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A 90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring electron scale turbulence (k⊥ρs ∼ 40). A metal lens relay antenna is used for in-vessel beam focusing, and a beam diameter of less than 40 mm is achieved in the plasma core region. This antenna can be steered at an angle of 159° ± 6°, which almost covers the plasma radius. The estimated size of the scattering volume is ∼105 mm at the edge and 135 mm at the core, respectively. A 60 m corrugated waveguide is used to achieve a low transmission loss of ∼8 dB. A heterodyne detection system for millimeter-wave circuits with probing power modulation can distinguish the scattered signal from background noise.

The plasma current is ramped up primarily by a 28 GHz electron cyclotron wave (ECW) in the Q-shu University experiment Steady-State Spherical Tokamak (QUEST), with multiple harmonic resonance layers from the second to the fourth stay in the plasma core. A steering antenna comprising two quasi-optical mirrors enhances the power density of ECWs. The ECW beam is injected obliquely from the low-field side where the parallel refractive index is N ∥ = 0.75 at the second-harmonic resonance layer. Analysis of the resonance condition has found that energetic electrons moving forward along the magnetic field resonate more effectively than those moving backward. Such symmetry breaking is consistent with the results of the current ramp-up experiment. The peak plasma current reaches I p > 70 kA, constantly injecting a beam of radio frequency power of 100 kW. Ray-tracing by the TASK/WR code demonstrates that the power of the 28 GHz extraordinary mode is absorbed by energetic electrons via single-pass cyclotron absorption.

We succeeded in increasing the radial observation points of the microwave frequency comb Doppler reflectometer system from 8 to 20 (or especially up to 45) using the high sampling rate of 40 GS/s digital signal processing. For a new acquisition system, the estimation scheme of the Doppler shifted frequency is constructed and compared with the conventional technique. Also, the fine radial profile of perpendicular velocity is obtained, and it is found that the perpendicular velocity profile is consistent with the E × B drift velocity one.

The spatial structure and Z dependence of transport coefficient were studied by injecting impurity pellets (C, Al, and Ti) and measuring high-spatial resolution bremsstrahlung in steady-state neutral beam heated plasmas on the Large Helical Device [A. Iiyoshi et al., Fusion Techol. 17, 169 (1990)]. The D and V were evaluated from a diffusive/convective model with minimizing the residual error between the experimental and computational bremsstrahlung intensities taking the recycling rate into account. As a result, the existence of an inward convection was observed at ρ>0.6, and no convection was required at ρ

An impurity pellet injector with three differential pumping systems has been developed to study the particle/impurity transport in combination with bremsstrahlung profile measurement in the Large Helical Device (LHD) plasma. For this purpose three different shaped pellets are tested; sphere, hemisphere, and cylinder. The characteristics of the pellets accelerated by pressurized helium gas were studied as a function of shape and mass of the pellets and He gas pressure. The velocity of the pellets was measured at two different positions using a He–Ne laser, two optical fibers, two lenses, and a slit with two narrow apertures. The delay time until the pellet begins to accelerate, and the effect of opening time of the vacuum valves for differential pumping on the amount of He gas leakage were also investigated. The difference in the velocities among the three different shaped pellets is discussed with the simple analytical model.

A four channel pulsed radar reflectometer system has been installed on the Large Helical Device (LHD). The complicated magnetic structure in LHD causes mode conversion and/or polarization rotation of the microwaves. Pulsed radar reflectometry is a suitable reflectometric technique, because it measures the delay time of the reflected wave, not the phase, and X-mode and O-mode polarized waves can be distinguished. By using X-mode operation of the pulsed radar reflectometer so that each pulse width is about 2 ns, and the repetition rate is up to 200 kHz, the critical density where the microwave is reflected is about 1×10^16 m^?3. Also it is found that the static natural island affects the X-mode reflectometric measurements.

