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The conceptual design of a heavy ion beam probe (HIBP) for HL-2M tokamak has been proposed to measure the plasma potential, electron density and their fluctuations to contribute to the understanding of advanced confinement modes and transport. Beam trajectory calculations are performed under 3D magnetic field and engineering conditions of the facility. By choosing the positions and angles of both primary and secondary beamlines, a wide region of observation area ( 0.1 r / a 0.9 ) is achieved with 2 MeV Tl + beam injection. The active trajectory control technique is used to obtain the poloidal sweep angle between − 10 ∘ and 10 ∘ . Then, the applied voltages of sweep plates are evaluated to be up to 25 kV. By integrating ionization and attenuation along beam paths, the detected signal level distribution in the plasma density condition n 0 = 5 × 10 19 m − 3 is higher than 100 nA with 100 μ A injection beam intensity in most of the plasma region, which ensures an acceptable signal-to-noise ratio for measurements.

Theories to understand the steep and localized radial electric field in the edge of toroidal plasma, which appears in conjunction with H-mode, is revisited based on the electric field bifurcation model. Key elements in the models of the L-H transition (including the toroidal effects on the dielectric constant and the effects of the curvature of radial electric field on turbulence suppression) are assessed. Results are applied to tokamak and helical plasmas, for which data with high-resolution have been obtained recently. The status of quantitative tests on various mechanisms through comparison with experimental observations is also addressed. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-regulation between structure and turbulence, which is a fundamental process in the complex system, has been widely regarded as one of the central issues in modern physics. A typical example of that in magnetically confined plasmas is the Low confinement mode to High confinement mode (L-H) transition, which is intensely studied for more than thirty years since it provides a confinement improvement necessary for the realization of the fusion reactor. An essential issue in the L-H transition physics is the mechanism of the abrupt “radial” electric field generation in toroidal plasmas. To date, several models for the L-H transition have been proposed but the systematic experimental validation is still challenging. Here we report the systematic and quantitative model validations of the radial electric field excitation mechanism for the first time, using a data set of the turbulence and the radial electric field having a high spatiotemporal resolution. Examining time derivative of Poisson’s equation, the sum of the loss-cone loss current and the neoclassical bulk viscosity current is found to behave as the experimentally observed radial current that excites the radial electric field within a few factors of magnitude.

A method for zonal flow study by using direct density fluctuation measurements is proposed. When ambient drift-wave turbulence is modulated by zonal flows (i.e. in the drift-wave-zonal flow systems), an envelope of the ambient density fluctuations has spectral peaks around zonal flow frequencies. A spectral peak at the geodesic acoustic mode (GAM) frequency is observed in the envelope of the ambient density fluctuations measured in edge plasma of the JFT-2M tokamak. The significant cross-bicoherence is also found between the ambient density fluctuations and its envelope in the GAM frequency. This result demonstrates that we can measure the GAM only by using density fluctuation data. This method provides a possibility of zonal flow research in burning core plasma by density fluctuation diagnostics such as microwave reflectometry.

Turbulence and zonal flow at a transport barrier are studied with twin heavy ion beam probes in a toroidal helical plasma. A wavelet analysis is used to extract turbulence properties, e.g. spectra of both density and potential fluctuations, the coherence and the phase between them and the dispersion relation. Particle transport estimated from the fundamental characteristics is found to clearly rise with their intermittent activities after the barrier is broken down. Time-dependent analysis reveals that the intermittency of turbulence is correlated with the evolution of the stationary zonal flow.

Wavelet and two-dimensional Fourier analyses of local potential and density signals were conducted. These signals were obtained in the core region of tokamak plasmas by a multiple-sample-volume heavy ion beam probe. In the low-density Ohmic plasma where the electron temperature is much higher than that of the ions, the high-frequency (HF) zonal flow in geodesic acoustic mode frequency region is more energetic than the low-frequency zonal flow from 10 to 1000 Hz in terms of integrated intensity. This result is in sharp contrast to the results of ITG simulations of the tokamak plasmas. The HF zonal flow has a short lifetime (a few cycles). The wavelet spectra of density are much more chaotic than those of potential turbulence, changing the amplitude and frequency of the dominant modes more rapidly within the lifetime of the HF zonal flows. It was also found that HF parts of the density fluctuations are highly modulated by the HF zonal flows.

