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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.

Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m2. We have clearly found the lower limit of electron current ratio depending on the arc discharge power. Certainly, the extracted electron current ratio increased in deuterium operation. However, we have had no trouble on the beam line components.Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m2. We have clearly found the lower lim...

The properties of cesium (Cs) recycling due to back-streaming ions have been investigated using an optical emission spec-troscopy in the hydrogen negative ion (H−) source with a slot-type grounded grid (GG). The slot-type GG performed well to enhance the beam performance, and to reduce the thermal loading on GG by high transparency. We clearly observed increase of Cs optical emission intensity during beam extraction owing to the increase of the Cs ions sputtered from the back plate of the source due to the back-streaming positive hydrogen ions. Increase of Cs is closely related with the extracted H− current, but it does not depend on the beam energy. Recycling Cs from the back plate is deeply relevant to the perveance condition of the H− beam, and is minimized at the optimum perveance. Strong Cs recycling from the back plate owing to the back-streaming ions is promoted in the high perveance condition with large divergence, which is consistent with the numerical calculation for the distribution of back str...

Total power of 16 MW has been successfully delivered to the plasma confined in the Large Helical Device (LHD) from three Neutral Beam Injectors (NBIs) equipped with negative hydrogen (H−) ion sources. However, the detailed mechanisms from production through extraction of H− ions are still yet to be clarified and a similar size ion source on an independent acceleration test bench called Research and development Negative Ion Source (RNIS) serves as the facility to study physics related to H− production and transport for further improvement of NBI. The production of negative-ion-rich plasma and the H− ions behavior in the beam extraction region in RNIS is being investigated by employing an integrated diagnostic system. Flow patterns of electrons, positive ions and H− ions in the extraction region are described in a two-dimensional map. The measured flow patterns indicate the existence a stagnation region, where the H− flow changes the direction at a distance about 20 mm from the plasma grid. The pattern also suggested the H- flow originated from plasma grid (PG) surface that turned back toward extraction apertures. The turning region seems formed by a layer of combined magnetic field produced by the magnetic filter field and the Electron-Deflection Magnetic (EDM) field created by magnets installed in the extraction electrode.

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.

On the Large Helical Device (LHD), improved confinement of fast ions by moving the magnetic axis location inward is theoretically predicted. This improvement is observed significantly in the fast-i...

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...

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.

An increase in the ion temperature due to transport improvement has been observed in plasmas heated with high-energy negative-ion-based neutral beam injection (NBI), in which the electrons are dominantly heated, in the Large Helical Device. When centrally focused electron-cyclotron-resonance heating is superposed on the NBI plasma, the ion temperature is observed to rise, accompanied by the formation of the electron internal transport barrier (electron-ITB). In the electron-ITB plasmas, an increase in a positive radial electric field is observed, and the transport analysis indicates that the ion transport in the half-radius region is improved with a reduction of the anomalous transport. Thus, this ion temperature rise is ascribed to the ion transport improvement by the transition to the neoclassical electron root. In high-Z plasmas, the ion temperature is increased with an increase in the ion heating power and reaches 13.5 keV. The central ion temperature increases with an increase in a gradient of the electron temperature in an outer plasma region of ρ = 0.8, suggesting the ion transport improvement in the outer plasma region induced by the neoclassical electron root. These results indicate the effectiveness of the electron-root scenario for obtaining high-ion temperature plasmas in helical systems.

A negative-ion-based neutral beam injection (NBI) system, in which large caesium-seeded negative-ion sources are utilized, has operated reliably in the Large Helical Device (LHD) since it was operational in 1998. The injection power of the hydrogen beam has been increased up to 13.1 MW with three injectors. In one injector with modified ion sources utilizing a multi-slotted grounded grid, the injection power reached 5.7 MW with an energy of 184 keV, which exceeds the designed value of 180 keV and 5 MW. Individual control of the local arc-discharge with independently divided arc and filament power supplies is effective in improving beam uniformity in a large negative ion source. The injection duration has been extended to 120 s, during which the LHD plasma is sustained by the NBI alone with a reduced power of 0.2–0.3 MW using one ion source with a cooled plasma grid. These results demonstrate that the injection performance of the negative-ion-based NBI is comparable to that of the conventional positive-ion-based NBI. The progress of the negative-NBI in the LHD is reviewed in detail from the point of view of the improvement of the negative ion sources.

