K. Kawahata

National Institutes Of Natural Sciences, Edo, Tōkyō, Japan

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Publications (639)730.08 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Review of Scientific Instruments 11/2014; 85(11):11D411. DOI:10.1063/1.4893430 · 1.61 Impact Factor
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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.
    Review of Scientific Instruments 10/2014; 85(11). DOI:10.1063/1.4885542 · 1.61 Impact Factor
  • 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.
    Review of Scientific Instruments 10/2014; 85(11). DOI:10.1063/1.4886777 · 1.61 Impact Factor
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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.
    Physics of Plasmas 05/2014; 21(5):055904. DOI:10.1063/1.4876619 · 2.14 Impact Factor
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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.
    Nuclear Fusion 03/2014; 54(3):033001. DOI:10.1088/0029-5515/54/3/033001 · 3.06 Impact Factor
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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.
    Nuclear Fusion 01/2014; 54(02):023006. DOI:10.1088/0029-5515/54/2/023006 · 3.06 Impact Factor
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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.
    Nuclear Fusion 11/2013; 53(11-11):113006. DOI:10.1088/0029-5515/53/11/113006 · 3.06 Impact Factor
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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.
    Nuclear Fusion 10/2013; 53(10):104015. DOI:10.1088/0029-5515/53/10/104015 · 3.06 Impact Factor
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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.
    Nuclear Fusion 10/2013; 53(10):104005. DOI:10.1088/0029-5515/53/10/104005 · 3.06 Impact Factor
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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.
    Plasma and Fusion Research 01/2013; 8:2402141-2402141. DOI:10.1585/pfr.8.2402141
  • Plasma and Fusion Research 01/2013; 8:1402092-1402092. DOI:10.1585/pfr.8.1402092
  • K. Nakayama · S. Okajima · T. Akiyama · K. Tanaka · K. Kawahata
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    ABSTRACT: A two color far-infrared (FIR) laser interferometer and polarimeter using 5.2- and 6.3-THz CH3OD lasers pumped by a 9R(8) CO2 laser has been developed for future plasma diagnostics. The pump 9R(8) CO2 laser has been stabilized by an external Stark-cell modulation. The performances are the power stability of 108 ± 0.6 W/hour and the frequency stability of ± 230 kHzp-p/hour at the line center.
    Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2013 38th International Conference on; 01/2013
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    ABSTRACT: Terahertz wave diagnostics have been developed for current and future high-temperature and high-density fusion plasma measurements. Since the combined system of a reflectometer and a delayometer requires a wide-band frequency source for the density profile measurements, a terahertz pulse is a possible candidate. A test system using terahertz time-domain spectroscopy has been constructed to develop the diagnostics. The system utilizes a femtosecond fiber laser as a pumping source and a bow-tie-type photoconductive antenna as a THz wave emitter. An output THz pulse with a frequency of up to 2 THz has been obtained. Some investigations involving measurement of the refractive index of a test material are performed, and the transmission dispersion effects of an optical fiber are determined.
    Plasma and Fusion Research 01/2013; 8:2402063-2402063. DOI:10.1585/pfr.8.2402063
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    ABSTRACT: “Fusion virtual laboratory (FVL)” is the experiments’ collaboration platform covering multiple fusion projects in Japan. Major Japanese fusion laboratories and universities are mutually connected through the dedicated virtual private network, named SNET, on SINET4. It has 3 different categories; (i) LHD remote participation, (ii) bilateral experiments’ collaboration, and (iii) remote use of supercomputer. By extending the LABCOM data system developed at LHD, FVL supports (i) and (ii) so that it can deal with not only LHD data but also the data of two remote experiments: QUEST at Kyushu University and GAMMA10 at University of Tsukuba. FVL has applied the latest “cloud” technology for both data acquisition and storage architecture. It can provide us high availability and performance scalability of the whole system. With a well optimized TCP data transferring method, the unified data access platform for both experimental data and numerical computation results could become realistic on FVL. The FVL project will continue demonstrating the ITER-era international collaboration schemes and the necessary technology.
    Fusion Engineering and Design 12/2012; 87(12):2189–2193. DOI:10.1016/j.fusengdes.2012.04.027 · 1.15 Impact Factor
