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We studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5 MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1 MeV D- ions for ≤1 h, 46A/870 keV H- ions for ≤1000 s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100 keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined. The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum. The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.

Improvement of deuterium injection power in the negative-ion-based NBIs (n-NBIs) for the Large Helical Device (LHD) are reported. Co-extracted electron current at acceleration of deuterium negative ions (D− ions) limits the injection power. The electron current is reduced by decreasing the extraction gap, and the injected D− current evaluated from the injection power increased from 46 to 55 A. Greater electron reduction was achieved by installing a structure named an ‘electron fence’ (EF), with which D− beam power was successfully improved from 2.0 MW to 3.0 MW. The injection power in three configurations − without EF, with EF of 5 mm and 7 mm distance from the plasma grid (PG) surface − have been compared in both cases of hydrogen and deuterium operations, and it was found that the configuration with the EF of 5 mm distance was the best to satisfy the performance for both of hydrogen and deuterium injections. Although the co-extracted electron current is reduced in the negative ion sources applied for JT-60SA and ITER by utilizing the PG filter, it is possible to achieve more effective electron reduction by combining the PG filter and the EF.

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.

The large RF negative hydrogen (deuterium) ion source at the ELISE test facility (half of the ITER-NBI source size) has been equipped with a Cavity Ring-Down Spectroscopy (CRDS) system, in order to measure the negative hydrogen (deuterium) ion density in the region in front of the plasma grid (first grid of the extraction system). The challenge of this diagnostic for ELISE relies on the large size of the source and therefore on the plasma length across which the measurements are performed as well as the long pulses at RF power, which can affect the cavity mirror reliability. A dedicated experiment on the mirror reliability was performed, ensuring the feasibility of measurements for long pulses (several hundred seconds) at high RF power. Two horizontal lines of sight were dedicated to CRDS: the measured density was in the range between 4 × 1016 and 1 × 1017 m-3, with a slightly higher density for the bottom lines of sight, for both the isotope hydrogen and deuterium. Different temporal evolution was observed for the two isotopes, showing a higher instability for the deuterium case: this is in correlation with the extracted negative ion current density and inversely correlated with the coextracted electron current density. The CRDS system allowed performing the first measurements of negative ion density for a long pulse (1000 s) in a large source: the temporal behavior and the effect of the beam extraction will also be discussed.

A compact ion source that produces hydrogen plasma with an electron cyclotron resonance (ECR) configuration combined with a 2-stage extraction system with a single aperture of 6 mm diameter has been designed and built to study the performance of different materials as the plasma electrode (PE) of a negative hydrogen ion source. The source has the capability to electrically bias the PE with respect to the ECR plasma. The first experiment with low ECR power input (less than 40 W) was carried out. The PE of the C12A7 electride showed the largest H- current among aluminum, molybdenum, and the C12A7 electride.

In recent deuterium experiments on the large helical device (LHD), we succeeded in expanding the temperature domain to higher regions for both electron and ion temperatures. Suppression of the energetic particle driven resistive interchange mode (EIC) by a moderate electron temperature increase is a key technique to extend the high temperature domain of LHD plasmas. We found a clear isotope effect in the formation of the internal transport barrier in high temperature plasmas. A new technique to measure the hydrogen isotope fraction was developed in the LHD in order to investigate the behavior of the isotope mixing state. The technique revealed that the non-mixing and the mixing states of hydrogen isotopes can be realized in plasmas. In deuterium plasmas, we also succeeded in simultaneously realizing the formation of the edge transport barrier (ETB) and the divertor detachment. It is found that resonant magnetic perturbation plays an important role in the simultaneous formation of the ETB and the detachment. Contributions to fusion reactor development from the engineering point of view, i.e. negative-ion based neutral beam injector research and the mass balance study of tritium, are also discussed.