Ion heating experiments have been carried out in the large helical device using ECH (82.5, 84.0, 168 GHz, ≤1 MW), ICRF (38.5 MHz, ≤2.7 MW) and NBI (H° beam: 160 keV, ≤9 MW). The central ion temperature has been observed from the Doppler broadening of Ti XXI (2.61 A) and Ar XVII (3.95 A) x-ray lines, which are measured using a newly installed crystal spectrometer with a charge-coupled device. Recently, in ECH discharges, on-axis heating became possible. As a result, a high Te(0) of 6–10 keV and a high ion temperature of 2.2 keV were obtained at ne = 0.6×1013 cm−3. A clear increment of Ti was also observed with the enhancement of the electron–ion energy flow when the ECH pulse was added to the NBI discharge. These results demonstrate the feasibility towards ECH ignition. A clear Ti increment was observed also in ICRF discharges at low density ranges of (0.4–0.6)×1013 cm−3 with appearance of a new operational range of Ti(0) = 2.8 keV > Te(0) = 1.9 keV. In low power ICRF heating (1 MW), the fraction of bulk ion heating is estimated to be 60% of the total ICRF input power, which means Pi>Pe. Higher Ti(0), up to 3.5 keV, was obtained for a combined heating of NBI ( Te(0), whereas the Ti(0) remained at relatively low values of 2 keV in H2 and He NBI discharges due to less Pi. The main reasons for the high Ti achievement in the Ne discharges are: (1) 30% increment of deposition power, (2) increase in Pi/ni (five times, Pi/niPe/ne, Pi

The five-year operation of the negative-ion-based neutral beam injector (N-NBI) system on the large helical device (LHD) is reviewed, and the prospects of negative-ion technology for applying it to future helical fusion reactors are discussed from these experiences. The N-NBI system was designed and constructed based on the results of R&D at NIFS. Using three beamlines, the total port-through injection power of 10.3 MW for 2 s has been achieved. Each beamline attained almost the same performance: ~3.5 MW of the port-through power with the beam energy of ~165 keV for hydrogen, which corresponds to the averaged negative ion current density of 25 mA cm−2 from each ion source. Recent modification of accelerators in one of the beamlines was very successful to increase beam energy up to 180 keV and port-through power, 4.4 MW. Long pulse injection heating was also pursued under the steady state LHD operation, and 110 s for 0.1 MW by one ion source, and 80 s for 0.5 MW by two ion sources (one beamline) were achieved. These facts show that the negative ion technology has been established to the same power level of conventional positive ion systems. Several technical improvements needed for obtaining these successful results are described, and the problems that limit the present performance are clarified. Some results of recent R&D to solve these problems are shown.

Characteristics of ionic plasmas, observed in a high-density hydrogen negative ion source, are investigated with a multi-diagnostics system. The ionic plasma, which consists of hydrogen positive- and negative-ions with a significantly low-density of electrons, is generated in the ion extraction region, from which the negative ions are extracted through the plasma grid. The negative ion density, i.e., the ionic plasma density, as high as the order of 1×1017m−3, is measured with cavity ring-down spectroscopy, while the electron density is lower than 1×1016m−3, which is confirmed with millimeter-wave interferometer. Reduction of the negative ion density is observed at the negative ion extraction, and at that time the electron flow into the ionic plasma region is observed to conserve the charge neutrality. Distribution of the plasma potential is measured in the extraction region in the direction normal to the plasma grid surface with a Langmuir probe, and the results suggest that the sheath is formed at the plasma boundary to the plasma grid to which the bias voltage is applied. The beam extraction should drive the negative ion transport in the ionic plasma across the sheath formed on the extraction surface. Larger reduction of the negative ions at the beam extraction is observed in a region above the extraction aperture on the plasma grid, which is confirmed with 2D image measurement of the Hα emission and cavity ring-down spectroscopy. The electron distribution is also measured near the plasma grid surface. These various properties observed in the ionic plasma are discussed.

Distributions of hydrogen Balmer-α (Hα) intensity and its reduction behavior close to a plasma grid (PG) surface have been observed by a spectrally selective imaging system in an arc discharge type negative hydrogen ion source in National Institute for Fusion Science. Hα reduction indicates a reduction of negative hydrogen ions because the mutual neutralization process between H+ and H− ions causes the dominant excitation process for Hα emission in the rich H− condition such as in ionic plasma. We observed a significant change in Hα reduction distribution due to change in the bias voltage, which is used to suppress the electron influx. Small Hα reduction in higher bias is likely because the production of negative ions is suppressed by the potential difference between the plasma and PG surface.

