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M. Willensdorfer, E. Wolfrum, R. Fischer, J. Schweinzer, M. Sertoli, B. Sieglin, G. Veres, F. Aumayr, . ASDEX Upgrade Team:
"Improved chopping of a lithium beam for plasma edge diagnostic at ASDEX Upgrade";
Review of Scientific Instruments, 83 (2012), 0235011 - 0235017.

The lithium beam diagnostic at ASDEX Upgrade routinely delivers electron density profiles in the
plasma edge by lithium beam impact excitation spectroscopy. An accurate background subtraction
requires a periodically chopped lithium beam. A new, improved chopping system was developed and
installed. It involves a voltage modulation for the extractor electrode and the beam deflection plates.
The modulation of the extractor electrode reduces the unused portion of lithium ions and improves the
stability of the beam with respect to its position. Furthermore, the data indicate an extended emitter
lifetime. The extractor chopping was also found to be insensitive to sparks. The deflection chopping
experiments demonstrated beam chopping in the kilohertz range. The significantly higher modulation
frequency of the deflection chopping improves background subtraction of fast transient events. It
allows a more accurate density measurements in the scrape off layer during impurity injections and
edge localized modes.