Detailed characterization of electron sources yielding first demonstration of European X-ray Free-Electron Laser beam quality

Abstract

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV=m at the photocathode and the other at about 60 MV=m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1:26 ` 0:13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad

Nanometer emittance ultralow charge beams from rf photoinjectors

“…This beam, with far lower charge than the original LCLS design working point, has enabled a new regime of operation of the x-ray laser characterized by fs pulses and short free-electron laser gain length. More recently, different groups have reported comparable brightness [11][12][13][14]. One should point out here that measurement systematics (in the technique, in the image processing, etc.)…”

Generation and measurement of velocity bunched ultrashort bunch of pC charge

“…The results reported here suffer from the large fluctuations of the beam parameters during the multi-shot measurement technique. This is not a fundamental limitation of the velocity compression scheme, since much better reliability and stability of the system have been demonstrated elsewhere [41,42], nevertheless it points out that achieving ultrashort pulse durations is deeply linked to the progress in the stability of the rf and laser system. In applications where tight focusing of the beam is not required, the same simulation model yields a pulse duration as short as 16 fs, since the space charge effect on bunch lengthening is much alleviated compared to the 3-dimensional focused and compressed case.…”

Overview on superconducting photoinjectors

“…For further comparison, the chart was expanded by the DC and NCRF high-current sources summarized in [47] and the three existing high brightness injectors of Spring8 [48], Flash [49], and Linac Coherent Light Source [50]. All parameters refer to the exit of each injector.…”