To investigate the flows of charged particles in front of the plasma grid (PG) in a negative hydrogen ion source, the information of the local densities of electrons and negative hydrogen ions (H-) are necessary. For this purpose, the laser photodetachment is applied for pure hydrogen plasmas and Cs-seeded plasma in a 1/3-size negative hydrogen ion source in NIFS-NBI test stand. The H- density obtained by photodetachment is calibrated by the results from cavity ring-down (CRD). The pressure dependence and PG bias dependence of the local H- density are presented and discussed. The results show that H- density increases significantly by seeding Cs into the plasma. In Cs-seeded plasma, relativity exists between the H- ion density and plasma potential.

A simultaneous high ion temperature (Ti) and high electron temperature (Te) regime was successfully extended due to an optimized heating scenario in the LHD. Such high-temperature plasmas were realized by the simultaneous formation of an electron internal transport barrier (ITB) and an ion ITB by the combination of high power NBI and ECRH. Although the ion thermal confinement was degraded in the plasma core with an increase of Te/Ti by the on-axis ECRH, it was found that the ion thermal confinement was improved at the plasma edge. The normalized ion thermal diffusivity ${{\chi}_{\text{i}}}/T_{\text{i}}^{1.5}$ at the plasma edge was reduced by 70%. The improvement of the ion thermal confinement at the edge led to an increase in Ti in the entire plasma region, even though the core transport was degraded.

The ICRF heating experiments conducted in 1999 in the third experimental campaign on LHD are reported, with an emphasis on the optimization of the heating regime. Specifically, an exhaustive study of seven different heating regimes was carried out by changing the radiofrequency relative to the magnetic field intensity, and the dependence of the heating efficiency on H minority concentration was investigated. It was found in the experiment that both ion and electron heating are attainable with the same experimental set-up by properly choosing the frequency relative to the magnetic field intensity. In the cases of both electron heating and ion heating, the power absorption efficiency depends on the minority ion concentration. An optimum minority concentration exists in the ion heating case while, in the electron heating case, the efficiency increases with concentration monotonically. A simple model calculation is introduced to provide a heuristic understanding of these experimental results. Among the heating regimes examined in this experiment, one of the ion heating regimes was finally chosen as the optimized heating regime and various high performance discharges were realized with it.

Recent large helical device experiments revealed that the transition from ion root to electron root occurred for the first time in neutral-beam-heated discharges, where no nonthermal electrons exist. The measured values of the radial electric field were found to be in qualitative agreement with those estimated by neoclassical theory. A clear reduction of ion thermal diffusivity was observed after the mode transition from ion root to electron root as predicted by neoclassical theory when the neoclassical ion loss is more dominant than the anomalous ion loss.

An ion cyclotron range of frequency (ICRF) heating experiment was conducted in the third campaign of LHD in 1999. 1.35 MW of ICRF power were injected into the plasma and 200 kJ of stored energy were obtained, which was maintained for 5 s by ICRF power only after the termination of ECH. The impurity problem was so completely overcome that the pulse length was easily extended to 68 s at a power level of 0.7 MW. The utility of a liquid stub tuner in steady state plasma heating was demonstrated in this discharge. The energy confinement time of the ICRF heated plasma has the same dependences on plasma parameters as those of the ISS95 stellarator scaling with a multiplication factor of 1.5, which is a high efficiency comparable to that of NBI. Such an improvement in performance was obtained by various means, including: (a) scanning of the magnetic field intensity and minority concentration, (b) improvement of particle orbits due to a shift of magnetic axis and (c) reduction of the number of impurity ions by means of titanium gettering and the use of carbon divertor plates. In the optimized heating regime, ion heating turned out to be the dominant heating mechanism, unlike in CHS and W7-AS. Owing to the high quality of the heating and the parameter range being extended far beyond that of previous experiments, the experiment can be regarded as the first complete demonstration of ICRF heating in stellarators.