The electrostatic and density fluctuation are measured simultaneously with a heavy ion beam probe. The electrostatic fluctuation with the geodesic acoustic mode (GAM) frequency is observed in L-mode plasmas and not in H-mode plasmas. The poloidal and radial structure is consistent with the GAM. So the fluctuation is concluded to be the GAM.The amplitude of the GAM changes in the radial direction; it is small near the separatrix, has a maximum at 3 cm inside the separatrix and decreases again to 5 cm inside the separatrix.The GAM and the temporal behaviour of the ambient density fluctuation show a significant coherence, and the phase of modulation of the ambient density fluctuation tends to delay the potential oscillation of the GAM. It is clearly verified that the GAM affects ambient fluctuation and also the local particle transport through modulation of the amplitude of the ambient fluctuation.

The characteristics of geodesic–acoustic-mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the . The causality between the GAM and the modulation of the density fluctuation is revealed.

Compatibility between plasma and reduced activation ferritic steel, which is the leading candidate for the structural material of a fusion demonstration reactor, has been investigated in the Advanced Material Tokamak EXperiment (AMTEX). Ferritic plates (FPs) were installed progressively in the JFT-2M tokamak. The effect of ferromagnetism on plasma production, control, confinement, and stability has been investigated. Impurity release behavior has also been investigated. Even when the inside vacuum vessel wall was fully covered with the FPs and the tokamak plasma was operated close to the wall, no deleterious effect was observed, and the normalized beta could be increased up to ~3.5. Thus, encouraging results are obtained for application of this material to the demonstration reactor.

The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.

Large potential oscillations were detected in JIPPT-IIU tokamak plasmas in a wide range of plasma cross-sections in measurements using a multi-sample-volume heavy ion beam probe. These oscillations have large amplitudes reaching a few hundreds of volts and their frequencies are in the range of the geodesic acoustic mode (GAM). They are found over a wide range of plasma cross-sections and commonly have m = 0 structures. As they were Fourier analysed, it was found that the central frequency is higher in the core of the plasma and lower in the edge of the plasma. These observations agree with the properties of theoretically predicted GAM oscillations. It was also found that the frequency spectrum is peaked in the core and broad in the edge, which may have something to do with damping mechanisms of the GAM. The phase relation between the density and the electric field fluctuations was studied extensively in terms of the cross-correlation function. The level of the density fluctuation was low as it should be, and the expected 90° phase difference was found in a limited radial domain.

The large helical device (LHD) is the world largest helical system having all superconducting coils. After completion of LHD in 1998, six experimental campaigns have been carried out successfully. The maximum stored energy, central electron temperature, and volume averaged beta value are 1.16 MJ, 10 keV, and 3.2%, respectively. The confinement time of the LHD plasma appears to be equivalent to that of tokamaks. One of the most important missions for LHD is to prove steady-state operation, which is also significant to international thermonuclear experimental reactor (ITER) and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 150 s. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady-state operation is whether divertor function to control particle and heat flux is effective enough. Relevant to this, LHD diagnostics should be consistent with the following: 1) continuous operation of main diagnostics during long-pulse operation for feedback control and physics understanding; 2) measurement of fraction of H, He, and impurities in the plasma; 3) heat removal and measure against possible damage or surface erosion of diagnostic components inside of the vacuum chamber; 4) data acquisition system for handling real time data display and a huge amount of data. Although there are already some achievements on the above subjects, there remain still several issues to be resolved. On the other hand, the long-pulse operation of the plasma gives benefits to the diagnostics. For example, the polarizing angle of ECE emission can be changed during the discharge, and the intensity dependence on the polarizing angle has been obtained. The spatial scanning of the neutral particle analyzer and the spectrometer can supply the spatial profiles of the fast neutral particle flux and the specific impurity lines. In this paper, the present status of these issues and future plans are described.

