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We report on the characteristics of the beam-generated plasma in the multibeamlet case of a hydrogen negative ion beam at NIFS. The plasma potential, and the energy of secondary particles in the drift region of an ion beam, offer an insight into the mechanisms that allow beam transport in low pressure gasses. The first measurements reported here were made by means of a four-gridded retarding field energy analyzer, combined with the measurement of the drain current at the beam dump, and with infrared beamlet monitoring technique. The retarding field analyzer measures the energy distributions of particles emitted radially from the beam-generated plasma [1]. The beam is dumped onto a graphite calorimeter, which is electrically insulated, so that it can be electrically biased and the collected current can be measured. The thermal image of the calorimeter is acquired during the beam pulse, thus offering a quantitative estimation of the single beamlet optics and multibeamlet focusing. Finally, the influence of the neutral gas density is studied by puffing hydrogen gas in this drift region. The measured ion and electron energy distribution functions are strongly affected by the electric bias of the calorimeter. For a 47kV, 0.3A beam, an electron temperature of 3eV at the plasma edge was found. The energy distribution of the ions for a pressure of about 0.03Pa is presented, from which the plasma potential can be inferred.

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...

As one of candidate materials for a low work function plasma grid, a new inorganic material, C12A7 electride having the low work function and high electrical conductance, was studied experimentally. The photoelectric current from the material under irradiations with 325 nm He-Cd laser and 405 nm, 450 nm, 532 nm diode lasers was measured. The measured work function was less than 2.7 eV. Exposure to the C12A7 electride to an atomic hydrogen (H0) flux has produced electric current corresponding to negative hydrogen ion (H-) current from the surface. Similar characteristics were obtained for a low-work-function bi-alkali material covered molybdenum surface (∼2.3 eV). Nearly the same negative current signals were observed under the same exposure condition of atomic hydrogen flux.

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.

Balmer-α light emission from the extraction region of the LHD one-third ion source has shown a characteristic Doppler broadening in the wavelength spectrum detected by a high resolution spectrometer. The spectrum resembles Gaussian distribution near the wavelength of the intensity peak, while it has an additional component of a broader foot. The measured broadening near the wavelength of the intensity peak corresponds to 0.6 eV hydrogen atom temperature. The spectrum exhibits a larger expansion in the blue wing which becomes smaller when the line of sight is tilted toward the driver region from the original observation axis parallel to the plasma grid. A surface collision simulation model predicts the possibility of hydrogen reflection at the plasma grid surface to form a broad Balmer-α light emission spectrum.

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.

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.

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.

Characteristics of radio frequency (RF) plasma production were investigated using a FET inverter power supply as a RF generator. High density hydrogen plasma was obtained using an external coil wound a cylindrical ceramic tube (driver region) with RF frequency of lower than 0.5 MHz. When an axial magnetic field around 10 mT was applied to the driver region, an electron density increased drastically and attained to over 10{sup 19} m{sup -3} in the driver region. Effect of the axial magnetic field in driver and expansion region was examined. Lower gas pressure operation below 0.5 Pa was possible with higher RF frequency. H{sup -} density in the expansion region was measured by using laser photo-detachment system. It decreased as the axial magnetic field applied, which was caused by the increase of energetic electron from the driver.

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.

Pure volume production of D− negative ions is studied in rectangular negative ion source equipped with an external magnetic filter (MF). Plasma parameters (ne and Te) in D2 plasmas are varied mainly in the downstream region by changing the magnetic field intensity of the MF (i.e., BMF). Production and control of D2 plasma is nearly the same as that of H2 plasmas, although the values of ne and Te in D2 plasma are slightly higher than that of H2 plasmas in both the source and the extraction regions. On D−/H− production, however, it appears some different points. By varying the BMF, H− production in the extraction region is remarkably changed corresponding to the variation of ne and Te in the extraction region. On the other hand, D− production is not so varied under the same discharge conditions in H2 plasmas. This difference in D− production is not well explained only variation of ne and Te in D2 plasmas. Optimum BMF and gas pressure for D− production is slightly higher than that for H− production.

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...

Using a mesh grid bias method for plasma parameter control, volume production of hydrogen negative ions H− is studied in pure hydrogen rf plasmas. Relationship between the extracted H− ion currents and plasma parameters is discussed. Both high and low electron temperature Te plasmas are produced in the separated regions when the grid is negatively biased. In addition, with changing grid potential Vg, values of ne increase while Te decrease in their values. The negative ion production depends strongly on the grid potential and related plasma conditions. It is also confirmed that grid bias method is more effective than the so‐called magnetic filter method to optimize plasma parameters for H− production. Production of deuterium negative ions D− is also studied briefly.

