Measurements Of Density Fluctuations Research Articles

Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra

Backscattering of a microwave beam close to the cut-off allows for measurement of density fluctuations at a specified wave-number, selected by the scattering geometry , where ki is the beam wave-number at the reflection layer. On the Doppler reflectometry system installed on Tore Supra, both the scattering wave-number k⊥ and the scattering localization (r/a) can be changed during the shot owing to the steppable probing frequency and the motorized antenna. Operating in O mode, the spatial and wave-number ranges depend essentially on density profile, typically probing 0.5 < r/a < 0.95 and 2 < k < 15 cm−1. Wave number spectra are similar to those obtained with conventional scattering systems. The perpendicular fluctuation velocity in the laboratory frame is obtained from the Doppler shift of the frequency spectrum Δω = k⊥v⊥. It is dominated by the plasma Er × B velocity. In the core, the latter is mainly due to the projection of the toroidal velocity, as this is shown by comparison with measurements by charge exchange recombination spectroscopy. In the set of analysed Tore Supra ohmic and ICRH plasmas, the observed rotation is consistent with a poloidal velocity in the electron diamagnetic direction and/or a toroidal velocity in the counter current direction. The detailed structure of the velocity profile, at the edge and in different plasma regimes, allows us then to get information on the radial electric field distribution. The dynamics of the fluctuation velocity can be studied from the time frequency analysis of the signal, for investigating intermittent behaviour and transient regimes.

A dual source D-band reflectometer for density profile and fluctuations measurements in Tore-Supra

The stepped fixed frequency X-mode reflectometer operating between 105 and 155 GHz first designed for density fluctuation measurement has been upgraded to also measure the density profile. It runs now with two sources in parallel: a fast swept source and a frequency synthesizer to measure during the same shot the density profile and density fluctuations. The accuracy of the density profile from reflectometry reduces uncertainties on fluctuation profile measurement. This double operating mode also offers two ways to probe MHD modes and the micro-turbulence.

Thermal radiation and thickness fluctuations in freely suspended liquid films

Thermal convection in a vertically suspended soap film subjected to a vertical temperature gradient is marked by intense density fluctuations δρ2, uncommon to laboratory Rayleigh-Bénard convection (RBC). Such large fluctuations result from stratification in the film under the influence of gravity. Herein we present the first direct measurement of two-dimensional density fluctuations in a free-standing soap film using a single-point infrared detector. The radiation densitometer is nonintrusive and responds instantaneously to local density variations. Measurements of the power spectrum Γ(f) of δρ2 are carried out using a variety of sample geometries of the aspect ratio of unity. In all cases, Γ(f) scales with the frequency as f−1.4±0.1 in the low frequency regime, which is in good agreement with the Bolgiano’s theoretical prediction for a stably stratified fluid. Combining thermal imaging and particle tracking, we also show that it is feasible to measure the full-field density flux j(x,y)=ρ2(x,y)v(x,y) in the film. Despite that individual snapshots of j(x,y) appear random, the time-averaged flux forms a close loop similar to the large-scale circulation commonly seen in RBC, when a sufficiently large temperature gradient is present.

Influence of reentry turbulent plasma fluctuation on EM wave propagation

Electromagnetic wave propagation in turbulent plasma media during reentry is being investigated. Emphasis is placed on the effects of electron density fluctuations on electromagnetic wave propagation. The objective of this paper is to specify the source of noise in electromagnetic signal reception due to turbulence in the flow about a high velocity flight vehicle. A review of existing ground and flight test data is conducted, including electron density fluctuation measurements. Analytical algorithms deduced from first principles are being developed and validated with experimental measurements. A Navier–Stokes model, three-dimensional parabolized Navier–Stokes, and a coupled boundary-layer and inviscid flow codes are used to estimate the mean and variance of the electron density on the frustum and at the vehicle’s base region. Analytical and numerical solutions solving Maxwell’s equations and a commercial off the shelf code are used to estimate the phenomena of electromagnetic wave propagation in inhomogeneous plasma media. Results include the estimation of signal attenuation and phase shift induced by the mean electron density and by electron density fluctuations in the flowfield. Comparisons are made between the predictions and available data.

