Akinobu Matsuyama

Experimental analyses and predictive simulations of toroidal rotation driven by the neoclassical toroidal viscosity in rippled tokamaks

A cooperation framework for analyses and predictions of the neoclassical toroidal viscosity (NTV) and the resultant toroidal flow is developed among the TOPICS, VMEC and FORTEC-3D codes. With the real geometry in JT-60U taken into account, it is found that the NTV is one of the cardinal torque sources especially in the edge region irrespective of the insertion of the ferritic steel tiles (FSTs) that reduce the toroidal field ripple amplitude and is essential to numerically reproduce the measured toroidal rotation profile in the edge. The up–down asymmetric component of the NTV is damped due to the FSTs and the NTV profile correlates with the profile of the radial electric field Er. Predictive simulations for JT-60SA H-mode scenarios are also performed to investigate the effects of the NTV on toroidal rotation. The NTV reversal is observed in the pedestal region where the steep pressure gradient is formed, due to the dependence of the NTV on Er.

Design and performance of a punch mechanism based pellet injector for alternative injection in the large helical device

A low speed single barrel pellet injector, using a mechanical punch device has been developed for alternative injection in the large helical device. A pellet is injected by the combined operation of a mechanical punch and a pneumatic propellant system. The pellet shape is cylindrical, 3 mm in diameter and 3 mm in length. Using this technique the speed of the pellet can be controlled flexibly in the range of 100-450 m/s, and a higher speed can be feasible for a higher gas pressure. The injector is equipped with a guide tube selector to direct the pellet to different injection locations. Pellets are exposed to several curved parts with the curvature radii R(c) = 0.8 and 0.3 m when they are transferred in guided tubes to the respective injection locations. Pellet speed variation with pressure at different pellet formation temperatures has been observed. Pellet intactness tests through these guide tubes show a variation in the intact speed limit over a range of pellet formation temperatures from 6.5 to 9.8 K. Pellet speed reduction of less than 6% has been observed after the pellet moves through the curved guide tubes.

Drift resonance effect on stochastic runaway electron orbit in the presence of low-order magnetic perturbations

During major disruptions, an induced loop voltage accelerates runaway electrons (REs) towards high energy, being in the order of 1–100 MeV in present tokamaks and ITER. The stochastization mechanisms of such high-energy RE drift orbits are investigated by three-dimensional (3D) orbit following in tokamak plasmas. Drift resonance is shown to play an important role in determining the onset of stochastic drift orbits for different electron energies, particularly in cases with low-order perturbations that have radially global eigenfunctions of the scale of the plasma minor radius. The drift resonance due to the coupling between the cross-field drift motion with radially global modes yields a secondary island structure in the RE drift orbit, where the width of the secondary drift islands shows a square-root dependence on the relativistic gamma factor γ. Only for highly relativistic REs (γ 1), the widths of secondary drift islands are comparable with those of magnetic islands due to the primary resonance, thus the stochastic threshold becoming sensitive to the RE energy. Because of poloidal asymmetry due to toroidicity, the threshold becomes sensitive not only to the relative amplitude but also to the phase difference between the modes. In this paper, some examples of 3D orbit-following calculations are presented for analytic models of magnetic perturbations with multiple toroidal mode numbers, for both possibilities that the drift resonance enhances and suppresses the stochastization being illustrated.

Observation of three-dimensional motion of the pellet ablatant in the Large Helical Device

Application of two-point stereoscopic diagnostics using a fast camera and bundled fibre has enabled the observation of the three-dimensional nature of the pellet trajectory in the Large Helical Device. It has been observed that the pellet trajectory deviates from its injection direction, toroidally and vertically depending on the direction of the tangentially applied neutral beams. The magnitude of the toroidal deviation is similar in the clockwise as well as counter-clockwise neutral beam directions and is of 15–20 cm with a deflection speed of up to 400 m s−1. In contrast, an asymmetry in trajectory deflection has been observed in the vertical direction. Experimental results also indicate that the starting radius of the pellet trajectory bending in the counter-clockwise neutral beam case is more inward to that of the plasma. Collectively this leads to less penetration of the pellet inside the plasma and is prominent in the case of the clockwise neutral beam. Additionally, this fact supports evidence that the fast ion plays an important role in the pellet ablation process in the Large Helical Device. The pellet deflection is explained by the rocket effect due to unilateral ablation by the fast ions. The possible cause of the difference in the vertical deflection is explained by considering the geometrical aspects of the magnetic field structure.

