Masashi Kisaki

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.

For the ITER experimental fusion reactor up to 3 heating neutral beams (HNBs) are foreseen. Each HNB will generate 1280 individual beamlets of H ^- /D ^- ions which will be accelerated up to 870 keV 1 MeV ⁻¹ , neutralized and focused into the tokamak plasma, to provide up to 16.5 MW of heating power. To this purpose, divergence and aiming of each beamlet are required to be ⩽7 mrad and within ±2 mrad, respectively. An accurate compensation is thus essential, both for the magnetic deflection induced by the magnets embedded in the extraction grid (EG) (necessary to immediately deflect the co-accelerated electrons) and for the Coulomb repulsion among the beamlets. The Asymmetric Deflection Compensation Magnets, the solution developed for the full-scale ITER HNB prototype called MITICA, was recently tested, for the first time on a MeV class beam, at the MegaVolt Test Facility (MTF) at QST (Naka, Japan). In these experiments, a MITICA-like EG was built and installed on MTF. In this work, the beam pattern measured on a calorimeter made of unidirectional carbon fibre composite is compared with the predictions from simulations by the numerical code IBSimu, in order to develop methodologies and identify limitations for its application for MITICA exploitation and improvements. The model accuracy varied with experimental conditions; in all the cases, however, the proposed methodology proved very valuable in providing trends and dependencies.

High-energy negative ion-based neutral beam injectors (N-NBIs) are commonly equipped with filament-arc or radio frequency (RF) ion sources. Both types are based on the tandem concept, which envisages the presence of a static and oriented magnetic field, called the filter field (FF), for preventing negative ion destruction by energetic electrons. As a consequence, plasma parameters strongly depend on the position inside the ion source. To this end, a water-cooled movable Langmuir probe (LP) was developed at Consorzio RFX (Padua, Italy) and operated in the negative ion test stand (NITS) facility in Quantum Science and Technology (QST) (Naka, Japan), specifically in a filament-arc-based negative ion source, the so-called Kamaboko. This study presents a spatially resolved characterization along the axial direction of plasma parameters such as plasma density, electron temperature, and plasma potential. In particular, both the arc and extraction regions were explored under varying experimental conditions, including source pressure and wall polarization. For the same scans, the peak intensity trends of the principal hydrogen Balmer emission lines ( $\mathrm {H}_{\alpha }$ , $\mathrm {H}_{\beta }$ , and $\mathrm {H}_{\gamma }$ ) measured along two lines of sight are also provided.

In large beam sources for neutral beam injectors (NBIs) operating at high energies, negative hydrogen ions (NI) are extracted from a caesiated plasma discharge. H $^{-}$ ions are generated from volume and surface processes; surface production is enhanced by cesium deposition on the source walls and in particular on the plasma-facing electrode of the accelerator. The mechanisms of negative ion production and of extraction in the magnetic field present at the extraction apertures influence the formation of the single beamlets and their optics. A newly developed compact retarding field energy analyzer (CRFEA) was constructed and installed at the upstream surface of the plasma grid (PG) of a filament-arc source, the research negative ion source (RNIS) at the National Institute of Fusion Science. It was used to characterize the negative ions in the plasma source with and without cesium. The first electrode of the analyzer has a conical shape, resembling the PG apertures from which the beam is extracted. The measured saturation current was analyzed and correlated with the source parameters and the current of the negative-ion beam. The energy distribution of collected ions depends on the origin of the negative ions, and on possible energy exchange processes; but it is found to be also strongly influenced by the curvature of the meniscus (the plasma edge where the beam is formed). This compact retarding field analyzer proved to be effective for the study of the formation of a negative ion beam. The limits of the present design, and possible improvements are discussed, in particular to minimize the dependence of the transmission on perveance and extend the dynamic range of the diagnostic.

The shape of a plasma meniscus is a key factor to determine the beam focusing. The physics model of the meniscus formation for hydrogen negative ion sources has not been established yet. A backward trajectory calculation based on experimental observation is performed in order to derive the particle information at the meniscus. It is observed that the negative ion density is spatially nonuniform in the direction parallel to the magnets for suppression of co-extracted electrons. A nonuniformity of the negative ion density in the vicinity of the meniscus is taken into account in the forward trajectory calculation. It reveals that the nonuniform negative ion distribution leads to degradation of the beam focusing and the beam splitting in phase space. The importance of the spatial distribution of negative ions on meniscus modelling is discussed with a comparison to uniform extraction model.

Beam focusing is one of the most important elements for the stable and safe operation of high power negative ion beams, such as neutral beam injection into magnetically confined fusion plasmas. In order to investigate impacts of the source plasma fluctuation on beam focusing, a simultaneous measurement of the source plasma fluctuation and the beam current profile has been carried out in the research-and-development negative ion source at the National Institute for Fusion Science. The responses of beam width and of the beam centre deviation are observed for the first time, indicating the importance of the source plasma stability for the negative ion beam focusing.

Simultaneous measurement of negative ion source plasma and extracted beam is carried out in order to clarify a key plasma parameter governing the meniscus formation in negative ion sources for fusion. The plasma discharge is performed with various discharge powers at different bias voltages in order to vary the plasma parameters. It is shown that the beam width changes along the same curve with respect to the negative ion density at any bias voltage while it varies along different curves with other plasma parameters depending on the bias voltage. This implies that the mechanism of meniscus formation in negative ion sources could be described along the similar manner as positive ion sources.

Negative ion beam focusing is a key element for advanced applications of negative ion beams such as accelerators for particle physics, compact accelerators for medical fields, and plasma experiments for nuclear fusion because complicated magnetic fields exist both inside of the source plasma and the grid system. In order to understand the beam focusing, phase space structure measurements for a single beamlet have been performed with a research-and-development negative ion source at the National Institute for Fusion Science. A complicated phase space structure is observed in the direction parallel to the filter magnetic field in the vicinity of the plasma grid, while a single-Gaussian beamlet structure is observed in the direction perpendicular to the filter field. Detailed analyses for the phase space structure of the single beamlet reveal that the complicated structure can be identified as a combination of three beam components with different beam axes. The shifts of each axis are also observed to depend on the ratio of the acceleration voltage for the extraction voltage, which may significantly degrade the beamlet focusing.

Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to I_e/I_acc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m². The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign.

Plasma grid bias has been utilized for reducing an electron density in the vicinity of a plasma grid in negative ion sources for fusion, resulting in achievement of low co-extracted electron current. In this study, the effectiveness and feasibility of the plasma grid bias on beam optics optimization are demonstrated. It is shown that the beam optics strongly depends on the bias voltage and is successfully optimized at different bias voltages depending on the discharge power. Responses of the beam properties such as the perveance and the current ratio to the plasma grid bias are also shown for both hydrogen and deuterium beams.