Time-resolved measurements of the density fluctuations in ion phase space

A two-point correlation function measurement of the ion density fluctuations is presented. Using two laser-induced fluorescence (LIF) detection systems, the density fluctuation is resolved in time, ion parallel velocity, and space (along the magnetic field). The measurements reveal two components of the density fluctuations, one of which is explained by fluid theory. The other component is ion-velocity-dependent and is newly identified. In addition to the density fluctuation measurements, a velocity-resolved estimate of the fluctuation-induced transport flux using correlations between a Langmuir probe and LIF is reported.

Water vapour flux profiles in the convective boundary layer

Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar. The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The eddy-correlation method was applied, and error estimates of ±50 W/m2 for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between 60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease.

Temporal variation of density fluctuation and transport in reversed shear plasmas on JT-60U

Further transport reduction is induced by a pellet or electron cyclotron (EC) wave injection after the formation of an internal transport barrier in JT-60U reversed shear plasmas. After pellet injection, the O-mode reflectometer signal drastically decreases, indicating reduction in the electron density fluctuation level. The ion thermal diffusivity decreases by one order of magnitude and reaches the neoclassical level. The effective particle diffusivity also decreases by one order of magnitude, but the electron thermal diffusivity remains constant. After EC wave injection, when no reduction of the O-mode reflectometer signal is observed, the electron thermal diffusivity decreases by a factor of ∼2–3, while decreases in the ion thermal diffusivity and the effective particle diffusivity are small. These results indicate that the ion heat transport and particle transport were coupled with the measured density fluctuation, but electron heat transport was decoupled. Ion heat and particle transport and electron heat transport were dominated by different types of fluctuation.

Observations of Density Fluctuations in an Elongated Bose Gas: Ideal Gas and Quasicondensate Regimes

We report in situ measurements of density fluctuations in a quasi-one-dimensional 87Rb Bose gas at thermal equilibrium in an elongated harmonic trap. We observe an excess of fluctuations compared to the shot-noise level expected for uncorrelated atoms. At low atomic density, the measured excess is in good agreement with the expected "bunching" for an ideal Bose gas. At high density, the measured fluctuations are strongly reduced compared to the ideal gas case. We attribute this reduction to repulsive interatomic interactions. The data are compared with a calculation for an interacting Bose gas in the quasicondensate regime.

Nonlinearly driven second harmonics of Alfvén cascades

In recent experiments on Alcator C-Mod [J. A. Snipes et al., Phys. Plasmas 12, 056102 (2005)], measurements of density fluctuations with phase contrast imaging through the plasma core show a second harmonic of the basic Alfvén cascade (AC) signal. The present paper presents a theory that describes the second-harmonic perturbation as a nonlinear sideband produced by the AC eigenmode via quadratic terms in the magnetohydrodynamic equations. It is shown that in a low-pressure plasma the nonlinear coupling to compressional Alfvén and acoustic perturbations can be neglected when calculating the second-harmonic density. The derived expressions for this density perturbation can potentially be used together with experimental measurements to determine the AC amplitude inside the plasma, rather than just at the edge as with magnetic probes.

Structure and scaling properties of the geodesic acoustic mode

Characteristics and scaling properties of the geodesic acoustic mode (GAM), a coherent, radially-sheared high frequency (∼15 kHz) zonal flow oscillation, are studied systematically using time-delay-estimation techniques applied to localized, multi-point density fluctuation measurements obtained by beam emission spectroscopy on DIII-D. The GAM amplitude is shown to increase strongly with increasing safety factor, q95, and to likewise become undetectably small for q95 < 4.2, qualitatively consistent with theoretical predictions based on collisional damping as well as simulations. The radial structure of the GAM exhibits peak amplitude in the radial range 0.88 < r/a < 0.95 with a rapid amplitude reduction inside and outside this region. The measured frequency is close to the predicted frequency, though some deviation to higher frequency is observed at lower q. The GAM amplitude is also shown to increase with plasma elongation, κ, while its frequency decreases.

Geodesic–acoustic-mode in JFT-2M tokamak plasmas

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.

