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Optical chopper based mechanically Q-switched ∼3 μm Er:YAP single-crystal laser
Get PDFWe developed a compact and low-cost diode-pumped ∼3 μm mechanically Q-switched Er:YAP laser with high-power using an optical chopper. At 2.4 W pumping and 7 kHz modulation frequency, a stable single-pulse repetitively Q-switched laser with a wavelength of 2.796 μm has been observed. We recorded the pulse duration, the peak power, and the pulse energy of 264 ns, 22 W, and 5.9 μJ, respectively. As increasing pumping power to 7.4 W at a frequency of 10 kHz, the lasing wavelength shifted to 2.921 μm and a stable multiple-pulse laser was observed. The pulse with 75 ns duration and 34 μJ energy has been observed, corresponding to a peak power of 456 W. The peak power and the pulse energy are the highest value ever reported for an actively Q-switched mid-infrared (IR) Er:YAP laser. Thus, this laser has the potential to be utilized for industrial and science fields as high-power mid-IR light source.journal articl
Theoretical analysis of energetic-ion-driven resistive interchange mode stabilization strategies using a Landau closure model
Get PDF0000-0002-6114-0539The aim of the present study is to perform a theoretical analysis of different strategies to stabilize energetic-ion-driven resistive interchange mode (EIC) in Large Helical Device (LHD) plasma. We use a reduced MHD for the thermal plasma coupled with a gyrofluid model for the energetic particles (EP) species. The hellically trapped EP component is introduced through a modification of the drift frequency to include their precessional drift. The stabilization trends of the 1/1 EIC observed experimentally with respect to the thermal plasma density and temperature are reproduced by the simulations, showing a reasonable agreement with the data. The LHD operation scenarios with stable 1/1 EIC are identified, leading to the stabilization of the 1/1 EIC if the thermal plasma density and temperature are above a given threshold. The 1/1 EIC are also stabilized if the rotational transform is modified in a way that the 1/1 rational surface is located further away than 0.9 times the normalized radius, or the magnetic shear in the plasma periphery is enhanced. Also, LHD discharges with large magnetic fields show a higher EIC destabilization threshold with respect to the thermal plasma density. If the perpendicular NBI deposition region is moved further inward than 0.875 times the normalized radius the 1/1 EIC are also stabilized. In addition, increasing the perpendicular NBI voltage such that the EP energy is higher than 30 keV stabilizes the 1/1 EIC. Moreover, deuterium plasmas show a higher stability threshold for the 1/1 EIC than hydrogen plasmas. The experimental data shows a larger time interval between EIC events as the power of the tangential NBI is increased providing that the perpendicular NBI power is at least 13 MW. This implies a stabilizing effect of the tangential NBI.journal articl
Current Distribution Optimization in Electromagnet: Application to Superconducting Linear Acceleration System
Get PDF0000-0003-4355-8293The current distribution in the electromagnet is optimized so as to maximize the acceleration performance of a superconducting linear acceleration (SLA) system. As a novel pellet injection system for a fusion reactor, the SLA system has been proposed recently. The SLA system is composed of the electromagnet and the pellet container to which a high-temperature superconducting (HTS) film is attached. The pellet container is accelerated by using the Lorentz force between the HTS film and the electromagnet. In the present study, the current distribution in the electromagnet is represented as a set of the filaments by the on-off method. Moreover, the current distribution optimization is performed by using the non-dominated sorting genetic algorithm II (NSGA-II). Furthermore, the dynamic motion of the pellet container is determined by solving the equivalent-circuit model and Newton's equation of motion. According to the numerical results, the acceleration performance is improved by partly applying the electric current to the electromagnet. Compared with the case of the homogeneous current distribution, the pellet velocity for the optimized current distribution increases by 26%.journal articl
Basic research of HTS coil cooling assist technology by magnetic refrigeration
Get PDFThe fundamental research of HTS coil cooling assist technology by magnetic refrigeration has been studied. Using magnetic refrigeration technology, we are focusing on numerical analysis to see if the cooling efficiency around 20 K of high-temperature superconducting coils can be enhanced. The feature of the proposed cooling method is that it utilizes the leakage magnetic field from the HTS coil. Magnetic refrigeration technology needs to apply a change of magnetic field to a certain type of magnetic material, and the configuration of applying a change of magnetic field to the material in the cryostat is the point of the proposed cooling assist method. We confirm a system that uses a magnetic refrigerator in combination with a refrigerant circulation cooling system by utilizing the leakage magnetic field generated when using a superconducting coil and producing a magneto caloric effect has higher refrigeration performance than the refrigerant circulation cooling system alone.journal articl
Island divertor configuration design for a quasi-axisymmetric stellarator CFQS