Standard diagnostics for fundamental plasma parameters and for plasma physics are routinely utilized for daily operation and physics studies in the large helical device (LHD) with high reliability. Diagnostics for steady-state plasma are under intensive development, especially for Te, ne (yttrium–aluminium garnet (YAG) laser Thomson, CO2 laser polarimeter), data acquisition in steady-state and heat-resistant probes. To clarify the plasma properties of the helical structure, two- or three-dimensional diagnostics are being aggressively developed: tangential cameras (fast SX TV, photon counting CCD, Hα TV); tomography (tangential SX CCD, bolometer); imaging (bolometer, ECE, reflectometer). Divertor and edge physics are important key issues for steady-state operation. Diagnostics for neutral flux (Hα array, Zeeman spectroscopy) and ne (fast scanning probe, Li beam probe, pulsed radar reflectometer) are also in advanced stages of development. In addition to these, advanced diagnostics are being intensively developed in LHD through domestic and international collaborations.

Compatibility between the plasma and low activation ferritic steel, which is a candidate material for fusion demonstration reactors, has been investigated step by step in the JFT-2M tokamak. We have entered the third stage of the Advanced Material Tokamak EXperiment (AMTEX), where the inside of the vacuum vessel wall is completely covered with ferritic steel plates ferritic inside wall (FIW). The effects of a FIW on the plasma production, impurity release, the operation region, and H-mode characteristics have been investigated. No negative effect has been observed up to now. A high normalized beta plasma of βN ~ 3, having both an internal transport barrier and a steady H-mode edge was obtained. A remarkable reduction in ripple trapped loss from 0.26 MW m−2 (without ferritic steel) to less than 0.01 MW m−2 was demonstrated by the optimization of the thickness profile of FIW. A code to calculate fast ion losses, taking into account the full three-dimensional magnetic structure was developed, and values obtained using the code showed good agreement with experimental results. Thus, encouraging results are obtained for the use of this material in the demo-reactor.

Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012 and the total injection power of ECRH reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature T{sub e0} = 13.5 keV with n{sub e} = 1×10{sup 19}m{sup −3}. In the LHD, an electron-internal-transport barrier (e-ITB) related to the production of high T{sub e} plasmas has been realized by strongly centre-focused ECRH. The electron thermal confinement clearly improved inside the e-ITB. The radial electric field was measured using the heavy ion beam probe. The formation of the positive E{sub r} was observed in the core region, which well agreed with the prediction of the neoclassical transport theory. The energy confinement characteristics have been investigated in the ECRH plasmas. It was found that higher plasma stored energy and lower radiation power was realized in the outward configuration. The plasma stored energy of 530more » kJ with n{sub e} = 3.2×10{sup 19}m{sup −3}, which is the 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.« less

This paper reports on the behaviour of high-energy ions created by ion cyclotron range of frequency (ICRF) heating on the Large Helical Device (LHD). In the third experimental campaign conducted in 1999, it was found that minority heating has good heating performance, and high-energy particles were observed. In the fourth campaign in 2000, the temporal behaviour of high-energy ions was investigated in the minority heating regime using turnoff or modulation of ICRF power. The time evolution of the high-energy particle distribution was measured using a natural diamond detector (NDD) and a time-of-flight neutral particle analyser (TOF-NPA). It was found that the count number of higher-energy particles declines faster than that of lower-energy particles after ICRF turnoff. In the modulation experiments, the phase difference of the flux of high-energy particles with respect to the ICRF power modulation increased with energy. These results were explained qualitatively by the Fokker-Planck equation with a simple model. The pitch-angle dependence of the distribution function was also measured in the experiment by changing the line of sight of the TOF-NPA, and an anisotropy of the high-energy tail was found. This anisotropy was reproduced by solving the bounce-averaged Fokker-Planck equation. The second harmonic heating was conducted successfully for the first time in the LHD in high-β plasma, and high-energy particles were also detected in this heating regime.

Potential and density/temperature fluctuations at the L-H transition are measured by the heavy ion beam probe on JFT-2M. It has been observed that the timescale of the potential change is as fast as 10-100??s when the input power (Pin) is larger than the L-H threshold power (Pth). When Pin ~ Pth, the confinement is improved gradually step by step with sawtooth crashes. After a few sawtooth crashes, the potential drops rapidly to the level of the ELM free H mode. From the gradual change of the potential, assuming that dEr/dr is a key to forming and sustaining the transport barrier, the criterion of dEr/dr is less than (1.2?0.4) ? 103?kV/m2. At an ELM just before the H-L transition, the potential inside the separatrix also shows a rapid positive jump. The timescales of the positive jump and of the recovery of the potential to its negative value are about 40 and 150??s, respectively. Before the H-L transition, the time between ELMs and/or dithering transition becomes shorter and the plasma finally goes back to the L?mode.

