T. Tokuzawa

Plasma confinement in the GAMMA 10 tandem mirror

The central cell density and the diamagnetic signal were doubled as a result of plug potential formation by ECRH in hot ion mode experiments on the GAMMA 10 tandem mirror. In order to obtain these remarkable results, the axisymmetrized heating patterns of ECRH and ICRF heating were optimized. Furthermore, conducting plates were installed adjacent to the surface of the plasma along the flat shaped magnetic flux tube located in the anchor transition regions; the plates may contribute to the reduction of some irregular electric fields produced possibly with ECRH in these thin flux tube regions. The conducting plates contributed to reducing the radial loss rate to less than 3% of the total particle losses, along with improvements in the reproducibility of the experiments and the controllability of the potential confinement. The increases in central cell density and diamagnetism in association with the increase in plug potentials scaled well with increasing ECRH power. A plug potential of 0.6 kV and a density increase of 100% were achieved using an ECRH power of 140 kW injected into both plug regions. The plasma confinement was improved by an order of magnitude over a simple mirror confinement owing to the tandem mirror potential formation.

Measurement of Alfvén ion-cyclotron wave using both X- and O-mode reflectometers

Alfven ion‐cyclotron wave excited in the central‐cell plasma of the GAMMAu200910 tandem mirror is investigated by a reflectometer system. The Fourier amplitude of the instability strongly depends on the anisotropy of the ion velocity distribution function and the plasma β value. The ratio 0.2–0.3 between O‐mode and X‐mode reflectometer signals suggests that a magnetic fluctuation level similar to the density fluctuation level is excited in the plasma due to the instability.

Preliminary electron density profile and fluctuation measurements on GAMMA 10 using ultrashort-pulse reflectometry

The use of ultrashort pulses (full width at half-maximum=1–100 ps) as a source of broadband, time-resolved radiation has high potential for application in microwave diagnostics for fusion plasmas. Here we report on the ultrashort-pulse reflectometry system developed for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. The four-channel system (selectable frequencies of 8–17 GHz at 1 GHz intervals) measures the double-pass time-of-flight from the vacuum window to the cutoff layer. The hardware has an uncertainty of ±40 ps and can be operated at a repetition rate of 400 kHz. The system is operational, however, data from plasma experiments is unavailable at the moment. Results from a single-channel O-mode system show time-of-flight measurements consistent with values calculated from profiles obtained by a scannable single-channel interferometer. Fluctuation measurements are also consistent with results from a Fraunhofer diffraction diagnostic. These results suggest that wit...

Development of millimeter-wave two-dimensional imaging array

A millimeter-wave two-dimensional imaging array has been developed for diagnostics of magnetically confined plasmas. It is applied to the plug cell of the GAMMA 10 tandem mirror, and measures time evolution of radial and axial line-density and electron cyclotron emission (ECE) profiles in one plasma shot. The monolithic-type GaAs Schottky diode detector having an antenna and built-in amplifier on a GaAs substrate is newly fabricated using monolithic microwave integrated circuit (MMIC) technology. This novel detector is developed to attain high frequency response up to submillimeter-wave region and wideband heterodyne characteristics, and is expected to be applied to ECE imaging in a large helical device. The response of the detector is compared with that of the hybrid-type detector using beam-lead Schottky barrier diodes. The first experimental result of the ECE measurement is reported using a heterodyne radiometer with the MMIC detector.

MILLIMETER-WAVE TWO-DIMENSIONAL IMAGING ARRAY FOR THE GAMMA 10 TANDEM MIRROR

A millimeter-wave two-dimensional imaging array has been fabricated in order to diagnose the plug-cell plasma of the GAMMA 10 tandem mirror. The heterodyne system with two separate sources enables the measurement of density and electron cyclotron emission (ECE) in one plasma shot. The interferometer successfully measures the time evolution of the axial and radial line-density profiles with phase resolution less than 1/200 fringe and spatial resolution of 16 mm in the vertical direction and 5 mm in the horizontal one. It is shown that the Abel-inverted density profiles are changed due to the injection of electron-cyclotron-resonance heating at the plug/barrier cells and central cell. The axial and radial dependence of the second-harmonic ECE emitted from hot electrons produced in barrier region is observed simultaneously. The fluctuation spectra of the ECE signals are compared with those obtained from the far-forward scattering diagnostic system.

