Ryosuke Seki

Steady-state operation using a dipole mode ion cyclotron heating antenna and 77 GHz electron cyclotron heating in the Large Helical Device

The steady-state operation of high-performance plasmas in the Large Helical Device (LHD) has progressed since the 2010 IAEA Conference in Korea by means of a newly installed ion cyclotron heating (ICH) antenna (HAS antenna) and an improved electron cyclotron heating (ECH) system. The HAS antenna can control the launched parallel wave number and heat the core plasma efficiently in the case of dipole mode operation. Understanding of the physics and technology of wave heating, particle and heat flow balances, and plasma?wall interactions in LHD has also improved. The heating power of steady-state ICH and ECH exceeded 1?MW and 500?kW, respectively, and a higher density helium plasma with minority hydrogen ions was maintained using the HAS antenna and new 77?GHz gyrotrons. As a result, plasma performance improved, e.g. electron temperature of more than 2?keV at a density of more than 2???1019?m?3 became possible for more than 1?min. Heat flow balance and particle flux balance of steady-state operation were evaluated. Particle balance analysis indicated that externally fed helium and hydrogen particles were mainly absorbed by the chamber wall and divertor plates, even after the 54?min operation.

Physics analyses on the core plasma properties in the helical fusion DEMO reactor FFHR-d1

Physics assessments on magnetohydrodynamics equilibrium, neoclassical transport and alpha particle confinement have been carried out for the helical fusion DEMO reactor FFHR-d1, using radial profiles extrapolated from the Large Helical Device. Large Shafranov shift is foreseen in FFHR-d1 due to its high-beta property. This leads to deterioration in neoclassical transport and alpha particle confinement. Plasma position control using vertical magnetic field has been examined and shown to be effective for Shafranov shift mitigation. In particular, in the high-aspect-ratio configuration, it is possible to keep the magnetic surfaces similar to those in vacuum with high central beta of ~8% by applying a proper vertical magnetic field. As long as the Shafranov shift is mitigated, the neoclassical heat loss can be kept at a level compatible with the alpha heating power. The alpha particle loss can also be kept at a low level if the loss boundary of alpha particles is on the blanket surface and the plasma position control is properly applied. The lost positions of alpha particles are localized around the divertor region that is located behind the blanket in FFHR-d1.

Use of a twisted 3D Cauchy condition surface to reconstruct the last closed magnetic surface in a non-axisymmetric fusion plasma

The three-dimensional (3D) Cauchy condition surface (CCS) method code, ?CCS3D?, is now under development to reconstruct the 3D magnetic field profile outside a non-axisymmetric fusion plasma using only magnetic sensor signals. A new ?twisted CCS? is introduced, whose elliptic cross-section rotates with the variation in plasma geometry in the toroidal direction of a helical-type device. Independent of the toroidal angle, this CCS can be placed at a certain distance from the last closed magnetic surface (LCMS). With this new CCS, it is found through test calculations for the Large Helical Device that the numerical accuracy in the reconstructed field is improved. Furthermore, the magnetic field line tracing indicates the LCMS more precisely than with the use of the axisymmetric CCS. A new idea to determine the LCMS numerically is also proposed.

Integrated physics analysis of plasma start-up scenario of helical reactor FFHR-d1

1D physics analysis of the plasma start-up scenario of the large helical device (LHD)-type helical reactor FFHR-d1 was conducted. The time evolution of the plasma profile is calculated using a simple model based on the LHD experimental observations. A detailed assessment of the magnetohydrodynamic equilibrium and neo-classical energy loss was conducted using the integrated transport analysis code TASK3D. The robust controllability of the fusion power was confirmed by feedback control of the pellet fuelling and a simple staged variation of the external heating power with a small number of simple diagnostics (line-averaged electron density, edge electron density and fusion power). A baseline operation control scenario (plasma start-up and steady-state sustainment) of the FFHR-d1 reactor for both self-ignition and sub-ignition operation modes was demonstrated.

