Diffraction Patterns of the Millimeter Wave with a Helical Wavefront by a Triangular Aperture

In this paper, for the first time, we developed the measurement method of the topological charge (TC) in millimeter wave with helical wavefront by a triangular aperture. The millimeter wave with helical wavefront was passively generated by conversion from a Gaussian beam using a spiral mirror, which we also developed. Each diffraction pattern depended on TC, and negative TC and positive TC were symmetric pairs under reflection. These diffraction patterns were perfectly compatible with the calculation results

Development of the calibration method for a fast steering antenna for investigating the mode conversion window used in EBW heating in the LHD plasma

In this study, we developed a calibration method for a fast steering antenna for investigating the mode conversion window used in electron Bernstein wave heating in the large helical device. The calibration was carried out in under-dense plasma against a line-of-sight with an optical thickness which varied spatially. Although multi-reflected background radiation becomes dominant in optically thin lines-of-sight, we succeeded in calibrating the fast steering antenna by including the effect of multi-reflected background radiation in the solution of the radiation transfer equation as the constant by which the temperature of the center of the plasma is multiplied. In addition, we report the initial results of experiments investigating the mode conversion window in over-dense plasma using the calibrated antenna

Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ∼10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described

W-band millimeter-wave back-scattering system for high wave number turbulence measurements in LHD

A 90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring electron scale turbulence (kρs ∼ 40). Ametal lens relay antenna is used for in-vessel beam focusing, and a beam diameter of less than 40mm is achieved in the plasma core region.This antenna can be steered at an angle of 159○ ± 6○, which almost covers the plasma radius. The estimated size of the scattering volume is ∼105mm at the edge and 135mm at the core, respectively. A 60m corrugated waveguide is used to achieve a low transmission loss of ∼8 dB. A heterodyne detection system for millimeter-wave circuits with probing power modulation can distinguish the scattered signal frombackground noise

Isotope effect in transient electron thermal transport property and its impact on the electron internal transport barrier formation in LHD

In this study, we perform a comprehensive comparison of the transport hysteresis width in deuterium (D) plasmas, hydrogen (H) plasmas, and D-H mixed plasmas. The core focused modulation electron cyclotron resonance heating (MECH) is applied as the heat source perturbation, and the heat flux is evaluated using the energy conservation equation with the measured electron temperature response and the ECH deposition profile calculated by the ray-tracing scheme. Systematic density scan in plasmas with different ion mass reveals that there is no significant isotope effect in their hysteresis width. It is found that plasmas with heavier isotope mass can easily form the electron internal transport barrier. As the hysteresis width is insensitive to the isotope mass, the classical part of the diffusivity is considered to be responsible for the isotope effect in the transport barrier formation

Improved performance of electron cyclotron resonance heating by perpendicular injection in the Large Helical Device

A real-time interlock system for power injection in electron cyclotron resonance heating(ECRH) was developed to be applied to Large Helical Device (LHD) plasma. This systemenabled perpendicular injection, thus improving the performance of ECRH more than has everbeen achieved before in LHD. Perpendicular propagation of the electron cyclotron wave at77 GHz became more insensitive to the effect of refraction in comparison to the conventionaloblique propagation. The achieved central electron temperature in the case of perpendicularinjection was approximately 2 keV higher than that in the case of standard oblique injectionfor a central electron density of 1 × 1019 m−3 by 1 MW injection.With such improvedperformance of ECRH, high-density ECRH plasma of 8 × 1019 m−3 was successfullysustained after the injection of multiple hydrogen ice pellets for the first time in LHD

増強されたECHアンテナシステムを用いたLHDにおけるECCD適用性の向上

The power injection system for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) was modified and upgraded. An outside horizontal port 2-O on the Large Helical Device (LHD) was furnished with two antenna systems for the EC-waves of the frequencies of 77 and 154 GHz, respectively. In addition to them, two new antenna systems for 77 and 154 GHz waves were installed in the 2-O port. Each antenna in the 2-O port has wide range of EC-wave beam direction control so that these are suitable for ECCD which requires toroidally oblique EC-wave beam injection. In the LHD 18th experimental campaign in 2014-2015, an ECCD experiment with second harmonic resonance condition, on-axis magnetic field of 1.375 T for 77 GHz waves, was performed in which some combination patterns of two 77 GHz ECCDs were applied. The discharges of dual co- and dual counter-ECCDs showed remarkable plasma currents of ∼±26 kA in both of the co- and counter-directions, by 6 s pulse duration and injection powers of 366 and 365 kW. The new antenna has nearly the same capability for ECCD with that of the existing antenna. The improvement in the flexibility of the ways of applying plural ECCDs will offer a highly useful tool for investigations on the phenomena concerning with the plasma current such as magnetohydro-dynamics

Extended capability of the integrated transport analysis suite, TASK3D-a, for LHD experiment

The integrated transport analysis suite, TASK3D-a (Analysis), has been developed to be capable for routine whole-discharge analyses of plasmas confined in three-dimensional (3D) magnetic configurations such as the LHD. The routine dynamic energy balance analysis for NBI-heated plasmas was made possible in the first version released in September 2012. The suite has been further extended through implementing additional modules for neoclassical transport and ECH deposition for 3D configurations. A module has also been added for creating systematic data for the International Stellarator–Heliotron Confinement and Profile Database. Improvement of neutral beam injection modules for multiple-ion species plasmas and loose coupling with a large-simulation code are also highlights of recent developments

Development and application of a ray-tracing code integrating with 3D equilibrium mapping in LHD ECH experiments

The central electron temperature has successfully reached up to 7.5 keV in large helical device(LHD) plasmas with a central high-ion temperature of 5 keV and a central electron density of1.3×1019 m−3. This result was obtained by heating with a newly-installed 154 GHz gyrotronand also the optimisation of injection geometry in electron cyclotron heating (ECH). Theoptimisation was carried out by using the ray-tracing code ‘LHDGauss’, which was upgradedto include the rapid post-processing three-dimensional (3D) equilibrium mapping obtainedfrom experiments. For ray-tracing calculations, LHDGauss can automatically read the relevantdata registered in the LHD database after a discharge, such as ECH injection settings (e.g.Gaussian beam parameters, target positions, polarisation and ECH power) and Thomsonscattering diagnostic data along with the 3D equilibrium mapping data. The equilibrium mapof the electron density and temperature profiles are then extrapolated into the region outsidethe last closed flux surface. Mode purity, or the ratio between the ordinary mode and theextraordinary mode, is obtained by calculating the 1D full-wave equation along the directionof the rays from the antenna to the absorption target point. Using the virtual magnetic fluxsurfaces, the effects of the modelled density profiles and the magnetic shear at the peripheralregion with a given polarisation are taken into account. Power deposition profiles calculatedfor each Thomson scattering measurement timing are registered in the LHD database. Theadjustment of the injection settings for the desired deposition profile from the feedbackprovided on a shot-by-shot basis resulted in an effective experimental procedure

Stable sustainment of plasmas with electron internal transport barrier by ECH in the LHD

The long pulse experiments in the Large Helical Device has made progress in sustainment of improved confinement states. It was found that steady-state sustainment of the plasmas with improved confinement at the core region, that is, electron internal transport barrier (e-ITB), was achieved with no significant difficulty. Sustainment of a plasma having e-ITB with the line average electron density ne_ave of 1.1 × 1019 m−3 and the central electron temperature Te0 of ∼3.5 keV for longer than 5 min only with 340 kW ECH power was successfully demonstrated