Speaker
Miklos Vecsei
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1002.pdf
Edge density profile and turbulence measurements with an alkali beam
diagnostic on Wendelstein 7-X
M. Vécsei1 , G. Anda1 , O. Asztalos2 , D. Dunai1 , S. Hegedűs1 , M. Otte3 , G. I. Pokol2 , B. Tál1 ,
S. Zoletnik1 and the W7-X team3
1 Department of Plasma Physics, Wigner Research Centre for Physics, Budapest, Hungary
2 Institute of Nuclear Techniques, Budapest University of Technology and Economics,
Budapest, Hungary
3 Max-Planck-Institute for Plasma Physics, Greifswald, Germany
The Alkali Beam Emission Spectroscopy (A-BES) system is a recently installed diagnostic
instrument at the Wendelstein 7-X stellarator. In comparison to the conventional Lithium-based
BES (Li-BES) systems, the shorter lifetime of the relevant excited state of the Na atoms fa-
cilitates a more localized analysis of the density profiles. This is a major advantage for the
analysis of transport processes at the plasma edge, especially at the steep gradients expected at
the banana-shaped cross section of the W7-X plasma.
The diagnostic consists of a 60 keV Sodium atomic beam injector [1] which can provide about
1 mA ion equivalent neutral current in a ∼ 2 cm FWHM beam. The beam emission is observed
from the poloidal direction with a high-etendue 40 channel optical system, where each channel
collects light from a 4 × 0.5 cm (toroidal × radial) area of the beam. The light is detected
by an Avalanche Photodiode (APD) system with 2 MHz sampling rate. Despite the 500 kHz
analogue bandwidth the system has a peak signal-to-noise ratio up to 50, enabling the study
of fast transients and turbulence. In addition to the CII background radiation, a considerable
amount of light is also generated by Sodium gas originating from the beam neutraliser. The
latter has a significant contribution to the detected light profiles inside the SOL. Resolving this
necessitates the implementation of high-frequency modulation (chopping) of the atomic beam.
A-BES has been operational since December, 2017. The experimental data have been utilized
for the reconstruction of electron density profiles near the LCFS of the plasma. The results
imply A-BES to be robust, even at a time resolution of a few 10µs. The detected light profiles
show evidence for turbulent transport at the location of the beam. Notably, there is also a clear
indication of the presence of a magnetic island, according to expectations. The results have been
compared with the available experimental data of various plasma diagnostic tools.
References
[1] Anda, G., Dunai, D., Lampert, M., Krizsanóczi, T., et al., Review of Scientific Instruments, 89(1), 013503
(2018).
