DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 103: Turbulence and Transport dependence on Mach number in Hybrid discharges | ||
| Name: | George R. McKee ( |
Affiliation: | University of Wisconsin, Madison |
| Research Area: | Transport | Presentation time: | Requested | Co-Author(s): | C. Petty, T. Rhodes, D. Schlossberg. L. Schmitz, M. Shafer |
| Description: | Perform a Mach number scan (M=v_tor/c_s) in hybrid discharges using the co/counter neutral beam capability and measure turbulence characteristics along with transport variation. BES turbulence measurements, of particular interest for this experiment, could not be obtained during a previous similar experiment due to a shutter problem. | ||
| Experimental Approach/Plan: | Hybrid discharges very similar to those already developed by C. Petty et al. would be used, with the exception that the neutral beams used for beta feedback will need to be changed to allow for the BES measurements (which require the 150 left beam on steady). Other co and counter beams will be used in feedback to maintain beta and rotation. Similar discharges with modulated 30/330 beams required CER/MSE data will also be performed (150 likely in feedback mode). | ||
| Background: | Transport in Hybrid scenario discharges has been shown to depend sensitively on the toroidal Mach number (M = v_tor /c_s). By varying the injected neutral beam torque into hybrid plasmas and simultaneously maintaining beta constant via feedback control, it has been demonstrated that the "H-factor" decreases by approximately 20% as the Mach number is reduced from about M=0.5 to M=0.1 (Luce, IAEA-2006). This has been shown to be consistent with a reduction in ExB shearing at lower Mach number from GLF23 modeling. Previous measurements of turbulence characteristics in hybrid discharges with the upgraded BES diagnostic, showed that turbulent eddies exhibit a strongly tilted structure in these plasmas. This is in sharp contrast to the more radially-poloidally symmetric eddy structure typically observed in the core of L-mode discharges. The direction of this tilted eddy structure is consistent with the ExB shear flow in these plasmas, although it was questionable as to whether the shear magnitude could bring about such a strong tilt or shear.
An experiment to systematically study turbulent eddy structure as a function of Mach number in hybrid discharges could help address these issues by directly measuring eddy structure, magnitude, decorrelation rates, and radial & poloidal correlation lengths, in these hybrid discharges with the recently expanded upgraded BES system and the Doppler reflectometer system. The long-duration, stationary hybrid discharges make these an excellent platform in which to study the turbulence characteristics. The low-amplitude of fluctuations in the core of hybrid plasmas makes their study more difficult, but the stationary discharges (several seconds) allow for ensemble-averaging of the characteristics with good resulting signal-to-noise. This will allow us to examine the improved transport in hybrid discharges, and specifically the Mach number dependence, as well as to more broadly and generally examine the ExB shear effects on turbulence and transport. | ||
| Resource Requirements: | All 7 neutral beam sources | ||
| Diagnostic Requirements: | BES, Doppler Reflectometry, FIR | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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