The deuterium experiment started from March 2017 on the large helical device (LHD) as a part of the LHD high-performance upgrade project. The objectives of the deuterium experiment are: 1) to realize the high-performance operation; 2) to explore the physics of isotope effect; 3) to demonstrate the confinement capability of energetic particles in helical devices; and 4) to explore the research on plasma–material interactions using the benefit of stable steady-state operation ability of LHD. As preparations for deuterium experiment, the positive-ion-based neutral beam injectors (NBIs) are upgraded to increase their injection power in deuterium operation. On the other hand, the injection power of negative-ion-based NBI is deteriorated due to the isotope effect of negative-ion source. The neutron diagnostic and the exhaust detritiation system are newly installed for the deuterium experiment. The commissioning of LHD for the deuterium experiment is quite successful. The high ion temperature operation region is extended by the deuterium experiment. Ion temperature exceeding 9 keV is achieved with the deuterium operation of LHD.

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

A prototype negative ion source has been designed and constructed, in order to initialize investigations on physical and engineering problems of a negative ion source which is a key component of a negative-ion-based neutral beam injector. Driven by filament-arc discharge, experiments have been carried out to validate that this ion source is able to produce and extract negative ions. In pure hydrogen discharge without Cesium (Cs), diagnostics by a single Langmuir probe indicates that electron temperate is around 0.5-0.7 eV at the beam extraction region and the strength of the filter field is enough to cool electrons down. By applying extraction and acceleration voltages to the accelerator, beam extraction experiment has been performed. The maximum acceleration current with Cs reaches 1.22 A, corresponding a beam current density of 6.42 mA/cm2. The current ratio of electrons to negative ions is 0.28 – 0.44 depending on the arc power. The arc efficiency is affected by the partial pressure of water, increment in which brings decrement in arc efficiency.

In order to investigated the dynamics of H− ions and understand the extraction process inside filament-arc-driven plasmas in a Cs-seeded negative ion source, diagnostic experiments using a directional Langmuir probe combined with photodetachment measurement have been conducted. Two-dimensional flow pattern of H− ion flow has been obtained as well as the profile of H− ion density. The result shows that H− ions come from the direction of plasma grid surface, flow to the plasma in the extraction region. During beam extraction, H− ion flow turns to the beam direction. The stagnation point of H− ion flow is at ∼20 mm apart from the plasma grid. Around the stagnation point, the maximum decrement of H− density caused by beam extraction appears. The results indicate that H− ions are extracted from the plasma volume. The extraction process occurs in a region near the plasma grid aperture with boundary of ∼20 mm apart from the plasma gird.

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 Ie/Iacc(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.

To evaluate negative ion beam properties, a dual beamlet monitor system has been developed. The dual beamlet monitor system has two diagnostics in one hexagonal box. One diagnostic is a “fast beamlet monitor” for measuring the time evolution of beamlet current profiles with the time resolution of up to 25 MHz. The other diagnostic is a “pepper-pot-type phase space analyzer,” which is applied for the evaluation of a phase space structure of the negative ion beamlet. The dual beamlet monitor system is applied to the measurement of the beamlet in the Neutral Beam Test Stand at National Institute for Fusion Science (NIFS-NBTS), in which the beam accelerator is almost identical to those of working beam injectors in the large helical device. It is demonstrated that the overlapping components from the neighboring beamlet can be eliminated, and the phase space structure can be obtained for the single beamlet.To evaluate negative ion beam properties, a dual beamlet monitor system has been developed. The dual beamlet monitor system has two diagnostics in one hexagonal box. One diagnostic is a “fast beamlet monitor” for measuring the time evolution of beamlet current profiles with the time resolution of up to 25 MHz. The other diagnostic is a “pepper-pot-type phase space analyzer,” which is applied for the evaluation of a phase space structure of the negative ion beamlet. The dual beamlet monitor system is applied to the measurement of the beamlet in the Neutral Beam Test Stand at National Institute for Fusion Science (NIFS-NBTS), in which the beam accelerator is almost identical to those of working beam injectors in the large helical device. It is demonstrated that the overlapping components from the neighboring beamlet can be eliminated, and the phase space structure can be obtained for the single beamlet.

