R. Yoshino

Generation and termination of runaway electrons at major disruptions in JT-60U

The operation conditions to avoid runaway electron generation at the major disruption have been investigated in JT-60U tokamak plasmas. It has been found that runaway electrons are not observed for low Bt of ? 2.2?T or low plasma current quench rates (I? ? -(dIp/dt)/Ip) of <50?s-1. Furthermore, they are not observed for low effective safety factors defined at the plasma edge (qeff) of ? 2.5 even for high I? of 300-400?s-1, which is the case for uncontrolled disruptions accompanied by large plasma displacements (e.g., vertical displacement events (VDEs)). On the other hand, in controlled disruptions with small plasma shifts, qeff easily increases above 8, and runaway electrons are observed even for low current quench rates of 50-100?s-1. Furthermore, it has been found that in these position controlled disruptions the runaway current tail can rapidly decay even for zero or weakly positive plasma surface voltages. These observations of the avoidance and termination of runaway electrons suggest an anomalous loss mechanism for runaway electrons.

Runaway electrons in magnetic turbulence and runaway current termination in tokamak discharges

The behaviour of runaway electrons in three types of magnetic turbulence in tokamak discharges is reviewed: (a) micromagnetic turbulence, (b) low-m/n magnetic islands in a sea of stochasticity, (c) macroscale magnetic turbulence. The confinement of runaway electrons is much better than that of bulk thermal electrons in (a) and (b), but is greatly degraded in (c). Spontaneous and intrinsic termination of runaway current, which will be favourable for tokamak fusion reactors in order to reduce the heat flux on the first wall, was first found in JT-60U by decreasing the safety factor at the plasma surface qs to around 2 or 3 by three different methods: (i) controlled inward plasma shift, (ii) a vertical displacement event, (iii) plasma current rampup.

Fast plasma shutdown by killer pellet injection in JT-60U with reduced heat flux on the divertor plate and avoiding runaway electron generation

A killer pellet is an impurity pellet that is injected into a tokamak plasma in order to terminate a discharge without causing serious damage to the tokamak machine. In JT-60U neon ice pellets have been injected into OH and NB heated plasmas and fast plasma shutdowns have been demonstrated without large vertical displacement. The heat pulse on the divertor plate has been greatly reduced by killer pellet injection (KPI), but a low-power heat flux tail with a long time duration is observed. The total energy on the divertor plate increases with longer heat flux tail, so it has been reduced by shortening the tail. Runaway electron (RE) generation has been observed just after KPI and/or in the later phase of the plasma current quench. However, RE generation has been avoided when large magnetic perturbations are excited. These experimental results clearly show that KPI is a credible fast shutdown method avoiding large vertical displacement, reducing heat flux on the divertor plate, and avoiding (or minimizing) RE generation.

Characteristics of internal transport barriers in JT-60U reversed shear plasmas

The characteristics of internal transport barrier (ITB) structures are studied and active ITB control has been developed in JT-60U reversed shear plasmas. The following results are found. Outward propagation of ITBs with steep Ti gradients is limited to the minimum safety factor location ρqmin. However, ITBs with reduced Ti gradients can move to the outside of ρqmin. The lower boundary of the ITB width is proportional to the ion poloidal gyroradius at the ITB centre. Furthermore, active control of the ITB strength based on modification of the radial electric field shear profile is successfully demonstrated by toroidal momentum injection in different directions or an increase of heating power by neutral beams.

Fast plasma shutdown scenarios in the JT-60U tokamak using intense mixed gas puffing

Fast plasma shutdown without runaway electron generation by gas puffing is investigated in the JT-60U tokamak. Argon-only injection enables a fast shutdown; however, it induces runaway electron generation. Hydrogen-only injection generates much less runaway electrons; however, the shutdown time is considerably longer. Mixed injection of hydrogen and argon achieves a fast plasma shutdown without runaway generation. Argon atoms contribute to radiate the energy of plasma leading to a fast plasma shutdown, whilst hydrogen atoms contribute to increase the electron density for avoiding runaway electron generation.