Recent results of energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System (CHS) and Large Helical Device (LHD) are presented. Alfven eigenmodes (AEs) and fishbone-like burst modes (FBs) destabilized by energetic ions were observed in NBI heated plasmas of CHS. The AEs are toroidicity induced Alfven eigenmodes (TAEs) and global Alfven eigenmodes (GAEs), where the identified toroidal mode numbers are n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than, at most, half of the minimum TAE gap frequency and do not exhibit the obvious density dependence related to Alfven velocity. The modes have characteristic features of the energetic particle modes or the resonant TAEs excited by circulating energetic beam ions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as in FBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBs and about 25% in TAEs. In several shots, the power spectrum of the TAEs is split into multiple peaks having the same toroidal mode number through non-linear evolution of TAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2 FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal mode number. This research has been extended to LHD plasmas heated by neutral hydrogen beams with about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively low density plasmas at low toroidal magnetic field (Bt = 1.5 T).

We have achieved long-pulse plasma heating using a negative-ion-based neutral beam injector (NBI) in the large helical device (LHD), where the confinement magnetic field is generated by only external superconducting coils. In the initial long-pulse experiments at lower power than that in short-pulse experiments, 80 keV–1.1 MW NBI heating lasted for 10 s with a little increase in the plasma density at the pulse end. Almost steady-state plasma heating was achieved for 21 s with 66 keV–0.6 MW NB injection. Plasma relaxation oscillation phenomena at a period of 1–2 s were also observed for 20 s. Above 1 keV plasma was easily sustained with a long-pulse NBI heating in LHD, without the current drive nor the disruption in tokamaks. Negative ion source operation was stable and the cooling water temperature rise of beam accelerator grids was nearly saturated with a temperature rise below 10 °C. For a higher power injection, the pulse duration is determined by the beam blocking, where the reionization loss is expon...

The influence of filament positions on the arc discharge characteristics of a negative hydrogen ion source for a neutral beam injector were studied by simulations and experiments. We have made a simulation code named “PRIMELOC2,” which calculates the orbits of primary electrons emitted from cathode filaments, including collisions with H2 gas molecules filled in the source, to obtain the spatial distribution of ionization points. We applied this code to a large negative hydrogen ion source having an external magnetic filter for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). As a result of the calculations, we have found that the moving area of emitted primary electrons differs markedly according to the filament positions. Calculation results agreed well with the experimental results of high-power arc discharge and the spatial distribution of plasma parameters obtained by Langmuir probe measurements. By applying PRIMELOC2 to filament–arc ion sources, we can choose appropriate areas in a...

High power negative-ion-based neutral beam injectors for the LHD were constructed and started in operation in September 1998. Two beam lines with a pair of ion sources are installed tangentially in opposing direction. The maximum energy of the beam is 180 keV for hydrogen, and the total injection power is designed to be 15 MW with the pulse length of 10 seconds. One of the specific features of the negative-ion-based NBI system is the requirement on the vacuum pressure in order to avoid the stripping loss of negative ions as well as to realize the optimum gas line density for neutralization, which results in the long beam line. The system was then designed carefully by considering the focal length of the ion sources, neutralizing efficiency, and geometrical and re-ionizing loss of the beam. It is also considered the error magnetic field produced by the ferromagnetic materials that are indispensable for NBI for the magnetic shield over the long neutralizing region.

A negative-ion-based neutral beam injection (NBI) system is planned for plasma heating of the Large Helical Device (LHD). We have developed a negative ion source, which is 1/3 the scale of the source for the NBI. A magnetic filter held was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm/spl times/62 cm) for efficient H/sup -/ production. We investigated the magnetic field configuration and found a low electron temperature high density plasma ( 15 A: 1/3 current of LHD ion source). Based on the results, we are designing a negative ion source for the LHD.

A new experimental technique to investigate the particle confinement of energetic ions during their slowing-down process is developed for charge exchange neutral particle diagnostics. The method is based on the maximum entropy and the maximum likelihood method and is used for the neutral beam short pulse experiment. The method assumes that the energy loss of the fast ions is given by the classical deceleration rate at their birth orbit. It will not require the bulk neutral distributions and the energetic ion distribution in plasmas. The method provides the spatial distribution of confinement time of energetic particles on their orbit during the slowing-down processes. The method of the analysis is described and the application to the experimental data is shown in this article.

Characteristics of negative-hydrogen ion (H−) density in the vicinity of plasma grid (PG) which is a boundary electrode between plasma and beam were experimentally investigated in cesium-seeded H− source. The H− density was measured with Cavity Ring Down method (CRD). Our CRD system has been upgraded from fixed line measurement to movable one which provides a profile measurement of the H− density. The H− density above the PG aperture is lower than that above the PG metal surface, and this density structure become to disappear in further region from the PG surface. The H− density decreases with positive bias voltage where an arc discharge chamber is higher potential than the PG. On the other hand, the H− density does not largely change with negative bias voltage. Reduction of the H− density was observed when a beam extraction voltage is applied. The reduction occurs in the case of lower bias voltage close to plasma potential. The extraction voltage influences H− density to a greater degree than bias voltage in low bias voltage region.

