Kunihiro Ogawa

Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities.

Progress in development of the neutron profile monitor for the large helical device.

The neutron profile monitor stably operated at a high-count-rate for deuterium operations in the Large Helical Device has been developed to enhance the research on the fast-ion confinement. It is composed of a multichannel collimator, scintillation-detectors, and a field programmable gate array circuit. The entire neutron detector system was tested using an accelerator-based neutron generator. This system stably acquires the pulse data without any data loss at high-count-rate conditions up to 8 × 10(5) counts per second.

Wide dynamic range neutron flux monitor having fast time response for the Large Helical Device

A fast time response, wide dynamic range neutron flux monitor has been developed toward the LHD deuterium operation by using leading-edge signal processing technologies providing maximum counting rate up to ∼5 × 10(9) counts/s. Because a maximum total neutron emission rate over 1 × 10(16) n/s is predicted in neutral beam-heated LHD plasmas, fast response and wide dynamic range capabilities of the system are essential. Preliminary tests have demonstrated successful performance as a wide dynamic range monitor along the design.

Triton burnup measurements in KSTAR using a neutron activation system

Measurements of the time-integrated triton burnup for deuterium plasma in Korea Superconducting Tokamak Advanced Research (KSTAR) have been performed following the simultaneous detection of the d-d and d-t neutrons. The d-d neutrons were measured using a 3He proportional counter, fission chamber, and activated indium sample, whereas the d-t neutrons were detected using activated silicon and copper samples. The triton burnup ratio from KSTAR discharges is found to be in the range 0.01%-0.50% depending on the plasma conditions. The measured burnup ratio is compared with the prompt loss fraction of tritons calculated with the Lorentz orbit code and the classical slowing-down time. The burnup ratio is found to increase as plasma current and classical slowing-down time increase.

Magnetic configuration effects on TAE-induced losses and a comparison with the orbit-following model in the Large Helical Device

Fast-ion losses from Large Helical Device (LHD) plasmas due to toroidal Alfven eigenmodes (TAEs) were measured by a scintillator-based lost fast-ion probe (SLIP) to understand the loss processes. TAE-induced losses measured by the SLIP appeared in energy E ranges of around 50–180 keV with pitch angles χ between 35°–45°, and increased with the increase in TAE amplitudes. Position shifts of the magnetic axis due to a finite plasma pressure led not only to an increase in TAE-induced losses but also to a stronger scaling of fast-ion losses on TAE amplitudes. Characteristics of the observed fast-ion losses were compared with a numerical simulation based on orbit-following models in which the TAE fluctuations are taken into account. The calculation indicated that the number of lost fast ions reaching the SLIP increased with the increase in the TAE amplitude at the TAE gap. Moreover, the calculated dependence of fast-ion loss fluxes on the fluctuation amplitude became stronger in the case of large magnetic axis shifts, compared with the case of smaller shifts, as was observed in the experiments. The simulation results agreed qualitatively with the experimental observations in the LHD.

Current Status of Large Helical Device and Its Prospect for Deuterium Experiment

Abstract Achievement of reactor relevant plasma condition in Helical type magnetic devices and exploration in its related plasma physics and fusion engineering are the aim of the Large Helical Device (LHD) project. In the recent experiments on LHD, we have achieved ion-temperature of 8.1 keV at 1 × 1019 m−3 by the optimization of wall conditioning using long pulse discharge by Ion Cyclotron Heating (ICH). The electron temperature of 10 keV at 1.6 × 1019 m−3 was also achieved by the optimization of Electron Cyclotron Heating (ECH). For further improvement in plasma performance, the upgrade of the Large Helical Device (LHD), including the deuterium experiment, is planned. In this paper, the recent achievements on LHD and the upgrade of LHD are described.

Progress in development of neutron energy spectrometer for deuterium plasma operation in KSTAR.

Two types of DD neutron energy spectrometer (NES) are under development for deuterium plasma operation in KSTAR to understand behavior of beam ions in the plasma. One is based on the state-of-the-art nuclear emulsion technique. The other is based on a coincidence detection of a recoiled proton and a scattered neutron caused by an elastic scattering of an incident DD neutron, which is called an associated particle coincidence counting-NES. The prototype NES systems were installed at J-port in KSTAR in 2012. During the 2012 and 2013 experimental campaigns, multiple shots-integrated neutron spectra were preliminarily obtained by the nuclear emulsion-based NES system.

High detection efficiency scintillating fiber detector for time-resolved measurement of triton burnup 14 MeV neutron in deuterium plasma experiment

The behavior of the 1 MeV triton has been studied in order to understand the alpha particle confinement property in the deuterium operation of toroidal fusion devices. To obtain time evolution of the deuterium-tritium (D-T) neutron emission rate where the secondary DT neutron emission rate is approximately 1012 n/s, we designed two high detection efficiency scintillating fiber (Sci-Fi) detectors: a 1 mm-diameter scintillation fiber-based detector Sci-Fi1 and a 2 mm-diameter scintillation fiber-based detector Sci-Fi2. The test in an accelerator-based neutron generator was performed. The result shows that the directionality of each detector is 15° and 25°, respectively. It is found that detection efficiency for DT neutrons is around 0.23 counts/n cm2 for the Sci-Fi1 detector and is around 1.0 counts/n cm2 for the Sci-Fi2 detector.

Nonlinear excitation of subcritical fast ion-driven modes

In collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase-space. The growth of such structures is a nonlinear, kinetic mechanism, which provides a channel for free-energy extraction, different from conventional inverse Landau damping. However, such nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave–wave coupling can provide a seed, which can lead to subcritical instability by either one of two mechanisms. Both mechanisms hinge on a collaboration between fluid nonlinearity and kinetic nonlinearity. If collisional velocity diffusion is low enough, the seed provided by the supercritical mode overcomes the threshold for nonlinear growth of phase-space structure. Then, the supercritical mode triggers the conventional subcritical instability. If collisional velocity diffusion is too large, the seed is significantly below the threshold, but can still grow by a sustained collaboration between fluid and kinetic nonlinearities. Both of these subcritical instabilities can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. These results were obtained by modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave–wave coupling equations. This model is applied to bursty onset of geodesic acoustic modes in an LHD experiment. The model recovers several key features such as relative amplitude, timescales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, with sustained collaboration between fluid and kinetic nonlinearities.

Study on in situ calibration for neutron flux monitor in the Large Helical Device based on Monte Carlo calculations

Neutron monitoring is important to manage safety of fusion experiment facilities because neutrons are generated in fusion reactions. Monte Carlo simulations play an important role in evaluating the influence of neutron scattering from various structures and correcting differences between deuterium plasma experiments and in situ calibration experiments. We evaluated these influences based on differences between the both experiments at Large Helical Device using Monte Carlo simulation code MCNP5. A difference between the both experiments in absolute detection efficiency of the fission chamber between O-ports is estimated to be the biggest of all monitors. We additionally evaluated correction coefficients for some neutron monitors.