Images of turbulence in a tokamak plasma
IEEE Transactions on Plasma Science
Two-dimensional measurements of density fluctuations in the DIII-D tokamak, obtained with the Beam Emission Spectroscopy diagnostic, have allowed for construction of images and movies of plasma turbulence which are greatly aiding an understanding of tokamak turbulence phenomena. The measurements are obtained in the radial and poloidal plane at a spatial resolution of near 1 cm (about five ion gyroradii). Assembly of synchronized discrete measurements provides a low spatial resolution, high
... solution, high sensitivity, high throughput, high time resolution (1 = 1 s) imaging system. These measurements have shed light on such processes as flow shear, turbulent eddy interaction and shearing, and provide direct visualization of long wavelength turbulence in a hot plasma. Index Terms-Density fluctuations, image analysis, plasma measurement, plasma turbulence, tokamak. T HE LARGE pressure gradients in magnetically confined fusion-grade plasmas result in significant plasma turbulence. This turbulence is essentially a two-dimensional (2-D) phenomenon in tokamaks, and so its full characterization requires measurement of fluctuations in these relevant dimensions. Plasma turbulence in tokamaks is of great scientific interest because of the large anomalous cross-field transport of energy, momentum, and particles that results from correlated density, temperature, and electrostatic and/or magnetic fluctuations  . Turbulence images are desired for a number of scientific objectives: visualization of turbulent structures, measurement of shear flows and the turbulent velocity field, and ultimately to quantify the nonlinear transfer of energy within the turbulent spectrum . The turbulence is considered 2-D because the correlation lengths in the radial and poloidal directions are typically short, of order five to ten ion gyroradii ( 2 cm), while the toroidal correlation length is typically comparable to the device size ( 1 m) and so can be ignored. To address these issues, the Beam Emission Spectroscopy diagnostic system  , which measures spatially localized density fluctuations, has been deployed on the DIII-D tokamak and configured to provide 2-D measurements in the relevant radial and poloidal directions  . Density fluctuations are observed by measuring the collisionally-induced emission of the Doppler-shifted 3 2 transition (Balmer series at 656.1 nm) of the high-energy deuterium neutral beams that are injected to heat, fuel, and rotate tokamak plasmas. ). C. Fenzi was with the University of Wisconsin-Madison, Madison, WI 53706. She is now with CEA, Association Euratom-CEA, Cadarache 13108, France. R. J. Fonck is with the University of Wisconsin-Madison, Madison, WI 53706 USA. Publisher Item Identifier S 0093-3813(02)03074-6. The neutral beam and optical viewing geometry provide measurements at a spatial resolution of cm with roughly a 1-cm channel-to-channel separation. Thirty-two spatial channels are deployed allowing for 2-D measurements on a 5-cm 6-cm grid that can be radially scanned. Measurements are sampled at 1 MHz and the broadband density fluctuations typically exist up to 250 kHz in the outer regions of the plasma discussed here. Each channel has very high throughput and high efficiency, as required to measure these relatively small fluctuation signals, and provides a 0.5 s time record. Thus, such measurements cannot be readily achieved with higher resolution, lower throughput, high-speed cameras. Two-dimensional turbulence images have also been obtained in relatively cold plasmas using Langmuir probe arrays  . A temporal sequence of images of plasma turbulence obtained in a low-confinement mode plasma are shown in Fig. 1 . Each frame, integrated for s, is separated in time by 10 s. The measurements were obtained in the spatial region 0.9-1.05 at the outer midplane ( is the normalized toroidal magnetic flux radial coordinate). The images are constructed from the raw data as follows. The turbulence exists over the frequency range 250 kHz. This data was, therefore, frequency-filtered over 1-250 kHz (maintaining constant phase) to minimize extraneous electronic and photon noise in the signal at higher frequencies, as well as to avoid low-frequency beam noise ( 1 kHz). The 5 6 grid of data is then interpolated to a 50 60 grid using a minimum curvature surface 2-D spline method, and finally the data is linearly interpolated to a 250 300 pixel image for display. The color scheme is such that green represents the equilibrium density and red and blue represent positive and negative fluctuations about the equilibrium. The RMS magnitude of density fluctuations, , range from 4% to 15% over the radial region sampled, with densities in the range 1.5-2.5 10 m , ion temperatures near 200 eV, and electron temperatures near 100 eV. Movies are generated from the 2-D data by assembling a temporal sequence of images, each separated by 1 s. Radial and poloidal correlation lengths of the turbulenceare in the range of 1-3 cm, and decorrelation times are of order 10 s. The time-averaged eddy motion is in the poloidal (vertical) direction as a result of equilibrium and diamagnetic flows [indicated by white arrows in Fig. 1(a) ], with essentially no net steady-state radial motion. Typical poloidal velocities are of order 5-10 km/s and have a significant radial gradient. These velocities are measured using time-delay correlation analysis between poloidally separated spatial channels. A radial shear in the poloidal flow velocity is typically evident in this edge region of the plasma. Such flow shears can attain values of 5 km/s/cm or 5 10 s , comparable to the measured nonlinear decorrelation rates of the turbulence.