The first results of saturation-currents polar distribution, which is measured with a directional Langmuir probe, at the beam extraction region of our caesium (Cs) seeded negative ion source have been reported. The line from maximum to minimum of the distribution tilts by 40° to the normal direction of the plasma grid (PG) surface. The maximum intensity is one order of magnitude larger than the minimum one. Depth distribution of the saturation currents is also measured along the axis of the PG aperture and the middle line between a pair of the axes of nearest neighbor PG apertures. Ionic plasma with quite low electron density is generated within a distance of 10 mm from PG. With a bias voltage lower than plasma potential, the difference between negative to positive saturation currents, which corresponds to electron current, increases rapidly beyond the boundary of ionic plasma. The electron current decreases by a factor of 6 with increasing the bias voltage from 2.8 to 6.4 V.

Intense H− beams have been extracted from a large multicusp plasma source operated with cesium seeding. The H− beams were accelerated up to 100 keV by a single‐stage or a two‐stage electrode system. Spatial profiles of the beams are measured calorimetrically and the beam divergence angle is obtained from half of the e‐folding width. A minimum beam divergence angle of 5 mrads is achieved at a H− current density of 30 mA/cm2 with a beam energy of 100 keV. The ratio of acceleration current to H− current increases abruptly when a H− current saturates in the space charge limited region. This enhancement is mainly due to secondary electrons caused by the intersection of H− beams with an extraction grid. When the operating gas pressure decreases, the ratio of the acceleration current to the H− current decreases. This is related to a stripping loss of H− ions in the electrodes. A beam divergence angle reaches a minimum when a ratio of Vacc to Vext is set at an optimum value of 1.6 in the single‐stage acceleration...

Ion internal transport barrier (ITB) was formed in both hydrogen discharge and helium dominated discharge. The central ion temperature was investigated as a function of the hydrogen density ratio (nH/(nH+nHe)). The central ion temperature increases with the decrease of the hydrogen density ratio, while the electron temperature does not change significantly. The experimentally observed ion temperature was not reproduced by the prediction of TASK-3D modeling. The dominant activity in the density fluctuation measured by the phase contrast imaging is consistent with ion temperature gradient mode and they are almost identical between the hydrogen dominated and helium dominated plasmas.

An intense negative hydrogen ion source has been developed, which has a strong external magnetic filter field in the wide area of 35 cm×62 cm produced by a pair of permanent magnet rows located at 35.4 cm separation. The filter strength is 70 G in the center and the line‐integrated filter strength is 850 G cm, which keeps the low electron temperature in the extraction region. Strong cusp magnetic field, 1.8 kG on the chamber surface, is generated for improvement of the plasma confinement. These resulted in the high arc efficiency at the low operational gas pressure. 16.2 A of H− ion current with the energy of 47 keV was obtained at the arc efficiency of 0.1 A/kW at the gas pressure of 3.8 mTorr in the cesium‐mode operation. The magnetic field in the extraction gap is also strong, 450 G, for the electron suppression. The ratio of the extraction current to the negative ion current was less than 2.2 at the gas pressure of 3 mTorr. The two‐stage acceleration was tested, and 13.6 A of H− ion beam was accelerat...

A large negative hydrogen ion source with a rod-type magnetic filter was investigated for the construction of a neutral beam injection (NBI) system for the large helical device (LHD). A 1 3 - scale device was constructed and investigated. Three main phenomena are observed on seeding a small amount of cesium into the ion source. These are enhancement of the hydrogen negative ion (H−) current, a reduction in the electron beam extracted with the H− beam, and a decrease in the operating gas pressure. The enhancement factor of the H− current is 3–4, and a total H− current of 8.5 A is obtained from 20 × 40 extraction apertures 9 mm in diameter. This current corresponds to a current density of 17 mA cm−2. A multi-wall structure forming a lattice, called an egg-box cell, is installed between the magnetic filter rod and the plasma grid. The electron current with this cell is half as low as the electron current without the cell, but the H− current is hardly affected. This phenomenon is observed when cesium is seeded in the source chamber. The electron temperature and its density increase by a factor of 2 on strengthening the cusp field. Experiments to obtain a higher H− current density were carried out using the improved are plasma. A hydrogen negative ion current of 54 mA cm−2 was extracted from a restricted aperture of 36 extraction holes. The H− current was extracted from 20 × 40 full apertures of the ion source with strong cusp field. A total H− current of 10.4 A was obtained from these apertures. The optimum filling pressure is less than 1.0 Pa.