Drift wave instability in the Helimak experiment

Electrostatic drift wave linear stability analysis is carried out for the Helimak configuration and compared against experimental data. Density fluctuation and cross-spectrum measurements show evidence of a coherent mode propagating perpendicular to the magnetic field which becomes unstable at k⊥ρs∼0.15. By comparing the experimental results with the wave characteristic of linear two-fluid theory, this mode is identified as an unstable resistive drift wave driven by the density gradient and magnetic grad-B/curvature present in an otherwise magnetohydrodynamic stable steady-state equilibrium.

Reduced divertor heat loads, plasma shape effects, and radial electric field structures in JFT-2M HRS H-mode plasmas

A key feature of the ‘high recycling steady’ (HRS) H-mode regimes is the reduction of the transient heat load to the divertor target due to large ELMs. The heat flux to the divertor targets measured by means of a Langmuir probe in the scrape-off layer on the target plate is about ∼0.3 MW m−2 in the typical HRS plasmas, which is very small in comparison with the transient heat load in the ELMy H-mode discharge. In addition, the effects of the plasma shaping on the access to and improved plasma performance of HRS H-mode regimes have been investigated. Furthermore, simultaneous measurements of both density fluctuations and potential profiles at the plasma edge region using a heavy ion beam probe are also presented.

Rapid density fluctuations in the solar wind

Abstract. Electron density fluctuations (up to 2.5 Hz) in the solar wind have been studied, using the EFW experiment on the Cluster spacecraft, which measures density through measurements of the biased probe potentials relative to the spacecraft. The density fluctuation spectra obtained from the EFW probe potential variations are compared to earlier, OGO 5, measurements of ion density fluctuations and ISEE measurements of electron density fluctuations, and are consistent with them. The electric fields corresponding to the electron density fluctuations are extremely small compared with what would be obtained if the electron fluctuations were not cancelled out by nearly equal ion density fluctuations. This is consistent with the nature of ion acoustic waves. In agreement with ISEE work, the fluctuations are proportional to the ambient density. Correlation with magnetic fluctuations is weak, essentially nonexistent during part of the period studied. This might be expected as magnetic fluctuations are known to be nearly incompressible, but even the correlation with fluctuations in the magnitude of B is very small. However, many structures which apparently are pressure balance structures are found. Pressure balance structures are the nearly perpendicular propagation limit of ion acoustic waves. As ion acoustic waves are strongly damped in plasmas like the solar wind at least if the plasma is taken as Maxwellian, it has always been a puzzle as to why they are found there. We speculate that these waves are created by mode conversion from magnetic fluctuations, and may represent part of the dissipation process for these.

In situmeasurements of density fluctuations and compressibility in silica glasses as a function of temperature and thermal history

In this paper, small-angle x-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility ${\ensuremath{\chi}}_{T}^{0}$ in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain, respectively, the temperature and fictive temperature dependences of ${\ensuremath{\chi}}_{T}^{0}$. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations.

Measurement of Flow Conductivity and Density Fluctuations in Supersonic Nonequilibrium Magnetohydrodynamic Flows

A new blowdown nonequilibrium plasma magnetohydrodynamic (MHD) supersonic wind tunnel operated at complete steady state has been developed and tested at Ohio State. The wind tunnel can be operated at Mach numbers up to M = 3-4 and mass flow rates of up to 45 g/s at a stagnation pressure of 1 atm

Two-dimensional density and density fluctuation diagnostic for the edge plasma in fusion devices

A technique is described for the two-dimensional measurement of electron density profile and fluctuations in edge regions of magnetically confined fusion plasmas. The method is based on existing lithium beam beam emission spectroscopy technique, two-dimensional resolution is achieved by electrostatically scanning the beam. If scanning is performed faster than the lifetime of the turbulent structures in the plasma, the diagnostic is capable of measuring the structure of electron density fluctuations as well. The beam strength of currently available beams makes the detection of single fluctuation events impossible, but the full two-dimensional spatial structure of correlations can still be determined. The article describes the technique and fast beam deflection tests up to 250kHz. The capabilities of such a diagnostic for fluctuation measurement are explored by simulating measurement signals. Measurement of both the two-dimensional density profile, fluctuation correlation function and poloidal flow velocity are demonstrated at the Wendelstein 7-AS stellarator. The shape of the density profile, the radial and poloidal correlation lengths and the flow velocity are in agreement with expectations and previous Langmuir probe measurement.