Ultrafast broadband frequency-modulation reflectometer for density profile measurements in GAMMA 10

A broadband microwave reflectometer was applied to the GAMMA 10 tandem mirror using a fast-sweep hyperabrupt varactor-tuned oscillator (HTO) that can be swept over the full band (12–18 GHz) in less than 5 μs. Clear fringes are observed during a plasma shot when the resultant frequencies from the reflectometer output are much larger than the ion cyclotron range of frequency (3–10 MHz). The density profile measurement is performed for various sweep times of HTO from 1 ms to 5 μs in which two reconstruction methods are utilized, zero-cross counting of the fringes and frequency analysis using the maximum entropy method. The reliability of the profile measurement seems to be improved when the sweep time is faster than 10–30 μs. The integrated value of many reconstructed density profiles agrees well with the profile obtained from a scanning interferometer. The fast-sweep reflectometer is also applied to the measurement of density fluctuations such as plasma movement as well as density profiles.

Measurement of ICRF waves in the GAMMA10 tandem mirror using reflectometers

Microwave reflectometry has been applied to the central cell of the GAMMA 10 tandem mirror for measurements of fluctuations in the ion cyclotron range of frequency (ICRF). The instability with frequency ω<ωci is identified as the Alfven ion cyclotron wave driven by plasma β and ion pressure anisotropy. Internal magnetic fluctuations with δB/B∼10−4 as well as density fluctuations of the same level for this wave are determined by a combination of X‐ and O‐mode reflectometers. The fluctuations associated with the ICRF heating wave (ω=ωci) are also observed by three separated X/O mode reflectometers to study the physics of wave excitation and propagation. The radial profile of the fluctuation level is expected to give an estimation of the deposition profile of the heating power. The results will be applied in order to achieve efficient ion heating and to help the power balance study of central‐cell plasma.

Cross-polarization scattering from low-frequency electromagnetic waves in the GAMMA10 tandem mirror

A cross-polarization scattering (CPS) diagnostic method was applied to the GAMMA10 tandem mirror in order to study electromagnetic plasma waves with frequency less than 10 MHz. In the CPS process, an incident ordinary (extraordinary) wave is converted to an extraordinary (ordinary) wave by magnetic fluctuations in a plasma. The frequency spectra, and the density and magnetic fluctuations of the waves can be evaluated from both the CPS and reflectometry. The converted wave propagates through the cutoff layer and reaches a different diagnostic port with a receiver horn. The observed waves are identified as the electromagnetic drift wave assuming the wave number matching condition. The low frequency waves nonlinearly coupled with the Alfven ion cyclotron waves are also observed with increase in the plasma pressure.

Measurement of plasma density using wavelet analysis of microwave reflectometer signal

A new method of plasma density profile reconstruction in microwave reflectometry is proposed and implemented on an X-mode broadband reflectometer of the GAMMA 10 mirror device with an ultrafast sweep rate of 10–20 μs. The proposed method makes use of the wavelet transform of the detected signal. Excellent resolution in the time-frequency domain, inherent to wavelet analysis, allows one to obtain a radial electron density profile for every frequency sweep. The electron density reconstruction algorithm, besides the wavelet transform of the reflectometer signal, also includes the calibration, profile initialization, and the solution of an integral equation, ultimately yielding the local values of the electron density. Calibration of the measured signal phase and profile initialization is performed using the independent results of microwave interferometry. Inversion of the integral equation is implemented utilizing the gradient method, numerically stable even for plasma regions with steep density gradients an...

WAVELET ANALYSIS OF PLASMA FLUCTUATIONS IN MICROWAVE REFLECTOMETRY

A time–frequency analysis of plasma turbulence is performed for the GAMMA10 tandem mirror plasma. The method is based on the decomposition of the reflectometer signal into a set of wavelets. Excellent resolution in the time–frequency domain, inherent to wavelet analysis, allowed us to observe fast changes in the instantaneous spectra of fluctuations, which would be averaged out if treated using conventional Fourier analysis. In order to estimate the propagation velocity of turbulent fluctuations, a wavelet cross-correlation function was calculated. Comparison of wavelet spectra, corresponding to different reflectometer channels, demonstrates the existence of short-time flashes of fluctuation intensity, as well as elongation of low-frequency disturbances along the magnetic field lines and their rotation in the E×B direction. Plasma fluctuations in the Alfven range of frequencies sometimes propagate from the center towards the magnetic plugs with velocities close to 1000 km/s. In general, the wavelet spectr...