K. Kawahata

National Institutes Of Natural Sciences, Edo, Tokyo, Japan

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Publications (675)752.99 Total impact

  • K. Nakayama · S. Okajima · T. Akiyama · K. Tanaka · K. Kawahata
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    ABSTRACT: Simultaneously oscillated 48-μ m and 57-μ m CH3OD lasers pumped by a 9R(8) CO2 laser have been developed as optical sources of interferometer and polarimeter for fusion plasma diagnostics. The objective of this paper is to present a method for stabilizing the two-color far-infrared (FIR) laser outputs. A stable FIR laser requires a stable pump CO2 laser and a prevention of a back reflection from the FIR laser cavity. The frequency of the pump CO2 laser has been stabilized by an external Stark-cell modulation. The back reflection has been reduced by an optical isolator using a quarter-wave plate and an absorbing thin-film reflector. The two-color FIR laser outputs have been stabilized by controlling the cavity length. As a result of these measures, we have achieved output stabilities of ± 1.2%/h at the 48-μ m laser and ± 1.3%/h at the 57-μ m laser.
    No preview · Article · Dec 2015 · Journal of Instrumentation
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    ABSTRACT: A CO2 laser dispersion interferometer (DI) has been developed for both magnetically fusion plasmas and high pressure industrial plasmas. The DI measures the phase shift caused by dispersion in a medium. Therefore, it is insensitive to the mechanical vibrations and changes in the neutral gas density, which degrade the resolution of the electron density measurement. We installed the DI on the Large Helical Device (LHD) and demonstrated a high density resolution of 2× 1017 m−3 without any vibration-free bench. The measured electron density with the DI shows good agreement with results of the existing far infrared laser (a wavelength of 119 μ m) interferometer. The DI system is also applied to the electron density measurement of high-pressure small-scale plasmas. The significant suppression of the phase shift caused by the neutral gas is proven. The achieved density resolution was 1.5× 1019 m−3 with a response time of 100 μ s. A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics
    No preview · Article · Sep 2015 · Journal of Instrumentation
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    ABSTRACT: This paper describes a conceptual design of the density measurement system on the helical reactor FFHR-d1 based upon its quantitative operation scenario. The density measurement is necessary to satisfy the reactor design, and to have a high density resolution of the order of 1017 m-3 with a time resolution of 10 ms. High reliability that avoids measurement failures is also necessary. A combined system of 'a dispersion interferometer' and a polarimeter is designed and a prototype of the dispersion interferometer with an Nd:YAG laser is tested. The dispersion interferometer with a CO2 laser is also installed on LHD, which can realize a demo relevant density plasma. The prototypes demonstrate the feasibility on a demo reactor.
    No preview · Article · Sep 2015 · Nuclear Fusion
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    ABSTRACT: An analysis method for unfolding the spatially resolved wave-number spectrum and phase velocity from the 2D CO2 laser phase contrast imaging system on the large helical device is described. This is based on the magnetic shear technique which identifies propagation direction from 2D spatial Fourier analysis of images detected by a 6 × 8 detector array. Because the strongest modes have wave-number at the lower end of the instrumental k range, high resolution spectral techniques are necessary to clearly resolve the propagation direction and hence the spatial distribution of fluctuations along the probing laser beam. Multiple-spatial point cross-correlation averaging is applied before calculating the spatial power spectrum. Different methods are compared, and it is found that the maximum entropy method (MEM) gives best results. The possible generation of artifacts from the over-narrowing of spectra are investigated and found not to be a significant problem. The spatial resolution Δρ (normalized radius) around the peak wave-number, for conventional Fourier analysis, is ∼0.5, making physical interpretation difficult, while for MEM, Δρ ∼ 0.1.
    No preview · Article · Sep 2015 · Review of Scientific Instruments
  • K. Nakayama · S. Okajima · T. Akiyama · K. Tanaka · K. Kawahata
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    ABSTRACT: A two-color interferometer and polarimeter using simultaneously oscillated 48- and 57-μm CH3OD lasers pumped by a 9R(8) CO2 laser have been developed for future plasma diagnostics. Using a frequency-stabilized pump CO2 laser and a twin type far-infrared (FIR) laser, the output power and the beat frequency have been stabilized by controlling the FIR laser cavity with a stepper motor. The stable two-color laser oscillation was enabled. The output power and beat frequency stabilities of the 48-μm laser were ±2.8 %/hour and 600±18 kHzp-p/hour, respectively. The performances of the 57-μm laser were ±3.0 %/hour and 1200±18 kHzp-p/hour.
    No preview · Article · Nov 2014
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    ABSTRACT: We propose a new interferometer system for density profile measurements. This system produces multiple measurement chords by a leaky-wave antenna driven by multiple frequency inputs. The proposed system was validated in laboratory evaluation experiments. We confirmed that the interferometer generates a clear image of a Teflon plate as well as the phase shift corresponding to the plate thickness. In another experiment, we confirmed that quasi-optical mirrors can produce multiple measurement chords; however, the finite spot size of the probe beam degrades the sharpness of the resulting image.
    No preview · Article · Nov 2014 · Review of Scientific Instruments
  • T. Akiyama · R. Yasuhara · K. Kawahata · S. Okajima · K. Nakayama