High performance reversed shear plasmas with a large radius transport barrier in JT-60U

The operation of reversed shear plasmas in JT-60U has been extended to the low-q, high-Ip region keeping a large radius transport barrier, and a high fusion performance has been achieved. Record values of deuterium-tritium (DT)-equivalent power gain in JT-60U have been obtained: QDTeq = 1.05, τE = 0.97 s, nD(0) = 4.9 × 1019 m-3 and Ti(0) = 16.5 keV. A large improvement in confinement resulted from the formation of an internal transport barrier (ITB) with a large radius, which was characterized by steep gradients in electron density, electron temperature and ion temperature just inside the position of qmin. Large negative shear regions, up to 80% of the plasma minor radius in the low-qmin regime (qmin~2), were obtained by plasma current ramp-up after the formation of the ITB with the pressure and current profiles being controlled by adjustment of plasma volume and beam power. The ITB was established by on-axis beam heating into a low density target plasma with reversed shear that was formed by current ramp-up without beam heating. The confinement time increased with the radius of the ITB and the decrease of qmin at a fixed toroidal field. High H factors, up to 3.3, were achieved with an L mode edge. The effective one fluid thermal diffusivity χeff had its minimum in the ITB. The values of H/q95 and βt increased with the decrease of q95, and the highest performance was achieved at q95 ~3.1 (2.8 MA). The performance was limited by disruptive beta collapses with βN~2 at qmin~2.

Effect of Halo Current and Its Toroidal Asymmetry During Disruptions in JT-60U

A poloidal halo current due to a vertical displacement event (VDE) is observed in experimentally simulated VDE discharges and density limit disruptions in the JT-60U tokamak. In the case of a clockwise I{sub p} and B{sub T} discharge, the halo current flows into the vacuum vessel from the inside separatrix and goes back to the plasma from the outside separatrix. A maximum halo current is produced by a change in the poloidal flux generated by plasma current decay. A toroidal asymmetry factor of 2.5 is estimated from the requirements of the fracture of the carbon-fiber composite tiles. The toroidal asymmetry is caused by the poloidal field (PF) that is produced by the toroidal field (TF) ripple, the deformation of the vacuum vessel, the setting error between the vacuum vessel and the TF and PF coils, the low-n mode during current quench, etc. To consider this asymmetry, in JT-60U, one must estimate the total halo current as nearly 26% of the plasma current just before a current quench. 25 refs., 10 figs.

Study of plasma termination using high-Z noble gas puffing in the JT-60U tokamak

Argon, krypton and xenon were puffed with and without simultaneous hydrogen gas puffing into Ohmically heated plasmas of the JT-60U tokamak with low plasma currents in order to study the capability of disruption mitigation. It was found that krypton gas puffing can provide a plasma termination with smaller amounts of runaway electrons in comparison to argon and xenon gas puffing.

The softening of current quenches in JT-60U

Current quenches caused by density limit disruptions have been investigated in JT-60U divertor plasmas in order to develop general methods of reducing the current decay rate and of suppressing the generation of runaway electrons. The reduction of the impurity influxes at an energy quench and the direct neutral beam (NB) heating of the plasma core during a current quench were beneficial for reducing the speed of the current quench. The low stored energy just before the energy quench and the high safety factor at the plasma edge had the advantage of decreasing the impurity influxes at the energy quench. The detached plasma state was useful for degrading the energy confinement for both joule and NB heated plasmas within a short time period. Runaway electrons were not generated at plasma densities of more than 5*1019 m-3 measured just before the energy quench. Fast controlled plasma shutdown with energy quenches but without a current quench was demonstrated successfully from 2 MA to zero, with a value of dIp/dt of -6 MA/s. This shutdown was obtained by lowering the stored energy within a short time period of 20 ms, actively using the detached plasma state produced by intense helium gas puffing

Mechanism of vertical displacement events in JT-60U disruptive discharges

Enhanced vertical displacement events (VDEs), which are frequently observed in JT-60U disruptive discharges, are investigated using the Tokamak Simulation Code (TSC). The rapid plasma current quench can accelerate the vertical displacement, owing to both the up/down asymmetry of the eddy current distribution arising from the asymmetric geometry of the JT-60U vacuum vessel and the degradation of magnetic field decay index n, leading to high growth rates of positional instability. For a slightly elongated configuration (n=-0.9), the asymmetry of attractive forces on the toroidal plasma plays a dominant role in the VDE mechanism. For a more elongated configuration (n=-1.7), the degradation of field decay index n plays an important role on VDEs, in addition to the effect of asymmetric attractive forces. It is shown that the VDE characteristics of a highly elongated configuration with a rapid plasma current quench can be dominated by the field decay index degradation. It is also pointed out that both the softening of current quenches as was experimentally developed in the JT-60U tokamak, and the optimization of the allowable elongation of the plasma cross-section are critical issues in the development of a general control strategy of discharge termination