Measurements Of Density Fluctuations Research Articles

Recent results on turbulence and MHD activity achieved by reflectometry

Over the last years, owing to hardware progress and the development of new methods, reflectometry has become a common diagnostic on plasma fusion devices. This paper presents some results obtained with reflectometry on transport, turbulence and magnetohydrodynamic (MHD). The emphasis is put on some new results from Tore-Supra. Combining the density profile and fluctuation measurement, it was shown on Tore-Supra that the particle pinch inside the q = 1 surface is close to the neoclassical value in ohmic plasma, while the observed small diffusion is in agreement with a very low level of density fluctuations inside the q = 1 surface. In β scaling experiments, no change in the fluctuation levels was found on Tore-Supra, in agreement with the observation of weak confinement degradation with increasing β. Zonal flows have been detected by Doppler reflectometry in ASDEX-U and with correlation reflectometry in T-10. On Tore-Supra, a fast decrease in the density fluctuation level at high poloidal wavenumbers was measured with Doppler reflectometry, suggesting a minor role of electron temperature gradient driven modes. Various forms of Alfvén eigenmodes (toroidal Alfvén eigenmodes, Alfvén cascades and possibly beta Alfvén eigenmodes) have been detected with reflectometry in TFTR, JET and Tore-Supra. The density fluctuations induced by the mode were found to be higher on the high-field side.

Comparison of density fluctuation measurements between O-mode and X-mode reflectometry on Tore Supra

Reflectometry is a versatile diagnostic which allows both electronic density profile and density fluctuation measurements. Fast sweep heterodyne technique is particularly suitable for precise measurement of the phase of the reflected signal, which records the story of the wave propagation through the plasma up to the cutoff layer, including the density fluctuations. The present article exhibits a comparison of the density fluctuation radial profile measurements between fast sweep frequency technique, both using O-mode and X-mode polarizations, and fixed frequency technique. The correct agreement between all measurements of the relative values of the density fluctuation profiles reinforces the validity of the approximations used.

Density fluctuation measurement using motional Stark effect optics in JT-60U

The multichannel motional Stark effect (MSE) diagnostic system in JT-60U has been upgraded to measure density fluctuation profile. A 16-channel fast-sampling digitizer has been added in order to measure photomultiplier-tube signals at measurement frequency of 0.5–1MHz. The new system works as a MSE and beam emission spectroscopy diagnostic. Spatially resolved electron density fluctuation profile measurement in various operation regimes is presented. In the core plasma, density fluctuation induced by rotation of tearing mode islands was observed. Temporal evolution of the fluctuation frequency agrees with that measured by Mirnov coils (poloidal and toroidal mode numbers: 2 and 1, respectively). The phases of the fluctuations on either side of the q=2 surface are inverted, which is consistent with electron cyclotron emission. These measurements show that the density fluctuation is caused by a rotating magnetic island structure induced by the tearing mode. In the scrape-off layer of a H-mode plasma with edge-localized-mode (ELM), i. e., ELMy H-mode outward propagation of strong intermittent emission corresponding to ELM crash was also observed. The propagation velocity is 0.69–2.2km∕s along the MSE measurement points, the time lag and distance between adjacent channels being 67±35μs and 70mm, respectively.

Microwave imaging reflectometry in LHD

A multichannel reflectometry with an imaging optical system is under development for the measurement of the electron density fluctuations in the Large Helical Device (LHD). The right-hand cutoff layer is utilized as a reflection surface. The angle of an ellipsoidal mirror installed inside the vacuum chamber is remotely adjustable with the ultrasonic motor in order to optimize the illumination angle for the wider range of the plasma parameters. An oscillation due to density fluctuation was observed using the microwave imaging reflectometry for the first time in LHD plasma experiment.

X -mode reflectometry measurements in the JET plasma core region

The X-mode radial correlation reflectometry diagnostic on JET has long suffered from strong signal attenuation in the original transmission lines. To reach higher performances, new low-attenuation corrugated circular waveguides and a new cluster of six antennas have been successfully installed in Autumn 2005. The increase of the signal-to-noise ratio using these new transmission lines is found to be on the order of magnitude of 20 dB, thus enabling good quality of reflectometry measurement. With the benefit of this improvement, first X-mode reflectometry measurements of density fluctuations in the core region of the JET plasma are reported in this article.

Velocity fluctuation analysis via dynamic programming

A new method of calculating one-dimensional velocity fluctuations from spatially resolved density fluctuation measurements is presented. The algorithm uses vector-matching methods of dynamic programming that match structures, such as turbulent fluctuations, in two data sets. The associated time delay between data sets is estimated by determining an optimal path to transform one vector to another. This time-delay-estimation (TDE) method establishes a new benchmark for velocity analysis by achieving higher sensitivity and frequency response than previously developed methods, such as time-resolved cross correlations and wavelets. TDE has been successfully applied to beam emission spectroscopy measurements of density fluctuations to obtain poloidal flow fluctuations associated with such phenomena as the geodesic acoustic mode. The dynamic programming algorithm should allow extension to high frequency velocity fluctuations associated with underlying electrostatic potential and resulting E×B fluctuations.