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    ABSTRACT: This paper presents a system called “TSMAP” that maps electron temperature profiles to flux coordinates for the Large Helical Device (LHD). Considering the flux surface is isothermal, TSMAP searches an equilibrium database for the LHD equilibrium that fits the electron temperature profile. The equilibrium database is built through many VMEC computations of the helical equilibria. Because the number of equilibria is large, the most important technical issue for realizing the TSMAP system is computational performance. Therefore, we use multiple personal computers to enhance performance when building the database for TSMAP. We use virtual machines on multiple Linux computers to run the TSMAP program. Virtual machine technology is flexible, allowing the number of computers to be easily increased. This paper discusses how the use of virtual machine technology enhances the performance of TSMAP calculations when multiple CPU cores are used.
    Fusion Engineering and Design 12/2012; 87(12):2076–2080. DOI:10.1016/j.fusengdes.2012.02.127 · 1.15 Impact Factor
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    ABSTRACT: This paper proposes a clever usage of bispectral analysis for extracting or reconstructing a quasi-coherent and quasi-periodic structural evolution of plasma fluctuations. The method has been applied on electron temperature fluctuation signals measured with a multi-point electron cyclotron emission (ECE) radiometer in the Large Helical Device (LHD). The method successfully reconstructs the averaged spatiotemporal evolution of fluctuating coherent structure (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), while the structure is buried in (or comparable in power to) background fluctuations. The bicoherence analysis has found the faint fluctuating structure consisting of a fundamental mode and the harmonic modes from the existence of significant couplings between them. The reconstructed spatiotemporal structure with the proposed method is compared with that obtained with the lock-in (conditional) average. Three cases of spatiotemporal evolution of the non-sinusoidal waveform are presented.
    Plasma Physics and Controlled Fusion 11/2012; 54(11-11):115004. DOI:10.1088/0741-3335/54/11/115004 · 2.19 Impact Factor
  • T Tokuzawa · A Ejiri · K Kawahata · K Tanaka · I Yamada · M Yoshinuma · K Ida · C Suzuki
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    ABSTRACT: In order to measure the poloidal rotation velocity, a Doppler reflectometer has been developed in Large Helical Device (LHD). A remotely controlled antenna tilting system has been installed in an LHD vacuum vessel. A synthesizer is used as the source, and the operation microwave frequency ranges are ka-band and V-band. In LHD last experimental campaign we obtained the Doppler shifted signal, which was consistent with CXRS measurements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4733736]
    The Review of scientific instruments 10/2012; 83(10):10E322. DOI:10.1063/1.4733736 · 1.61 Impact Factor
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    ABSTRACT: A novel configuration of the multi-pass Thomson scattering (TS) system is proposed to improve the time resolution and accuracy of electron temperature measurements by use of a polarization control technique. This configuration can realize a perfect coaxial multi-passing at each pass, and the number of round trips is not limited by the optical configuration. To confirm the feasibility of the new method, we installed this system in the GAMMA 10 plasma system. As a result, the integrated scattering signal of the double-pass configuration is about two times larger than that of the single-pass configuration. These results are in good agreement with the design.
    The Review of scientific instruments 10/2012; 83(10):10E326. DOI:10.1063/1.4734495 · 1.61 Impact Factor
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    ABSTRACT: In the GAMMA 10 tandem mirror, the typical electron density is comparable to that of the peripheral plasma of torus-type fusion devices. Therefore, an effective method to increase Thomson scattering (TS) signals is required in order to improve signal quality. In GAMMA 10, the yttrium-aluminum-garnet (YAG)-TS system comprises a laser, incident optics, light collection optics, signal detection electronics, and a data recording system. We have been developing a multi-pass TS method for a polarization-based system based on the GAMMA 10 YAG TS. To evaluate the effectiveness of the polarization-based configuration, the multi-pass system was installed in the GAMMA 10 YAG-TS system, which is capable of double-pass scattering. We carried out a Rayleigh scattering experiment and applied this double-pass scattering system to the GAMMA 10 plasma. The integrated scattering signal was made about twice as large by the double-pass system.
    The Review of scientific instruments 10/2012; 83(10):10E333. DOI:10.1063/1.4734490 · 1.61 Impact Factor

Publication Stats

3k Citations
730.08 Total Impact Points


  • 2008–2014
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
  • 1990–2014
    • 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
  • 1976–2011
    • Nagoya University
      • • Department of Energy Engineering and Science
      • • Department of Clinical Engineering
      Nagoya-shi, Aichi-ken, Japan
  • 2006
    • Japan Synchrotron Radiation Research Institute (JASRI)
      Tatsuno, Hyōgo, Japan
  • 2005
    • Daido Institute of Technology
      Koromo, Aichi, Japan
    • Budker Institute of Nuclear Physics
      Novo-Nikolaevsk, Novosibirsk, Russia
  • 1991–2005
    • Chubu University
      • College of Engineering
      Касугай, Aichi, Japan
  • 2001
    • Princeton University
      Princeton, New Jersey, United States
  • 1983
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan
  • 1982
    • Osaka University
      • Department of Applied Physics
      Suika, Ōsaka, Japan