Abstract
With the improvement of acceleration techniques, the intrinsic emittance of the cathode has a strong impact on the final brightness of a free electron laser. The systematic studies presented in this paper demonstrate for the first time in a radiofrequency photocathode gun a reduction of the intrinsic emittance when tuning the laser photon energies close to the effective work function of copper. The intrinsic emittance was determined by measuring the core slice emittance as a function of the laser beam size at laser wavelengths between 260 and 275 nm. The results are consistent with the measured effective work function of the cathode. Slice emittance values normalized to the laser beam size reached values down to 500 nm/mm, close to that expected from theory. A 20% reduction of the intrinsic emittance was observed over the spectral range of the laser.
Highlights
The recent advent of 4th generation free electron lasers (FELs) delivering femtosecond x-ray pulses at intensities beyond 1014 W=cm2 has opened new opportunities in exploring ultrafast molecular and atomic phenomena [1,2,3]
The systematic studies presented in this paper demonstrate for the first time in a radiofrequency photocathode gun a reduction of the intrinsic emittance when tuning the laser photon energies close to the effective work function of copper
The intrinsic emittance was determined by measuring the core slice emittance as a function of the laser beam size at laser wavelengths between 260 and 275 nm
Summary
The recent advent of 4th generation free electron lasers (FELs) delivering femtosecond x-ray pulses at intensities beyond 1014 W=cm has opened new opportunities in exploring ultrafast molecular and atomic phenomena [1,2,3]. Beyond the current FELs future light sources are aiming for higher repetition rates and reduced machine length and cost Applications, such as ultrafast, single-shot, single-molecule diffraction imaging [4,5] call for ultrashort, bright x-ray pulses at high repetition rate. To meet these demands a growing number of compact FELs are being developed with optimized, low emittance electron beams [6,7,8,9], including SwissFEL at the Paul Scherrer Institute (PSI) [10]. The intrinsic emittance is close to the theoretical predictions when using the effective work functions determined experimentally in SITF and in excellent agreement with theoretical expectations [20]
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