H. Oohara

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 and Development of the 500-keV Negative-Ion-Based Neutral Beam Injection System for JT-60U

The 500-keV negative-ion based neutral beam injector for JT-60U started operation in 1996. The beam power has been increased gradually through optimizing operation parameters of the ion sources and conquering many troubles in the ion source and power supplies caused by a high voltage break-down in the accelerator. However, some issues remain to be solved concerning the ion source for increasing further the beam power and the beam energy. The most serious issue of them is non-uniformity of source plasma in the arc chamber. Various countermeasures have been implemented to improve the non-uniformity. Some of those countermeasures have been found to be partially effective in reducing the non-uniformity of the source plasma, and as the result the ion source, so far, has accelerated negative-ion beams of 17.4A at 403keV with deuterium and 20A at 360keV with hydrogen against the goal of 22A at 500keV. The neutral beam injection power into the plasma has reached 5.8MW at 400keV with deuterium. Further efforts to reach the target of 10MW at 500keV have been continued.

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.

Construction of a 500 keV/negative-ion-based NBI system for JT-60U

The construction of 500 keV NBI system using negative-ions has started for studying of mega-ampere level NB current drive and plasma core heating experiments with a high density reactor-like plasma In JT-60U. The specification of the NBI system is; a beam energy of 500 keV, an injection power of 10 MW, a beam duration time of 10 sec with a duty cycle of 1/60, a beam species of deuterium or hydrogen. The power of 10 MW is injected toward tangential/co-direction with one beamline that has two negative-ion sources. The construction of the NBI system will be completed in 1996.

Operation and Development on the Positive-Ion Based Neutral Beam Injection System for JT-60 and JT-60U

The positive-ion based neutral beam injection (NBI) system for JT-60, which consists of 14 beamline units and has a beam energy of 70 to 100 keV, started operation in 1986 with hydrogen beams and injected a neutral beam power of 27 MW at 75 keV into the JT-60 plasma. In 1991, the NBI system was modified to be able to handle deuterium beams as part of the JT-60 upgrade modification. After executing some research and developments, deuterium beams of 40 MW at 95 keV were injected in 1996. As a result, NBI has contributed to the achievement of the highest performance plasmas, a DT-equivalent fusion power gain of 1.25 and a fusion triple product of 1.55 × 1021 keVs/m3, in the world on JT-60U.

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.