This paper presents an overview of recent results from JT-60U. Topics we particularly focus on are (1) the mechanism determining the rotation profile and the effect of rotation on/from transport and ...stability, (2) the edge localized mode (ELM) physics and active ELM control and (3) plasma–wall interactions. An analysis of the momentum transport showed that rotation with a high pressure gradient can be reproduced by introducing a residual stress term proportional to the momentum diffusivity and the pressure gradient. Also, the momentum diffusivity in an internal transport barrier (ITB) region was reduced to an order similar to that of the ion thermal diffusivity. A comparison of the edge pedestal characteristics between JT-60U and JET with matched shape and operational parameters showed that the edge pressure did not change with increasing toroidal field (TF) ripple up to 1%, whereas a linear shift of the rotation velocity to the counter-direction was observed with increasing TF ripple. The absolute evaluation of tungsten accumulation in the core plasma by a spectroscopic method clarified that tungsten accumulation increased with increasing toroidal rotation in the counter-direction while an H-mode was sustained even at a tungsten density of 10
−3
times the electron density. Active control of neoclassical tearing mode (NTM) islands by the electron cyclotron current drive showed that the growth of NTM islands decelerated the plasma rotation. A transition to a low-rotation frequency state occurred for larger NTM islands. A statistical analysis of the precursor of type I ELM showed a small growth rate of γ/ω
A
∼ 10
−3
(where γ and ω
A
are the growth rate and Alfvén angular frequency, respectively). The measurement of the precursor at different toroidal locations showed that the toroidal mode number was 8–10 or 14–16. Energetic-particle-driven wall modes (EWMs) were found to trigger ELMs and decrease the ELM amplitude to half of that without EWMs. Enhancement of the ELM frequency by electron cyclotron heating in the edge region of the high-field side was observed, and the frequency increase was four times than that of neutral beam injection cases having the same absorption power. Laser scattering measurement of carbon dust generation showed that the dust distribution had a peak in the far scrape-off layer and penetration into the core plasma was not significant. Depth profile measurements of deuterium and carbon in tungsten-coated tiles (tungsten thickness: 50 µm) determined that the ratio of deuterium to carbon was 0.06 ± 0.02 over a 20 µm depth, suggesting deuterium trapping by carbon in the tungsten layer. Furthermore, new records for both the beam energy of negative-ion-based neutral beams (507 keV) and the output power of 110 GHz electron cyclotron waves (1.5 MW for 4 s from one gyrotron) were achieved, which confirmed and extended the heating and current drive capabilities of ITER and JT-60SA.
Abstract
A large superconducting machine, JT-60SA has been constructed to provide major contributions to the ITER program and DEMO design. For the success of the ITER project and fusion reactor, ...understanding and development of plasma controllability in ITER and DEMO relevant higher beta regimes are essential. JT-60SA has focused the program on the plasma controllability for scenario development and risk mitigation in ITER as well as on investigating DEMO relevant regimes. This paper summarizes the high research priorities and strategy for the JT-60SA project. Recent works on simulation studies to prepare the plasma physics and control experiments are presented, such as plasma breakdown and equilibrium controls, hybrid and steady-state scenario development, and risk mitigation techniques. Contributions of JT-60SA to ITER and DEMO have been clarified through those studies.
•Layout of the JT-60SA MGI system inside and outside of the vacuum vessel.•Detailed description of the MGI valves.•Calculations done for the design of the MGI valve.•Design of the peripheral system ...(gas preparation, vacuum feed through, control system).•Results of first component tests.
Disruption mitigation is of high priority for future tokamaks like ITER and DEMO. Massive gas injection (MGI) has proven to be an effective method in medium size machines and will likely be part of future disruption mitigation systems. For further research, the large superconducting tokamak JT-60SA will be equipped with a MGI system as an experimental equipment. This system will consist of two in-vessel MGI valves, which are mounted in opposite segments of the machine, vacuum feed throughs, a gas preparation system and an industrial PLC for control. The MGI valves are a scaled version of the spring-driven valve used in ADSEX Upgrade with an internal gas reservoir of 815 cm³, a maximum mitigation gas pressure of 6.5 MPa, a closing pressure of about 2 MPa, a nozzle diameter of 28 mm and an opening time below 2 ms. CFD simulations with common gas mixtures indicate a peak flow rate of 3.8 kg/s after 1.6 ms. The valve has a size of 140 mm x 110 mm x 292 mm. The gas preparation system allows easy and reproducible mixing of two gases by using an electronic pressure controller.