A new electron Bernstein wave emission diagnostic consisting of a heterodyne radiometer and a reflectometer was installed on TST-2. The instrument has a frequency range from 5 to 12 GHz which covers partially the fundamental and fully covers the second and third harmonic emissions. X-mode emission perpendicular to the flux surface, generated by B–X–FX mode-conversion scenario, is measured. The mode-conversion efficiency is calculated using a one-dimensional full-wave code with the density profile obtained simultaneously from the reflectometer, and the electron temperature (Te) is estimated. The levels corresponding to the 100–150 eV range are measured from the plasma core region, and the mode-conversion efficiency is calculated to be 0.5–0.8, leading to Te of approximately 200 eV. In addition, it is found that the reflectivity measured by the reflectometer is consistent with the calculated mode-conversion efficiency. This property can be used for a direct measurement of the mode-conversion efficiency.

In order to measure fine scale fluctuations of visible emission in plasmas, the four-beam correlation method is proposed and its feasibility is studied. This is an extension of the conventional cross beam techniques and uses four beams (i.e., four lines of sight), which lie in two parallel planes. While two beams in the same plane provide the fluctuation levels at the intersection, two beams in different planes provide correlation as a function of the distance between the planes. Various cases are considered, and the method feasibility is shown. This method is useful for reconstructing a two- or three-dimensional wave-number spectrum. In addition, it can be used to measure the magnetic field direction assuming that the parallel correlation length is longer than the perpendicular one.

Measurements of edge plasma parameters such as current density, electron density, and electron temperature were performed during internal reconnection events in TST-2 Ohmic plasmas. The measured current density consists of two components: a slowly varying component and a spiky bipolar component. The magnitude of the slowly varying component is comparable to the mean current density averaged over the poloidal cross section, and it seems to represent the global transport from the core to the edge. The spiky bipolar component is about an order of magnitude larger than the slowly varying component, but the spatial structure seems to be localized and its effect on plasma confinement is not catastrophic.

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer has been developed and installed on the Compact Helical System (CHS). This MLM based spectrometer is a new approach to devising a soft x-ray spectrometer with medium time and energy resolutions. The spectrometer mainly consists of a MLM as the dispersive element and a 20-channel (p-i-n) pin diode array for the detection of the soft x-rays. Both the MLM and the pin diodes have been calibrated at the KEK (High Energy Accelerator Research Organization) Photon Factory for an energy range of 300–1200 eV. The reflectivity of the mirror was found to increase from 3% (photon energy of 335 eV) to 25% (photon energy of 1050 eV). The pin diodes were found to have almost 100% efficiency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temperature measurement and study of fast magneto-hydrodynamic (MHD) events occurring in plasmas. The experiments show that in...

The Large Helical Device (LHD) is the largest helical machine with superconducting coils. Key diagnostics issues for LHD are: (a) capability for multidimensional measurements because of the nonaxisymmetric toroidal plasma; (b) measurements of the electric field; (c) cross check of fundamental parameters using different methods; (d) advanced measurements appropriate for steady-state operation; and (e) a satisfactory data acquisition system. The design and research and development of plasma diagnostics were carried out taking these issues into consideration. As a result, the present status of diagnostics is described: diagnostics for LHD operation, fundamental diagnostics for plasma performance, diagnostics for physics subjects, innovative diagnostics and diagnostics for long-pulse operation. The LHD experiment started in March, 1998. Since then, the development of diagnostics has kept pace with the experimental campaigns.

Pulsed radar reflectometry is a suitable reflectometric technique with which to study the effect of strong magnetic shear for the polarization of microwaves in the Large Helical Device. Because pulsed radar reflectometry measures the delay time of the reflected wave, not the phase, X-mode and O-mode polarized waves can be distinguished. At a preliminary stage of X-mode operation it is found that the position of the ergodic edge layer is steady in spite of the increased density in the core region when a neutral beam is injected. If the electron density does not reach the critical cutoff density, the pulsed radar system could be used as a delayometer. The measured delayometer signal is almost in agreement with the numerical calculation under the assumption that polarization of the wave propagated into the plasma is decided at the edge region.

