Leonid G. Bruskin

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.

Relationship between particle and heat transport in JT-60U plasmas with internal transport barrier

The relationship between particle and heat transport in an internal transport barrier (ITB) has been systematically investigated in reversed shear (RS) and high βp mode plasmas of JT-60U. The electron effective diffusivity is well correlated with the ion thermal diffusivity in the ITB region. The ratio of particle flux to electron heat flux, calculated on the basis of the linear stability analysis, shows a similar tendency to an experiment in the RS plasma with a strong ITB. However, the calculated ratio of ion anomalous heat flux to electron heat flux is smaller than the experiment in the ITB region. Helium and carbon are not accumulated inside the ITB even with ion heat transport close to a neoclassical level, but argon is accumulated. The helium diffusivity (DHe) and the ion thermal diffusivity (χi) are 5–15 times higher than the neoclassical level in the high βp mode plasma. In the RS plasma, DHe is reduced from 6–7 times to a 1.4–2 times higher level than the neoclassical level when χi is reduced from 7–18 times to a 1.2–2.6 times higher level than the neoclassical level. The carbon and argon diffusivities estimated assuming the neoclassical inward convection velocity are 4–5 times larger than the neoclassical value, even when χi is close to the neoclassical level. Argon exhaust from the inside of the ITB is demonstrated by applying electron cyclotron heating (ECH) in the high βp mode plasma, where both electron and argon density profiles become flatter. The flattening of the argon density profile is consistent with the reduction of the neoclassical inward convection velocity due to the reduction of the bulk plasma density gradient. In the RS plasma, the density gradient is not decreased by ECH and argon is not exhausted. These results suggest the importance of density gradient control in suppressing impurity accumulation.

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...

Two-dimensional microwave scattering by fluctuations of plasma density and magnetic field

A system of 2D full wave equations is solved for the case of microwave scattering by plasma density and magnetic field fluctuations near the cut-off layer. The 2D field structure in ordinary-to-ordinary scattering (O to O), extraordinary-to-extraordinary scattering (X to X) and O to X, X to O mode conversion processes is studied as well as the influence of refraction, side scattering, diffraction on both X- and O-mode reflection near the cut-off layer. For the case of relatively large antenna size the expressions obtained reveal high sensitivity of the scattered field to the plasma disturbance wavenumber in the transverse direction. For the horn size of 1-2 wavelengths the sensitivity is weak. Numerical examples exhibit multisource spatial distribution of the scattered field. Resonance features of modes scattered by wave-shaped fluctuations were found to be similar to the 1D case.

Analytical study of ultra-short pulse reflectometry

The results of an analytical treatment of the time-dependent 2D full-wave equation are presented here for the case of ultra-short pulse (USP) reflectometry. We consider several models of the plasma geometry, namely linear and nonlinear slab models, as well as a 2D plasma density profile with cylindrical symmetry. The latter model is more realistic when compared to the 1D stratified plasma models previously employed in all the analytical, and most numerical, treatments, since the plasma in fusion toroidal devices, mirror machines and plasma processing chambers can often be considered axially symmetric on the scale relevant to microwave reflectometry. Based on the results of analytical modelling, a signal record analysis method of profile reconstruction is proposed. The method has the advantage of using raw signal records instead of poorly localized frequency modes, which makes it robust for the profile measurements using USP reflectometry.

Microwave Measurement of Heart Beat and Analysis Using Wavelet Transform

Microwave reflectometric measurement is applied to diagnose vital signal of a human. The reflectometer signal is processed by a quadrature phase detector in order to obtain both phase and amplitude components of the signal. The phase component is analyzed by using fast-Fourier transform and wavelet transform to evaluate the frequency spectrum of the heartbeat and respiration. Various applications of the microwave measurement are discussed.

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.

Microwave scattering near the cut-off layer by density and magnetic field fluctuations

The reflection and mode conversion of microwaves are investigated analytically for the case of relatively high field distortion by density and magnetic fluctuations. The polarization ratio of two extraordinary wave components is obtained. A solution of the full-wave equation is derived for the 1D cross-scattering problem. Peculiarities of cross-polarization scattered waves are discussed as well as possible applications to reflectometry of fusion devices.

Numerical study of microwave imaging reflectometry for measurements of density fluctuations in a tandem mirror plasma

In this paper, numerical experiment is used to study the imaging properties of microwave imaging reflectometry for the case of tandem mirror device geometry. First of all, the alignment of the experimental setup is performed and then the imaging system is applied for density fluctuations measurements. Fluctuations employed in this study have a nonshifted Gaussian wavenumber spectrum with equal poloidal and radial widths. The size of the optics is shown to be a main parameter limiting the performance of the imaging system. Space-imaging and time-imaging modes are considered. In the latter case, for model conditions imaging and conventional (without optics) reflectometers demonstrate comparable performance in general. When root-mean-squared amplitude of the density fluctuations is large (σ n = 0.06 and 0.09) the imaging system shows a wider range of measurable parameters.

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...