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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.

A powerful neutral beam system with six high current H− ion sources achieved a total power of 10.3 MW in the large helical device (LHD). We describe the present status of the ion sources, and the results of studies of cesium consumption, tungsten filament lifetime, and the velocity spectrum of the beam. Maximum power and beam energy in those ion sources range from 3.5 to 4.4 MW and 165 to 180 keV (design energy), respectively. The maximum H− current is ∼25 A. Much progress on beam acceleration and plasma uniformity was facilitated by a new accelerator and new arc power supply system with careful Cs seeding. Cs consumption for 1/2 year of operation ranges from 3.9 to 10.6 g per one source (over 2×104 shots). It is converted to the rate of 0.17–1.5 mg per one shot per source. It is considered that Cs deposition on the inside surface of the plasma source could be from 0.11 to 1.0 monolayer per one shot per source. This is of the same order as the expected optimum coverage of ∼0.7. Filament weight loss was me...

In this article we describe the first results of beam injection using an improved negative ion source, which consists of a set of steering and multislot grounded grids. The set of the grids is designed to achieve high power beam injection for the large helical device (LHD). The main purpose in replacing the conventional multiaperture with multislot grids is to reduce the heat load onto the grid. By adopting the multislot grid, the frequency of voltage breakdown at the acceleration gap decreased considerably and the maximum beam energy attained was 180 keV, which is one of the target values of the LHD neutral beam injector (NBI). The maximum injection power reached 4.4 MW, consequently. The heat load onto the multislot grid was water calorimetrically measured and the load is suppressed by 50% compared to the multiaperture case. In the condition for maximum injection power, the beam profile obtained in the accelerator with the multislot grounded grid becomes vertically elongated due to the asymmetric electric field close to the grid slot. Although some part of this elongated beam is lost at the injection port, the multislot grid has enough performance for beam injection.

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.

The Large Helical Device-Neutral Beam Injector (LHD-NBI) system uses 40 A×180 keV (10 s) negative hydrogen ion sources. Three studies of the H− ion source are reported, i.e., conditioned status, co-accelerated electron beam component, and weight loss of tungsten filaments. Total injection numbers through experimental cycles sum up to ∼8000 shots with four ion sources on two beam lines. Injection power reached a maximum of 5.2 MW. The maximum energy achieved was 171 keV. A pulse length up to 80 s was reached with 0.5 MW. Electron fraction in the accelerated H− beam was evaluated by measuring the heat load on the electron beam dump. Total fraction of the power in the electron beam component deposited on the dump was evaluated to be 1.5%–6% with up to ∼1 Pa of H2. Stripped electrons which were produced inside acceleration gap comprised the majority compared to electrons extracted from source plasma. The weight loss of tungsten filament was 0.01–0.103 mgr/shot/filament. The total loss was ∼0.5% after ∼6500 sh...

The Large Helical Device (LHD) is the only machine that relies upon negative ion-based neutral beam injection system (NBI) as the principal heating power source. Since the first injection in 1998, NBI has been used to heat over 3000 shots for 2 years. Performance has progressed step by step to reach an injection power of 4.5 MW, with beam energy up to 165 keV, and pulse lengths up to 80 s at 0.5 MW. System operating level has reached ∼30% of the design power and ∼65% of the H− design current. This tangential high-energy NBI system based on negative ions has proved to be as efficient a heating system in a helical device as in a tokamak. The ratio of injection neutral beam power to high voltage drain power (injection power efficiency) was typically about 0.3. This is a low efficiency compared to the design one. For long pulse injection beam blocking (such as observed on PLT) was encountered, but it did not limit the operation. In order to achieve the design output of 15 MW, a great R&D effort is underway on the negative ion test stand to improve the performance of 40 A 180 keV negative hydrogen ion source.