A novel remote dry processing of specific reactive negative ions with a deflection and focusing system is being developed to precisely control their energy, flux and reaching position on the nanoscale. The trajectory simulations of the negative ion extraction and transport for the design of this processing apparatus have been carried out using SIMION software. It is found from the simulation results that the ion beams with good focusing characteristics, high directivity and appropriate current amount can be successfully extracted from the plasma source and transported to the reactor chamber through the deflection and focusing system. The simulation results of O− ion beam exhibit that the extraction and transport characteristics of the flat intermediate electrode are superior to those of the oblique one.

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.

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.

Density distributions of negative hydrogen (H−) ions and negative deuterium (D−) ions were measured with the laser photodetachment method in the extraction region of the negative ion source. The distribution of H− ion density peaks at the center of the ion source, while that of the D− ion shows a flatter profile in the direction parallel to the plasma grid. The positive ion densities of hydrogen and deuterium estimated from the positive saturation current indicate similar profiles with different amounts close to the grid. The difference in the H− ion and D− ion distributions can be explained by the difference in the negative ion yield and the survival probability of the ions due to the isotope effect.

Tantalum was compared with tungsten as filament material for negative ion extraction in the discharge of hydrogen and deuterium. For hydrogen discharge, tantalum filament operation showed about 40 % increase of negative hydrogen ion current and decrease of co-extracted electron current by 10 % against tungsten filament operation. Tantalum filament operation with deuterium indicated lower intensity of co-extracted electron current compared with tungsten while negative deuterium ion current showed slightly higher intensity. Dependence of extracted currents upon the plasma electrode bias voltage was linked to the local plasma parameters measured by a Langmuir probe. The effects due to the filament materials in hydrogen and deuterium plasmas are discussed.Tantalum was compared with tungsten as filament material for negative ion extraction in the discharge of hydrogen and deuterium. For hydrogen discharge, tantalum filament operation showed about 40 % increase of negative hydrogen ion current and decrease of co-extracted electron current by 10 % against tungsten filament operation. Tantalum filament operation with deuterium indicated lower intensity of co-extracted electron current compared with tungsten while negative deuterium ion current showed slightly higher intensity. Dependence of extracted currents upon the plasma electrode bias voltage was linked to the local plasma parameters measured by a Langmuir probe. The effects due to the filament materials in hydrogen and deuterium plasmas are discussed.

The consumption rate of cesium (Cs) for negative hydrogen (H-) ion source increases when the source operation gas is changed from hydrogen to deuterium. There was observed a clear indication that a deuterium discharge erodes Cs atoms on the plasma grid (PG) surface to increase work function and the co-extracted electron current. We have proposed a model, that the enhanced sputtering yield of Cs from the PG due to deuterium ions, which carries more kinetic energy to Cs adsorbed on the surface directly or indirectly, is the main reason for this fast dissipation of Cs. Introduction of helium (He) into discharge can verify the enhanced sputtering effect due to the lager mass ions in the source discharge. Comparing the effect due to seeded Cs before and after the He injection into discharge through Cs OES signals as well as the H- density measured with cavity ring-down method, the sputtering/evaporation enhancement due to He is estimated. Neutral atoms and positive ions in the He discharge should cause enhanced sputtering like deuterium, while the system does not generate neutron under the induction of acceleration voltage to diagnose the extracted negative ion beam. Plasma parameters of the H2 and He plasmas are investigated by diagnostics tools installed on NIFS-RNIS (National Institute for Fusion Science, Research and development Negative Ion Source) together with H- density measurement by cavity ring down. Enhanced consumption rate of Cs is compared with proposed sputtering yield data to predict the rate for deuterium operation of negative ion sources.

Recent advances in the high power and steady state heating system and experiment results of the Large Helical Device (LHD) are reviewed in this paper. Plasma performance is extended largely through...

