The Large Helical Device (LHD) is a heliotron-type device employing large-scale superconducting magnets to enable advanced studies of net-current-free plasmas. The major goal of the LHD experiment ...

Goal and Achievements of Large Helical Device Project

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 ? 1021?m?3 due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2?keV at the density of 1.6 ? 1019?m?3, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

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Development of net-current free heliotron plasmas in the Large Helical Device

ICRF heating in the Large Helical Device is studied applying two global simulation codes; a drift kinetic equation solver, GNET, and a wave field solver, TASK/WM. Characteristics of energetic ion distributions in the phase space are investigated changing the resonance heating position; i.e. the on-axis and off-axis heating cases. A clear peak of the energetic ion distribution can be seen in the off-axis heating case because of the stable orbit of resonant energetic ions. The simulation results are also compared with experimental results evaluating the count number of the neutral particle analyzer and a relatively good agreement is obtained.

https://iopscience.iop.org/article/10.1088/0029-5515/46/7/S05/pdf

A global simulation study of ICRF heating in the LHD

The deuterium experiment started from March 2017 on the large helical device (LHD) as a part of the LHD high-performance upgrade project. The objectives of the deuterium experiment are: 1) to realize the high-performance operation; 2) to explore the physics of isotope effect; 3) to demonstrate the confinement capability of energetic particles in helical devices; and 4) to explore the research on plasma–material interactions using the benefit of stable steady-state operation ability of LHD. As preparations for deuterium experiment, the positive-ion-based neutral beam injectors (NBIs) are upgraded to increase their injection power in deuterium operation. On the other hand, the injection power of negative-ion-based NBI is deteriorated due to the isotope effect of negative-ion source. The neutron diagnostic and the exhaust detritiation system are newly installed for the deuterium experiment. The commissioning of LHD for the deuterium experiment is quite successful. The high ion temperature operation region is extended by the deuterium experiment. Ion temperature exceeding 9 keV is achieved with the deuterium operation of LHD.

Preparation and Commissioning for the LHD Deuterium Experiment

The deuterium operation was initiated in the LHD in 2017. In the first campaign of the deuterium experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The confinement characteristics of ITB plasmas were compared between hydrogen and deuterium discharges. The ion thermal diffusivity was reduced in the ion-ITB plasmas with deuterium compared with the plasmas without deuterium. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case.

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Realization of high T i plasmas and confinement characteristics of ITB plasmas in the LHD deuterium experiments

Confinement of energetic ions from neutral beam injection heating is investigated by changing the magnetic field configuration of the Large Helical Device from a classical heliotron configuration t...

Effect of Neoclassical Transport Optimization on Energetic Ion Confinement in LHD

The confinement of high‐energy particles produced by ICRF heating was studied in LHD experimentally setting magnetic axis at 3.6m and 3.75m in two methods. It was concluded that the confinement of high‐energy particles with Rax=3.6 m was better than that with Rax=3.75m. Based on this result the long pulse experiments were conducted in the magnetic configuration of Rax=3.6m. The plasma was sustained for 150sec. The total input power reached 71MJ though the pulse length was limited by the collapse of plasma due to density increase. The second harmonic experiment was done also with Rax=3.6m. The stored energy increased and the particles with the high‐energy were observed.

T. Ozaki

Papers

Effect of Neoclassical Transport Optimization on Energetic Ion Confinement in LHD

The Analysis of ICRF Heating Experiment in View of the Confinement of High‐energy Particles in LHD