Publications (17)6.92 Total impact
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Article: Reconstruction of edge density profiles on Large Helical Device using ultrashort-pulse reflectometry.
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ABSTRACT: Reflectometry has been expected to be one of the key diagnostics to measure density profiles. We have applied an ultrashort-pulse reflectometry (USPR) system to Large Helical Device in the National Institute for Fusion Science. Wide frequency band system is required to obtain wide density profile since an incident wave is reflected at the density layer corresponding to its cutoff frequency. The reflectometry utilizes an impulse with less than 30 ps pulse width as a source. Since the bandwidth of an impulse has an inverse relation to the pulse width, we can cover the frequency range of micro- to millimeter waves (18-40 GHz) with a single source. The density profiles can be reconstructed by collecting time-of-flight (TOF) signals for each frequency component of an impulse reflected from the corresponding cutoff layer. We utilize the signal record analysis (SRA) method to reconstruct the density profiles from the TOF signal. The effectiveness of the SRA method for the profile reconstruction is confirmed by a simulation study of the USPR using a finite-difference time domain method.The Review of scientific instruments 11/2008; 79(10):10F112. · 1.52 Impact Factor -
Article: Fast ion charge exchange spectroscopy measurement using a radially injected neutral beam on the large helical device.
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ABSTRACT: An experimental technique to investigate fast ion confinement based on charge exchange spectroscopy of H(alpha)-light was applied to evaluate the confinement property of perpendicular fast ions in large helical device (LHD). Sensitivities of the H(alpha) spectra to the pitch angles of injected neutral beams (NBs) and these to the angle between the sight line of the measurement and NB injection path are examined. The energy dependence of the charge exchange cross section significantly affects the observed spectra since the driving NB is injected perpendicular to the magnetic field lines in the geometry of LHD. The measured spectra are compared to the spectra of GNET simulation results and the simulated spectra agreed well with the experimental measurement when we take into account the contribution of halo neutrals. Although it is difficult to obtain the fast ion distribution functions directly, this technique provides useful experimental data in benchmarking simulation codes.The Review of scientific instruments 11/2008; 79(10):10E519. · 1.52 Impact Factor -
Article: Remote experiment of ultrashort-pulse reflectometry for large helical device plasmas
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ABSTRACT: An ultrashort-pulse reflectometer (USPR) has been applied to the large helical device plasmas in National Institute for Fusion Science for edge density profile measurement. Remote control system using super science information network has been introduced to the USPR system. The remote console at Kyushu University having graphical user interface is prepared to control the instruments of the USPR via the general-purpose interface bus. The operations such as the adjustment of supply voltage fed to amplifiers and the frequency doubler, timing control of the impulse, data acquisition, and monitoring can be performed from the remote site. The position of transmitter and receiver antennas can also be controlled remotely in order to observe the cutoff layer depending on various plasma conditions. The directly recorded signal by a sampling scope is analyzed and reconstructed by means of the signal record analysis method.Review of Scientific Instruments 11/2006; · 1.37 Impact Factor -
Article: Development of short‐wavelength far‐infrared lasers and measurement of optical constants
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ABSTRACT: A powerful short-wavelength far-infrared (FIR) laser from 40µm to 100µm in wavelength is required for the optical source of diagnostics of high-density and large-volume plasmas, and a production of -rays by inverse Compton scattering. In order to design the optical system, we have measured the optical constants (refractive index and absorption coefficient) of crystal quartz, CVD-diamond, and silicon for the short-wavelength FIR lasers. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(3): 1–8, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20082Electrical Engineering in Japan 08/2005; 153(3):1 - 8. · 0.09 Impact Factor -
Article: Ultrashort-pulse reflectometer on LHD
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ABSTRACT: We have applied an ultrashort-pulse reflectometer to large helical device plasmas for density profile measurement. The frequency range of the impulse is up-converted into the R band, which corresponds to the cutoff frequency of edge plasma region in the ordinary mode. The reflected wave from plasma is received and directly recorded by a sampling scope. This system can be controlled and monitored from remote site (Kyushu University) by using ultrawideband science information network (super-SINET) that ideal bandwidth reaches up to 1 Gbps . As a result of the measurement, we have confirmed reflection wave from the plasma when the plasma density reaches steady state during the measurement.Review of Scientific Instruments 11/2004; · 1.37 Impact Factor -
Article: Development of an Ultrashort-Pulse Reflectometer for Density Profile Measurements on GAMMA 10
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ABSTRACT: An ultrashort-pulse reflectometer is under development for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. At the heart of the system is an impulse generator which creates an 8 V, 65 ps full-width half-maximum pulse. Data was taken at the midplane of the GAMMA 10 central cell. The incident wave was launched with ordinary-mode polarization and frequency components of the reflected wave in the range 8 10 GHz were detected. The observed values of the time-of-flight are in rough agreement with those obtained from calculations using the Abel-inverted profile obtained from a 70 GHz microwave interferometer. These results suggest that the present system can be expanded to yield a reliable diagnostic for electron density profile measurements.Japanese Journal of Applied Physics 02/1998; 37:L300-L302. · 1.06 Impact Factor -