Hydrogen ion reflection properties have been investigated following the injection of H+, H2+ and H3+ ions onto a polycrystalline W surface. Angle- and energy-resolved intensity distributions of both scattered H+ and H− ions are measured by a magnetic momentum analyzer. We have detected atomic hydrogen ions reflected from the surface, while molecular hydrogen ions are unobserved within our detection limit. The reflected hydrogen ion energy is approximately less than one-third of the incident beam energy for H3+ ion injection and less than a half of that for H2+ ion injection. Other reflection properties are very similar to those of monoatomic H+ ion injection. Experimental results are compared to the classical trajectory simulations using the ACAT code based on the binary collision approximation.

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.

Proton beams at 0.3, 0.5 and 1 keV energies were injected onto a Mo surface at shallow incidence angle, and the angle- and energy-resolved intensity distributions of reflected negative/positive ions were measured. The negative to positive intensity ratio increased as the incident angle and the reflected angle became shallower and the incident energy became lower. A numerical simulation calculation, Atomic Collision in Amorphous Target (ACAT), has been carried out to get the angular distribution of total reflection particle flux. Experimentally measured angular distributions of reflected H- ion intensity showed maxima at smaller angles than the calculated results, indicating that the negative ion formation has a maximum at v⊥ of 0.03 - 0.04 atomic unit.

A radiofrequency oscillation is successfully superimposed on a plasma potential of a filamented source plasma in an ion beam source while maintaining a constant plasma density, in order to investigate effects of oscillating source plasma potential on an extracted ion beam. The experiment is preliminarily performed with a positive argon ion beam source. A class-D amplifier operational over a wide range of a frequency from a few tens of kHz to several MHz is installed; leading the oscillation of the plasma potential in the plasma source for the frequency range being tested. The beam current profile downstream of the extraction grids shows an oscillation of the beam current at the peripheral region of the ion beam; implying that the oscillation of a beam halo is induced by the potential oscillation of the source plasma.

Plasma parameters were compared in the extraction region of a negative hydrogen (H−) ion source between the discharge sustained by tungsten (W) and tantalum (Ta) high temperature cathodes. A hydrogen plasma driven by the Ta filament showed lower electron temperature and higher ratio of positive ion saturation current to negative ion/electron saturation current than a plasma excited by the W filament. Plasma potential indicated different spatial distribution along the direction of H− ion extraction corresponding to the voltage biased on the plasma electrode. These results are correlated to the intensities of the H− ion extraction current.

This study aims to develop a novel flat oxygen plasma source containing a magnetic filter. The aspect ratio of the diameter to the length of the source vessel was approximately four. A pair of small permanent magnets was inserted in a metal flange to generate the magnetic filter at the exit hole of the plasma source. Measured currentvoltage characteristics indicated that a clear decrease in the electron temperature was successfully attained across the magnetic filter, which was adequate for producing negative oxygen ions via dissociative electron attachment.

The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current () reaches 46.2 A from two beam sources with the averaged current density being 190 A m−2 for 2 s, and the extracted electron to accelerated ion current ratio () increases to 0.39 using 5.6 V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIFS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, increases to 55.4 A with the averaged current density being 233 A m−2 for 1.5 s using the shot extraction gap length. The low of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below.

To understand the physics of the cesium (Cs) recycling in the large Cs-seeded negative ion sources relevant to ITER and JT-60SA with ion extraction area of 45-60 cm × 110-120 cm, the time evolution of the negative ion profile was precisely measured in JT-60SA where the ion extraction area is longitudinally segmented into 5. The Cs was seeded from the oven at 180 °C to the ion source. After 1 g of Cs input, surface production of the negative ions appeared only in the central segment where a Cs nozzle was located. Up to 2 g of Cs, the negative ion profile was longitudinally expanded over full ion extraction area. The measured time evolution of the negative ion profile has the similar tendency of distribution of the Cs atoms that is calculated. From the results, it is suggested that Cs atom distribution is correlated with the formation of the negative ion profile.

