Speaker
Wolfgang Schneider
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1008.pdf
Neutron monitoring and in-situ detector calibration
at the Wendelstein 7-X stellarator
W. Schneider1, C. Biedermann1, R. Burhenn1, F. Grünauer2, T. Richert1,
H. Schuhmacher3, B. Wiegel3, M. Zboril3, A. Zimbal3 and the W7-X Team1
1
Max-Planck-Institut für Plasmaphysik, Greifswald, Germany
2
Physics Consulting, Zorneding, Germany
3
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
The neutron monitor system of the W7-X stellarator consists of three sets with up to five
neutron detector tubes of different sensitivities to thermal neutrons in a dedicated moderator
geometry in order to realize a nearly constant response independent of the neutron energy.
The monitors have been designed to cover the expected neutron yields in deuterium plasmas
from 1011 up to 1016 neutrons per second with a time resolution of 5 ms and a statistical
uncertainty of better than 15 %. This corresponds to neutron fluence rates from 103 cm−2 s−1
to 109 cm−2 s−1 at the location of the monitors. The central monitor is located 3.9 m above
the equatorial mid-plane of W7-X, the two peripheral monitors are placed outside the cryostat
at a distance of 0.8 m from the vessel at a height of 1.6 m above the equatorial plane directed
towards the plasma axis. The monitors have been characterized in the neutron reference fields
of the Physikalisch-Technische Bundesanstalt (PTB) to validate the specified properties.
The objectives of the neutron monitors of W7-X combine the documentation of the total
neutron emission per year and the monitoring of the maximum neutron emission rate as well
as the determination of the neutron flux rates measured at different positions around the W7-
X verifying physical plasma parameters such as the ion temperature and deuterium density.
In order to determine the neutron emission produced by D(d,n)3He fusion reactions in the
plasma an in-situ calibration with a neutron source of known source strength is required. The
results of such a calibration procedure depend on the entire scattering behavior of the neutrons
by all W7-X materials and its environment, starting inside the plasma up to the detector tubes
inside the neutron monitors. This is why we have performed two in-situ calibrations, one
before the first operation of W7-X in a limiter configuration without graphite tiles in January
of 2015 and one after the installation of the divertor targets including uncooled graphite tiles
in March of 2017. The procedures of these in-situ calibrations will be described and the
results compared to predictions of Monte-Carlo calculations (using the MCNP code) of the
neutron propagation from the location of the neutron source to the neutron monitors using a
simplified model of W7-X.
