M. Nagata

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 × 1022u2009m−3u2009s−1 after a delay of 100–200u2009µs. 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 = 1.1u2009m 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 250–350u2009kHz and propagates in the toroidal direction at the Alfven speed of the order of 106u2009mu2009s−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 high-resolution ion Doppler spectrometer for quartz ultraviolet line emissions

A high-resolution quartz ultraviolet spectrometer based on a single compact 16-channel multianode photomultiplier tube (MAPMT) has been developed and tested on coaxial helicity injection and ohmic transformer current drive plasmas generated on the Helicity Injected Torus at the University of Washington. The use of the compact MAPMT allows a simple design with high light collection and high resolution for an ion Doppler shift diagnostic. The high quality and low cost of the design make it uniquely attractive for university scale plasma experiments. The Doppler shift of impurity ion emission lines from two chords of equal impact parameter but of opposite rotational sense is used to determine chord-averaged toroidal flow velocity. The width of the Doppler broadening is used to calculate the ion temperature. A detailed description of the ion Doppler spectrometer measurement system and some typical results are presented.

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.

Instantaneous current and field structure of a gun-driven spheromak for two gun polarities

The instantaneous plasma structure of the SPHEX spheromak is determined here by numerically processing data from insertable Rogowski and magnetic field probes. Data is presented and compared for two modes of gun operation: with the central electrode biased positively and negatively. It is found that while the mean-, or even instantaneous-, field structure would give the impression of a roughly axisymmetric spheromak, the instantaneous current structure does not. Hundred per cent variations in J measured at the magnetic axis can be explained by the rotation of a current filament that has a width equal to half of the radius of the flux-conserving first wall. In positive gun operation, current leaves the filament in the confinement region leading to high wall current there. In negative gun operation, wall current remains low as all injected current returns to the gun through the plasma. The plasma, in either instance, is strongly asymmetric. We discuss evidence for the existence of the current filament in other gun-driven spheromaks and coaxial plasma thrusters.

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.

Influence of vacuum toroidal field on two-fluid flowing equilibria of helicity-driven spherical torus plasmas

Two-fluid flowing equilibrium configurations of a helicity-driven spherical torus (HD-ST) in the realistic confinement region, including a flux conserver and a coaxial helicity source, are numerically determined by means of the combination of the finite difference and the boundary element methods. It is found from the numerical results that electron fluid near the central conductor is tied to a vacuum toroidal field and ion fluid is not. The magnetic configurations change from the high-q HD-ST (safety factor, q>1) with a paramagnetic toroidal field and low-β (volume average β value, ⟨β⟩≈2%) through the helicity-driven spheromak and reversed-field pinch to the ultra-low-q HD-ST (0<q<1) with a diamagnetic toroidal field and high-β (⟨β⟩≈18%) as the vacuum toroidal field at the inner edge regions decreases and reverses the sign. The two-fluid effects are more significant in this equilibrium transition when the ion diamagnetic drift has the same direction as the E×B one.

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.