Ion cyclotron heating has been established as one of the heating schemes in nuclear fusion research and its use in steady state plasma heating in various devices is being considered. The optimal technology for steady state ion cyclotron range of frequency heating has not been firmly established. This article reports on the liquid stub tuner which was newly developed in research and development activities on the large helical device. It demonstrated high performance in real use in experiments. Two different impedance-matching systems based on the liquid stub tuner are studied: one is a triple liquid stub tuner system and the other is a single stub tuner system with a liquid phase shifter. The characteristics of the two systems are compared from the points of view of how wide a frequency range is covered, and how great the reduction of the voltage in the transmission line is.

Ion cyclotron range of frequency (ICRF) heating on the large helical device (LHD) is characterized by high power (up to 12 MW) and steady state operation (30 min). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6 m) with superconducting coil windings (l=2, m=10). The main purpose of physical research is to investigate currentless and disruption-free plasma. Research and development for steady state ICRF heating has been carried out in recent years: A high rf power transmission system consisting of stub tuners, a ceramic feedthrough, and an ion cyclotron heating loop antenna has been developed. In addition, steady state operation of a rf oscillator has been achieved at a power higher than 1 MW. A liquid stub tuner has been proposed as an innovation. The liquid stub tuner makes use of the difference between the rf wavelengths in liquid and in gas due to the different relative dielectric constants. The liquid stub tuner has been experimentally proved to be a reliable rf com...

Steady-state ion cyclotron range of frequency (ICRF) heating technologies have been developed to heat plasma for >30 min in the Large Helical Device (LHD). Steady-state-operation tests of high volt...

Ion Cyclotron Range of Frequency (ICRF) heating on Large Helical Device (LHD) is characterized by its high power (up to 12MW) and by steady state operation (30 minutes). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6m) with super-conducting coil windings (l=2, m=10). The main physical purpose is to investigate currentless and disruption-free steady state plasmas. Research and development for ICRF heating have been carried out in recent years. A high RF power transmission system has been developed, which consists of stub tuners, ceramic feed-through and ICRF heating loop antenna. The RF transmission system was tested and withstood 58kV for 10 seconds and 40kV for 30 minutes. The RF voltage corresponds in the case of a plasma loading resistance, 5Ω to a transmitted RF power capability of 3.4MW and 1.6MW. In addition, a pre-matching stub tuner was very effective in reducing the RF voltage. The reduction rate was confirmed to be one third, which leads to a higher ICRF injection efficiency because of reduction of RF power loss in the transmission system. Furthermore a proper procedure to effectively carry out aging of the RF antenna was found in terms of selecting the RF pulse length, repetition rate and RF voltage.

Beam-driven Alfvén eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B 0 = 0.95 T, 〈R〉 = 1.5 m, 〈a〉 = 0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (P NBI ⩽ 1.0 MW, E NBI = 32 keV, P ECRH ⩽ 0.6 MW). In low-density plasmas in the range n e ¯ = (0.3–1.5) × 1019 m−3, a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50 kHz f AE ι is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of ι intervals within which the AEs studied are chirping or steady-state is determined.

Vertical-cavity surface-emitting lasers (VCSELs) emitting at 1.3-μm are potentially attractive for optical interconnects. The advantages of such lasers include their lower driving current, a surface-mountable device structure, and a narrow circular output beam that is ideal for a polymer multimode waveguide on a printed circuit board [1]. Accordingly, there has been a widespread interest in developing 1.3-μm VCSELs [2,3] and a novel type of VCSEL, which is a lens-integrated surface-emitting laser that consists of a short InGaAlAs multiple-quantum-well (MQW) distributed feedback laser (DFB) active stripe integrated with a 45° mirror and an aspheric lens [4,5]. Here, we propose a novel 1.3-μm quantum dot (QD) surface emitting laser (SEL), which consists of a QD laser section and a polymer optical circuit (POC) section. A QD laser emitting at 1.3 μm needs to have a lower operation power and temperature insensitive operation because of the discrete energy state. A suitable POC design can be used to control the emitting place, direction, and beam profile of a laser. Moreover, POCs are easy to fabricate and are less expensive owing to the low cost of polymer material. In this work, we propose a QD SEL using a POC, which consists of a polymer optical waveguide, spot-size converter, and 45° metal mirror. We designed the polymer spot-size converter to match the laser beam profile using the optical polymer waveguide. We were able to realize the operation of a QD SEL using a POC.

