Experimental investigations of backward transition radiation characteristics in extreme ultraviolet region

Abstract

ABSTRACT This report summarizes the results of an experiment dedicated to the observation of backward transition radia-tion in the EUV spectral region. This radiation was ge nerated by an 855MeV electron beam at a molybdenumtarget. The radiation characteristics in the EUV region are compared to those in the optical region. It wasshown that the radiation measured in the EUV region was more intense than theoretically predicted. As aresult the EUV radiation yield seems to be suci ent for standard beam pro“le diagnostics.Keywords: EUV, Transition radiation, Beam diagnostics 1. INTRODUCTION Transverse pro“le diagnostics in mo dern electron linear accelerators as F ELs or injector linacs is mainly basedon optical transition radiation (OTR) as standard technique which is observed in backward direction when acharged particle beam crosses the boundary between two media with dierent dielectric properties. Advantagesof OTR are the radiation generation directly at the screen boundary in an instantaneous emission process, alinear response, and the rather high light output emitted in a small lobe with an opening angle de“ned by thebeam energy. However, there are two physical limitations that make the method ineect ive for modern beamdiagnostics.The experience from modern linac based light sources showed that OTR diagnostics might fail even for highenergetic electron beams because o f coherence eects in the OTR emission process. Cause of this coherentemission is the Microbunching Instability, i.e. some unstable micro structures in the electron bunch thatcompromise the use of OTR monitors as reliable diagnostic and may as well interfere with the FEL processin a malfeasant way. Coherent OTR (COTR) was observed e.g. at the Linac Coherent Light Source LCLS inStanford (USA)