Spatial transfer function for the beam emission spectroscopy diagnostic on DIII-D

The spatial transfer function for the beam emission spectroscopy (BES) diagnostic is critical to quantitatively interpret local density fluctuation measurements. A three-dimensional geometrical calculation of the spatial transfer function is presented for the upgraded BES diagnostic on DIII-D to determine its spatial resolution and wave-number sensitivity. The spatial transfer function calculation for the BES system on DIII-D incorporates the high speed (f∕2) collection optics, an optical fiber bundle, neutral beam-sight line geometry, the neutral beam cross-section intensity profile, magnetic field pitch angle, as well as atomic physics of the finite atomic transition time of the collisionally excited beam atoms. The resulting imaged volumes for each BES channel typically have ∼1–2cm radial and poloidal resolutions. In addition, the viewing volume is nominally aligned along a magnetic field line to minimize spatial smearing of the field-aligned turbulent eddies. This calculation is crucial for performing the proper deconvolution of measured signals to determine correlation lengths and wave-number spectra of the measured density fluctuations, and is also required for applying synthetic diagnostics to nonlinear turbulence simulation codes (e.g., GYRO) to relate code output to measured signals.

Application of precise phase detector for density profile and fluctuation measurements using CO2 imaging heterodyne interferometer on LHD

The two-color CO2∕YAG laser imaging heterodyne interferometer was recently equipped with a precise digital phase counter (eight channels, 0.5mrad resolution, 1MHz carrier frequency and sampling rate, 500kHz bandwidth, ±250rad recording range, and a maximum data record of 8.4×106 samples/channel). This modification enables detailed measurements of plasma density dynamics during pellet ablation and, in addition, observations of density fluctuations. Examples of fast rising density peaks in pellet discharges as well as spatial and spectral characteristics of plasma density fluctuations obtained with the heterodyne interferometer are presented.

Radial Structure of Alfvén Eigenmodes in the DIII-D Tokamak through Electron-Cyclotron-Emission Measurements

The spatial structure of toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in DIII-D is obtained from electron-cyclotron-emission measurements. Peak measured temperature perturbations are of similar magnitude for both toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes and found to be deltaT(e)/T(e) approximately equal to 0.5%. Simultaneous measurements of density fluctuations using beam-emission spectroscopy indicate deltan(e)/n(e) approximately equal to 0.25%. Predictions of the measured temperature and density perturbation profiles as well as deltaT(e)/deltan(e) from the ideal magnetohydrodynamic code NOVA are in close agreement with experiment.

Frequency- and space-resolved measurement of local density fluctuations in air by laser vibrometry

The objective of this work was the application of interferometry as a tool for non-intrusive measurement of frequency spectra in turbulent flows. Two laser vibrometer systems together with a signal analyser were used to obtain the frequency spectra of density fluctuations across a fully developed turbulent free jet. Since laser vibrometry is based on interferometric techniques the derived signals are path integrals along the measurement beam. To derive local information on the frequency spectra of density fluctuations long-time averaged measurements from each of the two systems were treated using correlation functions and cross spectra. Results were compared to spectral data from a pitot-mounted pressure sensor.

Experimental Investigation of Reynolds and Favre Averaging in High-Speed Jets

Recent advancements in a molecular Rayleigh scattering based diagnostic technique allowed for simultaneous measurement of velocity and density fluctuations with high sampling rates. The technique was used to investigate unheated high subsonic and supersonic fully expanded free jets in the Mach number range from 0.8 to 1.8. The difference between the Favre-averaged and the Reynolds-averaged axial velocity and axial component of the turbulent kinetic energy is found to be small. On average, estimates based on Morkovin’s strong Reynolds analogy are found to underpredict turbulent density fluctuations.

Status and prospects for mm-wave reflectometry in ITER

Reflectometry with wavelengths in the centimetre to millimetre-wave range will be used in ITER to measure the density profile in the main plasma and divertor regions and to measure the plasma position and shape in order to provide a reference for the magnetic diagnostics in long pulses. In addition, it is expected to provide key information for the measurement of density fluctuations. A set of reflectometers to meet the relevant ITER measurement requirements has been included in its present outline as part of the ITER design since 2001 and is being adapted to the present ITER baseline and to accommodate progress with reflectometry techniques and measurement capabilities. It comprises low and high field side (HFS and LFS, respectively) ordinary (O-) mode systems for the measurement of the density profile in the gradient regions, a LFS extraordinary (X-) mode system for the detailed study of the edge profile, an HFS X-mode system operating in the left hand cutoff to measure the core profile, a dedicated O-mode system for plasma-wall gap measurement and a multi-band, multiple line of sight O-mode system to measure divertor density profiles. This paper describes the evolution of the design, in particular some recent improvements in the engineering implementation and improvements aimed at enhancing the measurement capability. It concludes with a brief assessment of the likely measurement performance against the ITER measurement requirements for the parameters of interest and the overall confidence that the technique will be implanted on ITER.

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 (σn = 0.06 and 0.09) the imaging system shows a wider range of measurable parameters.

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.