In the large helical device (LHD), fundamental electron cyclotron resonance heating (ECRH) by the electron Bernstein wave (EBW) excited via the ordinary-extraordinary–EBW (O-X-B) mode conversion process was performed with high power (~1MW) launching. Profiles of increase of the electron temperature ( T e ) and the soft X-ray signals during the power injection suggest power absorption in the core region. Effects of the local modification of the rotational transform l / 2π (= 1 / q ) by electron cyclotron current drive (ECCD) on the formation and sustainment of the electron internal transport barrier (e-ITB) was investigated for the first time. Co ECCD raised l / 2π close to 0.5 in the core region and caused the flattening of the T e profile. Additional ECRH power is required to form the e-ITB. On the contrary, counter (cntr.) ECCD separates l / 2π from 0.5 in the core region and avoids the flattening of the T e profile. The e-ITB can be formed and sustained without additional ECRH. Analysis of the heat pulse transport with use of the modulation ECRH (MECH) shows the good confinement region extends to the l / 2π =0.5 rational surface in the case of cntr. ECCD.

Reversed-shear Alfven eigenmodes were observed for the first time in a helical plasma having negative q{sub 0}{sup ''} (the curvature of the safety factor q at the zero shear layer). The frequency is swept downward and upward sequentially via the time variation in the maximum of q. The eigenmodes calculated by ideal MHD theory are consistent with the experimental data. The frequency sweeping is mainly determined by the effects of energetic ions and the bulk pressure gradient. Coupling of reversed-shear Alfven eigenmodes with energetic ion driven geodesic acoustic modes generates a multitude of frequency-sweeping modes.

Optical-phonon spectra were investigated for single crystals of the parent materials of various cuprate superconductors and related materials. Comparing the spectra, all the observed phonons were unambiguously assigned. It was found that the main phonon modes in the CuO{sub 2} plane, the Cu-O stretching and bending modes, exhibit completely different dependences of their frequency on the Cu-O bond distance {ital a}/2; that is, the former shows strong {ital a} dependence while the latter is relatively insensitive to {ital a}. From a quantitative analysis of the spectra, the highly covalent character of these layered cuprates was confirmed. The remarkable change of the stretching-mode frequency against {ital a} indicates a direct coupling of this phonon with the Cu-O charge-transfer electronic excitation.

The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti).

The magnitude of geodesic acoustic modes (GAMs) is analyzed, which are driven by the oscillating Reynolds stress force in poloidal direction in the limit cycle oscillation (LCO) of tokamaks near the condition of H-mode transition. Although the frequency of LCO is much lower than the natural frequency, the forced oscillation of GAMs at the LCO frequency is excited. The amplitude of excited GAMs can be similar to that of low frequency zonal flows. The result is compared with the observation of the JFT-2M tokamak.

This article presents a radial electric field measurement by a heavy ion beam probe in the JFT-2M tokamak, during the L-H transition. An abrupt increase (time scale of O(100 µs)) of the strong edge radial electric field (localized in the radius with FWHM ~7 mm) results in the increase of density gradient and turbulence reduction. Rapid inward propagation of the turbulence suppression front is observed at the transition. After the transition, the electric field structure in the tiny edge localized modes (ELMs) is analyzed. Transport self-regulation events observed in the vicinity of the L-H transition, i.e. the limit cycle oscillation (LCO) in the L-mode, the tiny ELM in the H-mode, as well as the L-H transition itself, are summarized in a single Lissajous diagram in the electric field-density gradient space, which provides a comprehensive explanation of the transition dynamics.

