Detailed studies of low-frequency microturbulence are presented. We utilize a unique multichannel far-infrared (\ = 1.22 mm) laser scattering system capable of measuring the entire S(k±,a)) spectrum during a single tokamak discharge. A statistical dispersion is unfolded for k 9 which exhibits a phase velocity -3v De at low k 9 with a distinct rolloff for k 9^9 cm -1 . In addition, a strong spatial asymmetry is observed in the k 9 density-fluctuation distribution which reverses with toroidal current direction.PACS numbers: 52.35.Kt, 52.35.Mw A nonperturbing technique for investigation of plasma microturbulence, such as the low-frequency density fluctuations potentially responsible for anomalous transport 1,2 in tokamaks, utilizes collective scattering of electromagnetic radiation. 3 " 6 The turbulent nature of the phenomenon to be explored along with discharge irreproducibility dictate the need to determine the complete time-resolved frequency and wave-number spectra during a single tokamak discharge. In this paper, the successful use of a unique multichannel scattering apparatus to perform detailed single-shot studies of tokamak microturbulence is reported. New results concerning the spectrum and spatial distribution of density fluctuations are presented. In particular, a statistical dispersion is unfolded for poloidal fluctuations together with a strong up-down spatial asymmetry in the fluctuation level which is observed to invert with reversal of the toroidal current direction.The measurements were performed on the Texas Experimental Tokamak (TEXT), a device of R = 1 m major radius and a =0.27 m minor radius. The be discussed has 7^=300 kA, cm" 3 , 5 r = 2.8 T, 7; 0 = 950 eV, r /0 = 600 eV, Z eff -1.8, and r E -12 ms. Data are taken during the plateau region (I p , B T , n e ) of sawtoothing discharges. The plasma current and toroidal field directions are parallel unless otherwise noted.The multichannel far-infrared (FIR) laser scattering system employed for these measurements is described in detail by Park et al 1 The apparatus simultaneously collects the frequency-shifted scattered radiation at six discrete angles (0 ^/c ±^1 5 cm -1 ) thereby enabling the entire S(k lf co) spectra 3 to be monitored throughout the duration of a single tokamak discharge. By translation of the system both horizontally and vertically the scattering volume may be centered at virtually any position within the plasma cross section. Single-channel heterodyne measurements have been performed (in addition to multichannel homodyne detection) in order to establish conclusively the wavedischarge to /L, = 3.5xl0 13 propagation direction. The output of the probe laser (P 0 -S mW, \ 0 = 1.22 mm) is weakly focused along a vertical chord to a beam waist of -2 cm resulting in a measured wave-number resolution of A/c ± =±l cm -1 . The length of the scattering volume along the incident-beam direction is dependent upon the scattering angle and varies from a chord average at k L = 0 to ±8 cm (e~l points of the scattered power) at A: ± = 12 cm -1 .F...

Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.

No abstract

No abstract