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.
Highlights
The performance of high-brilliance, short-wavelength free-electron lasers is critically dependent on the quality of the electron beam driving the free-electron laser (FEL) process
With the use of this automatic program, the gun prototype 3.1, which was installed at PITZ in early 2006 and foreseen as a replacement gun for the FLASH machine, has been conditioned up to the maximum requirements of FLASH: a maximum gradient of at least 40 MV=m with rf pulse length of 900 s at 10 Hz repetition rate
This paper has described the detailed characterization of two laser-driven rf guns at the Photoinjector Test Facility at DESY, Zeuthen site (PITZ)
Summary
The performance of high-brilliance, short-wavelength free-electron lasers is critically dependent on the quality of the electron beam driving the free-electron laser (FEL) process. The subject of this paper is the intermediate extension stage PITZ-1.6, which includes a preliminary booster cavity (called the TESLA booster), about 60% of the diagnostics foreseen for the PITZ-2 stage and a laser system capable of producing pulses with temporal flattop profile of 20 ps in length, with about 6–7 ps rise and fall times. This setup was used to characterize and optimize two gun cavities at two different operating conditions: 43 MV=m and $60 MV=m peak accelerating gradient at the photocathode. The European XFEL requires a duty factor of 32 500 pulses per second (1 nC micropulses at 5 MHz in 650-s trains produced at 10 Hz), but the demonstration of this high current operation will be the subject of a separate paper
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