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    ABSTRACT: Since a dispersion interferometer is insensitive to mechanical vibrations, a vibration compensation system is not necessary. The CO2 laser dispersion interferometer with phase modulations on the Large Helical Device utilizes the new phase extraction method which uses modulation amplitudes and can improve a disadvantage of the original dispersion interferometer: measurement errors caused by variations of detected intensities. The phase variation within ±2 × 1017 m-3 is obtained without vibration compensation system. The measured line averaged electron density with the dispersion interferometer shows good agreement with that with the existing far infrared laser interferometer. Fringe jump errors in high density ranging up to 1.5 × 1020 m-3 can be overcome by a sufficient sampling rate of about 100 kHz.
    No preview · Article · Oct 2014 · Review of Scientific Instruments
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    ABSTRACT: A multi-pass Thomson scattering (TS) has the advantage of enhancing scattered signals. We constructed a multi-pass TS system for a polarisation-based system and an image relaying system modelled on the GAMMA 10 TS system. We undertook Raman scattering experiments both for the multi-pass setting and for checking the optical components. Moreover, we applied the system to the electron temperature measurements in the GAMMA 10 plasma for the first time. The integrated scattering signal was magnified by approximately three times by using the multi-pass TS system with four passes. The electron temperature measurement accuracy is improved by using this multi-pass system.
    No preview · Article · Oct 2014 · Review of Scientific Instruments
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    ABSTRACT: We have studied two types of spatio-temporal turbulence dynamics in plasmas in the Large Helical Device, based on turbulence measurements with high spatial and temporal resolution. Applying conditional ensemble-averaging to a plasma with Edge-Localized Modes (ELMs), fast radial inward propagation of a micro-scale turbulence front is observed just after ELM event, and the propagation speed is evaluated as ∼100 m/s. A self-organized radial electric field structure is observed in an electrode biasing experiment, and it is found to realize a multi-valued state. The curvature of the radial electric field is found to play an important role for turbulence reduction.
    No preview · Article · May 2014 · Physics of Plasmas
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    ABSTRACT: In the Large Helical Device (LHD), H-mode plasmas are produced, having large amplitude edge-localized modes (ELMs) which are induced by resistive interchange modes (RICs) at the iota/2 pi = 1 rational surface near the foot of the edge transport barrier (ETB). These large ELMs expel a large fraction of plasma stored energy, up to 20% of the stored energy (Wp). The ETB and the iota/2 pi = 1 surface are thought to be in the stochastic field region (SFR) generated intrinsically in three-dimensional magnetic configuration on LHD, because plasma shielding effects for such stochastic fields are not significant due to low electrical conductivity sigma and moderate angular frequency of plasma rotation. in the ETB and the field penetration depth is estimated to be comparable to the ETB width. Even if the ETB is in such intrinsic SFR where electron mean free path is much shorter than the connection length, large amplitude ELMs are excited. Such ELMs were mitigated by externally applied m = 1/n = 1 resonant magnetic perturbations (RMPs), for the first time, in a stellarator/helical plasma, where m and n are the poloidal and toroidal mode numbers, respectively. The RMPs reduce the amplitude, enhancing the repetition frequency significantly. The energy loss fraction Delta W-p/W-p is reduced less than similar to 5% by the mitigation. This mitigation is realized without terminating the H-mode and large penalty to global energy confinement. The RMPs preferentially decrease electron density in ETB indicating enhanced particle transport, while electron and ion temperature profiles are nearly unchanged. Plasma shielding effects for the applied RMPs is also not significant because of low sigma and moderate omega in ETB. Noticeable decrease in the gradients of electron density and pressure in the ETB and also at the iota/2 pi = 1 surface is induced by the penetration of the applied RMPs. Enhanced ELM frequency indicates that MHD stability of ETB for RICs is degraded by RMPs. Potential mechanisms of ELM mitigation on LHD are discussed.
    No preview · Article · Mar 2014 · Nuclear Fusion
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    ABSTRACT: A collective Thomson scattering (CTS) diagnostic was developed and used to measure the bulk and fast ions originating from 180 keV neutral beams in the Large Helical Device (LHD). Electromagnetic waves from a gyrotron at 77 GHz with 1 MW power output function as both the probe and electron cyclotron heating beam. To clarify the diagnostic applicability of the gyrotron in the 77 GHz frequency band, we investigated the dependence of the probe and receiver beam trajectories in plasmas with high electron densities of (4–5) × 10 19 m −3 and low electron densities of (1–2) × 10 19 m −3 . At high density, a stray radiation component was observed in the CTS spectrum whereas it was negligibly small at low density. The CTS spectrum was measured and analysed after the in situ beam alignment using a beam scan. Qualitatively, the CTS spectrogram shows consistent response to ion temperatures of 1–2 keV for electron densities of (1–2) × 10 19 m −3 and electron temperatures of 2–4 keV. The measured CTS spectrum shows an asymmetric shape at the foot of the bulk-ion region during the injection of 180 keV fast ions. This shape is explained by the fast-ion distribution in the velocity space (v || , v ⊥) based on Monte Carlo simulation results. The analysis method of the CTS spectra is used to evaluate the ion temperature and fast-ion velocity distribution from the measured CTS data.
    No preview · Article · Jan 2014 · Nuclear Fusion
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    ABSTRACT: A multi-pass Thomson scattering (TS) system has the advantage of enhancing scattered signals. We constructed a multi-pass TS system modeled on the GAMMA 10 TS system; the new system has a polarization-based configuration with an image relaying system. For the first time, we used the new system to measure electron temperatures in the GAMMA 10 plasma. By using the multi-pass TS system with four passes, the integrated scattering signal was magnified by approximately a factor of three.
    Full-text · Article · Jan 2014 · Plasma and Fusion Research