As a key component of a strontium based optical clock, we have developed an apparatus which allows laser cooling and trapping of more than 10/sup 7/ /sup 88/Sr atoms to temperatures below 1 /spl mu/K. Using this system, we have performed the first high accuracy measurement of the /sup 88/Sr /sup 1/S/sub 0/-/sup 3/P/sub 1/ clock transition using an ultra-cold sample. Detailed descriptions of both the measurement technique and corresponding systematic uncertainties is provided. In addition, by varying the density of the ultra-cold /sup 88/Sr sample over a range of three orders of magnitude we obtain the first definitive measurement of a density-related frequency shift and linewidth broadening of an optical clock transition in an ultra-cold alkaline earth system. Spectroscopy of the /sub 1/S/sub 0/-/sup 3/P/sub 0/ transition is currently underway using this narrow probe laser and an octave spanning femtosecond laser. Our most recent results are presented. The proposed measurement technique, accuracy, and stability of an optical clock based on the EIT (electromagnetically-induced-transparency) measurement in /sup 88/Sr are described.

Two types of electrostatic modes with small-poloidal wave numbers ($\ensuremath{\sim}1$ and $10--15\text{ }\text{ }\mathrm{kHz}$) are observed in the edge region of Ohmically heated plasmas in the JFT-2M tokamak. The envelope of the higher frequency coherent mode is modulated at the frequency of the lower frequency mode. A bispectral analysis revealed that a significant nonlinear coupling among the two types of fluctuations and the broadband background turbulent potential fluctuations occurs inside the last closed magnetic flux surface, suggesting that a nonlinear process such as the parametric-modulational instability is involved.

Summary form only given, as follows. The Large Helical Device, LHD, is the world largest helical system having all superconducting coils. After completion of LHD in 1998, 6 experimental campaigns have been carried out successfully. The maximum stored energy, electron temperature, and beta value are 1.2 MJ, 10 keV, 3.2 %, respectively. The confinement time of the LHD plasma appears to be equivalent to that, of tokamaks. One of the most important missions for LHD is to prove steady state operation, which is also significant to ITER and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 127 sec. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady state operation is whether divertor function to control particle and heat flux is effective enough.

Diagnostics for helical systems are reviewed. A helical system is expected to be a disruption-free steady-state reactor since no plasma current is required. Different types of medium-sized helical systems such as CHS1, W7-AS2, Heliotron E3 and J4, H-1NF Heliac5 and HSX have been developed and constructed. Recently, a large helical device (LHD) has been constructed in Japan and another large one (W7-X8) is under construction in Germany. These two devices are fully equipped with superconducting magnets, modern diagnostics and high power heating systems. The plasma parameters are expected to be comparable to large tokamaks. While most plasma diagnostics are similar to those for tokamaks, there are also specific features: (1) measurement of vacuum magnetic flux surfaces; (2) intrinsic 3-D structure including divertor; (3) intensive diagnostics for the electric field; (4) diagnostics to verify device design elements; (5) high density diagnostics; (6) steady state operation. W7-X optimization principles call for sensitive current diagnostics (Pfirsch-Schluter and bootstrap currents) and tomographic means to verify the reduced Shafranov shift as well as fast particle confinement diagnostics11. Steady state or long pulse operation fulfills the demands of correlation diagnostics as Te fluctuation measurements, it allows for coherent averaging and it enables scans of various diagnostics. Examples are: i) polarization scans of ECE, ii) wavelength scans of spectrometers, and iii) spatial scans of several diagnostics. The special features of diagnostics for helical systems are described in the following chapters.

In the JFT-2M tokamak (JFT standing for JAERI Fusion Torus), the limit cycle oscillation (LCO), together with several variables, i.e., the electrostatic potential, radial electric field, electron density, turbulence intensity, and Dα emission from the divertor region, is observed before the L-to-H transition. The spatiotemporal dynamics of the LCO is analysed in detail. Zonal flows are not seen, while modulation is observed in the edge-localized poloidal flow and density gradient. Modulation is also seen in the Reynolds stress, caused by that in the turbulence intensity and turbulence wavenumber. However, flow acceleration cannot be explained by the modulation in the Reynolds stress. Rapid inward propagation is also observed for the density gradient and turbulence packet. The characteristics of the propagation are verified by means of turbulence spreading theory and diffusion theory.

