Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment
- MIT-Plasma Science and Fusion Center, Cambridge, Massachusetts 02139 (United States)
- Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (United States)
- National Fusion Research Institute, Daejeon (Korea, Republic of)
- University of California at Davis, Davis, California 95616 (United States)
- University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
- Nova Photonics, Inc., Princeton, New Jersey 08540 (United States)
Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k{sub ⊥}ρ{sub e} ≲ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed in NSTX with a high-k microwave scattering system [D. R. Smith et al., Rev. Sci. Instrum. 79, 123501 (2008)]. Here we report on stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, which is shown to be consistent with linear stabilization of ETG modes due to the density gradient using the analytical ETG linear threshold in F. Jenko et al. [Phys. Plasmas 8, 4096 (2001)] and linear gyrokinetic simulations with GS2 [M. Kotschenreuther et al., Comput. Phys. Commun. 88, 128 (1995)]. We also found that the observed power of electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient, suggesting density gradient as a nonlinear stabilizing mechanism. Higher density gradients give rise to lower values of the plasma frame frequency, calculated based on the Doppler shift of the measured density fluctuations. Linear gyrokinetic simulations show that higher values of the electron density gradient reduce the value of the real frequency, in agreement with experimental observation. Nonlinear electron-scale gyrokinetic simulations show that high electron density gradient reduces electron heat flux and stiffness, and increases the ETG nonlinear threshold, consistent with experimental observations.
- OSTI ID:
- 22489923
- Journal Information:
- Physics of Plasmas, Vol. 22, Issue 12; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BEAM INJECTION
DOPPLER EFFECT
ELECTRON DENSITY
ELECTRON TEMPERATURE
FLEXIBILITY
FLUCTUATIONS
HEAT FLUX
HEAT TRANSFER
H-MODE PLASMA CONFINEMENT
MICROWAVE RADIATION
NSTX DEVICE
PLASMA
PLASMA SIMULATION
SCATTERING
SPHERICAL CONFIGURATION
TEMPERATURE GRADIENTS
TURBULENCE