468 research outputs found
NSTX tangential divertor camera
Strong magnetic field shear around the divertor x-point is numerically predicted to lead to strong spatial asymmetries in turbulence driven particle fluxes. To visualize the turbulence and associated impurity line emission near the lower x-point region, a new tangential observation port has been recently installed on NSTX. A reentrant sapphire window with a moveable in-vessel mirror images the divertor region from the center stack out to R 80 cm and views the x-point for most plasma configurations. A coherent fiber optic bundle transmits the image through a remotely selected filter to a fast camera, for example a 40500 frames/sec Photron CCD camera. A gas puffer located in the lower inboard divertor will localize the turbulence in the region near the x-point. Edge fluid and turbulent codes UEDGE and BOUT will be used to interpret impurity and deuterium emission fluctuation measurements in the divertor
Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data
Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocity analysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measured velocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity.National Science Foundation (U.S.) (1122374
Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the BES system on MAST
The mean motion of turbulent patterns detected by a two-dimensional (2D) beam
emission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST)
is determined using a cross-correlation time delay (CCTD) method. Statistical
reliability of the method is studied by means of synthetic data analysis. The
experimental measurements on MAST indicate that the apparent mean poloidal
motion of the turbulent density patterns in the lab frame arises because the
longest correlation direction of the patterns (parallel to the local background
magnetic fields) is not parallel to the direction of the fastest mean plasma
flows (usually toroidal when strong neutral beam injection is present). The
experimental measurements are consistent with the mean motion of plasma being
toroidal. The sum of all other contributions (mean poloidal plasma flow, phase
velocity of the density patterns in the plasma frame, non-linear effects, etc.)
to the apparent mean poloidal velocity of the density patterns is found to be
negligible. These results hold in all investigated L-mode, H-mode and internal
transport barrier (ITB) discharges. The one exception is a high-poloidal-beta
(the ratio of the plasma pressure to the poloidal magnetic field energy
density) discharge, where a large magnetic island exists. In this case BES
detects very little motion. This effect is currently theoretically unexplained.Comment: 28 pages, 15 figures, submitted to PPC
Comparison of Gas Puff Imaging Data in NSTX with the DEGAS 2 Simulation
Gas-Puff-Imaging (GPI) is a two dimensional diagnostic which measures the edge Dα light emission from a neutral D2 gas puff nears the outer mid-plane of NSTX. DEGAS 2 is a 3-D Monte Carlo code used to model neutral transport and atomic physics in tokamak plasmas. In this paper we compare measurements of the Dα light emission obtained by GPI on NSTX with DEGAS 2 simulations of Dα light emission for specific experiments. Both the simulated spatial distribution and absolute intensity of the Dα light emission agree well with the experimental data obtained between ELMs in H-mode. _________________________________________________
A Model of Turbulence in Magnetized Plasmas: Implications for the Dissipation Range in the Solar Wind
This paper studies the turbulent cascade of magnetic energy in weakly
collisional magnetized plasmas. A cascade model is presented, based on the
assumptions of local nonlinear energy transfer in wavenumber space, critical
balance between linear propagation and nonlinear interaction times, and the
applicability of linear dissipation rates for the nonlinearly turbulent plasma.
The model follows the nonlinear cascade of energy from the driving scale in the
MHD regime, through the transition at the ion Larmor radius into the kinetic
Alfven wave regime, in which the turbulence is dissipated by kinetic processes.
The turbulent fluctuations remain at frequencies below the ion cyclotron
frequency due to the strong anisotropy of the turbulent fluctuations,
k_parallel << k_perp (implied by critical balance). In this limit, the
turbulence is optimally described by gyrokinetics; it is shown that the
gyrokinetic approximation is well satisfied for typical slow solar wind
parameters. Wave phase velocity measurements are consistent with a kinetic
Alfven wave cascade and not the onset of ion cyclotron damping. The conditions
under which the gyrokinetic cascade reaches the ion cyclotron frequency are
established. Cascade model solutions imply that collisionless damping provides
a natural explanation for the observed range of spectral indices in the
dissipation range of the solar wind. The dissipation range spectrum is
predicted to be an exponential fall off; the power-law behavior apparent in
observations may be an artifact of limited instrumental sensitivity. The
cascade model is motivated by a programme of gyrokinetic simulations of
turbulence and particle heating in the solar wind.Comment: 29 pages, 14 figure
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