THE high-transition-temperature (high- Tc) copper oxide superconductors are highly anisotropic in their electrical properties; this is one of the key concerns for applications of these materials, and may also provide a clue to the mechanism of the superconductivity. Although the resistivity in the CuO2 planes, ρab, shows a metallic temperature dependence, the resistivity parallel to the c axis (ρc) has been reported to be both non-metallic1,2 and metallic3,4. Here we present systematic data for ρc in a number of high-Tc materials, obtained from well characterized single crystals. Both the magnitude and the temperature dependence of ρc are strongly dependent on crystal structure, and on the concentration of charge carriers. We find that ρc is non-metallic (dρc/dT< 0) in most superconducting compounds, suggesting an unconventional conduction mechanism. Fully oxygenated YBa2Cu3O∼7 (with Tc≈90K) is the only exception, with a relatively small ρc and positive dρc/d T that may arise from the crystal structure specific to this material.

The final goal of Ion Cyclotron Range of Frequency (ICRF) heating on the Large Helical Device (LHD) is characterized by its high power (up to 12MW) and by steady state operation (30 minutes). Initial ICRF heating experiments were carried out using a pair of loop antemas in the 2nd experimental campaign in 1998. The ICRF heating power was applied to an ECH-produced plasma at an RF power level of 300 kW for 0.2 seconds. An applied frequency of f=25.6 MHz was selected. A cyclotron resonance layer of hydrogen ions was located at the half minor radius during operation at B=1.5 T. A mode conversion layer of a He plasma with a minority of hydrogen ions was located between the magnetic axis and the last closed magnetic flux. The plasma stored energy was observed to increase to twice that of the ECH plasma (PECH=300 kW). The plasma stored energy of the ECH target plasma was 11-13 kJ at an average electron density of ne=8–9×1018 m−3 and a central electron temperature of Te0=400 eV. The plasma stored energy increase...

Alfven eigenmode (AE) activity driven by NBI-produced fast ions is observed in TJ-II plasmas. A two-step response of the measured AEs to electron cyclotron heating (ECH) power is seen. In a first step, the continuous character of the unstable AEs changes to a chirping character of the marginally unstable AEs when moderate values of ECH power are applied to the NBI-only-heated plasma. In a second step, a significant reduction of the AE amplitude is observed when the ECH power is doubled. This stabilizing effect has been experimentally confirmed both on a shot-by-shot basis and along a single discharge by means of ECH modulation. The observed stabilizing effect is stronger with on-axis ECH than with off-axis ECH power injection.

A central ion temperature of 7 keV in a neutral beam injection (NBI)-heated plasma and a central-electron temperature of 20 keV in an electron cyclotron resonance heating plasma were achieved in the Large Helical Device (LHD) using an upgraded heating system with a newly installed perpendicular-NB injector and gyrotrons. The values of Ti and Te significantly exceeded 5.6 and 15 keV, obtained in previous experiments, respectively. High-Ti plasma was obtained using a carbon pellet injection and the kinetic-energy confinement improved by a factor of 1.5. Transport analysis of the high-Ti plasmas showed that the ion-thermal diffusivity and the viscosity were reduced after the pellet injection. Dynamic-transport analysis is applied and the transition to the ion-internal-transport barrier and back transition are discussed.

As the clinical use of positron emission tomography technology grows, so does the demand for noninvasive measurement of cerebral blood flow (CBF). This chapter presents the development of a compact radiation monitor composed of dual β-detectors suitable for the detection of radiation from superficial tissue. One detector is placed over the wrist above the radial artery to monitor arterial radioactivity, whereas another was placed on an adjacent skin area to measure the background. The monitor worked fairly well except on obese subjects, for whom an arterial count curve was not recoverable. A model has been developed to restore the arterial input function comparing the two-detector response with time that incorporated the superficial tissue flow, mainly of skin and the subcutaneous tissue. Twenty-four studies on five normal volunteers are conducted with [ 15 O]-water slow bolus injections, which are controlled by an automatic injection system. A significant correlation is obtained between the calculated CBF using restored input function by the system and the CBF using an actual arterial input curve by arterial cannulation. Further, refinement of the procedures that include the calibration of restored arterial input to well counter and accurate correction of dispersion in the restored arterial input may increase the accuracy of this noninvasive method.

