4,570 research outputs found
Realization of high T<sub>i</sub> plasmas and confinement characteristics of ITB plasmas in the LHD deuterium experiments
No full textThe deuterium (D) operation was initiated in the LHD in 2017. In the first campaign of the D experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature (Ti) of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The thermal confinement characteristics of ITB plasmas were compared between hydrogen and D discharges. The effective ion thermal diffusivity of the ion-ITB plasmas was found to be smaller in the D discharges than that in the H discharges. The profiles of the Ti, the electron density, and the impurity of the high Ti plasmas strongly depended on the magnetic configuration and these profiles tended to peaked in the inward-shifted configuration. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case. The GKV simulation showed the linear growth rate of TEM/ITG reduced in the plasmas with D both for the ion ITB and the electron ITB plasmas and qualitatively agreed with the tendency of the change in the thermal diffusivity obtained from the power balance analysis.journal articl
Evaluation of Spatial Resolution of Neutron Profile Monitor in LHD
No full text0000-0003-1799-5406The vertical neutron camera (VNC) has been developed to measure neutron emission profile in deuterium plasmas of the large helical device (LHD). The experiment on spatial resolution evaluation for VNC was carried out in November 2016 by using a 252 Cf neutron source of 800 MBq. The neutron source was introduced into the LHD vacuum vessel through an aluminum pipe from an upper diagnostics port to the equatorial plane. The source was placed at the following two positions. The first position is just on the cylindrical collimator axis, and the second position is in the middle of two neighboring collimator axes. The stilbene scintillation detectors are aligned radially at the collimator end to detect unscattered fast-neutrons generated in a deuterium plasma. The 3-D neutron transport calculations by using a general-purpose Monte Carlo N-particle code 6 reproduced the experimentally obtained neutron counting rate, indicating that VNC has sufficient spatial resolution and the crosstalk between two neighboring collimators is a fairly small fraction. It is expected that the VNC on LHD will function as a tool for the study on the radial transport of energetic ions.journal articl
Extended investigations of isotope effects on ECRH plasma in LHD
No full textIsotope effects of ECRH plasma in LHD were investigated in detail. A clear difference of transport and turbulence characteristics in H and D plasmas was found in the core region, with normalized radius ρ < 0.8 in high collisionality regime. On the other hand, differences of transport and turbulence were relatively small in low collisionality regime. Power balance analysis and neoclassical calculation showed a reduction of the anomalous contribution to electron and ion transport in D plasma compared with H plasma in the high collisionality regime. In core region, density modulation experiments also showed more reduced particle diffusion in D plasma than in H plasma, in the high collisionality regime. Ion scale turbulence was clearly reduced at ρ < 0.8 in high collisionality regime in D plasma compared with H plasma. The gyrokinetic linear analyses showed that the dominant instability ρ = 0.5 and 0.8 were ion temperature gradient mode (ITG). The linear growth rate of ITG was reduced in D plasma than in H plasma in high collisionality regime. This is due to the lower normalized ITG and density gradient. More hollowed density profile in D plasma is likely to be the key control parameter. Present analyses suggest that anomalous process play a role to make hollower density profiles in D plasma rather than neoclassical process. Electron scale turbulence were also investigated from the measurements and linear gyrokinetic simulations.</p
Extended capability of the integrated transport analysis suite, TASK3D-a, for LHD experiment
No full textThe 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.journal articl
Feedback control of plasma density and heating power for steady state operation in LHD
No full text0000-0003-0755-2433For steady state operation, the feedback control of plasma density and heating power system was developed in the Large Helical Device (LHD). In order to achieve a record of the long pulse discharge, stable plasma density and heating power are needed. This system contains the radio frequency (RF) heating power control, interlocks, gas fueling, automatic RF phase control, ion cyclotron range of frequency (ICRF) antenna position control, and graphical user interface (GUI). Using the density control system, the electron density was controlled to the target density and using the RF heating power control system, the RF power injection could be stable. As a result of using this system, we achieved the long pulse up to 48 min in the electron density of more than 1 × 1019 m−3. Further, the ICRF hardware experienced no critical accidents during the 17th LHD experiment campaign in 2013.journal articl
Effects of the Stochasticity on Transport Properties in High-β LHD
No full textEffects of the stochasticity of magnetic field lines on transport properties are investigated. In a high-β LHD plasma, the structure of field lines in the peripheral region becomes stochastic by finite-β effects but the finite pressure gradient exists in that region. The radial diffusion coefficient and the Kolmogorov length of stochastic field lines are estimated. In the stochastic region, the radial diffusion of stochastic field lines becomes large and the Kolmogorov length becomes short due to increasing β. In that region, the radial heat transport becomes large due to the stochasticity of field lines.journal articl
Observation of Enhanced Radial Transport of Energetic Ion due to Energetic Particle Mode Destabilized by Helically-trapped Energetic Ion in the Large Helical Device
No full text0000-0003-4555-1837A deuterium experiment was initiated to achieve higher-temperature and higher-density plasmas in March 2017 in the Large Helical Device (LHD). The central ion temperature notably increases compared with that in hydrogen experiments. However, an energetic particle mode called the helically-trapped energetic-ion-driven resistive interchange (EIC) mode is often excited by intensive perpendicular neutral beam injections on high ion-temperature discharges. The mode leads to significant decrease of the ion temperature or to limiting the sustainment of the high ion-temperature state. To understand the effect of EIC on the energetic ion confinement, the radial transport of energetic ions is studied by means of the neutron flux monitor and vertical neutron camera newly installed on the LHD. Decreases of the line-integrated neutron profile in core channels show that helically-trapped energetic ions are lost from the plasma.journal articl
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