  • No preview · Article · Jan 2014 · Plasma and Fusion Research
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    ABSTRACT: We report observations of the dynamic response of micro-fluctuations and turbulent flux to a low-frequency heating power modulation in the Large Helical Device. The responses of heat flux and micro-fluctuation intensity differ from that of the change in temperature gradient. This result violates the local transport model, where turbulence is determined by the local temperature gradient. A new relationship between flux, gradient and turbulence is found. In addition to the temperature gradient, the heating rate is proposed as a new, direct controlling parameter of turbulence to explain the fast response of turbulence against periodic modulation of heating power.
    No preview · Article · Nov 2013 · Nuclear Fusion
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    ABSTRACT: The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.
    No preview · Article · Oct 2013 · Nuclear Fusion
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    ABSTRACT: Since the first H-mode discharges in 2010, the duration of the H-mode state has been extended and a significantly wider operational window of plasma parameters has been attained. Using a second neutral beam (NB) source and improved tuning of equilibrium configuration with real-time plasma control, a stored energy of W-tot similar to 450 kJ has been achieved with a corresponding energy confinement time of tau(E) similar to 163 ms. Recent discharges, produced in the fall of 2012, have reached plasma beta(N) up to 2.9 and surpassed the n = 1 ideal no-wall stability limit computed for H-mode pressure profiles, which is one of the key threshold parameters defining advanced tokamak operation. Typical H-mode discharges were operated with a plasma current of 600 kA at a toroidal magnetic field B-T = 2 T. L-H transitions were obtained with 0.8-3.0MW of NB injection power in both single-and double-null configurations, with H-mode durations up to similar to 15 s at 600 kA of plasma current. The measured power threshold as a function of line-averaged density showed a roll-over with a minimum value of similar to 0.8 MW at (n) over bar (e) similar to 2 x 10(19) m(-3). Several edge-localized mode (ELM) control techniques during H-mode were examined with successful results including resonant magnetic perturbation, supersonic molecular beam injection (SMBI), vertical jogging and electron cyclotron current drive injection into the pedestal region. We observed various ELM responses, i.e. suppression or mitigation, depending on the relative phase of in-vessel control coil currents. In particular, with the 90 degrees phase of the n = 1 RMP as the most resonant configuration, a complete suppression of type-I ELMs was demonstrated. In addition, fast vertical jogging of the plasma column was also observed to be effective in ELM pace-making. SMBI-mitigated ELMs, a state of mitigated ELMs, were sustained for a few tens of ELM periods. A simple cellular automata ('sand-pile') model predicted that shallow deposition near the pedestal foot induced small-sized high-frequency ELMs, leading to the mitigation of large ELMs. In addition to the ELM control experiments, various physics topics were explored focusing on ITER-relevant physics issues such as the alteration of toroidal rotation caused by both electron cyclotron resonance heating (ECRH) and externally applied 3D fields, and the observed rotation drop by ECRH in NB-heated plasmas was investigated in terms of either a reversal of the turbulence-driven residual stress due to the transition of ion temperature gradient to trapped electron mode turbulence or neoclassical toroidal viscosity (NTV) torque by the internal kink mode. The suppression of runaway electrons using massive gas injection of deuterium showed that runaway electrons were avoided only below 3 T in KSTAR. Operation in 2013 is expected to routinely exceed the n = 1 ideal MHD no-wall stability boundary in the long-pulse H-mode (>= 10 s) by applying real-time shaping control, enabling n = 1 resistive wall mode active control studies. In addition, intensive works for ELM mitigation, ELM dynamics, toroidal rotation changes by both ECRH and NTV variations, have begun in the present campaign, and will be investigated in more detail with profile measurements of different physical quantities by techniques such as electron cyclotron emission imaging, charge exchange spectroscopy, Thomson scattering and beam emission spectroscopy diagnostics.
