Speaker
Elisabeth Wolfrum
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1024.pdf
First results from the thermal Helium beam diagnostic at ASDEX Upgrade
E. Wolfrum1, M. Griener1, M. Cavedon1, J.M. Muñoz Burgos2, O. Schmitz3, U. Stroth1 and
the ASDEX Upgrade Team
1
Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany
2
Astro Fusion Spectre, astrofusionspectre@gmail.com
3
Engineering Physics Department, University of Wisconsin-Madison, Madison, USA
Line ratio spectroscopy on thermal Helium is a diagnostic method allowing the determination
of electron density and electron temperature simultaneously [1,2,3,4]. The line ratio of two
singlet transitions is mainly dependent on density, while the ratio of a singlet and a triplet
transition is dominantly dependent on electron temperature. Evaluable signal of He I line
radiation can only be collected in plasmas restricted to certain combinations of density and
temperature. At the low end of both quantities the signal is too weak due to the low excitation
rate and towards higher electron densities and temperatures the neutral Helium density is
strongly attenuated. Such a diagnostic has recently been implemented at the tokamak ASDEX
Upgrade. It is very well suited to investigate the plasma edge, with the measurable radial
region from the far scrape-off layer (SOL) to the near SOL and in low density cases even
across the separatrix into the confined region.
A piezo valve [5], mounted at the vessel
optical head for line ratio spectroscopy wall very close to the plasma is used to
2 lenses
inject neutral helium into the plasma. As
shown in the figure, the lines of sight,
53 lines of sight (LOS)
optimised for radial resolution (~ 4 mm),
dip tube for 2D GPI cover a radial range of 8 cm in the plasma
edge region, with additional ones for
poloidally resolved measurements. The
line resolved emission intensities of four
He I lines are measured simultaneously
with a newly developed 32 channel
piezo valve [5] polychromator system, based on dichroic
mirrors, small band interference filters and
linear array photomultiplier tubes. With a
data acquisition rate of 900 kHz this
helium cloud, measured with GPI diagnostic provides not only a good spatial
but also an excellent temporal resolution.
The capabilities of the diagnostic are demonstrated in selected examples. The characteristics
of electron density and temperature profiles in the near and far SOL are measured across
regime transitions, such as I-phase to H-mode or L-mode to I-mode. The effect of a regime
transition can be seen across the whole SOL from the separatrix to the wall. Because of the
high spatial and temporal resolution not only changes in profiles can be determined but also
the propagation velocity of fast transient events such as bursts and blobs can be measured.
1 B. Schweer et al, J. Nucl. Mater. 198 (1992) 174
2 O. Schmitz et al, Plasma Phys. Control. Fusion 50 (2008) 115004
3 U. Kruezi et al, Rev. Sci.Instrum. 83, 065107 (2012).
4 M. Griener et al, Plasma Phys. Control. Fusion 60 (2018) 025008
5 M. Griener et al, Rev. Sci.Instrum. 88 (2017) 033509
