24A10
Study of Turbulence-Zonal Flow Interactions in Magnetically Confined Plasmas
M. Xu1,2, P. Manz1,2, G. R. Tynan1,2, P. H. Diamond1,3,4, K. J. Zhao5, J. Q. Dong5, 6, C. Holland2, N.
Fedorczak1,2, S. Chakraborty Thakur1,2, S. Muller1,2, J. H. Yu2, J. Cheng5, W. Y. Hong5, L. W. Yan5

1

Center for Momentum Transport and Flow Organization

2

Center for Energy Research & Department of Mechanical and Aerospace Engineering
3

Center for Astrophysics and Space Science & Department of Physics,
University of California at San Diego, California 92093, USA

4

WCI Center for Fusion Theory, National Fusion Research Institute, Gwahangno 113, Yusung-gu, Daejeon 305-333,Korea
5Southwestern Institute of Physics, P. O. Box 432, Chengdu, China

6Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou, China

This talk gives a brief summary of several recent experimental studies on the zonal flow-turbulenceinteractions, particularly with the focus on the energy transfer measurements in both laboratory and fusion
plasmas. It will also present some on-going work in a linear plasma device and in the HL-2A tokamak.

The interaction of sheared flows withgradient-turbulence in magnetically confinedplasmas is a problem of fundamental importance inmagnetic fusion, and also bears striking similarities
to related problems in geophysical andastrophysical systems and thus is also of broad
scientific interest. This talk gives a brief summaryof several recent experimental studies on thisproblem, updated with some on-going work in thelinear plasma device CSDX (Controlled Shear
Decorrelation Experiment) and in the HL-2A
tokamak.
First, in CSDX the development of nonlinear
energy transfer during a controlled transition to astate of weak drift-interchange turbulence is studied.
When magnetic field strength is increased thesystem undergoes several changes from aquasi-periodic to a phase locked to a weakly
turbulent regime [1]. The polarization drift
nonlinearity transfers kinetic energy to larger scales.
G G
The E × B drift nonlinearity provides access to thefree energy by coupling density fluctuations at
smaller scales to the larger scale potential structures,
resulting in a formation of a very robust
phase-locked regime and acting as a mechanism of
self-sustainment in the weakly turbulent regime.
Although the interplay between these nonlinearities
is different in the different regimes, it results in a
successive increase in the degrees of freedom in all
regimes, which is the most important feature of atransition to turbulence. At high magnetic fieldkinetic energy transfer results in the formation of asheared flow as indicated by other time-domain
studies.

In fusion devices, correlation studies have been
conducted to characterize the spatial structures ofthe low frequency ZFs (zonal flows) and GAMs
(geodesic acoustic modes) [2, 3, 4]. The results
clearly demonstrated their theoretical expected
symmetry properties.
Most recently, anexperiment aimed to measure the nonlinear kinetic
energy transfer at different heating power using afrequency-domain bispectral technique wasconducted on the HL-2A tokamak. I was found that
turbulent kinetic energy is transferred into both ZFs
and GAMs, as well as fluctuations with higher (>80kHz) frequencies, consistent with the observed
spectral broadening of 2D turbulence. It was alsoobserved that when the heating power wasincreased, the energy transfer from turbulence intoGAMs first increased then decreased while theenergy transfer into ZFs and the ZFs themselves
both became stronger monotonically, suggestingthat ZFs very likely play a leading role in the low tohigh (L-H) plasma confinement transition [5].
Experiments in the TJ-K torsatron provide thefirst study of turbulent spectral energy transfer inthe wavenumber domain [6]. Results show a dualtransfer of energy in which kinetic energy
accumulates in large scale zonal flows,
while density fluctuations are scattered to small
spatial scales corresponding to dissipation
scales. When a radial current is injected into theplasma with a biased electrode aligned with themagnetic flux surface a mean shear flow sustainedby non-turbulent processes is created [7]. Studiesof the nonlinear turbulent kinetic energy transfer
into the large spatial scales indicate that the strengthof the energy transfer is increased by the impositionof an externally driven flow. Such a response
should act to amplify the large scale shear flow andthus setup the conditions necessary for theformation of a state of strong shear flow and weakturbulent transport.
All of the above experiments are consistent
with the theory that ExB sheared flows can be
self-generated and amplified by turbulent stresses,and support the critical role that ExB sheared flows

play in turbulent transport regulation and in thetransition to the improved confinement.
References[1] M. J. Burin, G. R. Tynan, and G. Y. Antar et al., Phys.
Plasmas 12 052320 (2005)[2] Fujisawa A. et al, Phys. Rev. Lett. 93 165002 (2004)[3] K. J. Zhao et al., Phys. Rev. Lett. 96, 255004 (2006)[4] G. S. Xu, B. N. Wan, M. Song and J. G. Li, Phys. Rev.
Lett. 91, 125001 (2003)[5] M. Xu, G. R. Tynan, and P. H. Diamond et al., to besubmitted to Phys. Rev. Lett. (2011)[6] P. Manz, M. Ramisch, U. Stroth, Phys. Rev. Lett. 103,165004 (2009)[7] Y. Xu et al, Nucl. Fusion 51, 063020 (2011)