Get PDF0000-0002-8156-8233A magnetic field configuration of an island divertor for a quasi-axisymmetric stellarator (CFQS) is proposed. The configuration incorporates large islands surrounding the core confinement region. The interface between the core region and the peripheral region of the island divertor is a clear magnetic separatrix similar to a tokamak divertor. The structure of divertor magnetic field lines is very regular without stochasticity and the connection length is sufficiently long for good divertor performance. Such a divertor configuration is produced in the magnetic field configuration for the CFQS device, which is now under construction in China.journal articl
Isotope effect in transient electron thermal transport property and its impact on the electron internal transport barrier formation in LHD
Get PDF0000-0001-5669-19370000-0002-1606-32040000-0002-0585-45610000-0003-1682-1519In 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.journal articl
Modification of laser-induced fluorescence spectrum by additional azimuthal Doppler effect in optical vortex beams
Get PDF0000-0002-0602-0665Laser-induced fluorescence (LIF) method is a powerful tool to measure flow velocities of atomic and ionic species in a plasma. Optical vortex beams, which carry orbital angular momentum of light, have a potential to expand the capability of flow velocity measurements with LIF method, because an atom moving in an optical vortex beam experiences additional Doppler effect in the azimuthal direction. The LIF spectra obtained by substituting a commonly-used plane-wave-like beam with an optical vortex beam have been numerically evaluated. It is demonstrated that modification of the spectrum can be used to detect a fast flow perpendicular to the laser path, which is impossible to be observed by conventional method in principle. The use of standard deviation of the spectrum as an index of perpendicular flow velocity is also discussed.journal articl
Conceptual design of a collimator for the neutron emission profile monitor in JT-60SA using Monte Carlo simulations
Get PDF0000-0002-7371-9652Materials and structures of a collimator for a new neutron emission profile monitor in JT-60SA are examined through Monte Carlo simulations using the Monte Carlo N-Particle transport code. First, the shielding properties of various material combinations are compared in order to determine a combination with high shielding performances against both neutrons and gamma-rays. It is found that a collimator consisting of borated polyethylene and lead has a high shielding performance against neutrons. Moreover, a high shielding performance against gamma-rays is obtained when a lead pipe with a radial thickness of 0.01 m is inserted into a collimation tube. Second, we demonstrate that it is possible to improve the spatial resolution to a desired level by installing a thin tubular extension structure that fits into the limited space available between the main collimator block and the tokamak device. Finally, the collimator structures that meet both the targeted spatial resolutions (<10% of the plasma minor radius) and the targeted counting rate (105 cps order) are discussed.journal articl
Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma
Get PDF0000-0002-6114-0539The aim of the present study is to analyze the stability of the pressure gradient driven modes (PM) and Alfvén eigenmodes (AE) in the large helical device (LHD) plasma if the rotational transform profile is modified by the current drive of the tangential neutral beam injectors (NBI). This study forms a basic search for optimized operation scenarios with reduced mode activity. The analysis is performed using the code FAR3d which solves the reduced MHD equations describing the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations for density and parallel velocity moments of the energetic particle (EP) species, including the effect of the acoustic modes. The Landau damping and resonant destabilization effects are added via the closure relation. On-axis and off-axis NBI current drive modifies the rotational transform which becomes strongly distorted as the intensity of the neutral beam current drive (NBCD) increases, leading to wider continuum gaps and modifying the magnetic shear. The simulations with on-axis NBI injection show that a counter (ctr-) NBCD in inward shifted and default configurations leads to a lower growth rate of the PM, although strong n = 1 and 2 AEs can be destabilized. For the outward shifted configurations, a co-NBCD improves the AEs stability but the PM are further destabilized if the co-NBCD intensity is 30 kA T−1. If the NBI injection is off-axis, the plasma stability is not significantly improved due to the further destabilization of the AE and energetic particle modes (EPM) in the middle and outer plasma region.journal articl
Analysis of Hydrocarbons in the Exhaust Gas of a Fusion Test Device Using Infrared Absorption Spectroscopy
Get PDF0000-0001-9941-1958To investigate the behavior of hydrogen isotopes in a large fusion test device, a new technique using infrared absorption spectroscopy is applied for the monitoring of hydrocarbons in exhaust gas. Fourier transform infrared (FT-IR) spectroscopy combined with a gas cell, which has an optical path length of 16 m, was installed at the plasma exhaust line. In this configuration, the detection limit of CH4, C2H6, C2H4, and CO is at the level of sub-ppm. The exhaust gas observations were conducted during hydrogen glow discharge and during the regeneration operation of cryosorption pumps during the hydrogen plasma phase. As a result, hydrocarbons and carbon monoxide were detected in the exhaust gas, and the exhaust behavior and the ratio of gas components were determined. It is also demonstrated that light and the heavy hydrocarbons can be discriminated by the FT-IR system. Our infrared absorption spectroscopy study provides positive prospects for its application to exhaust gas analyses.journal articl