Abstract
Divertor monitoring pulses (DiMPle) have been run in JET from the C35 campaign onwards. They provide an opportunity to study the impurity contamination of the plasma when it is limited by different surfaces within the machine, as well as the longer term behaviour of the impurities. In these discharges the plasma is first limited on the outer wall, then on the inner wall and, subsequently, in the X-point configuration the outer strike point is positioned on the horizontal tile 5 of the machine followed by tile 6 and then the vertical tile 7. The present study details the impurity behaviour in the DiMPle pulses from JET-ILW campaigns C35 to C38, which ran from 2015 to 2019. The impurities can largely be divided into two groups. The first, including most gases, are present immediately after their use in the machine; the second group includes those elements that are retained on plasma facing surfaces within the vessel. Most of these are metals, for which a systematic behaviour is found. Influxes due to metallic dust particles behave more like the elements of the first group. The origin of the impurities where this is known is given as well as details of the systematic behaviour, including differences due to the line-of-sight of the observing spectrometer. A clear difference is seen when the discharge fuel is H and this has implications for tritium and deuterium-tritium operations.
Highlights
Impurity behaviour in plasma machines is best considered as a series of special cases, each depending on the previous history and the plasma configuration, as well as plasma parameters, such as density and temperature, which influence both
The previous history will determine the background impurity levels in the machine and the plasma configuration, the position of the plasma relative to plasma facing surfaces, is critical in that it affects the release of impurities either from the surface element itself or contaminants previously deposited on these surfaces
The present analysis describes the impurity behaviour in the Divertor monitoring pulses (DiMPle) pulses from campaign C35 (August–December 2015) to C38 (May 2018–December 2019), this including the hydrogen campaign, C37 (June–September 2016), which due to the lower fuel mass might be expected to lead to different impurity levels
Summary
Impurity behaviour in plasma machines is best considered as a series of special cases, each depending on the previous history and the plasma configuration, as well as plasma parameters, such as density and temperature, which influence both. They have been run from the start of the JET campaign C35, with table 1 giving information about the JET campaigns covered by this analysis Before this the impurity behaviour in the JET machine was assessed using Be monitoring pulses, in which, as with the DiMPle pulses, the same configuration and plasma parameters were used (Brezinsek et al 2013). Magnetic configuration in the divertor of the DiMPle pulse 91779 X-point phases with the outer strike point on tile 5 (13.75 s) (in blue) and tile 6 (20.25 s) (in green). Magnetic configuration in the divertor of the DiMPle pulse 91779 X-point phases with the outer strike point on tile 5 (13.75 s) (in blue) and tile 7 (23.77 s) (in magenta). Visible spectra give little information about mid- and high-Z elements, which emit mainly in the VUV and soft x-ray spectral regions
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