High-frequency Density Fluctuations Research Articles

Multi-Perspective Heterodyne Digital Holography For High-Speed Imaging Of Density Fluctuations In Supersonic Flow

Density fluctuation measurements with high spatio-temporal resolution provide meaningful insights into the dynamics of turbulent structures in shear layers, boundary layers, and disturbances that influence the transition to turbulence. The current contribution presents a new multiple perspective digital holographic imaging technique that aims to resolve high-frequency density fluctuations with spatial discrimination along the line of sight. A key element of the proposed method is the generation of a beamlet array of varying view angles that is spatially overlapped within the measurement zone. We present the first experimental realization of this approach using a 7-element hexagonally packed fly’s eye lens array and analyze results for a pair of mutually perpendicular and axially offset sonic jets. The experimental apparatus also incorporates heterodyne detection with a heterodyne frequency set to 1/4 of the imaging frame rate. The optical system performance is evaluated for a crossing jet flow and key design considerations for the multi-perspective optical system are discussed.

Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment.

Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field-reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations.

Investigation of zonal flows by using the collective scattering measurement of density fluctuations

The poloidal rotation velocities in the core plasma region are studied using the instantaneous frequency method (IFM) with the density fluctuations measured by the CO2 laser collective scattering diagnostics on the HT-7 tokamak. A coherent mode is observed in the fluctuations of poloidal velocities with the mode frequency from 10 to 20 kHz. It is identified as geodesic acoustic mode (GAM) zonal flow with poloidal symmetry (m = 0) and its mode frequency coinciding with the theoretical expected GAM frequency. The nonlinear interactions are investigated by applying the envelope analysis on the density fluctuations. The results confirm that the envelope modulation in the high frequency density fluctuations only comes from the shearing by GAM. The comparison between IFM and envelope analysis is also discussed.

Two-Point Measurement of High-Frequency Density Fluctuations by Reflectometry in GAMMA 10

To study the detailed radial structure of the Alfvén ion-cyclotron (AIC) waves excited in the GAMMA10 central cell, we upgraded the previously used reflectometer system so as to measure radially separated two density fluctuations simultaneously. Successful application of the new reflectometer system reveals existence of non-trivial radial structure of the AIC waves; from the core to about a half plasma radius, the density fluctuations arising from the AIC waves have common phase relationship, while in the external region, they have opposite phase relation with that of the core.

Use of the Far Infrared Tangential Interferometer/Polarimeter diagnostic for the study of rf driven plasma waves on NSTX

A rf detection system for waves in the 30 MHz range has been constructed for the Far Infrared Tangential Interferometer/Polarimeter on National Spherical Torus Experiment (NSTX). It is aimed at monitoring high frequency density fluctuations driven by 30 MHz high harmonic fast wave fields. The levels of density fluctuations at various radial chords and antenna phase angles can be estimated using the electric field calculated by TORIC code and linearized continuity equation for the electron density. In this paper, the experimental arrangement for the detection of rf signal and preliminary results of simulation will be discussed.

NSTX far infrared tangential interferometer/polarimeter electronics upgrade

New electronics for the multichannel far infrared tangential interferometer/polarimeter system employed on the National Spherical Torus Experiment (NSTX) have greatly extended its capability to monitor high frequency density fluctuations. Such measurements are essential in understanding transport physics issues in NSTX as well as for the coming ITER device. The electronics, which were previously limited to ∼250 kHz, have been upgraded with a video bandwidth that extends to 4 MHz when operating as an interferometry-only configuration, and to ∼500 kHz when operating as a simultaneous interferometer/polarimeter system. Experimental details and test results of the new electronics are presented.

Low- and high-mode separation of short wavelength turbulence in dithering Wendelstein 7-AS plasmas

In this article measurements of small scale electron density fluctuations in dithering high confinement (H)-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS (W7-AS) stellarator [H. Renner et al., Plasma Phys. Control. Fusion 31, 1579 (1989)]. The experimental setup and discharge properties are described. Hα-light observing an inner limiter was used to separate low confinement (L)- and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L- (H-) mode fluctuations dominate at high (low) frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L- and H-mode measurements are calculated and fitted by power-laws and exponential functions. The separated measurements can be fitted with the same exponents in L- and H-mode. Correlations between the density fluctuations, the Hα-signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows (several dithering periods) with time lag steps of 100 μs showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L- and H-mode correlations on a 20 μs time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 μs, while no correlation exists for H-mode.

