M. Hamabe

Compensation of beam deflection due to the magnetic field using beam steering by aperture displacement technique in the multibeamlet negative ion source

A beam steering technique using aperture displacement was examined to correct the negative ion beam deflection due to the magnetic field for electron suppression in a large-area multibeamlet H− source. The total deflection angle was estimated, including the effect of the deflection by the electron suppression magnetic field and the beam steering by the aperture displacement, both by linear optics theory and by three-dimensional beam trajectory simulation. Two methods were compared; one used the displacement both of the grounded grid (GG) apertures and of the exit part of the extraction grid (EG exit) apertures, and the other used the displacement only of the EG exit. The beam steering experiments were performed using a large-area multibeamlet H− source with both displacement methods, and the results were compared with the theoretical estimations. As a result, both methods were effective to correct the beam deflection. In particular, the displacement of only the EG exit with a simplified displacement struc...

Effect of argon additive in negative hydrogen ion sources

Using the photodetachment technique we investigated the hydrogen negative ion density in the extraction region of a magnetically filtered multicusp ion source when argon is added to hydrogen. We found that the negative-ion density goes up at most by a factor of 1.5 when argon is added to low base hydrogen pressure (0.05 or 0.1 Pa), but goes down when argon is added to higher initial hydrogen pressure (0.5 or 1 Pa). Adding argon did not enhance the Werner and Lyman bands in the vacuum ultraviolet, which indicates that argon addition does not increase the production rate of vibrationally excited H2 molecules. The increase of the negative-ion density by adding argon into a low-pressure hydrogen discharge is probably due to the increase of the low-energy electron density.

Simple emittance measurement of H− beams for neutral beam injectors in magnetic fusion

A novel emittance meter is developed using the pepper‐pot method. Kapton foils are used to detect intensity distributions of small beamlets at the ‘‘image’’ plane of the pepperpot. Emittance of H− beams for the neutral beam injector of the large helical device has been measured. The normalized emittance (95%) of a 6 mA H− beam with an emission current density of about 10 mA/cm2 is ∼0.59 mmu2009mrad. The present system is simple, and it eliminates many complexities of the existing schemes.

Development of a compact multicusp ion source of He

A filament-type helium ion source has been developed for the purpose of alpha particle diagnostics and as a test beam for electrostatic quadrupole transport experiments. We have obtained preliminary results of He+ beam current, ∼60 mA/cm2. The normalized brightness of this beam is ∼1010u200aA/(mu200arad)2. The normalized emittance (90%) measured by multislit faraday cup emittance meter is ∼0.6 mmu200amrad below 100 V, 10 A arc discharge. The effects of plasma parameters in the source on the beam emittance and on the beam perveance are discussed.

Optimization of Cs deposition in the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection system

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...

Improvement of a large negative ion source for the Large Helical Device neutral beam injector

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 independent 12 arc power supplies connected to 12 filament power supplies. The initial results show good controllability of the individual arc current distribution at a high arc power. The improvement of the ion source performance would lead to an increase in the injection power in the following LHD experimental campaign.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...

Performance of LHD-NBI H− ion source

The Large Helical Device-Neutral Beam Injector (LHD-NBI) system uses 40u2009A×180u2009keV (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...

Extraction physics in volume H- -ion sources

Recent hydrogen negative-ion sources (JT60, large helical device (LHD)) operate with a magnetic filter field extending up to the plasma electrode and extraction opening. As shown earlier such a magnetic field has a strong effect upon the value of the extracted negative ion and electron current. Measurements of the negative ion and electron density were performed in the ion source, both in the magnetic-field-free region and in the region near the plasma electrode. This work presents the experimental value of the ratio between the extracted negative ion and electron current in different sources. It is shown that the measured current ratio is considerably higher than what can be predicted from a simple thermal flux model based on the density ratio in the bulk of the plasma. We also calculated the ratio between the extracted negative ion and electron currents based on the measured ratio between the negative ion and electron density near the extraction opening using the different theoretical models for electro...

Cooling effect of hydrogen negative ions in argon gas mixture

The gas mixture effect upon the negative ion temperature in negative ion sources has been studied by the laser photodetachment technique. The ion source is operated at hydrogen pressure from 0.05 to 2.0 Pa. Then argon gas is added up to 100% of the hydrogen pressure. Maximum enhancement of the negative ion density is observed at around a 20%–40% argon to hydrogen mixture in the low hydrogen pressure region. The recovery time for the electron current signal after photodetachment becomes longer when more argon is added to hydrogen plasmas, indicating a lower negative ion temperature.

A comparative study of emittance measurements using different techniques

The pepper-pot method and two slit-and-collector type techniques are tested for their applicability to measure the emittance of low energy ion beams. The exposure of Kapton foil, used here as the detector in the pepper-pot method, has been found to show a nonlinear behavior of the image contrast with the local power density of He+ beam for energy less than 10 keV. Therefore, the emittance measurement with the pepper-pot method must be made with the calibration of the contrast of the Kapton film in advance. The measured values of emittance, obtained by the different diagnostic schemes, are similar. From the view point of the negative ion beam measurement, the pepper-pot type system using Kapton film and the multislit-and-Faraday-cup system are proved to be promising.