A new type of digital phase linearizer has been developed for real-time electron density measurement using a multichannel far-infrared (FIR) interferometer. The phase linearizer can measure phase shifts up to 640 fringes. The size of internal memory is 16 bits-2 Mwords and the sampling frequency of the waveform data is up to 1 MHz. The internal clock for the counter is selectable to 100 or 200 MHz and the frequencies of the input signals, i.e., the reference signal and the probe one, are around 1 MHz. In the performance test the resolution of the phase components is achieved at 1/100 fringes, which corresponds to the line integrated density of 9.0×1016 m−2. For real-time measurements the phase linearizer has digital-to-analog converter (DAC) output. The resolution of DAC output is 12 bits and the amplitude is ± 5 V. The density feedback system on the large helical device has been utilized.

A multilayer mirror (MLM) soft x-ray spectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature Te with fast time resolution using the MLM spectrometer.

Effects of magnetic shear on electron cyclotron resonance heating (ECRH) are studied in heliotron/torsatron configurations. In such configurations, the poloidal magnetic field is comparable to the toroidal magnetic field, and varies spatially along the minor radius, making a strong magnetic shear. When high power millimeter waves are launched into a plasma, it is coupled to propagating modes at the plasma peripheral region. The existence of a transition layer between the core plasma region and the vacuum region, where the magnetic field direction is largely changed, requires accurate polarization control for good single pass absorption. The mode conversion between the propagation modes due to the magnetic shear also affects the launching conditions. The polarization control experiment by using second harmonic ECRH in Heliotron E [T. Obiki, Fusion Technol. 17, 101 (1990)] are compared with the numerical calculation in which one dimensional second order coupled equations are solved. The polarization dependence experimentally measured is in good agreement with the numerical results including the magnetic shear terms.

In the National Institute for Fusion Science, the large helical device (LHD) (R=3.9 m, ap=0.65 m, B=3–4 T) was constructed in Toki City on January, 1998. The LHD is a torsatron/heliotron type device with a superconducting helical winding of pitch parameters of l/m=2/10 and three sets of superconducting poloidal coils. A two color millimeter-wave interferometer has been developed for the measurement of line integral electron density on the horizontal midplane of the LHD. To overcome the effects of the change in the transmission path length (∼100 m) during long pulse plasmas (10–1800 s), we have applied two color interferometry at 140 and 285 GHz.

A phase sensitive heterodyne polarimeter was installed on compact helical system (CHS). An HCN laser (337 μm) and Schottky barrier diodes are used for the radiation source and for detection, respectively. By using a phase sensitive heterodyne scheme, the Faraday rotation angle can be measured from the phase variations caused by the phase difference between the right- and left-handed waves in the magnetized plasma. The purpose of the polarimeter on CHS is for a stable density monitor which is free from fringe jump and for the measurements of the internal magnetic field induced by the plasma current. The present resolution of the Faraday rotation angle is 0.1° with 10 ms time resolution and 1° with 1 ms. These are mainly determined by the laser instability.

Extraordinary mode interferometry with the frequency lower than the electron cyclotron frequency has been applied to the core plasma of the JIPP TII-U tokamak. The system is operated as a reflectometer when the density exceeds the cutoff for the extraordinary mode. The measured fringe shift is in good agreement with the calculated fringe shift using the density profile obtained by a six-chord HCN laser interferometer. Advantages and disadvantages of extraordinary mode interferometry are shown numerically and experimentally. To overcome the disadvantages, a dual (ordinary and extraordinary) mode interferometer is proposed as a divertor diagnostic. There are two attractive choices for this scheme. One choice double the operational density range but does not compensate for changes in the length of the transmission line. The other can compensate for transmission line changes, but with a density range limited by the O-mode cutoff.

A conventional visible spectrometer system has been applied to velocity fluctuation measurements in plasmas. By measuring the number of photons, the effect of quantum noise is subtracted, and real velocity fluctuations are obtained. The first measurements in a reversed‐field‐pinch plasma show finite velocity fluctuations of about 1.7×103 m/s when the system views off‐center chords. When it views the central chord, finite fluctuations are not found. These results impose an upper limit to spurious Doppler broadening, which could arise from fast and large vibrations of cold plasma. The effect of the spurious Doppler broadening is found to be small.