The neutral beam (NB) shine-through profile is routinely monitored on the Large Helical Device (LHD) both to calibrate the port-through power of the NB and to evaluate the NB-deposition power to LHD plasmas. The profile is measured with a calorimeter (CM) array on an armor plate of the NB counter wall inside the LHD vacuum vessel. An infrared camera is also used to check the beam profile where CMs are not located, and measures a temperature increase of the armor plate due to the NB heat load. The measured beam profile is compared to the calculated NB profile at the armor plate. The measurement indicates that the beam is not uniform at the exit of the ion source and that the steering angle of the beam in the horizontal direction is not the same as the designed value. It is found that the monitoring of the NB shine-through profile is important to estimate the NB port-through power and the NB deposition power, especially when the neutral beam injector (NBI) is based on a large negative-hydrogen ion source.

A compact cesium deposition system was used for direct deposition of cesium atoms and ions onto the inner surface of the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection (LHD-NBI), system. A small, well defined amount of cesium deposition in the range of 3–200 mg was tested. Negative ion extraction and acceleration were carried out both in the pure hydrogen operation mode and in the cesium mode. Single Cs deposition of 3–30 mg to the plasma chamber has produced temporary 2–5 times increases of H− yield, but the yield was decreased within several discharge pulses to the previous steady-state value. Two consecutive 30 mg depositions done within a 3–5 h/60 shot interval, produced a similar temporary increase of H− beam, but reached a large H− yield steady-state value. Deposition of larger 0.1–0.2 g Cs portions with a 20–120 h/150–270 shot interval improved the H− yield for a long (2–5 days) period of operation. Directed depositions of Cs to the various walls of the p...

Accelerated electrons, which would lead to high thermal load of grids, have been suppressed in a high-current large hydrogen negative ion source. An extraction grid, with apertures shaped as the secondary electrons generated on the grid aperture surface would be shielded against the acceleration electric field, works well to prevent the secondary electrons from leaking to the acceleration gap, compared with a straight aperture extraction grid. Although the strong magnetic field at the extraction grid also lowers the electron leakage downstream, the aperture shaping of the extraction grid is more effective for the suppression of the accelerated electrons. The acceleration efficiency, defined by the ratio of the negative-ion current to the acceleration drain current, is improved to around 85%. There remains the accelerated electrons generated in the negative ion neutralization by collision with the residual neutral molecules during the acceleration. The direct interception of the accelerated negative ions w...

A high-power large negative ion source has been operated for a long pulse duration. A three-grid single-stage accelerator is used, where the extraction grid is shaped so that the secondary electrons generated on the extraction grid would be prevented from leaking into the acceleration gap. A stable long-pulse arc discharge with an arc power of 100 kW has been obtained over 15 s by balancing an individual arc current flowing through each filament. The cesium-seeded operation is not influenced by a temperature rise over 100 °C of the plasma grid during the long-pulse arc discharge. As a result, 330 kW (91 keV–3.6 A) of the negative ion beam was produced stably for 10 s from an area of 25 cm×26 cm, where the current density was 21 mA/cm2 and the negative ion power density was 1.9 kW/cm2. The neutralization efficiency of accelerated negative ions has been measured including the residual positive and negative ion ratios by the water calorimetry of the beam dumps. The result agrees well with the calculation result.

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.

Multipower supplies for the arc discharge are installed to improve beam uniformity of a large size of the neutral beam injection system in the large helical device (LHD). The plasma tends to shift on the upper side of the arc chamber when the same arc voltage is applied to all filaments. The arc discharge current distribution becomes uniform as a consequence of the individual voltage control, but about 20 V difference of the arc voltage between the top and bottom circuits is required. We have estimated the beam profile from each acceleration grid by comparing the calculation and measurement of the beam profile of Halpha and calorimeter. The uniformity of arc discharge has been improved in most regions without the discharge adjustment by a horizontal filament connection. The controllability of the arc discharge distribution of the horizontal filament connection is better than that of the vertical filament connection. The efficiency of the beam production is improved by about 10% in the case of the horizontal filament connection. As a result 3.7 MW of the beam injection has been achieved in the LHD 6th experimental campaign in 2003.