Integrated modelling of toroidal rotation with the 3D non-local drift-kinetic code and boundary models for JT-60U analyses and predictive simulations

The integrated simulation framework for toroidal momentum transport is developed, which self-consistently calculates the neoclassical toroidal viscosity (NTV), the radial electric field and the resultant toroidal rotation together with the scrape-off-layer (SOL) physics-based boundary model. The coupling of three codes, the 1.5D transport code TOPICS, the three-dimensional (3D) equilibrium code VMEC and the 3D drift-kinetic equation solver FORTEC-3D, makes it possible to calculate the NTV due to the non-axisymmetric perturbed magnetic field caused by toroidal field coils. Analyses reveal that the NTV significantly influences in JT-60U and holds the key to determine the NTV profile. The sensitivity of the profile to the boundary rotation necessitates a boundary condition modelling for toroidal momentum. Owing to the high-resolution measurement system in JT-60U, the gradient is found to be virtually zero at the separatrix regardless of toroidal rotation velocities. Focusing on , the boundary model of toroidal momentum is developed in conjunction with the SOL/divertor plasma code D5PM. This modelling realizes self-consistent predictive simulations for operation scenario development in ITER.

Stochastic Transport of Runaway Electrons due to Low-order Perturbations in Tokamak Disruption

During disruption events in tokamak devices, runaway electrons are often generated due to the toroidal loop voltage induced with increasing the resistivity. Experimentally, avoidance of runaway generation has been demonstrated by means of magnetic perturbations applied externally or induced spontaneously [1]. If runaways are escaped from the core region in the timescale much shorter than that of avalanche multiplication, the runaway current observed after the plasma current quench is expected to be suppressed significantly. However, necessary levels of magnetic perturbations were not yet fully understood [2], which requires detailed 3-D simulations of runaway electrons in mixed magnetic topologies including nested flux surfaces, island chains and stochastic volumes.

Imaging spectroscopy diagnosis of internal electron temperature and density distributions of plasma cloud surrounding hydrogen pellet in the Large Helical Device

To investigate the behavior of hydrogen pellet ablation, a novel method of high-speed imaging spectroscopy has been used in the Large Helical Device (LHD) for identifying the internal distribution of the electron density and temperature of the plasma cloud surrounding the pellet. This spectroscopic system consists of a five-branch fiberscope and a fast camera, with each objective lens having a different narrow-band optical filter for the hydrogen Balmer lines and the background continuum radiation. The electron density and temperature in the plasma cloud are obtained, with a spatial resolution of about 6 mm and a temporal resolution of 5 × 10(-5) s, from the intensity ratio measured through these filters. To verify the imaging, the average electron density and temperature also have been measured from the total emission by using a photodiode, showing that both density and temperature increase with time during the pellet ablation. The electron density distribution ranging from 10(22) to 10(24) m(-3) and the temperature distribution around 1 eV have been observed via imaging. The electron density and temperature of a 0.1 m plasma cloud are distributed along the magnetic field lines and a significant electron pressure forms in the plasma cloud for typical experimental conditions of the LHD.

Modelling of the pellet deposition profile and ∇B-induced drift displacement in non-axisymmetric configurations

Drift displacement during density homogenization is modelled for hydrogen pellets injected into the Large Helical Device (LHD). The pellet ablation and deposition profiles are simulated for neutral-beam injection heated plasmas and are shown to reproduce well the main characteristics of the observed drift displacement for both low-field side and high-field side (HFS) injected pellets. The model describes the parallel expansion of ionized ablated pellet particle cloudlets (plasmoid) in non-axisymmetric magnetic configurations and the associated evolution of the plasmoid drift acceleration force exerted by the average magnetic field gradient over the plasmoid length. It is shown that, during the ablation and early homogenization phases, plasmoids are strongly accelerated towards the inverse direction of the local magnetic field gradient. In the case of the LHD, its direction and magnitude depend mainly on the pellet launching location with respect to the external helical coils. While such an initial drift—induced near the ablation region—is efficiently damped by plasmoid internal currents as soon as the plasmoid length becomes comparable to a toroidal connection length, a weak drift acceleration force is maintained over the whole homogenization time, whose direction depends on whether the confining magnetic field possesses a magnetic well or hill structure. Simulations show that, in a strong magnetic hill configuration like the LHD, this small but long-term drift becomes significant and results in a radially outward displacement of the mass deposition even for pellets injected from the HFS.

High-order integration scheme for relativistic charged particle motion in magnetized plasmas with volume preserving properties

Abstract A numerical algorithm for solving full gyro orbit of relativistic charged particle motion in magnetized plasmas is presented. The algorithm developed here achieves the following features simultaneously. (1) The time advancement is explicit, and (2) the integration is performed with respect to the observation time in a laboratory frame. (3) It is suitable for accurate long time integration with its volume preserving properties in the phase space. (4) The algorithm can properly treat the E × B drift velocity in electromagnetic fields for large relativistic factors, and (5) can be extended to arbitrary high orders with the aid of symmetric composition methods. Because our algorithm is formulated in the Lorentz covariant form, explicit conservation of the Minkowski norm is not assumed. Nevertheless, no secular growth of the numerical errors in the norm does occur, and its influence can be minimized up to the levels of round-off errors when a high-order scheme is applied. Numerical results are compared with explicit and implicit Runge–Kutta methods. The numerical accuracy and the computational efficiency are discussed for long time integration in toroidal magnetic field configuration.