Article: Demonstration of Plasma Current Control by Using High Power Millimeter-Waves
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ABSTRACT: Active current drive is one of promising tools to improve plasma quality even in the helical devices through the control of the rotational transform or magnetic shear profiles and suppression of some magnetohydrodynamics activities. The current drive by electron cyclotron waves is the most suitable actuator for these purposes in terms of controllability of driven current with high density. Optimum conditions for efficient electron cyclotron current drive (ECCD) in large helical device (LHD) are explored using 3-dimensional ray-tracing code which can treat electron cyclotron waves with large parallel refractive index. In the experiment, inversion of plasma currents corresponding to injected ECCD modes is first demonstrated and the result can be elucidated by the Fisch-Boozer theory. Displacements of rotational transform are also verified by use of the motional stark effect polarimetry. -
Article: MHD instabilities with sharply peaked pressure profile after ice-pellets injection in the Large Helical Device
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ABSTRACT: The effect of peaked pressure profiles on the MHD behavior of the Heliotron type plasma is investigated. The pressure profile is more peaked with the ice-pellet injections than with normal gas-puffing. The degree of the peaking depends on the vacuum magnetic axis position. In inward-shifted plasma, where the interchange modes are unstable in the core region, larger levels of the MHD fluctuations are observed. Sawtooth-like relaxation events, which terminate the peaking, are also observed when the pressure gradient exceeds a threshold value. Existence of these MHD instabilities may explain the fact that we do not obtain internal density barrier discharge with inward shifted configurations. -
Article: Effect of Ellipticity on Thermal Transport in ECH Plasmas in LHD
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ABSTRACT: Effect of ellipticity on thermal transport was investigated in ECH plasmas in LHD. Ellipticity κ is scanned from 0.8 to 1.4 by controlling the quadrupole magnetic field. Experimental data of energy confinement time align with the scaling for all configurations; however, there exist systematic offsets. Performance τEexp /τEISS04 is summarized as 0.94 ± 0.02 for κ = 0.8, 1.41 ± 0.07 for κ = 1.0, and 0.91± 0.03 for κ = 1.4. Local transport analysis based on power balance indicates that the anomalous transport predominates the plasma transport. However, the observed anomaly shows correlation with the change in the effective helical ripple εeff. Physical background of this correlation and the dependence on the poloidal viscous damping rate Cp is discussed. The present experimental comparison suggests a negative evidence for the relevance of Cp. -
Article: Configuration Effects on Local Transport in High-Beta LHD Plasmas
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ABSTRACT: To distinguish between the beta effect and configuration effect due to an increment in beta on the gradual degradation of the global confinement property in high-beta plasmas on the Large Helical Device (LHD), the local transport characteristics in the high-beta plasmas are studied by considering the change in the major radius of the magnetic flux surface with the beta value. First, the influence of the change in the magnetic configuration on the local transport is studied in low-beta plasmas, and it is confirmed that the dependence of the local transport properties on the magnetic configuration is almost the same as that proposed in the ISS04 scaling in the entire plasma region. Next, the dependence of the local transport characteristics in high-beta plasmas on the major radial position of a geometric center of the magnetic flux surface is studied in comparison with that in low-beta plasmas. The dependence of the local transport in the peripheral region is correlated more with beta itself than the magnetic configuration effect, whereas the core transport appears to be correlated more with the configuration effect. -
Article: H-Mode-Like Discharge under the Presence of 1/1 Rational Surface at Ergodic Layer in LHD
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ABSTRACT: H-mode-like discharge was found in LHD with a full Bt field of 2.5 T at an outwardly shifted configuration of Rax = 4.00 m where the m⁄n = 1⁄1 rational surface is located at the ergodic layer. The H-mode-like discharge was triggered by changing the PNBI from 9 MW to 5 MW in a density range of 4-8 × 1013 cm-3, followed by a clear density rise, ELM-like Hα bursts, and a reduction of magnetic fluctuation. These H-mode-like features vanished with a small radial movement of the 1⁄1 surface. -
Article: Density Regimes of Complete Detachment and Serpens Mode in LHD
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ABSTRACT: In the Large Helical Device (LHD), the hot plasma column shrinks at the high-density regime and complete detachment takes place. Hydrogen volume recombination is observed at complete detachment. This phase is self-sustained under specific experimental conditions and called the Serpens mode (self-regulated plasma edge ‘neath the last-closed-flux-surface). The Serpens mode is achieved after either rapid or slow density ramp up, and either by hydrogen or helium gas puffing. The threshold conditions for complete detachment and the Serpens mode are experimentally documented in the parameter space of heating power and density. The threshold density for the Serpens mode transition increases with ∼ 0.4 power of the heating power. The total radiation is shown to be not adequate to describe the threshold conditions, since it mainly includes the information of very edge region outside the hot plasma column. The operational density limit in LHD, which is sustainable in steady state, has been extended to 1.7 times as high as the Sudo density limit, by applying pellet injection to the Serpens plasmas. -