This paper evaluates the accuracy of electron temperature measurements and relative transmissivities of double-pass Thomson scattering diagnostics. The electron temperature (Te) is obtained from the ratio of signals from a double-pass scattering system, then relative transmissivities are calculated from the measured Te and intensity of the signals. How accurate the values are depends on the electron temperature (Te) and scattering angle (θ), and therefore the accuracy of the values was evaluated experimentally using the Large Helical Device (LHD) and the Tokyo spherical tokamak-2 (TST-2). Analyzing the data from the TST-2 indicates that a high Te and a large scattering angle (θ) yield accurate values. Indeed, the errors for scattering angle θ = 135° are approximately half of those for θ = 115°. The method of determining the Te in a wide Te range spanning over two orders of magnitude (0.01-1.5 keV) was validated using the experimental results of the LHD and TST-2. A simple method to provide relative transmissivities, which include inputs from collection optics, vacuum window, optical fibers, and polychromators, is also presented. The relative errors were less than approximately 10%. Numerical simulations also indicate that the Te measurements are valid under harsh radiation conditions. This method to obtain Te can be considered for the design of Thomson scattering systems where there is high-performance plasma that generates harsh radiation environments.

Fluctuations are measured in the edge and scrape-off layer (SOL) of QUEST using fast visible imaging diagnostic. Electron cyclotron wave injection in the Ohmic plasma features excitation of low frequency coherent fluctuations near the separatrix and enhanced cross-field transport. Plasma shifts from initial high field side limiter bound (inboard limited, IL) towards inboard poloidal null (IPN) configuration with steepening of the density profile at the edge. This may have facilitated the increased edge and SOL fluctuation activities. Observation of the coherent mode, associated plasma flow, and particle out-flux, for the first time in the IPN plasma configuration in a spherical tokamak may provide further impetus to the edge and SOL turbulence studies in tokamaks.

A grating polarizer with rectangular grooves is designed for the 106.4 GHz electron cyclotron heating (ECH) system on the Heliotron‐E helical device. The polarizer is installed on mirrors of waveguide miter bends. In oversized waveguides, the HE11 mode can be treated by the plane wave theory. The polarization of the reflected wave is calculated as a function of the groove parameters and the mirror rotation angle. When launching the ECH power to fusion devices with moderate or strong magnetic shear such as heliotron/torsatron configurations, we should take into account the change of the wave polarization through the plasma. The effect of the magnetic shear can be investigated by using the polarizer. The dependence of the absorbed power on the mirror rotation angle is presented.

Magnetic and velocity fluctuations are studied in the REPUTE‐1 [Plasma Phys. Controlled Fusion 28, 805 (1986)] reversed‐field pinch (RFP). The first measurement of velocity fluctuation in an RFP plasma has been done using a Doppler shift of the O v(O4+, 278.1 nm) line. The fluctuation level increases as the radius of the viewing chord increases. Magnetic fluctuation measurements by an insertable probe reveal that the radial cross correlation of toroidal field fluctuation changes its sign at the radius slightly inside the reversal surface. The level of magnetohydrodynamic dynamo term is estimated from magnetic fluctuations at the surface correlation changes and oxygen velocity fluctuations measured with the chord distance of 115 mm. The dynamo term and that due to resistivity are the same level. This fact is consistent with Ohm’s law on which magnetohydrodynamic dynamo models are based.

A multichannel far-infrared laser interferometer system has been developed for the measurement of the spatial and temporal behaviors of the electron density on the Large Helical Device (LHD) at the National Institute for Fusion Science. Of several candidates for high power laser sources a 119 μm CH3OH laser was taken as a probing light. The optical configuration is of the Michelson interferometer type with 13 channels. The optical system of the interferometer is mounted on a massive stainless frame floated on three vibration isolating mounts. The resolution of the fringe counters is 1/100 fringes, and it corresponds to a line averaged density of 5.6×1016 m−3 at the central chord. Preliminary results have been obtained in the initial experiments on the LHD.

