Table of contents

The superconducting machine LHD has conducted long pulse experiments for four years to achieve long-duration plasmas with high performance. The operational regime was largely extended in discharge duration and plasma density. In this paper, the plasma characteristics, in particular, plasma performance and impurity behaviour in long pulse discharges are described. Confinement studies show that global energy confinement times are comparable to those in short pulse discharges. Long sustainment of high performance plasma, which is equivalent to the previous achievement in other devices, was demonstrated. Long pulse discharges enabled us to investigate impurity behaviour in a long timescale. Intrinsic metallic impurity accumulation was observed in a narrow density window (2–3×10¹⁹ m⁻³) only for hydrogen discharges. Impurity transport study by using active impurity pellet injection shows a long impurity confinement time and an inward convection in the impurity accumulation window, which is consistent with the intrinsic impurity behaviour. The pulsed neon gas injection experiment shows that the neon penetration into the plasma core is caused by the inward convection due to radial electric field. Finally, impurity accumulation control with an externally induced magnetic island at the plasma edge was demonstrated.

New observations of sub-cyclotron frequency instability in low aspect ratio plasmas in national spherical torus experiments are reported. The frequencies of observed instabilities correlate with the characteristic Alfvén velocity of the plasma. A theory of localized compressional Alfvén eigenmodes (CAE) and global shear Alfvén eigenmodes (GAE) in low aspect ratio plasmas is presented to explain the observed high frequency instabilities. CAEs/GAEs are driven by the velocity space gradient of energetic super-Alfvénic beam ions via Doppler shifted cyclotron resonances. One of the main damping mechanisms of GAEs, the continuum damping, is treated perturbatively within the framework of ideal MHD. Properties of these cyclotron instability ions are presented.

A new gyrokinetic toroidal particle code has been developed to study the ion temperature gradient (ITG) driven turbulence in reactor relevant tokamak parameters. We use a new method based on a canonical Maxwellian distribution F_CM(P_φ,ε,μ), which is defined by three constants of motion in the axisymmetric toroidal system—the canonical angular momentum P_φ, the energy ε, and the magnetic moment μ. A quasi-ballooning representation enables linear and nonlinear high-m,n global calculations to be carried out, with a good numerical convergence. Conservation properties are improved by using optimized particle loading. From comprehensive linear global analyses over a wide range of unstable toroidal mode numbers (n = 0–100) in large tokamak parameters (a/ρ_ti = 320–460), it is found that the reversed shear configuration produces an effective stabilizing effect on the ITG mode in the q _min region through global effects. In the nonlinear simulation, it is found that the new method based on F_CM can simulate a zonal flow damping correctly; and spurious zonal flow oscillations, which are observed in a conventional method based on a local Maxwellian distribution F_LM(ψ,ε,μ), do not appear in the nonlinear regime.

The mechanism for formation of a steep structure in the radial electric field is a key issue in plasma confinement. Properties of the radial electric field bifurcation are studied taking into account the effect of electrode biasing. The radial electric field structure is determined by the charge conservation equation. From the nonlinear mechanism associated with local current due to ion bulk viscosity, a transition can take place. Various types of radial electric field structures with multiple peaks are allowed for the same boundary condition. The ion orbit loss term breaks the symmetry of the radial current similarly to the ambipolar radial electric field. A radial current driven by the electrode plays the role of a control parameter in a transition similarly to the pressure gradient. A phase diagram is given in the spontaneous drive vs external drive space. Differences in the radial shape of solitary electric field structures are demonstrated in the presence of spatial varying components. This study clarifies the mechanisms of nonlinear structure formation in transport barriers.