Article: Progress in Impurity-Related Physics Experiments in LHD
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ABSTRACT: A variety of density profiles observed in the Large Helical Device (LHD) have suggested an interesting core impurity transport. The edge impurity transport in the ergodic layer formed by stochastic magnetic field lines with long connection length (10-2000 m) can also exhibit interesting phenomena in the competition of perpendicular and parallel transport. The LHD discharge is highly robust against impurity buildup, and operation is possible essentially up to the global power balance limit because current-driven instability does not principally exist. The LHD plasma has therefore provided information on many interesting physics issues closely related to impurities. Recent results of impurity-related physics experiments in the LHD are briefly reviewed. The specific contents presented here are (1) core impurity behavior with perpendicular transport, (2) edge impurity behavior with parallel transport, (3) high-Z discharges with high ion temperature, (4) impurity pellet injection with improved plasma performance, (5) impurity pellet ablation in the presence of energetic ions with high heat flux and (6) observation of magnetic dipole forbidden transitions for high-Z elements. A result from the Compact Helical System (CHS) is used only in the impurity pellet ablation study, because detailed data have not yet been obtained from the LHD. Finally, the results are summarized and future directions in these topics are noted. -
Article: Extension of Improved Particle and Energy Confinement Regime in the Core of LHD Plasma
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ABSTRACT: Recent two major topics of Large Helical Device (LHD) towards fusion relevant conditions, high-density operation and high-ion-temperature operation, are reported. Super dense core plasma was obtained by the combination of repetitive hydrogen ice pellet injection and high power neutral beam injection (NBI) heating. A very peaked density profile with the highest central density of 1.1 × 1021 m-3 was produced showing that the particle transport was suppressed very well in the plasma core. The spatial density profile varies as the position of magnetic axis (Rax), and the steepest profile is obtained at Rax = 3.95 m. The highest central ion temperature of 5.6 keV was obtained in hydrogen plasma at electron density of 1.6 × 1019 m-3 by NBI, where a peaked ion-temperature profile with internal ion energy transport barrier was observed. The profile of electron temperature did not change much and was broad even when the ion temperature had a peaked profile. The central ion temperature is higher than the electron temperature, which is a new operation regime of LHD. High central ion temperature accompanied strong toroidal rotation and an extreme hollow profile of carbon ions (impurity hole). These steep temperature profiles were obtained so far at around Rax = 3.6 m. The compatibility between particle and energy confinement is a new issue of LHD to explore a new operation regime for attractive fusion reactor. -
Article: Observation of the Internal Structure of Energetic Particle-Driven MHD Modes in a Large Helical Device
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ABSTRACT: For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfvén eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation. -
Article: Present Status in the Development of 6 MeV Heavy Ion Beam Probe on LHD
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ABSTRACT: In order to measure the potential in Large Helical Device (LHD), we have been developing a heavy ion beam probe (HIBP). For probing beam, gold beam is used, which is accelerated by a tandem accelerator up to the energy of 6 MeV. The experiments for calibration of beam orbit were done, and experimental results were compared with orbit calculations. The experimental results coincided fairly with the calculation results. After the calibration of the beam orbit, the potential in plasma was tried to measure with the HIBP. The experimental data showed positive potential in a neutral beam heating phase on the condition of ne ∼ 5 × 10^18 m^-3, and the increase of potential was observed when the additional electron cyclotron heating was applied to this plasma. The time constant for this increase was about a few tens ms, which was larger than a theoretical expectation. In the spatial position of sample volume, we might have an ambiguity in this experiment. -
Article: Development of a Plasma Diagnostics System Using an Impulse Waveform Voltage
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ABSTRACT: A new application using an electric impulse signal has been developed for electron density profile measurements in the Large Helical Device (LHD). Using an impulse waveform voltage, which has broadband frequency components, we constructed an ultrashort pulsed radar reflectometer with 28 channels in X-, Ka-, and U-band frequency components. The effect of the bandwidth of an inline band-pass filter was investigated, and was optimized. For a multiple-channel and multiple-frequency band system, a new switching technique and multiplexer were developed. Using the Abel inversion method, the reconstructed electron density profile was obtained from the delay time as a function of the probing frequency. Good agreement with other diagnostics was obtained.
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Institutions
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2005
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National Institute for Fusion Science
Toki, Gifu-ken, Japan
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