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...

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 cesiated hydrogen hollow cathode (CHC) was tested for plasma injection in the multicusp negative ion source (MS). The CHC arc with hydrogen feed and cesium seeding through the CHC volume was explored. One cathode unit (40 mm length, 19 mm in diameter, emission opening area 1–3 mm2) with no special cooling provided the MS discharge operation with direct current up to 30 A, and up to 60 A in the long-pulse mode. High efficiency of negative ion production in the MS discharge, driven by a CHC plasma injection was recorded.

For compensation of the magnetically deflected beamlets, a new type of grid system has been applied to a large negative ion source of a negative-ion-based neutral beam injector in the Large Helical Device (LHD) fusion experimental machine. Corresponding to deviation of the negative ion beamlet trajectory due to the electron suppression magnetic field at the extraction grid, the exit aperture of the extraction grid is displaced to cancel the beamlet deflection by electrostatic lens effect. As a result, the beamlet deflection was suppressed less than 2 mrad in a wide range of energy of 110–180 keV, and the port-through efficiency of the injected beam was improved to 90%–95% from 70%–75% in the case of the previous compensation by the grounded grid aperture displacement. The injected beam power was increased to 3.5 MW with an energy of 165 keV using one injector. To improve beam uniformity of the large area beam, individual arc currents through filaments are controlled for uniform plasma production, with ind...

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.

We describe the characteristic of stable beam injection in a neutral beam injector (NBI) for Large Helical Device (LHD) in high injection power of more than 6 MW. In the NBI, it takes a week after starting Cs seeding to finish the pre‐injection conditioning. The injection starts with the beam power ∼6.2 MW, and the maximum power reaches ∼7 MW. The Cs‐seeding rate affects the beam stability in such high power injections. By optimizing the rate to 0.65 mg/shot, the success ratio, which is defined as a ratio of actual pulse duration to setting one, increases to 85–90% in the power and energy range of more than 6.2 MW and 185 keV, respectively. The weights of Cs adsorbed on several surfaces in the ion sources of the NBI are measured by means of Inductively‐Coupled Plasma Atomic Emission Spectroscopy (ICP‐AES), and averaged surface densities are calculated by dividing with the several surface areas. The seeded Cs of 99.5% is condensed in the plasma generator, and very tiny amount of Cs reaches the surfaces of the accelerator grids. This very low amount of Cs on the grids is interpreted that most of the Cs atom evaporated from the inner walls is ionized during the arc discharges, and repelled to the source plasmas by the electrostatic field for H‐ extraction.

An additional beamline, BL5, equipped with four positive ion sources will be installed on Large Helical Device (LHD) in 2010. The performance of an ion source which generates 80 keV deuterium and 60 keV hydrogen beams was investigated. The structure of the ion source is based on that of a BL4 ion source on LHD. The main differences between the ion sources for the BL4 and BL5 are the acceleration voltages and the materials of plasma electrodes: copper and molybdenum, respectively. The molybdenum plasma electrode for BL5 has better performance than the copper plasma electrode of BL4. The integrated performance of the ion source for BL5 reached a value equivalent to approximately 58 A in the beam current of hydrogen positive ion at 60 keV in the beam energy.

A new multi-cusp H− ion source using a localized virtual magnetic filter of type I in the plasma electrode is investigated. A multi-pole arrangement with a spacing of 10 mm of the magnet bars holds an extraction hole, optimizing the efficient production of high H− current, and at the same time only a small electron component was co-extracted with the H− ions. The local filter arrangement separates the beam electrons at a low energy. It is shown that the coextracted total electron current is determined principally by the integrated magnetic field flux (Gcm) of the local filter with an extraction system at a constant extraction voltage. When the value of the Gcm is increased, the total electron component is reduced, while the H− electrical efficiency had a broad maximum around the optimized value of the Gcm. A thicker plasma electrode should be necessary for sufficient reduction of electron current. In pure hydrogen operation, the achieved current density of H− is 10 mA/cm2. When Cs is seeded in a filter op...

