Speaker
Jakub Seidl
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1103.pdf
Turbulent fluctuations in the scrape-off layer and edge plasma of the
COMPASS tokamak
J. Seidl1, K. Jirakova1,2, J. Adamek1, O. Grover1,2, J. Horacek1, M. Hron1, P. Vondracek1,3
1
Institute of Plasma Physics of the CAS, Prague, Czech Republic
2
FNSPE, Czech Technical University, Prague, Czech Republic
3
Faculty of Mathematics and Physics, Charles University in Prague, Prague, Czech Republic
This work brings an overview of properties of turbulent plasma fluctuations in the scrape-off
layer (SOL) and the edge of confined plasma in COMPASS ohmic and L-mode discharges,
measured using electrostatic probes. Amplitude and size of the structures, their ExB velocity,
phase shift between plasma potential and density and/or temperature and general fluctuation
statistics are studied across a range of plasma conditions (BT = 0.8-1.35 T, Ip = 80-290 kA, n =
2-10 1019m-3, κ = 1-1.8) in relation to the position and properties of the velocity shear layer
and the SOL collisionality. Moreover, their impact on the formation of radial plasma profiles
is discussed.
Generally, two types of coherent fluctuations are observed: a) broadband high frequency
branch (f ≈ 50-350 kHz) rotating in the electron diamagnetic direction and appearing mainly
in the confined region, pronounced particularly at the position of the largest pressure gradient,
but often protruding even to the SOL up to the near/far SOL boundary; b) low-frequency
blobs, probably of interchange origin, rotating mostly in an opposite poloidal direction,
formed in the vicinity of the radius of zero electric field and carrying significant particle and
energy flux outwards to the SOL. Nevertheless, spectral decomposition of the radial particle
flux shows that in the near SOL, where both types overlap, their contribution to the radial
transport can be comparable.
In the SOL, the radial transport is influenced by a short connection length that COMPASS
has, L||,LFS ≈ 3 m [1], implying small SOL collisionality and possible sheath dissipation of the
plasma potential [2, 3]. In the discharges with low-to-medium density the radial ExB velocity
tends to be out of phase with density fluctuations, thus reducing the effective radial transport.
At high densities this phase shift decreases, leading to increased radial transport.
[1] K. Jirakova, et al., 45th EPS Conference on Plasma Physics, 2018, Prague
[2] O.E. Garcia, et al., Physics of Plasmas 13, 082309 (2006)
[3] J.R. Myra, et al., Physics of Plasmas 18, 060501 (2006)