Error field optimization on DIII-D tokamak (Luxon J.L. 2002 Nucl. Fusion42 814) plasma discharges has routinely been done for the last ten years with the use of the external `n = 1 coil' or the `C-coil'. The optimum level of correction coil current is determined by the ability to avoid the locked mode instability and access previously unstable parameter space at low densities. The locked mode typically has toroidal and poloidal mode numbers n = 1 and m = 2, respectively, and it is this component that initially determined the correction coil current and phase. Realization of the importance of nearby n = 1 mode components m = 1 and m = 3 has led to a revision of the error field correction algorithm. Viscous and toroidal mode coupling effects suggested the need for additional terms in the expression for the radial `penetration' field B_pen that can induce a locked mode. To incorporate these effects, the low density locked mode threshold database was expanded. A database of discharges at various toroidal fields, plasma currents, and safety factors was supplemented with data from an experiment in which the fields of the n = 1 coil and C-coil were combined, allowing the poloidal mode spectrum of the error field to be varied. Using this technique to optimize the error field reveals a new and clear low density limit determined by the onset of runaway electrons, not locked modes. A multi-variate regression analysis of this new low density locked mode database was done to determine the low density locked mode threshold scaling relationship n_e∝B_T^−0.01q₉₅^−0.79B_pen and the coefficients of the poloidal mode components in the expression for B_pen. Improved plasma performance is achieved by optimizing B_pen by varying the applied correction coil currents.

In the vicinity of a magnetic island in tokamaks, the toroidal symmetry in the magnitude of the magnetic field |B| is broken. This leads to enhanced radial transport fluxes and momentum dissipation. The radial electric field can be determined from the quasineutrality condition, or equivalently the momentum equation, on the island magnetic surface. The equation that governs the radial electric field is nonlinear and can have bifurcated solutions. This may suppress turbulence fluctuations and improve plasma confinement. The theory remains valid for a rotating island with an appropriate re-interpretation of the radial electric field.

We investigated the differences between type I and type II edge localized modes (ELMs) in ASDEX Upgrade using the MHD stability analysis. When plasma conditions are changed from typical type I ELMy conditions to type II ELMy conditions, the character of the edge instabilities change. With increased triangularity and edge safety factor, the low-n peeling–ballooning mode becomes more stable and more localized to the edge region. We find the same mode localization effect in almost double null configuration. When the plasma density is increased, the access to the second stability of the high-n ballooning modes is closed. The changes in the stability properties give a qualitative explanation to the observed small amplitude of the type II ELMs.

During the preparation of the procurement specifications of ITER for long lead-time items, several detailed vacuum vessel (VV) design improvements are being pursued, such as elimination of the inboard triangular support, adding a separate interspace between inner and outer shells for independent leak detection of field joints, and revising the VV support system to gain more structural performance margin. Improvements to the blanket design are also under investigation, an inter-modular key instead of two prismatic keys and a co-axial inlet–outlet cooling connection instead of two parallel pipes. One of the most important achievements in the VV R&D has been demonstration of the necessary assembly tolerances. Further development of cutting, welding and non-destructive tests for the VV has been continued, and thermal and hydraulic tests have been performed to simulate the VV cooling conditions. In FW/blanket and divertor, full-scale prototypical mock-ups of the FW panel, the blanket shield block, and the divertor components, have been successfully fabricated. These results make us confident in the validity of our design and give us possibilities of alternate fabrication methods.

Plasmas heated by ICRF only in the JET tokamak show distinct structures in the toroidal rotation profile, with regions where dω/dr>0 when the minority cyclotron resonance layer is far off-axis. The rotation is dominantly co-current with a clear off-axis maximum. There is only a slight difference between a high-field side (HFS) or a low-field side position of this resonance layer: the off-axis maximum in the rotation profile is modestly higher for the HFS position. This is in contrast to the predictions of theories that rely mainly on the effects arising from ICRF-driven fast ions to account for ICRF-induced plasma rotation. The differences due to the direction of the antenna spectrum (co- or counter-) are small. A more central deposition of the ICRF power in L-mode and operation in H-mode both lead to more centrally peaked profiles, both in the co-direction. Strong MHD modes brake the rotation and lead to overall flat rotation profiles.