M. Kazawa

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.

Operation of the negative-ion based NBI for JT-60U

Abstract A beam injection experiment with the negative-ion based NBI system (N-NBI) started in March 1996 on JT-60U. After achieving the first neutral beam injection of 180 keV, ∼0.1 MW for 0.4 s into the JT-60U plasmas, the operation parameters of the ion source and power supply had been optimized for increasing the beam energy and beam current. In September 1996, a deuterium neutral beam of 2.5 MW at 350 keV was injected into JT-60U using two ion sources. In the operation with hydrogen at the beginning of 1997, a negative ion beam current of 18.4 A at 350 keV has been obtained, and a neutral beam of 3.2 MW at 350 keV for 1 s has been injected into the plasma with one ion source. A neutralization efficiency of negative ion beam has been confirmed to be about 60% at the beam energies of 250–385 keV as predicted theoretically.

Power flow in the negative-ion based neutral beam injection for JT-60

The negative ion based neutral beam injection system for JT-60 has operated since 1996 injecting neutral beam into JT-60 plasmas. A power flow measurement in the beam line and ion source with a water calorimeter had shown that 40%–50% of accelerated beam particles were intercepted on the two accelerator grids and the grounded grid at an ion source gas pressure of 0.2–0.3 Pa. Much of the beam loss was not caused by stripping loss of the negative ions, but rather by direct impingement of the negative ions onto the grids. After reducing the acceleration area by masking the edge area (about 13% of the extraction area) of the accelerator grid so as to minimize the edge effect of magnetic field in the arc chamber, the loss in the accelerator decreased by roughly 25%. In comparing a deuterium beam with a hydrogen beam, the neutral beam power with deuterium is lower by 30% than that of hydrogen at the same arc power, although the heat load onto the grounded grid does not change so much. The power deposition ratio...

Present status of the negative ion based neutral beam injector for JT-60U

The negative ion based neutral beam injector heating system for JT-60U has contributed to core plasma heating and noninductive current drive experiments on JT-60U. For increasing further the beam power and beam pulse duration, the serious issue is improvement of source plasma nonuniformity in the ion source. Various countermeasures have been devised to solve the nonuniformity. The first is to adjust the spatial distribution of the arc discharge through regulating the arc current limiting resistors to be connected in series to each of the filament groups. The second is to change the arc discharge mode through controlling the filament temperature. These measures have been found to be very effective.

Beam performance of negative-ion based NBI system for JT-60

The negative-ion based NBI system (N-NBI) for JT-60 is being operated for a high energy neutral beam injection into the JT-60 plasma, in parallel with increasing the beam power. The estimation of the beam characteristics is very important for the enhancement and optimization of the beam power. The following four items have been used as the characterization of the beam: neutron yield from the beamline, beam divergence, heat load on the beamline components and beam profile on the armor plate in the tokamak vacuum vessel. Since the beam power accelerated in the ion source is proportional to the neutron yield in the beamline components, the operation status of deuterium negative ion current can be monitored easily. The beam divergence estimated at the drift duct and ion dumps, and the heat load on the beamline components are used for the optimization of the ion source operation parameters and the evaluation of neutral beam injection power. The beam divergence measured at the drift duct is 4 mrad in the horizontal direction, 6 mrad in the vertical, which are in agreement with the design value of 5 mrad. The deposition profile on the armor plate is used for monitoring of the neutral beam profile.

Design of a 500 KEV negative-ion-based NBI system for JT-60U

Abstract The design of a negative-ion-based NBI system for JT-60U rated to inject neutral beams of 500 keV, 10 MW for 10 seconds is described. The neutral beam is injected tangentially from a beamline with two ion sources. The ion source is a cesium-seeded multicusp volume source. The acceleration current per source is 22 A with deuterium at a current density of 13 mA/cm2. The operating pressure of the plasma generator is less than 0.3 Pa. A three-stage multi-aperture electrostatic acceleration system is adopted as the accelerator. Residual ions are deflected horizontally by the combined magnetic fields produced by the deflecting coils and the stray field from the tokamak. The deflecting magnetic field is swept with a frequency of 2 Hz. An acceleration power supply of 490 kV, 64 A is inverter type power supply. The freauency of the inverters is 150 Hz. The system will be completed in 1996. The system will be the first negative-ion-based NBI system in the world.

