Tomography of the Electron Beam Transverse Phase Space at PITZ

Abstract

The operation of a Free Elector Laser, FEL, requires high ene rgy, high peak current electron beams with small transverse emittance. In the contemporary FELs, the e lectron beam is passed through a periodic magnetic structure - an undulator - which modifies the straig ht beam trajectory into a sinusoidal one, where FEL light is generated at each bend. According to the energy, the transverse emittance and the peak current of the beam and the parameters of the undulator, FEL radiatio n with wavelength in the range of nano- to micrometers can be generated. Studies and development of FELs are done all over the world. T he Free electron LASer in Hamburg, FLASH, and the international European X-ray FEL, XFEL [1], in Hamburg, Germany, are two leading projects of the Deutsches Elektronen SYnchrotron, DESY. Par t of the research program on FELs in DESY is realized in Zeuthen within the project Photo-Injector Te st Facility at DESY in Zeuthen, PITZ. PITZ is an international collaboration including Germany, Russia, I taly, France, Bulgaria, Thailand, United Kingdom. The Institute of Nuclear Research and Nuclear Energy, INRNE, at t he Bulgarian Academy of Sciences participates from bulgarian side. PITZ studies and optimizes the photo-injectors for FLASH and the XFEL. The research program em- phasizes on detailed measurements of the transverse phase- space density distribution [2]. Until 2010 the single slit scan technique has been used to measure the beam t ransverse distributions. At the end of 2010 a module for tomographic diagnostics has been installed whi ch extends the possibilities of PITZ to measure simultaneously the two transverse planes of a single microp ulse with improved signal-to-noise ratio. The difficult conditions of low emittance for high bunch charge an d low energy make the operation of the module challenging. This thesis presents the design considerations for the tomo graphy module, a number of reconstruction algorithms and their applicability to limited data sets, th e influence of the above mentioned challenges and approaches to solve them. The first measurements obtained wi th the device are shown. Using numerical particle tracking it is shown that the tomographic reconstr uction is consistent with the simulated data. Cross-check of some of the measured data with results obtain ed at different locations along the beamline, using single slit scans, prove that the measured phase-spac e distributions and the corresponding emittance values are consistent. Shot-to-shot fluctuations are revea led which would not be possible with the standard slit scans. Despite certain difficulties related to asymmetr y of the electron beam transverse profiles and the strong influence of space-charge forces, it is shown the modu le fulfills its purpose and improves the resolution of the measurements of the transverse phase spaces