Speaker
Til Ullmann
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1092.pdf
Enhancement of zonal flow drive through equilibrium shear flows
T. Ullmann1 , B. Schmid1 , P. Manz2 , M. Ramisch1
1 University of Stuttgart, IGVP, Pfaffenwaldring 31, 70569 Stuttgart, Germany
2 Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2 , 85748 Garching, Germany
Turbulence generated zonal flows (ZFs) are known to be part of regulating turbulent transport
and, therefore, are suspected to be involved in spontaneous transitions from low to high con-
finement regimes in toroidal fusion plasmas. ZFs are driven by radial gradients of the turbulent
Reynolds stress (RS) he
vθ ver i which de facto measures the tilt of vortices. Therefore, equilibrium
shear flows can constitute a seed flow for initially tilting vortices, initiating the ZF drive and
stimulating its self-amplification.
In this contribution the dependence of the RS on the background flow shearing rates is inves-
tigated experimentally. To this end, the poloidal E×B background flow in the stellarator TJ-K
is controlled via plasma biasing. A ring shaped electrode is positioned in the plasma and set
on a positive potential with respect to the vacuum vessel. The current drawn from the plasma
changes the equilibrium plasma potential profiles and therefore imposes strong E×B back-
ground flows. This application even allows to equalize the pressure driven E×B background
flow and, thus, to minimize flow shear. The plasma potential φ (r), from which the shearing rate
i.e. Ω = (RB0 )2 B−1 ∂r (RBθ )−1 ∂r φ is deduced [1], is measured with a radially moving emis-
sive probe. For measurements of the poloidal RS distribution and related radial gradients, a
specifically designed poloidal Langmuir probe array is employed, which consists of 128 probes
with 32 tips each of four adjecent flux surfaces, measuring potential fluctuations. This way, the
approximate flux surface averaged RS, the RS drive, and even zonal potential structures is anal-
ysed with respect to background flow shear experimentally.
Varying the bias voltages from 0 to 50V the shearing rates increase from negative to positive
values in the kHz range. With the shear approaching zero also the RS vanishes. At this point
the ZF power has a local minimum. The ZF power behaves similar to the RS and RS drive.
Shear induced changes in the RS are considered a consequence of a modification in the non-
linear wave coupling process, in which the amount of coupling modes is reduced as to increase
efficiency in the energy transfer into the zonal flow [2]. This paradigm is tested by means of
bispectral analysis carried out on the measured RS data.
References
[1] Burrell et al., PPCF 40, 1585 (1998).
[2] Ö. D. Gürcan, PRL 109, 155006 (2012).