Beam divergence and power loading on the beamline components of the negative-ion based NBI system for JT-60U

In JT-60U, a high-energy neutral beam injection (NBI) program using negative-ion based NBI (N-NBI) for non-inductive current drive and core plasma heating studies in high-density plasma has progressed. The target performance of the N-NBI is a neutral beam injection power of 10 MW for 10 s at 500 keV. A neutral beam power of 5.2 MW at 350 keV with deuterium has already been injected into JT-60U plasma. The beam divergence and power loading onto the beamline components are two important items to evaluate beam performance. The beam divergence, estimated roughly from heat loads at the beam drift duct in beam injection experiments into JT-60U plasma, was around 4 mrad for the horizontal direction and 6 mrad for the vertical direction, which were close to the design values of 5 mrad. The power loading on beamline components were reasonable as compared with the design value.

Study of plasma uniformity on JT-60U negative ion source

The negative ion source developed at JAERI for the N-NBI was intended to accelerate a 500 keV, 22 A D− beam for 10 s. Two of these ion sources are mounted in the beamline. It has been found that the spatial uniformity of the source plasma is not as good as expected in the design phase of the negative ion sources for JT-60U. The source plasma uniformity has been estimated through measuring spatial distributions of the arc current flowing into each filament group and also the ion saturation current with Langmuir probes placed at ten positions around the extraction area of the plasma grid. We then altered the relative values of the arc current-limiting resistors to the eight filament groups in each source to balance the arc. As a result of the optimization of the arc resistors, the nonuniformity of the source plasma has been reduced.

Recent progress of high-power negative ion beam development for fusion plasma heating

A negative-ion-based neutral beam injector (N-NBI) has been constructed for JT-60U. The N-NBI is designed to inject 500 keV, 10 MW neutral beams using two ion sources, each producing a 500 keV, 22 A D- ion beam. In the preliminary experiment using one ion source, a D- ion beam of 13.5 A has been successfully accelerated with an energy of 400 keV (5.4 MW) for 0.12 s at an operating pressure of 0.22 Pa. This is the highest D- beam current and power in the world. Co-extracted electron current was effectively suppressed to the ratio of Ie/I D - < 1. The highest energy beam of 460 keV, 2.4 A, 0.44 s has also been obtained. To realize 1 MeV class NBI system for ITER (International Thermonuclear Experimental Reactor), demonstration of ampere class negative ion beam acceleration up to I MeV is an important mile stone. To achieve the mile stone, a prototype accelerator and a I MV, I A test facility called MeV Test Facility (MTF) were constructed. Up to now, an H- ion beam was accelerated up to the energy of 805 keV with an acceleration drain current of 150 mA for 1 s in a five stage electrostatic multi-aperture accelerator.

Beam acceleration test in negative‐ion based NBI system for JT‐60U

Beam extraction and acceleration test in the Negative Ion Based Neutral Beam Injector for JT‐60U has been started using one ion source that is designed to produce a 500 keV, 22 A D− ion beam. Deuterium negative ions are produced in a cesium‐seeded semi‐cylindrical plasma generator and accelerated by a multi‐aperture three‐stage electrostatic accelerator. In the preliminary experiment of beam acceleration, the D− ion beam of 13.5 A was successfully accelerated to 400 keV for a pulse duration of 0.12 s. The negative ion beam power was 5.4 MW. The operating gas pressure in the plasma generator was as low as 0.22 Pa. The highest energy beam of 460 keV, 2.4 A, 0.44 s was also obtained. The ratio of extracted electron current to extracted negative ion current is estimated Ie/ID−<1. It was confirmed that the electron leak from the extractor to the accelerator is suppressed efficiently by the effects of biassing, electron trapping gap and magnetic field.