Cesium recovery from the polluted layers in the 1/3 scale hydrogen negative ion source for LHD-NBI system has tested. It was found that the cesium recovery can be produced by additional discharges as from the cesium layer, aged by tungsten and residual gas, so as from the cesium layers, polluted by an occasional water leak. The highest cesium recovery to negative ion production was produced by a xenon arc, while glow discharge and arcing in hydrogen were less effective. The mechanism of recovery is the ejection of cesium from the underlying enriched layer by the arc and its transport to the surface.

Negative‐ion‐based neutral beam injection (negative‐NBI) system has been operated for 10 years in Large Helical Device (LHD). The injection power has been increased year by year, according to the improvement of the negative ion sources. Up to now, every injector achieves the designed injection energy and power of 180 keV‐5 MW with hydrogen beams, and the total injection power exceeds 16 MW with three injectors. In the multi‐round aperture grounded grid (GG), the diameter of a round aperture has been enlarged for higher GG transparency. Then, the GG heat load is reduced, as well as in the multi‐slotted GG, and the voltage holding ability in the beam acceleration was improved. As a result, the beam energy is raised and the injection power is increased. To improve the anisotropic property of the beamlet convergence condition between the perpendicular and the parallel directions to the slots in the multi‐slotted GG, a round‐shape aperture of the steering grid (SG) has been changed to a racetrack shape. As a r...

A multi‐antenna radio‐frequency ion source with a Faraday shield is newly tested at a high RF power level in a large area negative ion source of 1/5th scale of the Large Helical Device‐NNBI ion source. Inductively coupled dense hydrogen plasmas were generated uniformly over an area of 25×25 cm2 at an RF input power up to 300 kW for a 10 ms pulse duration. A large negative plasma potential for the non‐Faraday shielded antenna was remarkably reduced by introducing a Faraday shield. The positive ion saturation current density measured by Langmuir probe reached 148 mA/cm2 at 174 kW around the center of the plasma. The optimal hydrogen filling pressure ranged around 0.13 Pa‐ 0.4 Pa for the positive ions. Ion beam extraction with a single hole (φ 0.5 cm) extractor has been studied systematically. A maximum H− ion beam current density of 1.6 mA/cm2 was obtained preliminarily. It was confirmed that the plasma profile was controllable by both the number and configuration of the antennas.

In the large area negative ion source for the LHD negative-ion-(H(-))-based neutral beam system, (I) we used the spectrometer to measure caesium lines in the source plasma during beam shots. (II) With Doppler-shifted measurements, the H(alpha) line at three different locations along the beam as well as the spectrum profile for cases of different plasma grid areas. (III) Caesium deposition monitor with a high speed shutter was tested to measure the weight of the deposited Cs layer. In the observation, cleaner spectra of Doppler-shifted H(alpha) line with only a small level of background light were obtained at a new observation port which viewed the blueshifted light in the drift region after the accelerator of a LHD ion source. Both the amounts of Cs I (852 nm, neutral Cs(0)) and Cs II (522 nm, Cs(+)) in the source plasma light rose sharply when beam acceleration began, and continued rising during a 10 s pulse. It was thought that this was because the cesium was evaporated/sputtered from the source back plate by the back-streaming positive ions. Cs deposition rate to the crystal sensor measured by adjusting the shutter open time was evaluated to be 2.9 nanograms/s cm(2) for preliminary testing. More neutral Cs tended to be evolved in the source after arc discharge. Much Cs could be consumed in a high rate-pulsed operation (such as LHD source).

