M. Mizuno

High energy negative-ion based neutral beam injection system for JT-60U

Abstract On the basis of recent progress in the research and development of a high current and high energy negative-ion source, the construction of a 500 keV negative-ion based neutral beam injection (NBI) system for JT-60U has begun to demonstrate a mega-amp level NB current drive at high plasma density and to study high energy beam heating in reactor-grade plasmas. The specification of the NBI system is as follows: a beam energy of 500 keV, an injection power of 10 MW, a beam duration time of 10 s, beam species of deuterium or hydrogen. The neutral beam of 10 MW is injected in a tangential codirection with a single beamline that has two negative ion sources. The construction of the negative-ion based NBI system will be completed in 1996, and NB current drive and plasma core heating experiments will start immediately in JT-60U.

dc voltage holding experiments of vacuum gap for high‐energy ion sources

dc voltage holding characteristics were investigated to obtain a data base for designing high‐energy and high‐power ion sources of neutral beam injectors. We confirmed that the voltage holding characteristics almost obey the clump theory in the experimental gap length of up to 50 mm. The magnetic field in the gap lowered the breakdown voltage in a gas discharge region higher than a pressure of 10−3 Torr. The breakdown voltage of 30% was reduced by seeding cesium on the electrode with one order higher density than that of actual ion source at the pressure region of lower than several mTorr.

Long pulse operation of a cesium‐seeded multicusp H− ion source

It is demonstrated that a cesium‐seeded volume H− ion source can be operated very stably for long pulse durations of up to 24 h. The source consists of a 20 cm cylindrical multicusp plasma generator and a 9 cm × 10 cm multiaperture extractor. By seeding a small amount of cesium, the source has produced 50 keV, 0.5 A, 1000 s H− ion beams with a current density of 14 mA/cm2. The cesium effect lasted for more than 24 h once 100 mg cesium was seeded before operation. Power flow measurement revealed that the heat loading of the ion source was low enough to operate the source in the dc mode.

A merging preaccelerator for high current H− ion beams

The high power ion beams used in the next generation thermonuclear fusion reactors require high current negative ion beams accelerated to high energy, with high efficiency. One way to meet these requirements is to merge multiple low current density H− beamlets into a single high current beam. The feasibility of a high current merging preaccelerator was demonstrated in this experiment by merging 19 beamlets of H− ions distributed over a circular area 80 mm in diameter from a Japan Atomic Energy Research Institute negative ion source. H− ions were extracted at a current density exceeding 10 mA/cm2 at the ion source which operates at 0.13 Pa (1 mTorr), with a low arc power density (70 V×250 A). Spherically curved grids (with built‐in magnetic electron suppression) were used in the preaccelerator to focus the extracted beamlets into a single 104 mA, 100 keV beam. The merged beam has a diameter of 23 mm and a converging angle of ±30 mrad at the beam envelope. The rms emittance of the 104 mA merging beam was 1....

Development of a high performance core snubber for high power neutral beam injectors

A high performance core snubber using Fe-based soft magnetic alloys composed of ultrafine grain structure cores has been developed to protect an ion source accelerator from electrical breakdowns. Dimension of each core is 900 mm in outer diameter, 400 mm in inner diameter, and 25.4 mm in thickness. Basic characteristics of the core has been investigated and confirmed that the core has a saturation magnetic flux density of 1.35 T with a high relative permeability of about 3500 for a high frequency pulse of 1 MHz. A total magnetic flux of the core snubber is 0.15 Wb with 13 cores and a biasing current. The size of the core snubber could be reduced to about 1/3 from the conventional one composed of Ni–Zn ferrite cores.

Design and experimental results of a new electron gun using a magnetic multipole plasma generator

A new electron gun utilizing a magnetic multipole plasma generator was designed and fabricated as the heat source of the high heat flux test facility, called JEBIS (JAERI electron beam irradiation stand). By changing the acceleration grids, this electron gun is able to produce a pencil to a sheetlike electron beams up to 4 A at 100 keV for 1 ms to continuous mode. In this electron gun, magnetic lens system is not adopted to focus the electron beam, but the space charge neutralization effect by the beam plasma produced downstream of the electron gun is utilized to prevent the blow‐up of the electron beam. In addition, high permeability metal is embedded in the first and the second grids to magnetically shield the earth field and the stray field from the beam bending magnet. It was experimentally demonstrated that wide range of heat flux from 0.2 MW/m2 to over 2000 MW/m2 can be realized at the test sample position about 1.7 m downstream of the electron gun.

Negative ion production in a large semicylindrical ion source

A large semicylindrical negative ion source, whose dimensions are 34 cm in diameter and 104 cm in length, has been developed. By optimizing a chamber depth and a filter strength, a H− current of 650 mA (7.7 mA/cm2) was obtained at an arc condition of 1.3 Pa, 70 V, and 1200 A. In order to increase the H− current and to reduce an operating pressure, a small amount of Cs was injected. As a result, the H− current was increased from 550 mA (6.5 mA/cm2) to 850 mA (10 mA/cm2) under an arc power of 70 V×800 A. The most significant feature of the cesium effect was the reduction of the operating pressure. With maintaining the sufficient current of 670 mA (8 mA/cm2), the operating pressure could be lowered to 0.03 Pa. At this pressure, the highest gas efficiency of 20% was achieved.

Recent progress on high power negative ion sources at JAERI

Abstract R & Ds on high power negative ion source at JAERI are reviewed. A method of cesium injection into volume source has drastically progressed in development of the high current source. The H− ion current was enhanced up to 10 A only by seeding cesium of a few hundreds mg into a volume production type magnetically filtered multicusp source. Reductions of the optimum gas pressure and the electron current were also observed in the operation of cesium seeded sources. These effects are helpful in stable ac-celeration of the ion beams. High energy negative ion beam acceleration was demonstrated in the cesium seeded sources. So far, we have succeeded to accelerate 0.13 A H− ion beam up to 350 keV for 1 s with good beam optics of the divergence ωl/e ∼ 5 mrad. Two methods of the beam focussing were attempted; i.e. aperture displacement and curved grids extraction. The beam focussing by aperture displacement was found to be useful for the high power beam steering, while the beam merge experiment were being p...

Acceleration of 100 mA of H− in a single channel electrostatic quadrupole accelerator

Neutral beams for the next generation tokamaks will be based on multiampere negative ion beams with a beam energy of about 1.0 MeV and pulse lengths of a thousand seconds. High intensity dc beams at these levels of beam energy will require extensive development in electrostatic accelerators. At Lawrence Berkeley Laboratory, a two‐module electrostatic quadrupole (ESQ) accelerator was built to accelerate ions to 200 keV. In this experiment, up to 100 mA of H− beam current was obtained from a Japan Atomic Energy Research Institute cesiated volume source using a multiaperture preaccelerator which merged 19 beamlets into a single circular beam at the entrance to the ESQ accelerator. The H− beam was accelerated by the ESQ to accelerate 200 keV without any significant beam loss or emittance growth.

Development of a multi‐ampere H− ion source at JAERI

Recent results of R&D on high current negative ion source at JAERI are ion beam production, the R&D work has been progressed toward longer pulse beam production and the beam optics. A long pulse operation for 24 hours was demonstrated stably with a small cesiated volume source. It was confirmed that the heat load in the source is a modest level allowable for the long pulse operation. The cesium effect lasted for 24 hours, once the cesium was seeded before operation. In beam optics study, we have produced a beam with very small divergent angle of 1.5 mrad. To take this advantage for a design of compact neutral beam system, beamlet focusing technique by aperture displacement was applied in the H− beam extractor.