Several improvements in the high‐voltage heavy‐ion‐beam probe (HIBP) are discussed. (1) It is clearly found that the precision slide mount of the detector plates 30° parallel to the base electrode is very effective for the determination of the in‐plane entrance angle of the beam in the analyzer to estimate the error in the potential measurement. (2) A two‐staged optical trap in the HIBP greatly reduced the effect of the UV radiation in the analyzer. (3) A multiple‐plate detector up to 13 measurement points clearly showed the direction of the propagation of the turbulence and path‐integral effects.

A Rogowski probe consisting of a small multi-layer Rogowski coil, five magnetic pick-up coils, and a Langmuir probe was developed to measure the local current density and its direction. It can be moved along the major radius and can be turned around its axis. This probe was used to measure the current density profile near the last closed flux surface of Ohmic plasmas in Tokyo Spherical Tokamak-2. The current density profile was measured successfully with a signal to noise ratio of greater than 20.

The multi-pass Thomson scattering (TS) scheme enables obtaining many photons by accumulating multiple TS signals. The signal-to-noise ratio (SNR) depends on the accumulation number. In this study, we performed multi-pass TS measurements for ohmically heated plasmas, and the relationship between SNR and the accumulation number was investigated. As a result, improvement of SNR in this experiment indicated similar tendency to that calculated for the background noise dominant situation.

In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms.

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.

A self-nonlinear process involving the geodesic acoustic mode (GAM) was analyzed. GAMs nonlinearly excite higher harmonics as quasimodes. In the fluid picture, the Reynolds stress is modeled by the drift wave action. Other nonlinear terms such as the coupling between parallel velocity and vorticity, and between density and vorticity, were also considered, and evaluated using linear eigenmodes. We considered nonlinearities up to the third order. Analytic expressions for the amplitude of the second harmonic GAM and the saturation level of the fundamental GAM were derived. The validity of the theory is discussed based on a comparison of experimental results with theoretical predictions.

The differential phase (DP) method was applied to microwave interferometry for the first time. Its advantage over standard interferometry is that the greatly reduced phase shift enables the unique determination of phase. Such a system was implemented on the Tokyo Spherical Tokamak-2 (TST-2), and demonstrated that the correct phase shift can be obtained even when the probing frequency is close to the cutoff density, and that very fast time evolutions of the line-integrated density can be measured.

Impurity transport of carbon has been studied using a new method combined carbon pellet injection with high-spatial resolution bremsstrahlung measurement on the Large Helical Device [A. Iiyoshi et al., Fusion Technol. 17, 169 (1990)]. The carbon pellets are injected into a steady phase in neutral beam heated discharges with a standard configuration of Rax=3.6m. The particle transport coefficients (diffusion coefficient D and convective velocity V) are inferred using a diffusive/convective model. The results are compared between hydrogen and helium plasmas. As a result, it is found that the analyzed D has a constant radial profile with almost closed values of 0.2m2∕s in both plasmas. On the other hand, the inward V is required only at the plasma outer region (ρ>0.6) where the electron density gradient exits, and the inward V in helium plasmas (−0.4m∕s at ρ=0.8 and ne∼4.0×1019m−3) is nearly half as much as that in hydrogen plasmas (−0.7m∕s). This difference of the inward V between hydrogen and helium plasma...

A convergence study of bispectral analysis on toroidal plasma turbulence is presented with a theoretical estimate of bicoherence for the system of the drift wave and zonal flow. Examples are chosen from the analysis of floating potential fluctuation in the edge region of the JFT-2M tokamak. The convergence properties of the bispectral functions are clarified for an oscillatory branch of zonal flow (or geodesic acoustic mode) and drift waves. The total squared bicoherence or the biphase converges inversely proportional to the realization number or to the square root of the realization number, respectively. The numbers of realizations to qualitatively converge and obtain a quantitatively significant value of bicoherence are discussed for the drift wave fluctuations and for the zonal flow.