T. Ohga

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.

Improvement of beam performance in the negative-ion based NBI system for JT-60U

The injection performance of the negative-ion based NBI (N-NBI) system for JT-60U has been improved by correcting beamlet deflection and improving spatial uniformity of negative ion production. Beamlet deflection at the peripheral region of the grid segment due to the distorted electric field at the bottom of the extractor has been observed. This was corrected by modifying the surface geometry at the extractor to form a flat electric field. Moreover, beamlet deflection due to beamlet–beamlet repulsion caused by space charge was also compensated for by extruding the edge of the bottom extractor. This resulted in a reduction of the heat loading on the NBI port limiter. As a result of the improvement above, continuous injection of a 2.6 MW H0 beam at 355 keV has been achieved for 10 s. Thus, long pulse injection up to the nominal pulse duration of JT-60U was demonstrated. This has opened up the prospect of long pulse operation of the negative-ion based NBI system for a steady-state tokamak reactor. So far, a maximum injection power of 5.8 MW at 400 keV, with a deuterium beam, and 6.2 MW at 381 keV, with a hydrogen beam, have been achieved in the JT-60U N-NBI. Uniformity of negative ion production was improved by tuning the filament emission current so as to direct more arc power into the region where less negative ion current was extracted.

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.

Quasi–dc extraction of 70 keV, 5 A ion beam

An ion source that produces a high energy ion beam in quasi‐dc mode has been fabricated and tested. The significant features of the source are effectively cooled accelerator grids with properly shaped apertures and a carefully cooled douPIGatron plasma generator. To opeate the source quasi‐continuously, the heats dissipated in the accelerator grids and in the back plate of the plasma generator were suppressed below permissible values by attaining low pressure in the arc chamber. Under such condition, a 70 keV, 5 A, 10 s ion beam was obtained repeatedly without any deleterious problems. The measurement by photo beam monitor indicates no change of beam divergence during 10 s. The pressure change in the beam drift region during the pulse has also been investigated.

The JT-60 neutral beam injection system

The JT-60 neutral beam injection system has been designed to inject a neutral hydrogen beam power of 20 MW at energies of 75–100 keV for 10 s. The system consists of 14 beamline units, 14 power supply units for the ion sources, a liquid helium and liquid nitrogen cryogenic system for the beamline cryopumps, a demineralized cooling system for heat dump materials, an auxiliary pumping system, and a computer aided control system. Each beamline unit is made with essentially the same geometry as that of the prototype injector unit, which was constructed in 1981 and tested from 1981 to 1983 to confirm unit performance. Each power supply unit provides a voltage regulated output of 100 kV, 90 A. The helium refrigerator has a cooling capacity of 3000 W at 3.6 K. Beam energy and the pulse timing of each unit can be set up independently. Since April 1984, each beamline unit has been tested and conditioned up to 75 keV, 70 A, 10 s at the prototype injector facility. Beamlines have been installed on JT-60 and completion of the total system is scheduled for July 1986.

Production of 75‐keV, 70‐A, 10‐s ion beams

High‐power long pulse ion sources were fabricated and tested at a prototype injector unit for JT‐60. Ion beams of 70 A at an energy of 75 keV were extracted repeatedly for up to 10 s. The heat loadings to each grid were within our design values and each grid turned out to be thermally stable during 10 s pulse. The neutral beam power deposited to the beam target was over 1.43 MW, which corresponds to the design value of the JT‐60 neutral beam injector. The e‐folding half‐width beam divergence angle was about 1.0° at optimum beam current and a proton ratio of about 80% was obtained. It was also confirmed that other beam line components, such as the ion beam dump and the cryopump, were sufficiently reliable.

100‐kV test of the prototype neutral beam injector for JT‐60

A prototype neutral beam injector for JT‐60 has demonstrated extraction of 100‐kV, 70‐A, 10‐s ion beams, delivering neutral beam power of 1.43 MW into the target chamber. The power‐flow measurements showed that all beam line components, including the ion sources, were operated successfully. This verified the validity of the design work related to the ion source and neutral beam cooling devices. No significant change in the beam divergence during the pulse has been observed up to the maximum rated beam extraction of 40 A at 100 kV for 10 s from each ion source. The measurement also indicated that the power distribution to the beam line components agreed well with independently obtained ion species ratio and gas pressure distribution. Efficiencies of 28% and 20% were obtained for the neutralization and neutral injection into the target, respectively, for 100‐kV, 70‐A, 10‐s operation.

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.