Large‐scaled hydrogen negative‐ion sources, in which cesium is introduced in the source plasma, have been developed for neutral beam injectors in Large Helical Device, and their operational characteristics are reviewed. For high‐efficient negative ion production, configuration of the magnetic filter field and the cusp magnetic field was optimized, resulting in a high arc efficiency for the negative ion production of 0.23A/kW. With use of a multi‐slotted grounded grid, the gas pressure in the acceleration gap is lowered, leading to reduction of the heat load of the grounded grid. As a result, the voltage holding ability is much improved, and the rated energy of 180 keV is achieved in a short conditioning period of 4 days. The injection power is increased linearly to the 5/2 power of the beam energy and reached 5.7MW with an energy of 184keV, which exceeds the specified value of 180keV–5MW. Beam uniformity has been improved with an individual control of the local arc discharge by adjusting 12‐divided output...

Two large helical device–neutral beam injector (LHD–NBI) ion sources were fabricated and tested in the test stand for producing a beam of 180 keV×40 A with H− ions. They are Cesiated multicusp ion sources with a rectangular discharge chamber and a single stage multihole accelerator. These are scaled up from the 16 A H− ion sources in the National Institute for Fusion Science (NIFS). A plasma source with a high aspect ratio was operated stably with an arc power up to ∼300 kW for 10 s, after balancing of the electron emission from the filaments was made. A satisfactorily dense and uniform plasma without mode flip was produced. Electrons accompanied by H− ions were reduced by an extraction grid with the electron trap, instead of straight holes. The electron beam component caused by the stripping of electrons from H− ions was detected with an array of calorimeters at the bottom of the connecting duct. At the first stage of the test, one of the five segment grids of the accelerator was installed. An H− ion cur...

A large hydrogen negative-ion source was constructed as a prototype one for the negative-ion-based neutral beam injection (NBI) system in large helical device. The ion source is designed to produce 180 keV-40 A of the negative ion beam with a current density of 40 mA/cm2, and is a cesium-seeded volume production source characterized by the external magnetic filter. The arc chamber of multicusp bucket is rectangular of 35 cm×145 cm in cross section and 21 cm in depth. The accelerator is a three-grid single-stage one, and the total grid area is 25 cm×125 cm, which is divided into five sections. The ion source was installed on the negative-ion-based NBI test-stand, and using one section of the grid 5.5 A of the negative-ion beam was produced at a gas pressure of 1.8 mTorr corresponding to a current density of 27.5 mA/cm2. The extracted electron current is low of 50% of the negative ion current, and the acceleration efficiency, defined as the negative ion current divided by the acceleration drain current, is ...

Compact cesiated hollow cathodes (CHC) has been developed and studied in two large multicusp H− ion sources at NIFS, aiming the long lifetime cathodes for plasma production in the neutral beam injectors. Results on hollow cathodes and power supplies improvement are described. Basic characteristics of multicusp source discharge, driven by several CHCs were studied. Key issues about simultaneous CHCs operation in high current discharge mode are discussed. Each of advanced CHC delivered stable and quiet income with current up to 70A for 5sec to multicusp source discharge. The hollow cathode operation with no Cs feed was obtained by using 30%Xe+70%H2 gas mixture.

A large vacuum‐immersed H− ion source has been operated on the negative‐ion‐based neutral beam teststand. The achieved level of the beam and the pulse duration in beam conditioning were limited by a high‐voltage breakdown in the vacuum vessel. A baffle plate at grounded potential for shielding completely from the charged particles was successful. A beam with an energy of up to 102 keV and the H− ion current of 0.55 A were achieved for 0.29 s without the breakdown. The ion current of ∼1.5 A was accelerated in cesium‐seeded operation. The corresponding ion current density was ∼7 mA/cm2. A magnetic filter (as Type I LV magnetic filter) on the plasma electrode was applied. The electron beam component which was extracted from the plasma source together with H− ions was found to be very reduced.