High Power Heating and Steady State Operation in the Large Helical Device

Recent advances in the high power and steady state heating system and experiment results of the Large Helical Device (LHD) are reviewed in this paper. Plasma performance is extended largely through high power NBI, ECH and steady state ICRF heating devices, and improved operation techniques. The NBI of a 28 MW has extended the plasma parameter regime such as ion ITB plasmas, has a central ion temperature of more than 8 keV, and the extremely high-density plasmas ten times higher than the tokamak limit. An ECH system with seven gyrotrons (total power of 4.6MW) has been operated for pre-ionization and plasma heating. The high electron temperature regime was extended toward a higher density regime and a central electron temperature of 13.5 keV was achieved with a line-averaged electron density of ne = 1 x 1019 m-3. Steady state operation plasma with ne = 1.2 x 1019 m-3, ion and electron temperature of 2 keV, and plasma sustainment time of 48 min was achieved with ICH and ECH heating power of 1.2 MW for majority helium with minority hydrogen.

Effect of re-entering fast ions on NBI heating power in high-beta plasmas of the Large Helical Device

We calculate the heating power of the neutral beam injection (NBI) in a 〈β〉 = 4.8% high-beta discharge achieved in the Large Helical Device (LHD). We investigate the difference of the heating efficiency and the heating power profile with and without the re-entering fast ion effects. When the re-entering fast ion effects are taken into account, the heating efficiency of the co-injection of the NBI (co-NBI case) is improved and it is about 1.8 times larger than that without the re-entering effects. In contrast, the heating efficiency with the re-entering effects in the counter-injection of the NBI (ctr-NBI case) scarcely differs from that without the re-entering ones. We also study the re-entering fast ion effects on the transport properties in the LHD high-beta discharges. It is found that the tendency of the thermal conductivities on the beta value is not so much sensitive with and without the re-entering effects. In addition, we investigate the difference in the re-entering fast ion effects caused by the field strength and the magnetic configuration. In the co-NBI case, the heating efficiency with the re-entering effect was improved with a decrease in the field strength. In contrast, in the ctr-NBI case, it barely differs by changing the field strength.

Three-dimensional Cauchy-condition surface method to identify the shape of the last closed magnetic surface in the Large Helical Device

Kuriharas Cauchy-condition surface (CCS) method, originally developed for axisymmetric tokamak plasma, has been expanded to reconstruct the 3D magnetic field profile outside the non- axisymmetric plasma in the Large Helical Device (LHD). The boundary integral equations (BIEs) in terms of 3D vector potential for magnetic field sensors, flux loops and points along the CCS are solved simultaneously. In the BIE for a flux loop, the portions related to the fundamental solution are integrated along the loop. The rotational symmetry of the plasma is incorporated into the formulation to reduce the number of unknowns. The reconstructed magnetic field caused only by the plasma current agrees fairly well with the reference solution for the LHD, while good agreement is observed when adding the coil current effect to the magnetic field. The magnetic field line tracing using the reconstructed field indicates the plasma boundary (the outer surface of the stochastic region) precisely and the last closed magnetic surface agrees fairly well with the reference one.

Improvement of Automatic Physics Data Analysis Environment for the LHD Experiment

Abstract The physical data of the Large Helical Device (LHD) project have been serviced by the Analyzed Data Server system, and approximately 600 kinds of physical data are served. In order to execute simulation programs for the LHD experiment, one must gather sets of physical data. Because the Automatic Analyzed Server (AutoAna) calculates the physical data automatically, it eases the scientist’s task of collecting these physical data. The AutoAna has provided better computing environments for the scientists. Thus, the scientists, having recognized its benefits, make various requests as issues arise. In this paper, the authors introduce the current status of the AutoAna system.

Performance of Impedance Transformer for High-Power ICRF Heating in LHD

There are two types of ion cyclotron range of frequencies antennas in the Large Helical Device. The handshake form (HAS) antenna has high heating efficiency. However, its loading resistance is small, and injection power is limited by the voltage of the transmission line. On the other hand, the field-aligned-impedance-transforming (FAIT) antenna has higher loading resistance than the HAS antenna despite having a smaller antenna head. However, the high voltage on the transmission line is again the bottleneck for high-power injection, as with the HAS antenna. We developed an ex-vessel impedance transformer for the HAS and FAIT antennas to decrease the voltage on the transmission lines by increasing loading resistance. The estimated enhancement factors of loading resistance were 1.65 and 2.50 for the HAS and the FAIT antennas, respectively, and the experimental result for the HAS antenna was consistent with the estimation. Therefore, higher power injection will be possible.