Mamoru Matsuoka

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.

Beam stops of JT-60 neutral beam injector

The JT-60 neutral beam injector consists of 14 beam line units and injects a rated power of 20 MW with an energy level of 70–100 keV for beam pulses up to 10 s. The total handling power of ion and/or neutral beams in the beam line unit amounts to as much as 8 MW per unit for a beam extraction of 100 keV/80 A, though each unit delivers a neutral beam power of about 1.4 MW. Accordingly, the beam stop components have to receive a high heat load. Another difficulty is that the beam stops must receive quasi continuous heat loadings for up to 10 s. The design procedures and the measured characteristics of the beam stop components irradiated with the beam are described. In the maximum rated operation of a 100 keV/80 A beam extraction for a beam pulse up to 10 s, the incident power to the beam stop components in the beam line unit has roughly reached the design value, and every component works well now.

Achievement of high fusion triple product in the JT-60U high βp H mode

Improvement of an enhanced confinement state in a high poloidal beta (βp) regime without sawtooth activity has been achieved in JT-60U. A confinement mode has been demonstrated where both the edge and the core confinement are improved. The attainable βp was also extended to higher values in this improved mode, because of its broader pressure profile. As a result of the improvement in confinement and in attainable βp, the highest value of the fusion triple product has been extended by a factor of 2.5 over that achieved in the 1992 experiments; it has reached (1.1 ± 0.3) × 1021 m-3.s.keV with a central ion temperature of about 37 keV. The D-D neutron emission rate has also been doubled in these experiments and has reached (5.6 ± 0.6) × 1016 s-1

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.

Effects of internal and external magnetic fields on the characteristics of a magnetic multipole plasma source

Influences of an externally produced axial magnetic field and the field produced by the filament current on the performance of a magnetic multipole plasma source were experimentally studied using a high‐proton‐yield ion source developed for the JT‐60 NBI. It was shown that both of the fields give significant effects on the arc discharge characteristics, the arc efficiency, and the plasma uniformity. The probe measurement showed that the change of the arc efficiency caused by the field is closely related to the change of the ion saturation current density distribution on the extraction grid. The change of the distribution can be explained by the effect of the magnetic field on the orbits of the primary electrons emitted from the filaments. On the basis of the experimental results, a permissible level of the external field in the source was evaluated to be ±5 G.

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.

Negative ion based neutral beam injector for JT‐60U

A negative-ion-based neutral beam injector system for JT-60U is under construction. The system injects a neutral beam of 10 MW at 500 keV for 10 seconds using one beamline with two large negative ion sources. The total system efficiency of the NBI system is estimated to be 40%. The construction will be completed by the beginning of 1996, following which an NBI current drive and a plasma core heating experiments with a high density plasma will start.

Geometrical efficiency of the prototype neutral beam injector unit for JT‐60

The geometrical efficiency of the prototype neutral beam injector unit for the JT‐60 was measured experimentally and was compared with numerical estimates. The measurements agreed well with the calculations in the wide range of perveance by taking into account the distortion of the beamlet intensity distribution function from a Gaussian distribution. Overdense operation of the ion source was preferable in that the high geometrical efficiency could be obtained at high perveance values.