field.

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

An isotope effect of negative ion motion in plasma near a plasma grid (PG), which is a plasma-beam boundary grid, has been investigated in a negative hydrogen-ion source with the surface production process of the negative ion on the PG surface. Negative deuterium-ion (D^-) density was higher than negative hydrogen-ion (H^-) density in the condition without extraction-grid bias (V _egb) and approached the H^- density in the condition with V _egb. Thus, the D^- density responded stronger than the H^- density to the V _egb. The density of the negative hydrogen-ion isotopes and the density response to the V _egb have been organized into respective identical trends by momenta of isotopes emitting from the PG. The Larmor motion can be a dominant mechanism of the negative ion transport from the PG to the plasma.

Well focused negative ion beams are required for neutral beam injection systems for heating and current drive in magnetically confined fusion plasma experiments. The control of a single negative ion beamlet divergence is a significant challenge with the use of a cesium-seeded negative ion source, where negative ions are mainly produced by conversion of hydrogen or deuterium atoms on a cesiated surface of a plasma grid. The single negative ion beamlet was found to be made by three-Gaussian components in our previous work. The origins of such multiple components are considered to be related to dynamics in the ion source and extraction processes of negative ions. This work has demonstrated a measurement of the abundance ratio of the three components (41%, 40%, and 19%) based on a full picture of their transverse velocity distributions, which is a powerful technique to investigate the origins of individual components and will contribute to improve the divergence of negative ion beamlet.

Neutral beam injectors are among the most important methods of plasma heating in magnetic confinement fusion devices. The propagation of the negative ions, prior to their conversion into neutrals, is of fundamental importance in determining the properties of the beam, such as its aiming and focusing at long-distances, so as to deposit the beam power in the proper position inside the confined plasma, as well as to avoid interaction with the material surfaces along the beam path. The final design of the ITER Heating Neutral Beam prototype has been completed at Consorzio RFX (Padova, Italy), in the framework of a close collaboration with European, Japanese and Indian fusion research institutes. The physical and technical rationales on which the design is based were essentially driven by numerical modelling of the relevant physical processes, and the same models and codes will be useful to design the DEMO neutral beam injector in the near future. This contribution presents a benchmark study of the codes used for this purpose, by comparing their results against the measures performed in an existing large-power device, hosted at the National Institute for Fusion Science, Japan. In particular, the negative ion formation and acceleration are investigated. A satisfactory agreement was found between codes and experiments, leading to an improved understanding of beam transport dynamics. The interpretation of the discrepancies identified in previous works, possibly related to the non-uniformity of the extracted negative ion current, is also presented.