A design for a power supply system for the neutral beam injection (NBI) test stand of the large helical device (LHD) is presented. The acceleration power supply of −250 kV, 50 A is composed of three power supplies connected in series to deliver two intermediate outputs. It has a −40 kV power supply, a −90 kV power supply and a −120 kV power supply. A beamline and the major part of the power supply system for the NBI test stand were constructed in 1992. The −120 kV power supply was moved from Higashiyama to Toki in 1994. Another power supply of −40 kV will be added for −250 kV acceleration in the future. A brief outline of the control system of the NBI test stand is also presented.

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.

The production efficiency of negative ions at tungsten surface by particle reflection has been investigated. Angular distributions and energy spectra of reflected hydrogen ions from tungsten surface are calculated with a Monte Carlo simulation code ACAT. The results obtained with ACAT have indicated that angular distributions of reflected hydrogen ions show narrow distributions for low‐energy incidence such as 50 eV, and energy spectra of reflected ions show sharp peaks around 90% of incident energy. These narrow angular distributions and sharp peaks are favorable for the efficient extraction of negative ions from an ion source equipped with tungsten surface as negative ionization converter. The retained hydrogen atoms in tungsten lead to the reduction in extraction efficiency due to boarded angular distributions.

A systematic study of the extraction probability of surface‐produced H− ions from the ion source has been done with H− and neutral Monte‐Carlo transport modeling. Without the effects of the flow velocity of background H+ ions and neutrals, the extraction probability (E.P.) depends strongly on the strength of the magnetic filter field BMF for the low gas pressure condition. On the other hand, the E.P. does not depend so much on BMF for the high gas pressure condition. Neutral transport modeling shows that H2 neutral possibly has a relatively high velocity (∼1 km/s). The effect on the E.P., however, is not significant even for the high gas pressure regime.

The potential distribution near the extraction region in negative ion sources for the plasma with the surface produced negative ions is studied analytically. The potential is derived analytically by using a plasma‐sheath equation, where negative ions produced on the Plasma Grid (PG) surface are considered in addition to positive ions and electrons. A negative potential peak is formed in the sheath region near the PG surface for the case of strong surface production of negative ions or for low energy negative ions. Negative ions are reflected by the negative potential peak near the PG and returned to the PG surface. This reflection mechanism by the negative potential peak possibly becomes a factor in negative ion extraction. It is also indicated that the potential difference between the plasma region and the wall decreases by the surface produced negative ions. This also has the possibility to contribute to the negative ion extraction.

A millimeter-wave interferometer with the frequency of 39 GHz ({lambda} 7.7 mm) was newly installed to a large-scaled negative ion source. The measurable line-integrated electron density (n{sub e}l) is from 2x10{sup 16} to 7x10{sup 18} m{sup -2}, where n{sub e} and l represent an electron density and the plasma length along the millimeter-wave path, respectively. Our interest in this study is behavior of negative ions and reduction of electron density in the beam extraction region near the plasma grid. The first results show the possibility of the electron density measurement by the millimeter-wave interferometer in this region. The line-averaged electron density increases proportional to the arc power under the condition without cesium seeding. The significant decrease of the electron density and significant increase of the negative ion density were observed just after the cesium seeding. The electron density measured with the interferometer agrees well with that observed with a Langmuir probe. The very high negative ion ratio of n{sub H-}/(n{sub e}+n{sub H-}) = 0.85 was achieved within 400 min. after the cesium seeding.

This paper describes the development of the neutral beam (NB) systems on JT‐60SA, where 30–34 MW D0 beams are required to be injected for 100 s. A 30 s operation of the NB injectors suggests that existing beamline components and positive ion sources on JT‐60U can be reused without the modifications on JT‐60 SA. The JT‐60 negative ion source was modified to improve the voltage holding capability, which leads to a successful acceleration of 2.8 A H− ion beam up to 500 keV of the rated acceleration energy for JT‐60SA.