A large hydrogen negative‐ion source with an external magnetic filter has been developed for a neutral beam injection (NBI) system in the Large Helical Device (LHD), and a high‐energy and high‐current H− ion beam has been produced. The ion source is operated at a high arc efficiency of 0.1 A/kW at an operational gas pressure of less than 3.5 mTorr, and produces 47 keV–16.2 A of a H− ion beam from a grid area of 25 cm×50 cm. With two‐stage acceleration, 13.6 A of a H− ion beam has been successfully accelerated to 125 keV. Multibeamlet focusing by the aperture displacement technique has been achieved 11.2 m downstream with a gross divergence angle of 9 mrad. The alternate beamlet deflection by the magnetic field at the extraction grid, which results in beam broadening in the deflection direction, was well compensated also by the aperture displacement technique. These results satisfy the specification of the negative‐ion‐based LHD–NBI system.

Large current negative hydrogen ions are extracted from a multicusp ion source with an extraction area of 25×44 cm2. By seeding a small amount of cesium vapor into a source chamber, the H− current is enhanced to several times larger than that in a pure hydrogen discharge, accompanied by a reduction of the electron current and the filling gas pressure. A H− current up to 16 A is obtained with a beam energy of 40 keV from 560 extraction holes of 9 mm in diameter each, where the current density is 45 mA/cm2. The H− current increases linearly with the input arc power.

Summary form only given. Magnetic multicusp ion sources, also known as bucket-type sources, are suitable for producing large-area ion beams and they have been developed as high-current ion sources in neutral beam injector (NBI) systems for nuclear fusion plasma heating. Neutral beams are produced by neutralization of ion beams in the systems. Recently, development of a negative-ion-based NBI system has become desirable for higher energy neutral beam production. The neutralization efficiency of a positive ion beam decreases as beam energy rises (20% at 100 keV, 15 A: 1/3 current of LHD ion source) was a condition of the design. The bucket plasma source has a 35 cm/spl times/62 cm area and a 20.6 cm depth. A large-area high-density plasma (>10/sup 12//cm/sup 3/) is produced by a high power are discharge (>100 kW) with tungsten filaments. For H/sup -/ production, a magnetic filter is used to lower the electron temperature of the negative ion production region near the beam extraction grid.

In this paper we report the first results on a prototype negative ion source for LHD-NBI. The ion source with a multi-cusp configuration consists of the external magnetic filter and has an inner volume of 140 cm in height ×35 cm in width ×25 cm in depth. Two cesium lines are installed to seed cesium vapor into arc plasma for enhancement of H− current. The H− ion beam was extracted from an area of 120 cm×25 cm and was accelerated via single-stage accelerator. An H− current of about 4 A with the energy of 73 keV was obtained at the arc power of 250 kW for a duration of 0.3 sec.

The acceleration characteristics of H− beam have been investigated using a single beam let accelerated up to 100 keV. The H− beam is extracted from a large negative ion source operated with Cs seeding and is accelerated by a single‐stage accelerating system. The currents and the spatial profiles of the H− beams are measured calorimetrically. The beam divergent angles are obtained by means of Gaussian fitting to the measured plots. A minimum divergent angle of 5 mrad is achieved with an H− current densities of 32 mA/cm2 and the beam energy of 94 keV. A ratio of acceleration current to H− current increases suddenly at the region where the H− current begins to saturate due to a space charge limitation. This is considered to be caused by emission of secondary electrons at an extraction electrode by collisions of the divergent H− beams. The dependence of the divergent angle on the acceleration voltage is measured, and an optimum ratio the electric fields for H− extraction and for the acceleration is determined...