    Full-text · Article · Oct 2013 · Nuclear Fusion
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    ABSTRACT: In this study, the effects of resonant magnetic perturbation (RMP) on particle transport are investigated in Large Helical device (LHD). The magnetic configuration is selected to be the outwardly shifted configuration, for which the magnetic axis position (Rax) is 3.9 m. At Rax = 3.9 m, the main plasma is surrounded by a thick ergodic layer, with width of about 30% of the plasma minor radius. The perturbation mode m/n = 1/1, where m and n are poloidal and toroidal mode numbers, is applied. The resonant layer is around the last closed flux surface. With RMP, a region in which both the connection and Kolmogorov lengths are finite and the magnetic field is ergodic forms; this region extends inside the main plasma. In the low-collisionality regime, where νh∗< 1(νh∗= 1 is the boundary between the 1/ν and plateau regimes in stellarator/helical neoclassical transport), there is no apparent difference in particle transport with and without RMP. However, in the high-collisionality regime (νh∗ > 1), a clear difference in particle transport is found. A clear difference in turbulence is also observed, suggesting that turbulence plays a significant role in particle transport in the high-collisionality regime both with and without RMP.
    Preview · Article · Jan 2013 · Plasma and Fusion Research
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    ABSTRACT: First mirrors in fusion devices have to retain the reflectivity and the degradation mechanism and dominant plasma operations that affect the reflectivity have to be understood. The reflectivity of visible laser light (635 nm) of a corner cube mirror is almost entirely determined by the initial hydrogen glow discharges for wall conditioning before conducting a main plasma experiment in the Large Helical Devices (LHD). The hydrogen glow discharge forms a carbon deposition layer on the mirror surface, which degrades its reflectivity. A neon glow discharge and subsequent main discharges had less effect.
    No preview · Article · Jan 2013 · Plasma and Fusion Research
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    ABSTRACT: For the quantitative analysis measured by the CTS diagnostic, we have improved the hardware mentioned in the above. The CTS spectra behave qualitatively, and the quantitative analysis is currently in progress to obtain the ion temperature.
    No preview · Article · Jan 2013
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    ABSTRACT: A notch filter for the rejection of stray light from gigahertz range heating sources was developed to protect a vulnerable microwave plasma diagnostic system. As one of the applications, we consider the installation of the notch filter into the receiver of a collective Thomson scattering diagnostic in the Large Helical Device. Experimental observations indicate that two types of notch filters are required for main and spurious mode rejection; they have very narrow, steep shapes to avoid disturbing the diagnostic signal. On the basis of numerically simulated results, notch filters were fabricated, and their performance was evaluated. An attenuation level of 35 dB at 74.746 GHz with a 3 dB bandwidth of 0.49 GHz is achieved by two pairs of resonator cavities. This attenuation is acceptable in our study.
    No preview · Article · Jan 2013 · Plasma and Fusion Research

Publication Stats

5k Citations
752.99 Total Impact Points


  • 2008-2015
    • National Institutes Of Natural Sciences
      Edo, Tokyo, Japan
  • 1990-2015
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 1990-2013
    • The Graduate University for Advanced Studies
      • Department of Fusion Science
      Миура, Kanagawa, Japan
  • 1974-2011
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya-shi, Aichi-ken, Japan
  • 2006
    • Japan Synchrotron Radiation Research Institute (JASRI)
      Tatsuno, Hyōgo, Japan
  • 1983-2006
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan
  • 2005
    • Budker Institute of Nuclear Physics
      Novo-Nikolaevsk, Novosibirsk, Russia
  • 1985-2005
    • Chubu University
      • College of Engineering
      Касугай, Aichi, Japan
  • 1980-1988
    • Osaka University
      • Department of Applied Physics
      Suika, Ōsaka, Japan