Alfvén eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density n e, electric potential φ and poloidal magnetic field B pol with frequencies 50 kHz f AE k θ , mode numbers m(m V θ ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfvén law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector k θ and mode number m remained constant, (ii) the cross-phases between the oscillations in B pol, n e and electric potential remained constant, having an individual value for each AE branch, (iii) V θ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in E pol and n e, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

The transition to the improved confinement mode by electrode biasing was observed for the first time in the Large Helical Device (LHD). A negative resistance was observed in the confinement mode sustained by the cold electrode biasing. The electrode current showed a clear decrease against an increase in the electrode voltage and was characterized by a hysteresis in the transition phenomena. The decrease in the electrode current suggested an improvement in the radial particle transport. An increase in the energy confinement time and remarkable suppression of density fluctuations, which correspond to the transition, were also observed. These results indicated that the electrode biased plasma in LHD showed similar improvements in confinement to those in the H-mode plasmas in tokamaks and stellarators.

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 × 1021 m−3 due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2 keV at the density of 1.6 × 1019 m−3, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

We present a detailed investigation of the geodesic acoustic eigenmode (GAEM) profile in the vicinity of the last closed flux surface (LCFS) in the JFT-2M tokamak. Envelope and spectral peak frequency of the GAEM vanish near the LCFS obtained by an equilibrium reconstitution code. Spatial changes in mean plasma gradients (the electron temperature, electron density and floating potential) are correlated with the boundary. This does not contradict that the GAEM can have a node at the LCFS. Discussions of an inverse problem and the application of this GAEM measurement to an experimental test of the LCFS location calculated by the equilibrium reconstitution code are presented.

Extremely hollow profiles of impurities (denoted as “impurity hole”) are observed in the plasma with a steep gradient of the ion temperature after the formation of an internal transport barrier (ITB) in the ion temperature transport in the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. The radial profile of carbon becomes hollow during the ITB phase and the central carbon density keeps dropping and reaches 0.1%?0.3% of plasma density at the end of the ion ITB phase. The diffusion coefficient and the convective velocity of impurities are evaluated from the time evolution of carbon profiles assuming the diffusion and the convection velocity are constant in time after the formation of the ITB. The transport analysis gives a low diffusion of 0.1?0.2 m2/s and the outward convection velocity of ~1 m/s at half of the minor radius, which is in contrast to the tendency in tokamak plasmas for the impurity density to increase due to an inward convection and low diffusion in the ITB region. The outward convection is considered to be driven by turbulence because the sign of the convection velocity contradicts the neoclassical theory where a negative electric field and an inward convection are predicted.

Present status of the analysis of HIBP data on zonal flows, streamers and drift-wave turbulence of the JIPPT-IIU tokamak plasmas, is reviewed. Limiting factors and improvements of heavy ion beam probe towards better understanding of tokamak turbulence, beam attenuation, beam quality of the probing beam, multiple energy analyzers for simultaneous measurement at various cross-sections of the plasmas, proposal for 2-dimensional measurement of the plasma turbulence, are discussed.

The most important feature of the “high recycling steady” (HRS) high-confinement mode (H-mode) regime in the JFT-2M tokamak [H. Ninomiya et al., Fusion Science and Technology (2002), Vol. 42, p. 7] is the stationary pedestal condition in the absence of the large edge localized modes (ELMs). Accompanying the HRS H-mode transition, the coherent magnetic fluctuations in the frequency range of the order of 10 – 100 kHz with significant variation are seen on the magnetic probes at the vessel wall. Above all, two types of edge magnetohydrodynamics (MHD) activities, which have associated toroidal mode number of n = 1 and n ∼ 7 , respectively, seem to be more important for the HRS H-mode plasmas. To investigate their interaction, bispectral analysis is applied for the magnetic probe data, exhibiting the phase-coupled oscillations between two types of edge MHD activities. Parameter similarities and differences with small ELMs and stationary ELM-free H-mode regimes in other devices are also discussed.

An MQW electroabsorption-modulator integrated DFB laser fabricated by using the in-plane bandgap energy control technique and a low-capacitance semi-insulating BH process is presented. This device has a threshold current as low as 5.4 mA and a modulation efficiency as high as 13 dB/2V. Modulation at 10 Gbit/s with a modulation voltage of only 1V peak to peak demonstrates the potential value of this device for electroabsorption modulation at 1.55 μm.