title: Improved chopping of a lithium beam for plasma edge diagnostic at ASDEX Upgrade.
abstract: 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.
authors.affiliation: Institute of Applied Physics, Vienna University of Technology, Association EURATOM-ÖAW, A-1040 Vienna, Austria. willensdorfer@iap.tuwien.ac.at
authors.forename: M
authors.initials: M
authors.lastname: Willensdorfer
authors.forename: E
authors.initials: E
authors.lastname: Wolfrum
authors.forename: R
authors.initials: R
authors.lastname: Fischer
authors.forename: J
authors.initials: J
authors.lastname: Schweinzer
authors.forename: M
authors.initials: M
authors.lastname: Sertoli
authors.forename: B
authors.initials: B
authors.lastname: Sieglin
authors.forename: G
authors.initials: G
authors.lastname: Veres
authors.forename: F
authors.initials: F
authors.lastname: Aumayr