The beam source of neutral beam heating/current drive system for ITER is needed to accelerate the negative ion beam of 40A with D- at 1MeV for 3600 sec. In order to realize the beam source, design and R&D works are being developed in many institutions under the coordination of ITER organization. The development of the key issues of the ion source including source plasma uniformity, suppression of co-extracted electron in D beam operation and also after the long beam duration time of over a few 100 sec, is progressed mainly in IPP with the facilities of BATMAN, MANITU and RADI. In the near future, ELISE, that will be tested the half size of the ITER ion source, will start the operation in 2011, and then SPIDER, which demonstrates negative ion production and extraction with the same size and same structure as the ITER ion source, will start the operation in 2014 as part of the NBTF. The development of the accelerator is progressed mainly in JAEA with the MeV test facility, and also the computer simulation of beam optics also developed in JAEA, CEA and RFX. The full ITER heating and current drive beam performance will be demonstrated in MITICA, which will start operation in 2016 as part of the NBTF.

The Indian program of the RF based negative ion source has started off with the commissioning of ROBIN, the inductively coupled RF based negative ion source facility under establishment at Institute for Plasma research (IPR), India. The facility is being developed under a technology transfer agreement with IPP Garching. It consists of a single RF driver based beam source (BATMAN replica) coupled to a 100 kW, 1 MHz RF generator with a self excited oscillator, through a matching network, for plasma production and ion extraction and acceleration. The delivery of the RF generator and the RF plasma source without the accelerator, has enabled initiation of plasma production experiments. The recent experimental campaign has established the matching circuit parameters that result in plasma production with density in the range of 0.5–1×1018/m3, at operational gas pressures ranging between 0.4–1 Pa. Various configurations of the matching network have been experimented upon to obtain a stable operation of the set up...

Recent progress in the development of a gas‐filled RF quadrupole ion cooler for cooling negative ions is reported. Experiments demonstrate that negative ion beams can be cooled to 2 eV FWHM energy spread with more than 50% transmission through the cooler. The RFQ cooler can potentially improve the purity of radioactive ion beams by magnetic mass separation. New developments on purifying negative ion beams by photodetachment in the RFQ cooler are presented. With a laser of proper photon energy, nearly 100% suppression of the unwanted negative ions in the RFQ cooler has been observed, while the desired ions remain mostly intact. A recent experimental study demonstrates that pure ground state negative ion beams can be obtained by state‐selective photodetachment in the RFQ cooler.

Optical emission spectroscopy (OES) and cavity ring‐down spectroscopy (CRDS) systems are installed in a 1/3‐scaled negative hydrogen‐ion source at the National Institute for Fusion Science testbed to investigate the dynamics of H− ions in the extraction region near the plasma grid. The signal form of the H− ion density rapidly drops after beam extraction on applying a low‐bias voltage. A similar signal drop appears in the intensity of the hydrogen Balmer‐line emission measured by OES and is caused by decreasing atomic hydrogen produced by mutual neutralization effects between H− and H+. Shot trend of the beam currents are similar to the H− density and Hα/Hβ in the extraction region, which increases twice as large immediately after Cs seeding. We observe a linear correlation between the H− density and the inclination of Hα/Hβ which allows for experimentally benchmarking the OES measurement with that of CRDS. Thus, this approach is used for estimating the H− density by OES in negative‐ion sources for high‐e...

The effect of the cesium on hydrogen negative ions is discussed using the vacuum ultraviolet emissions (VUV) from vibrational states of hydrogen molecules. The VUV spectrum from 90 to 165 nm is related to the H− production from the dissociative attachment of electrons to hydrogen molecules. The VUV spectra in hydrogen plasmas are measured in the extraction region of a negative ion source for neutral beam injector. Under the pure hydrogen discharge, the cesium vapor is introduced into the ion source to enhance the hydrogen negative ion density. When the same arc power of ∼99 kW is applied in both with and without cesium admixture cases, the ratio of the observed VUV spectrum without to with cesium is not changed clearly. Therefore the production process of H− related to the wall/electrode surface would be enhanced rather than the volume production process.

Unified description of the expansion (through a magnetic filter) and extraction regions, including also the driver, of volume‐production based sources of negative hydrogen ions is presented in the study. The model is one‐dimensional, developed within the fluid plasma theory. It covers description of both the second chamber of the tandem sources and a single inductively‐driven (with a planar coil) discharge of a matrix source. Four parameters have been varied in the analysis of the results: type, position and magnitude of the filter field and rf power applied for the discharge maintenance. The obtained results for the spatial distribution of the plasma parameters as well as for the electronegativity, for the negative ion beam current and its ratio to the electron current at the position of the extraction are discussed regarding optimum conditions for the source operation. The conclusion is that a magnetic field extended over a large region till the extraction favors locality in the discharge behavior and, respectively, the local production of the negative ions. This leads to high density of the extracted negative ion beam current at reasonable values of its ratio to the electron current density.