Effect of ion ∇B drift direction on density fluctuation poloidal flow and flow shear

The divertor magnetic geometry has a significant effect on the poloidal velocity and resulting velocity shear of turbulent density fluctuations in the outer region of L-mode tokamak plasmas, as determined via two-dimensional measurements of density fluctuations with beam emission spectroscopy on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. Plasmas with similar parameters, except that in one case the ion ∇B drift points towards the divertor X point (lower-single-null, LSN), and in the other case, the ion ∇B drift points away from the divertor X point (upper-single-null, USN), are compared. Inside r∕a=0.9, the turbulence characteristics (density fluctuation amplitude, flow direction, correlation lengths) are similar in both cases, while near r∕a=0.92, a dramatic reversal of the poloidal velocity of turbulent eddies relative to the core flow direction is observed in plasmas with the ion ∇B drift pointing towards the divertor X point. No such velocity reversal is observed in plasmas with the ion ∇B drift pointing away from the divertor X point. This poloidal velocity reversal results in a significantly larger local shear in the poloidal velocity of density fluctuations in plasmas with the ion ∇B drift pointing towards the divertor X point. Additionally, these plasmas locally exhibit significant dispersion with two distinct and counterpropagating turbulence modes. Likewise, the radial correlation length of the density fluctuations is reduced in these plasmas, consistent with biorthogonal decomposition measurements of dominant turbulence structures. The naturally occurring density fluctuation poloidal velocity shear in these LSN plasmas may facilitate the L-H transition that occurs at an input power of roughly one-half to one-third that of corresponding plasmas with the ion ∇B drift pointing away from the X point.

Time-average measurement of velocity, density, temperature, and turbulence velocity fluctuations using Rayleigh and Mie scattering

Measurement of time-averaged velocity, density, temperature, and turbulence velocity fluctuations in sparsely seeded gas flows using a non-intrusive, point-wise technique based on Rayleigh and Mie scattering is discussed. A Fabry-Perot interferometer (FPI) is used to spectrally resolve laser light scattered by molecules and particulates in gas flows. The spectral content of the scattered light provides information about velocity, density, and temperature of the gas. A CCD camera is used to record images of the fringes formed by scattered light passing through the interferometer. Models of the spectral components are used in a least squares fitting routine to estimate the parameters from fringe images. Flow measurements are presented for subsonic and supersonic jet flows. The application range for this technique is mostly for high velocity situations (>25 m/s). Velocity, density, temperature, and turbulence velocity fluctuations were determined with accuracies within 5 m/s, 4%, 2%, and 5 m/s, respectively.

Active and fast particle driven Alfvén eigenmodes in Alcator C-Mod

Alfvén eigenmodes (AEs) are studied to assess their stability in high density reactor relevant regimes where Ti≈Te and as a diagnostic tool. Stable AEs are excited with active magnetohydrodynamics antennas in the range of the expected AE frequency. Toroidal Alfvén eigenmode (TAE) damping rates between 0.5%&amp;lt;γ∕ω&amp;lt;4.5% have been observed in diverted and limited Ohmic plasmas. Unstable AEs are excited with a fast ion tail driven by H minority ion cyclotron radio frequency (ICRF) heating with electron densities in the range of n¯e=0.5–2×1020m−3. Energetic particle modes or TAEs have been observed to decrease in frequency and mode number with time up to a large sawtooth collapse, indicating the role fast particles play in stabilizing sawteeth. In the current rise phase, unstable modes with frequencies that increase rapidly with time are observed with magnetic pick-up coils at the wall and phase contrast imaging density fluctuation measurements in the core. Modeling of these modes constrains the calculated safety factor profile to be very flat or with slightly reversed shear. AEs are found to be more stable for an inboard than for central or outboard ICRF resonances in qualitative agreement with modeling.