Takemasa Shibata

High performance tokamak experiments with a ferritic steel wall on JFT-2M

Compatibility between the plasma and low activation ferritic steel, which is a candidate material for fusion demonstration reactors, has been investigated step by step in the JFT-2M tokamak. We have entered the third stage of the Advanced Material Tokamak EXperiment (AMTEX), where the inside of the vacuum vessel wall is completely covered with ferritic steel plates ferritic inside wall (FIW). The effects of a FIW on the plasma production, impurity release, the operation region, and H-mode characteristics have been investigated. No negative effect has been observed up to now. A high normalized beta plasma of βN ~ 3, having both an internal transport barrier and a steady H-mode edge was obtained. A remarkable reduction in ripple trapped loss from 0.26 MW m−2 (without ferritic steel) to less than 0.01 MW m−2 was demonstrated by the optimization of the thickness profile of FIW. A code to calculate fast ion losses, taking into account the full three-dimensional magnetic structure was developed, and values obtained using the code showed good agreement with experimental results. Thus, encouraging results are obtained for the use of this material in the demo-reactor.

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.

Effects of ergodization on plasma confinement in JFT-2M

A steady-state H-mode plasma with frequent ELMs (edge localized modes) was obtained by applying magnetic fields with high poloidal mode numbers resonant in the plasma edge. The induced ELMs limit the density and impurity accumulation normally observed during ELM-free H-mode. An EML coil with numerous resonant modes ( m 10) in the plasma edge appears the most effective. This method of control is extended to higher auxiliary heating power by increasing the magnitude of the magnetic perturbations.

Experimental studies of the dynamics of compact toroid injected into the JFT-2M tokamak

We present the first results from recent compact toroid (CT) injection experiments in the JFT-2M tokamak using the improved CT injector and diagnostics with fast time resolution. We have observed that the core line density increases rapidly at a maximum rate of ~1.3 × 1022 m−3 s−1 after a delay of 100–200 µs. This increment rate of the core density is about several times larger than that obtained so far. Interferometry measurement along the peripheral line chord of R = 1.1 m in the inboard side indicates that CT plasma reaches a region near the plasma core beyond the separatrix. Time-frequency and space distribution analyses of edge magnetic probe signals show that the magnetic fluctuation induced by the CT has the spectral peak at 250–350 kHz and propagates in the toroidal direction at the Alfven speed of the order of 106 m s−1. These results indicate the excitation of Alfven wave by CT injection. We have observed that the fluctuation level of the ion saturation current in the divertor and the Dα spectral line intensity decrease significantly after CT injection. Corresponding increase in the soft x-ray signals in the core region may suggest that the CT causes a transition to H-mode-like discharges.

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.

Progress of advanced material tokamak experiment (AMTEX) program on JFT-2M

Application testing of the low activation ferritic steel to plasma is in progress in the JFT-2M tokamak. The toroidal field ripple reduction with ferritic insertion between the vacuum vessel and the toroidal field coils (1st stage) was successfully demonstrated. So far, no deteriorating effects of ferritic boards (FBs) inside the vacuum vessel has been observed in the pre-testing on compatibility with plasma (2nd stage). The density limit was improved by more than a factor of 1.6 after boronization with inside FBs. Design and preparation works are in progress for the testing on compatibility with plasma (3rd stage), where inside wall of the vacuum vessel will be fully covered with ferritic steel.

Measurement of impurities in a long‐pulse, multiampere hydrogen beam

Impurity concentration in intense neutral beams extracted from both a 75 keV/6 A/10 s duoPIGatron ion source and a magnetic multipole line cusp ion source has been measured by magnetic deflection mass analyzer. After passage of 90% equilibrium neutralizer cell, the ion beam contains 1%–2% light impurities (such as C+, O+, H2O) and 0.02%–0.15% heavy impurities (such as Cu+, W+, Ag+). Taking into account the neutralization efficiency, the neutral beam is estimated to contain 3%–6% light impurities and 0.04%–0.3% heavy impurities. The impurities related to oxygen decrease with time during a 10 s pulse, while those related to carbon, copper, and tungsten increase slightly with time. The origin of these impurities is discussed and the source operation mode to reduce the impurity level is proposed for neutral beam injection.

Ion Collection from Laser-Induced Plasma Using Positively Biased Wire Electrode

In atomic vapor laser isotope separation (AVLIS), an ion collection method which has a short collection time and low applied potential is required. We demonstrated that by using a positively biased wire electrode, the ions are collected from the laser-induced plasma in a shorter time at the same applied potential or at lower electric potential in the same collection times compared with the conventional parallel electrode method. The ion collection times could be estimated using simple one-dimensional models for both the wire electrode method and the parallel electrode method.

An experimental investigation of the propagation of a compact toroid along curved drift tubes

Compact toroid (CT) injection is a viable technology for fuelling large tokamak reactors in the future. Experimental demonstration of CT injection has thus far been conducted using horizontal injection in the midplane of tokamak devices. However, recent analyses indicate adverse effects of the toroidal magnetic field on CT injection. In order to avoid these adverse effects, the CT would need to be injectable in any direction. We have therefore devised a curved drift tube to change the direction of CT propagation and have experimentally demonstrated its efficacy. It has been observed that a CT can be transported smoothly through curved drift tubes with 45° and 90° bends without any appreciable change in the CT parameters. The magnetic field, electron density and speed of CTs transported through both 45° and 90° bends are similar to those observed in a linear drift tube.