Measurements of the transverse emittance at the VUV-FEL

The TESLA Test Facility (TTF) linear accelerator (linac) at DESY has been extended to drive a new Free Electron Laser facility (VUV-FEL) in the wavelength range from the vacuum-ultraviolet (VUV) to soft Xrays. With the upgraded photo injector and increased electron beam energy up to 1 GeV, wavelengths down to 6 nm can be achieved. During the commissioning phase of the VUV-FEL the emphasis is on lasing with a wavelength of 30 nm. A high quality electron beam is required for the lasing process. At the VUV-FEL the design values are 2mmmrad normalized transverse emittance, 2500 A peak current, and 0.1% energy spread. In order to understand and optimize the electron beam, precise measurements of the beam properties are essential. In this diploma thesis, measurements of the transverse emittance during the commissioning of the VUV-FEL are presented. The transverse beam distribution is measured using optical transition radiation at four positions along the linac. The emittance is determined from the measured distributions using two methods: a fit of the Twiss parameters together with the emittance to the measured beam sizes, and as a second method, the tomographic reconstruction of the phase space density distributions using the Maximum Entropy Algorithm. Zusammenfassung Um eine neue Freie-Elektronen Laser Anlage zu betreiben, ist der Linearbeschleuniger der TESLA Test Anlage (TTF) am DESY erweitert worden. Mit dem verbesserten Photoinjektor und einer erhöhten Strahlenergie von bis zu 1GeV kann FEL-Strahlung im Wellenlängenbereich vom Vakuum-Ultraviolett (VUV) bis zu weicher Röntgenstrahlung (6 nm) erzeugt werden. Während der Inbetriebnahme des VUV-FEL wird ein Laserbetrieb mit einer Wellenlänge von 30 nm angestrebt. Eine hohe Strahlqualität ist wichtig für den Laserprozess. Am VUV-FEL liegen die Designwerte der normierten transversalen Strahlemittanz bei 2 mmmrad, des Spitzenstroms bei 2500A und der Energiebreite bei 0.1%. Für das Verständnis und zum Optimieren des Elektronenstrahls sind präzise Messungen der Elektronenstrahl-Eigenschaften essentiell. Im Rahmen dieser Diplomarbeit werden Messungen der transversalen Strahlemittanz während der Inbetriebnahme präsentiert. Die transversale Strahlverteilung wird an vier Positionen im Beschleuniger mit Hilfe von optischer Übergangsstrahlung gemessen. Aus diesen Verteilungen wird die Strahlemittanz über zwei verschiedene Methoden bestimmt: aus einem Fit der Twiss Parameter und der Strahlemittanz an die gemessenen Strahlbreiten und zum anderen aus der tomographischen Rekonstruktion der Phasenraum-Dichteverteilung unter Benutzung des Maximum-Entropie-Algorithmus. Florian Löhl Deutsches Elektronen-Synchrotron Notkestraße 85 22607 Hamburg, Germany florian.loehl@desy.de German title: Messungen der transversalen Emittanz am VUV-FEL Published under DESY report numbers: DESY-THESIS 2005-014 TESLA-FEL 2005-03 Printed by the DESY copy center. This document was typeset using LTEX and KOMA-Script.

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