Tadao Uyama

The internal magnetic field structures and current density profiles in the Helicity Injected Spherical Torus plasma driven by coaxial helicity injection

In the Helicity Injected Spherical Torus device [Nagata et al., Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference, Yokohama, 1998 (International Atomic Energy Agency, Vienna, 1998) CD-ROM, EXP4/10], internal magnetic field and current density structures of spherical torus (ST) plasmas sustained by coaxial helicity injection (CHI) have been revealed via intensive internal magnetic measurements. The internal magnetic configuration of the ST plasma formed by CHI is in good agreement with the results of numerical equilibrium fitting calculations. The generation of closed poloidal flux of ST has been verified by varying the external toroidal field strength in the same device. Interestingly, the transition of the current profile from hollow to peaked has been observed during the sustainment phase, which could be explained by inductive effects of currents on open field lines winding the central conductor.

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.

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.

Compact Toroidal Plasma with Toroidal and Poloidal Magnetic Fields

Compact toroidal plasma has been experimentally obtained in a drum-type copper vessel on injecting an annular plasma stream from a magnetized plasma gun. Toroidal and poloidal magnetic fields, B t and B p , of the plasma toroid show a typical exponential decay with 110 µs and no MHD instability is observed. Experimental data of B t and B p configuration for long confinement are described fairly well by the model, which is the fundamental-mode state of the minimum energy force-free equilibrium estimated from the low- β limit Grad-Shafranov equation. The configuration of the stable plasma is hence fully-determined by the metal vessel geometry.

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.

Partially relaxed magnetohydrodynamic equilibria with bias-flux leakage obtained in a helicity-driven spheromak

In the flux amplification compact torus experiment, the bias flux penetrates the wall of the flux conserver because it has been switched on a suitably long time prior to the production of a seed spheromak. The magnetohydrodynamic equilibrium configurations with a non-constant λ(≡µ 0 j · B / | B | 2 ) profile of a helicity-driven spheromak incorporating the situation of the bias-flux leakage out of the flux conserver are numerically determined by using the novel combination of the finite difference and the boundary element method. Here µ 0 , j and B are the permeability of vacuum, the current density and the magnetic field, respectively. The results of computations show that the effects of the bias-flux leakage cause a slight rise of the entire safety factor q in the case of I C / I P =0.4 ( I C and I P denote the total current of coil and plasma, respectively) because of the decrease in poloidal field over all space. Then, they also give rise to a 22.0% decrease in the magnetic flux inside the separatrix.

Formation and Sustainment of a Spheromak with BiasFlux by DC Helicity Injection

The formation and sustainment of a spheromak plasma with bias flux has been demonstrated successfully by “DC” or “electrostatic” helicity injection. The experimental results show that, as the helicity injection rate is increased, the self-generated toroidal plasma current and the toroidal flux increase significantly. By examination of the contours of poloidal flux of the sustained spheromak, we have verified the formation of the magnetic configuration with the external bias flux linking the region of the closed flux surfaces. In addition, it has been found that the λ spk ≡ j // / B value of the sustained spheromak configurations with a small amount of the closed flux in the flux conserver has been affected by the ratio of the electrode current to the bias flux.

MHD simulation of relaxation to a flipped ST configuration

The dynamics of spherical torus (ST) plasmas, when the external toroidal magnetic field is decreased to zero and then increased in the opposite direction, has been investigated using three-dimensional magnetohydrodynamic (MHD) numerical simulations. It has been found that the flipped ST configuration is self-organized after the ST configuration collapses because of the growth of the n = 1 mode in the open flux region and a following magnetic reconnection event. During the transition between these configurations, not only the paramagnetic toroidal field but also the poloidal field reverses polarity spontaneously.

CT Injection Experiment in JFT-2M

Abstract A compact toroid (CT) injector is considered to comprise one of the advanced refueling methods for the fusion reactor. In JFT-2M, it was demonstrated for the first time that a CT injected into a neutral beam-heated plasma penetrated deeply into the plasma and caused a rapid increase in the electron density. We also observed interesting motions such as shift and reflection of the injected CT plasma as well as magnetic fluctuations induced just after CT injection. A power spectrum analysis suggested that this fluctuation was related to magnetic reconnection between the CT plasmoid and the toroidal field. We also modified the shape of the CT injector electrodes to improve CT injection efficiency. As a result, the CT parameters were superior to those of previous experiments, and the operational window became broader. Finally, it has been successfully demonstrated that a CT could be transported smoothly through curved drift tubes, which is one of the key technologies to avoid adverse effects of the toroidal field and to extend flexibility of the design and layout of the CT injector to access larger devices.

A Two-Stage Ion Heating Obtained in a Rapid Theta-Compression Experiment. : II

The process of a two-staged ion heating of counter streaming plasma flows under a rapidly increasing magnetic field is studied by measuring plasma parameters and changes of fields. In the early stage of the compression, ions of the initial plasma are heated up to 300∼400 eV till 240 ns by the reflected ions from the magnetic piston existing near the wall. By a stable and strong azimuthal current sheet around the plasma column existed already in the encounter process, a large magnetic well is formed after 240 ns. By the interaction with this moving magnetic barrier, ions are accelerated effectively and the ion temperature reaches 3∼4 keV at last. The density and the temperature of electrons are (2∼3)×10 15 cm -3 and 300 eV, respectively.