To improve the performance of negative-ion based neutral beam injection on the Large Helical Device, the accelerator was modified on the basis of numerical investigations. A field limiting ring was installed on the upper side of a grounded grid (GG) support and a multi-slot GG was adopted instead of a multi-aperture GG. As a result, the voltage holding capability is improved and the heat load on the GG decreases by 40%. In addition, the arc efficiency is improved significantly only by replacing the GG.

Recent research results on negative-ion-rich plasmas in a large negative ion source have been reviewed. Spatial density and flow distributions of negative hydrogen ions (H - ) and positive hydrogen ions together with those of electrons are investigated with a 4-pin probe and a photodetachment (PD) signal of a Langmuir probe. The PD signal is converted to local H - density from signal calibration to a scanning cavity ring down PD measurement. Introduction of Cs changes the slope of plasma potential local distribution depending upon the plasma grid bias. A higher electron density H 2 plasma locally shields the bias potential and behaves like a metallic free electron gas. On the other hand, the bias and extraction electric fields penetrate in a Cs-seeded electronegative plasma even when the electron density is similar. Electrons are transported by the penetrated electric fields from the driver region along and across the filter and electron deflection magnetic fields. Plasma ions exhibited a completely di ff erent response against the penetration of electric fields.

At National Institute for Fusion Science (NIFS), a multi-ampere negative ion source is used to support the RandD on H- production, extraction, and acceleration. In this contribution, we study the characteristics of the acceleration system of this source, in order to characterize the beam optics at different operational conditions. A dedicated experimental campaign was carried out at NIFS, using as main diagnostic the infra-red imaging of the beam profiles. The experimental measurements are also compared with 3D numerical simulations, in order to validate the codes and to assess their degree of reliability. The simulations show a satisfactory agreement with the experimental results.

Based on high-density and high-temperature plasma experiments in the large helical device (LHD), conceptual design studies of the LHD-type helical DEMO reactor FFHR-d1 have been conducted by integrating wide-ranged R&D activities on core plasmas and reactor technologies through cooperative researches under the fusion engineering research project, which has been launched newly in NIFS. Current activities for the FFHR-d1 in this project are presented on design window analyses with designs on core plasma, neutronics for liquid blankets, continuous helical magnets, pellet fueling, tritium systems and plasma heating devices.

A cylindrical carbon pellet with a size of 1.2L × 1.2 mm to 1.8L × 1.8 mm and a velocity of 100 m/s to 300 m/s was injected into large helical device (LHD) for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of Δne = 1014 cm−3 is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet, e.g., 6.5 × 1016 cm−3 and 2.5 eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne = 2×1013 cm−3 to 4×1013 cm−3, and high ion temperature Ti(0) of about 6 keV is also observed with an internal transport barrier at ne = 1.2×1013 cm−3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas-puffed discharges, which typically ranges below 3 keV.

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.

High-power negative and positive ion-based neutral beam injectors (NBIs) are operated with high reliability in the Large Helical Device (LHD). The total injection power is >20 MW, and such high-pow...

A unique and reliable method of plasma initiation has been established in the Large Helical Device (LHD) by using neutral beam (NB) injection into vacuum. Since LHD is a superconducting machine, th...

A high production rate of negative hydrogen ions (H−) was observed from a nanoporous 12CaO 7Al2O3 (C12A7) electride surface immersed in hydrogen/deuterium low-pressure plasmas. The target was negatively biased at 20–130 V, and the target surface was bombarded by H3 + ions from the plasma. The production rate was compared with that from a clean molybdenum surface. Using the pseudo-exponential work-function dependence of the H− production rate, the total H− yield from the C12A7 electride surface bombarded at 80 V was evaluated to be 25% of that from a cesiated molybdenum surface with the lowest work-function. The measured H− energy spectrum indicates that the major production mechanism is desorption by sputtering. This material has potential to be used as a production surface of cesium-free negative ion sources for accelerators, heating beams in nuclear fusion, and surface modification for industrial applications.

Ion heating experiments have been intensively carried out in high- and low-Zeff conditions of Large Helical Device plasmas. In high-Zeff plasmas utilizing neon or argon gus puffing, the ion heating...