First divertor Thomson scattering measurements on MAST-U
MAST-U is equipped with a Super-X divertor which aims to promote detachment. Measurements
of plasma density and temperature in the Super-X chamber offer insight into the processes at
work in this type of divertor. First data has been obtained from the MAST-U divertor Thomson
scattering diagnostic [1] designed to measure these quantities. Following a Raman scattering
calibration in Nitrogen, the diagnostic operated over a number of plasma pulses in the first
physics campaign. Electron density and temperature measurements have been taken in attached
and detached conditions and as the strike leg has moved through the field of view of the
diagnostic. The system operated with a dedicated 30Hz laser with timing synchronised to 7
similar lasers installed in the core Thomson system.
Electron densities in the range 1 x 10 18 – 5 x 10 19 m -3 have been measured by the system
throughout these regimes. Although the system was specified to measure from 1eV-100eV [2],
electron temperatures in the Super-X divertor in the first campaign were low and measurement
down to 0.5eV has been critical, particularly close to the detachment front. This generation of
polychromators [1] have been designed with increased stray light rejection compared to those
used in the core system. This has proved successful with very low levels of stray light observed.
A range of different operational scenarios were carried out to investigate the capabilities of the
Super-X divertor. Magnetic geometry, gas fuelling and magnetic flux expansion were explored to
study the access to a detached regime. These measurements have been compared to other
diagnostics operating in the divertor such as Langmuir probes and spectral imaging systems and
have shown good agreement.
1. J. G. Clark et al, Rev. Sci. Instrum. 92, 043545 (2021)
2. J. Hawke et al, JINST 8, C11010 (2013)
This work has been carried out within the framework of the EUROfusion Consortium and has
received funding from the Euratom research and training programme 2014-2018 and 2019-2020
under grant agreement No 633053. The views and opinions expressed herein do not necessarily
reflect those of the European Commission. This work has been funded by the RCUK Energy
Programme [grant number EP/T012250/1] and supported by the Engineering and Physical
Sciences Research Council [EP/L01663X/1].