The enhancement of extracted negative ion current in cesiated sources is usually explained by the surface production of negative ions. In this contribution, the self‐consistent production and transport of H− in the extraction region of a radio‐frequency driven negative ion source is modelled by means of a parallel two‐dimensional Particle‐in‐Cell/Monte Carlo Collision simulation. It is shown that the number of surface‐produced negative ions extracted is regulated by a potential well developed in front of the plasma grid such that the extracted current does not proportionally increase with the flux of negative ions emitted at the surface.

This paper describes the accomplishments of the SNS H− ion source, which delivers routinely ∼50 mA at a 5.4% duty factor with ∼99% availability, enabling 1 MW beams for neutron production with ∼90% availability. It discusses the need for increasing reliability and beam current. But mostly it focuses on its unexpected feature: H− beams that are apparently persistent for up to 5 weeks without adding Cs after an initial dose of less than ∼5 mg. Thermal emission and sputtering are qualitatively evaluated, and appear consistent with a negligible Cs sputter rate after the initial dose disappears from the Cs plasma. It concludes with a list of future experiments that can shed more light on this apparently unique Cs cycle.

Voltage holding of -1 MV is an essential issue in development of a multi-aperture multi-grid (MAMuG) negative ion accelerator, of which target is to accelerate 200 A/m{sup 2} H{sup -} ion beam up to the energy of 1 MeV for several tens seconds. Review of voltage holding results ever obtained with various geometries of the accelerators showed that the voltage holding capability was about a half of designed value based on the experiment obtained from ideal small electrode. This is considered due to local electric field concentration in the accelerators, such as edge and steps between multi-aperture grids and its support structures. Based on the detailed investigation with electric field analysis, accelerator was modified to reduce the electric field concentration by reshaping the support structures and expanding the gap length between the grid supports. After the modifications, the accelerator succeeded in sustaining -1 MV for more than one hour in vacuum. Improvement of the voltage holding characteristics progressed the energy and current accelerated by the MeV accelerator. Up to 2010, beam parameters achieved by the MAMuG accelerator were increased to 879 keV, 0.36 A (157 A/m{sup 2}) at perveance matched condition and 937 keV, 0.33 A (144 A/m{sup 2}) slightlymore » under perveance.« less

The optimization of H− extraction from the source is one of the most important issues for developing negative ion sources. The following effects on the total extracted H− current IH− are systematically studied with the 2D3V PIC (Particle in Cell) modeling in a H− volume production source: 1) width of the extraction slit and 2) extraction grid voltage. The PIC modeling results make clear the dependence of IH− on these important parameters. The results also give useful physics understanding about the processes leading to the extraction of H− ions from the source and its optimization.

Due to the presence of a large transverse magnetic field (Bx and By where z is the extraction axis), the extraction of electrons from a negative ion source is likely to happen with a large angle with respect to z axis. The negative ion and electron sheaths are here studied both with kinetic and with fluid models. First, Vlasov‐Poisson models are reduced to one dimensional integrodifferential equations, discussing also trapped orbits. The integrodifferential equations for electron transport are analytically solved for a variety of extraction potentials (in 1D). Collision frequency dependency from electron flow speed and temperature is discussed. Then both ion and electron space charge and fluid motion are solved, using electron densities expression consistent with kinetic model. Results for the sheath charge profile and extraction field as a function of Bx are shown.

According to generally accepted models surface production of negative ions depends on ion escape velocity and work function of the surface. We have conducted an experimental study addressing the role of the ion escape velocity on H− production. A converter‐type ion source at Los Alamos Neutron Science Center was employed for the experiment. The ion escape velocity was affected by varying the bias voltage of the converter electrode. It was observed that due to enhanced stripping of H− no direct gain of extracted beam current can be achieved by increasing the converter voltage. The conversion efficiency of H− was observed to vary with converter voltage and follow the existing theories in qualitative manner. We present calculations predicting relative H− yields from different cesiated surfaces with comparison to experimental observations from different types of H− ion sources. Utilizing materials exhibiting negative electron affinity and exposed to UV‐light is considered for Cesium‐free H−/D− production.