High current hydrogen negative ion source development has been intensively carried out for neutral beam injector for Large Helical Device (LHD), in which ion source capable of delivering a negative ion beam of 45 A at a beam energy of 125 keV is required. A multicups ion source of 1/3 scale of the prototype source in NBI for LHD and a large rf ion source has been studied for negative ion production, beam extraction and acceleration. A total H− current of 16 A is obtained at the 1/3 scale source with Cs seeding operation. The H− ion current is proportional to the input arc power. A 13.6 A of H− ion beam is accelerated to 125 keV at the two stage acceleration. A divergent angle of 9 mrad is obtained at multi beam. Preliminary experimental results of r.f. ion source are described.

Fabrication of a 15 MW, 10 sec. negative‐ion‐based neutral beam injection system has been started for Large Helical Device (LHD). The system is composed of two tangential beam lines. Each beam line has two ion sources which are designed to deliver 40 A of negative hydrogen ions at the beam energy of 180 keV. The beam line has two vacuum chambers and a long gas neutralizer of 3.5 m between them. The first chamber is used as a gas expansion chamber of ion sources, and its gas pressure is controlled to keep the optimum line density for neutralization. The second chamber includes a magnetic bending system to deal with residual ions. The design is based on the results of our RD the high density negative hydrogen ion production, high power beam extraction, and low beam divergence. The commissioning of the system will start in 1997, and the operation is scheduled in 1998.

Present status of the hydrogen negative ion source development is reported for an NBI system in the Large Helical Device. An external-filter type negative ion source has produced a 16.2 A of H - ion beam with the energy of 47 keV at the arc efficiency of 0.1 A/kW and at the operational gas pressure of 3.8 mTorr. The H - ion beam of 13.6 A was successfully accelerated to 125 keV with two-stage acceleration. A rod-filter type negative ion source has also produced a 16 A of H - ion beam with the current density of 45 mA/cm 2 . A vacuum-immersed negative ion source has demonstrated stable operation on condition that the ion source is shielded from the beam-induced plasma. For the future long-lifetime operation, an RF-driven negative ion source has been developed, where an induction coil antenna is placed inside a metal-walled magnetic multicusp source.

Electron density measurements of a large-scaled negative ion source were carried out with a surface wave probe. By comparison of the electron densities determined with the surface wave probe and a Langmuir probe, it was confirmed that the surface wave probe is highly available for diagnostic of the electron density in H(-) ion sources. In addition, it was found that the ratio of the electron density to the H(-) ion density dramatically decreases with increase of a bias voltage and the H(-) ions become dominant negative particles at the bias voltage of more than 6 V.

We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H(-)) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H(-) density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H(-) density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H(-) charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H(-) density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.

A multiantenna rf ion source is studied. The new antenna system is developed to reduce the rf voltage on the antenna compared to that of the conventional loop antenna, which can reduce high voltage rf breakdown on antennas. The antenna was installed in a 1/6 scale ion source for the neutral beam injector (NBI) (H−) used in large helical devices. The characteristics of the ion density current of the plasma produced by using the segmented antennas were studied. A dense plasma with ion saturation current density of 92 mA/cm2 was generated at 45 kW of rf power at 9 MHz for the two-antenna system. At around 10 kW of rf power the plasma density jumped and became a high density mode. The threshold power for the density jump seems to be shifted by the antenna segments and the rf frequency. The current density of the positive ion beam with a small accelerator voltage is 26 mA/cm2 for 10.3 keV of extraction energy, 45 kW of rf power, and 0.16 Pa of plasma source pressure of hydrogen. This multiantenna rf source cou...

The velocity spectra of the negative-ion-(H−) based neutral beams are studied in high-performance large-area ion sources during injection into large helical device fusion plasmas. We are conducting systematic observations in standard neutral beam injection to correlate beam spectra with source operating conditions. Almost all of the transmitted beam power was at full acceleration energy (∼170keV). The small stripping beam component which was produced in the extraction gap was evaluated to be about 9%–22% by amplitude of the measured spectra for the sources in beam lines 1 and 2. H− production uniformity from the spectrum profile was 86%–90% for three sources. For the longest pulse injection during 74 and 128 s, a full energy component tended to decrease with time, while the accelerator gap stripping tail tended to increase slightly with time, which is attributed to beam-induced outgassing in the accelerator. A higher conductance multislot ground grid accelerator appeared to show little growth in the accel...