•Disruption predictor based on high-beta plasma experimental data in JT-60U.•Input plasma parameters are selected by sparse modeling using exhaustive search.•Performance of prediction is improved by ...extracting input parameters.•Key parameters including magnetic fluctuation and its time delivered are extracted.•False alarm rate is improved considering temporal change of parameters.
Disruption is a critical phenomenon in a tokamak reactor. Although disruption causes serious damage to the reactor, its physical mechanism remains unclear. To realize a tokamak reactor, it is necessary to understand and control the disruption phenomenon. The present research constructs a disruption predictor using experimental high-beta plasma data in the JT-60U tokamak. The predictor was constructed using a support vector machine as a linear discriminant, and we focus on a variable selection problem for the binary classification by sparse modeling, specifically, exhaustively searching the best combinations of variables which maximize the predictor performance. By the sparse modeling, we found that the six input parameters as the best combinations. The selected parameters were the n = 1 mode amplitude |Brn=1| and its time derivative d|Brn=1|/dt, the plasma density (relative to the Greenwald density limit) and its time derivative, and the time derivatives of the plasma internal inductance and plasma elongation. In particular, it was identified that the parameter d|Brn=1|/dt, plays a key role on plasma disruption. We should notice that the combination with other plasma parameters is indispensable and remarkably make it possible to improve the performance of disruption prediction.
Disruptions are dangerous events in tokamaks that require mitigation methods to alleviate its detrimental effects. A prerequisite to trigger any mitigation action is the existence of a reliable ...disruption predictor. This article assesses a predictor that relates in a linear way consecutive samples of a single quantity (in particular, the magnetic perturbation time derivative signal has been used). With this kind of predictor, the recognition of disruptions does not depend on how large the signal amplitude is but on how large the signal increments are: small increments mean smooth plasma evolution whereas abrupt increments reflect a non-smooth evolution and potential risk of disruption. Results are presented with data from the JT-60U tokamak and high-beta discharges. Two training methods have been tested: a classical approach in which the more data for training the better and an adaptive method that starts from scratch. In both cases the success rate is about 95%. It should be noted that predictors based on signal increments and their adaptive versions can be of big interest for next devices such as JT-60SA or ITER.
Minimum sets of in-vessel components were installed for initial operation of JT-60SA in order to product and control the plasma and to know the plasma basic information, including inboard first wall, ...upper divertor, protection limiter, glow electrodes and magnetic sensors. All of magnetic sensors were installed within the required accuracy with in-situ measurement with laser tracker. The inboard first wall and the upper divertor were installed for the limiter configuration at the plasma initiation and the divertor configuration, respectively. They have carbon tiles as the first wall, which must be installed with accuracy of ±1 mm in order to avoid the heat concentration. However, the vacuum-vessel wall had some deformation. Therefore, we measured in advance with laser tracker the position of the vacuum vessel where the bases of the inboard first wall and upper divertor would be welded. By using these position data and 3D CAD, we customized all bases. Owing to these procedures, we could install the first wall with required accuracy. Also, all other components were installed with required accuracy.
The plasma rotation necessary for stabilization of resistive-wall modes (RWMs) is investigated by controlling the toroidal plasma rotation with external momentum input by injection of tangential ...neutral beams. The observed threshold is 0.3% of the Alfvén velocity and much smaller than the previous experimental results obtained with magnetic braking. This low critical rotation has a very weak beta dependence as the ideal wall limit is approached. These results indicate that for large plasmas such as in future fusion reactors with low rotation, the requirement of the additional feedback control system for stabilizing RWM is much reduced.
Photo-control of material properties on femto- (10(-15)) and pico- (10(-12)) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ...ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr(14-x)Ca(x)Cu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature.
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