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.

Characteristics of radio frequency(rf) plasma production are investigated using a FET inverter power supply as an rf generator. The matching circuit in the inverter system is simple compared to a conventional 50 Ohm matching system and only an imaginary part of the impedance of rf transmission should be matched by adjusting operating frequency or capacitance of the circuit. An electron density over 1018 m−3 is produced in argon plasma with 1 kW rf power. Lower densities are obtained in helium and hydrogen plasmas compared to the argon plasma. Effect of axial magnetic field in driver region is examined. Electron density more than 1018 m−3 is obtained at the hydrogen gas pressure around 1 Pa with the help of the axial magnetic field.

A probing He0 beam for confined alpha particle measurement using a double charge exchange process is now under development. A proof of principle experiment for ground‐state He0 beam production will be performed on a test stand. Several methods are developed to measure the metastable fraction of a He0 beam. A full‐size strong‐focusing He+ source has been constructed and sufficient beam current was achieved with a beam size tolerable to be used on ITER.

This paper describes the mechanism of efficient H− production in a high-density hydrogen plasma produced by helicon wave discharge. An H− source using helicon wave discharge has been developed. Spatial and temporal variations of H− density in the helicon plasma were measured by laser photodetachment technique. A steep increase in H− density was observed in the afterglow. The H− ions were efficiently produced for the first 20μs after the termination of the discharge and mainly lost by mutual neutralization with positive ions. In the outer region of the plasma column, 20% of the negatively charged particles were H− ions. In the region of Te=1.5−2 eV, the effective attachment rate coefficient defined as the average value for hydrogen molecules of all energy levels was evaluated to be 0.5−1.3×10−11 cm3/s, of which the value is 3-order higher than that for attachment to the v=0 level of the ground state molecules.

We compare the negative ion fraction obtained for scattering of hydrogenous ions (H{sub x}{sup +}, x=1-3) from highly oriented pyrolytic graphite (HOPG), with that obtained for scattering from a polycrystalline graphite surface. In contrast to the HOPG surface, which has a negative ion yield of the order of 1-2%, the polycrystalline sample has yields of up to 30%.

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.

In this paper we report surface scattering experiments on graphite in an attempt to investigate fundamental charge exchange processes on divertor surfaces in plasma devices. A remarkably high H{sup {minus}} ion fraction (10{endash}20{percent}) as a function of outgoing angle of the scattered particles is found during scattering of positive hydrogenous ions (H{sup +}, H{sup +}{sub 2} and H{sup +}{sub 3}). These results cannot be explained with conventional theories of charge transfer. {copyright} {ital 1996 American Institute of Physics.}

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.

The 1/3‐scaled H− ion source for NBI‐system of Large Helical Device has been developed. By seeding cesium vapor into the multicusp plasma source, the H− ion current is enhanced 3 times larger than that in pure hydrogen operation. The reduction of electron current and operating gas pressure is also observed. Total H− ion current of 8.5 A has been obtained from 800 extraction holes of 9 mm in diameter arranging in a large area of 25×50 cm2. The ratio of electron to H− ion current can be reduced to 2 by using the EggBox Cell placed on the plasma grid. From 36 extraction holes, the H− ion current of 1.24 A has been obtained with the extraction voltage of 8.5 kV, which corresponds to the current density of 54 mA/cm2. The H− ion current density increases as an extraction voltage and an input arc power increase.

A negative‐ion‐based NBI test‐stand has been constructed in Toki site of National Institute for Fusion Science (NIFS). This Toki test‐stand has facilities to produce 2.5 MW neutral beam power with the energy of 125 keV for hydrogen and 250 keV for deuterium, which is utilized for developing a unit of neutral beam injection (NBI) system for Large Helical Device (LHD). All components of the Toki test‐stand are installed on the basis of conceptual design of the LHD‐NBI system. Development of a high‐current negative ion source with vacuum‐immersed structure is one of the main objectives of the test‐stand, as well as transport of a high‐energy beam with a small divergence angle. It is also an important subject to test all of hardware components such as beam dumps and cryo‐pumps. Experiments started in July.