A new multiantenna system for large area radio-frequency H− ion source using a filamentless ion source was tested to extract ions from dense plasmas with high rf input power. A 1∕6 scale large helical device ion source was equipped inside a plasma chamber with four linear antenna rods. Positive-ion extraction with a single hole extractor was studied in detail, while a negative one only showed preliminary results. The positive-ion current density of 34mA∕cm2 to the collector plate was achieved with a new multiantenna after optimization of the antenna number and source gas pressure, where the number of the antennae was 2 with ∼36kW of rf input power and with a gas pressure of ∼0.1Pa with H2. If the external magnetic filter optimized for negative-ion production in the multifilament discharge source is taken off, the current density is estimated to reach about 120mA∕cm2. This corresponded closely to the value measured with a Langmuir probe near the center. Two antennae appear to be required to generate unifor...

The injection duration has been extended beyond 100 s with a high-power hydrogen negative-ion source in a negative-ion-based neutral beam injector of the Large Helical Device superconducting fusion machine. The ion source is a cesium-seeded source with a thermally insulated plasma grid (PG), and optimized for a short-pulse operation of 2?3 s. The negative-ion production efficiency is strongly dependent on the PG temperature, and in the long-pulse operation it exceeds an appropriate temperature range of 200?300 °C, at which the optimum cesium coverage is formed on the PG surface. By making the PG temperature rise slower with a reduced arc power, the injection duration was extended to 110 s with an injection power of 110 kW. To extend the injection duration further with a higher injection power, stainless-steel cooling tubes have been mechanically attached to the PG for suppression of the PG temperature rise in the long-pulse operation. As a result, a long-pulse injection with an injection power of 200 kW was extended to 128 s until it was manually stopped due to the plasma collapse. However, the beam duration could be limited to around 3 min because the PG temperature rise was not saturated due to a low thermal conductivity with the thickness of the stainless-steel tube determined so that the short-pulse operation is also possible. On the other hand, the longitudinal beam distribution in a grid area of 25×125 cm^2 is observed to be more uniform than that with the uncooled PG. The temperature distribution of the individual grid parts becomes more uniform with the cooled PG, which should contribute to the improvement of the beam uniformity.

Fabrication of a 15 MW, 10 sec. negative-ion-based neutral beam injection system has been started for Large Helical Device (LHD). The system is composed of two beam lines. Each beam line has two ion sources which are designed to deliver 40 A negative hydrogen ions at the beam energy of 180 keV. The beam line has a long gas neutralizer and a magnetic bending system to deal with residual ions. The beam line has also a bending system for accelerated electrons, which we think necessary. Special attention is paid to reduce ferromagnetic materials in the beam line, because helical systems are sensitive to the error magnetic field. The design is based on the results of our R&D on the large negative hydrogen ion source, high density negative hydrogen ion production, high power beam extraction and low beam divergence.

Large-scaled hydrogen negative ion source development is reviewed for a negative ion based neutral beam injector (NBI) in the large helical device (LHD) fusion machine. The target performance of the ion source is characterized by a high current of 30–40 A with a relatively low energy of 120–180 keV. A series of negative ion source development is conducted with a one-dimensionally reduced size of ion sources which still have a large beam area of 25 cm×26 cm or 50 cm with multi apertures. We employed a cesium-seeded volume production source with an external magnetic filter for the source development. Improvement of the arc plasma confinement is effective to produce a high-current negative ion of 16 A with a current density of 31 mA/cm2 at a low operational gas pressure below 0.4 Pa. Suppression of the accelerated electrons is achieved both by strengthening the magnetic field at the extraction grid and by shaping the inside of the extraction grid aperture to shield the secondary electrons against the acceler...