Abstract
A 60 keV neutral Alkali beam system was designed, built and installed for beam emission spectroscopy measurement of edge plasma on W7-X.As the decay length of the first excited state of sodium is much shorter than lithium it was selected as beam species. This way the steep density gradient at the bean shaped cross section of the Wendelstein 7-X plasma can be resolved.Unexpectedly a thermal sodium beam was also observed in the far scrape off layer of W7-X plasma during the first measurements in 2017. It was found to originate from the recirculating neutralizer which uses sodium vapour. To resolve this issue the neutralizer material was changed to Potassium.The observation system consists of two parts: a 40 channel avalanche photo diode (APD) camera unit and a CMOS camera which run in parallel: 95% of the collected light goes to the APD unit which is digitized with 2 MHz sampling rate while the CCD camera is operated in the 100 Hz range. Chopping the beam up to 250 kHz is also possible for precise background measurement on the time scale of the turbulence.In this paper the main improvements of the sodium beam system using potassium for ion beam neutralization and the first measurement results are described.
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