Multichannel interferometer system for the helically symmetric experiment

A multichannel millimeter-wave interferometer system has been designed, fabricated and installed on the helically symmetric experiment (HSX), located at the University of Wisconsin, Madison. The interferometer system will view the plasma cross section along nine adjacent chords with 1.5 cm spacing. With this arrangement, coverage will span from the low-field side plasma scrape-off layer to well past the magnetic axis. For the plasma densities anticipated on HSX, a solid-state source operating at 288 GHz will be utilized. At this frequency refraction will be manageable, being less than the channel spacing. The source will be bias-tuned and modulated with a sawtooth wave form at 750 kHz in order to generate the intermediate frequency necessary for the heterodyne detection scheme. The signals will be measured using Schottky-diode corner-cube mixers. The interferometer will have sensitivity nedl≈8×1011 cm−2, being able to measure density changes <1%. Initially, the phase will be evaluated using analog electronics with bandwidth <10 kHz providing real-time line-integrated output. A digital phase comparator scheme will also be implemented whereby the measured wave forms are directly digitized and the phase evaluated using a software-based algorithm. This will increase the time response up to the modulation frequency of 750 kHz. Improved time response will permit measurement of high-frequency density fluctuations along with “fast changes in” the equilibrium profile.

Light-scattering study of slow and fast dynamics in a strong inorganic glass former

The dynamic properties of glassy and liquid ${\mathrm{As}}_{2}{\mathrm{O}}_{3}$ are investigated over a wide temperature range, in both the microscopic and macroscopic time domains by Brillouin scattering (BS) and photon correlation spectroscopy (PCS). The two characteristic properties of sound propagation, velocity, and attenuation were found to exhibit considerable, although unexpected, changes very close to the glass transition temperature ${T}_{g}.$ The high-frequency density fluctuations were quantitatively treated using a phenomenological formulation for the corresponding memory function, which considers both slow and fast processes. The obtained viscoelastic parameters were found to follow physically acceptable temperature dependencies. Both density and orientation autocorrelation functions show a very narrow distribution of relaxation times with a shape parameter close to 0.8. The peculiarities of the sound-velocity and the sound-absorption coefficient as well as the comparison between the PCS and the BS relaxation times confirmed the existence of two relaxation processes differing by 10 orders of magnitude near ${T}_{g}.$ The difference in activation energies, for the fast process, between strong and fragile glasses is discussed on the basis of the stability of asymmetric double-well potentials over a relaxation period. Evidence is provided conforming to the two fluid model predictions, invoking long-range density fluctuations. Pseudotransformations of chemically and topologically ``acceptable'' structures seem to be the driving force for low-energy excitations in network bonded glasses.

Interferometric measurement of high-frequency density fluctuations in Madison symmetric torus

As a consequence of recent improvements to the far-infrared (FIR) interferometer time response, from bandwidth of 10 kHz up to 1 MHz, high-frequency density fluctuations can now be resolved on the Madison symmetric torus (MST) reversed-field pinch. The phase measurement allows absolute calibration of the fluctuation amplitude while eleven chords generate information on the spatial distribution. Density fluctuations up to 200 kHz are observed. A unique feature of the MST interferometer is that six chords are toroidally displaced 5° from the remaining five. Hence, besides information on the poloidal structure, i.e., m=1, toroidal information is also available. By computing the cross power, coherence, and cross phase between two toroidally displaced chords, one can determine the toroidal rotation speed, dispersion, and correlation length.

Transport phenomena in Alcator C-Mod H-modes

Several interesting new results have come from studies of ICRF-heated, H-mode plasmas in Alcator C-Mod. Dimensionless scaling studies have found gyro-Bohm-like transport similar to that reported on other devices; however, the dependence on collisionality was surprisingly strong, with . Despite high edge temperatures and strong edge pressure gradients, type I edge-localized modes (ELMs) are not observed in C-Mod. Instead we obtain a regime that we have dubbed enhanced (EDA) which is accompanied by high-frequency density fluctuations. For all H-modes, core gradients were found to increase linearly with edge temperature, suggesting the importance of critical gradient/marginal stability behaviour. Comparisons with the IFS-PPPL model have begun, showing quantitative agreement in some cases. Impurity particle transport was studied via the laser blow-off technique with impurity confinement found to be effectively infinite for ELM-free discharges but reduced into the range 0.1-0.2 s for the EDA plasmas.

M=1/n=1 density fluctuations in currentless high-β plasma in heliotron E

High-frequency density fluctuations with m=1/n=1 have been observed in a currentless high-β plasma. The line density was measured with a 5-ch FIR laser interferometer. A high-β plasma with 〈β〉∼2% was obtained with about 2-MW neutral beam injection into a currentless target plasma produced by second-harmonic ECRH. With weak gas puffing [β(0)≳1%], sawteeth-like oscillations accompanied by high-frequency fluctuations of about 2–5 kHz were observed on the line density. These fluctuations coincide with the soft x-ray fluctuations. From both signals the mode number of this instability is estimated to be m=1 and n=1. During the fluctuations, rapid enhancement (10%–20%) in the edge ion saturation current was also measured, which indicates that this instability causes particle loss from the bulk to the edge. The instability is quenched by a change in density profile from peaked to broad with intense gas puffing. This behavior is consistent with a theoretical analysis.