Fermilab, Batavia, USA
The generation of the ultrabright beams required by modern free-electron lasers (FELs) has generally relied on chicane-based bunch compressions that often result in the microbunching instability. Following compression, spectral enhancements can extend even into the visible wavelengths through the longitudinal space charge impedances. Optical transition radiation (OTR) screens have been extensively used for transverse electron beam size measurements for the bright beams, but the presence of longitudinal microstructures (microbunching) in the electron beam or the leading edge spikes can result in strong, localized coherent enhancements (COTR) that mask the actual beam profile. We now have evidence for the effects in both rf photocathode-gun injected linacs* and thermionic-cathode-gun injected linacs**. Since the first observations, significant efforts have been made to characterize, model, and mitigate COTR effects on beam diagnostics. An update on the state-of-the-art for diagnosing these effects will be given as illustrated by examples at APS, LCLS, SCSS, SACLA, and NLCTA.
*A.H. Lumpkin et al.,Phys. Rev. ST Accel. Beams 12, 040704 (2009).
**H. Tanaka,"Commissioning of the Japanese XFEL at Spring8, Proceedings of IPAC2011, San Sebastián, Spain, 21-25 (2011).
PSI, Villigen PSI, Switzerland
LBNL, Berkeley, California, USA
The NGLS project consists in a 2.4 GeV superconducting linac accelerating sub-1 μm normalized emittance bunches used to produce high intensity soft X-ray short pulses from multiple FEL beamlines. The 1 MHz bunch repetition rate, and the consequent high beam power, present special challenges, but also opportunities, in the design of the various electron beam diagnostic devices. The wide range of beam characteristics, from the photoinjector to the undulators, require the adoption of different diagnostics optimized to each machine section and to the specific application of each individual measurement. In this paper we present our plans for the NGLS beam diagnostics, discussing the special requirements and challenges.
DESY Zeuthen, Zeuthen, Germany
IAP/RAS, Nizhny Novgorod, Russia
JINR, Dubna, Moscow Region, Russia
Cathode laser pulse shaping is one of the key issues for high brightness photo injector optimization. A flat-top temporal profile of the cylindrical pulses reduces significantly the transverse emittance of space charge dominated beams. As a next step towards further improvement in photo injector performance a 3D pulse shaping is considered. An ellipsoid with uniform photon density is the goal of studies in the frame of a Joint German-Russian Research Group, including the Institute of Applied Physics (Nizhny Novgorod), Joint Institute of Nuclear Research (Dubna) and the Photo Injector test facility at DESY, Zeuthen site (PITZ). The major purpose of the project is the development of a laser system capable of producing 3D quasi ellipsoidal bunches and supporting a bunch train structure close to the European XFEL specifications. The laser pulse shaping is realized using the spatial light modulator technique. The laser pulse shape diagnostics based on a cross-correlator is under development as well. Experimental tests of the new laser system with electron beam production are foreseen at PITZ. First results on the quasi ellipsoidal laser pulse shaping will be reported.
DESY, Hamburg, Germany
SLAC, Menlo Park, California, USA
A RF gun photocathode test facility has been commissioned at SLAC. The facility consists of a S-band gun, high power RF, a UV drive laser and beam diagnostics. Here we report on the capabilities of the facility demonstrated during commissioning. Currently the facility is being used to study in-situ laser processing of copper photocathodes. In the future the facility will be used to study fundamental gun and photocathode performance limitations and enhancement strategies. Eventually it is envisioned to integrate a load lock and plug into the gun enabling the evaluation of high performance surface sensitive semiconductor photocathodes and the incorporation of ex-situ surface science analytical techniques.
University of Dundee, Nethergate, Dundee, Scotland, United Kingdom
Cockcroft Institute, Warrington, Cheshire, United Kingdom
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
Electro-optic diagnostics with a target time resolution of 20fs RMS, and with intrinsically improved stability and reliability, are being developed. The new system is based on explicit temporal measurement of an electro-optically upconverted pulse, following interaction of the bunch with a quasi-CW probe pulse. The electro-optic effect generates an “optical-replica” of the longitudinal charge distribution from the narrow-bandwidth probe, simultaneously up-converting the bunch spectrum to optical frequencies. By using Frequency Resolved Optical Gating (FROG), an extension of autocorrelation, the optical replica can then be characterised on a femtosecond time scale. This scheme therefore bypasses the requirement for unreliable femtosecond laser systems. The high pulse energy required for single-shot pulse measurement via FROG will be produced through optical parametric amplification of the optical-replica pulses. The complete system will be based on a single nanosecond-pulse laser – resulting in a reliable system with greatly relaxed timing requirements.
Cockcroft Institute, Warrington, Cheshire, United Kingdom
The University of Liverpool, Liverpool, United Kingdom
The detailed diagnostics of the shortest beam pulses in free-electron lasers still pose significant challenges to beam instrumentation. Electro-optical methods are a promising approach for the non-intercepting measurement of electron bunches with a time resolution of better than 50 fs, but suitable optical materials need to be better understood and carefully studied. In addition, adequate timing systems with stability in the fs regime based on mode-locked fibre laser optical clocks, and actively length-stabilised optical fibre distribution require further investigation. Within the EU-funded LA³NET project these important problems are being addressed by an international consortium of research centres, universities, and industry partners. This contribution gives an overview of the LA3NET project and results from initial studies in both areas. It also describes the events, such as schools, topical workshops and conferences that LA3NET will organize.
SLAC, Menlo Park, California, USA
LBNL, Berkeley, California, USA
LLNL, Livermore, California, USA
The Linac Coherent Light Source is an X-ray free-electron laser at the SLAC National Accelerator Laboratory. It produces coherent soft and hard X-rays with peak brightness nearly ten orders of magnitude beyond conventional synchrotron sources and a range of pulse durations from 500 to <10 fs. The facility has been operating at X-ray energy from 500 to 10,000eV. Users have expressed great interest in doing experiments with X-Rays near the carbon absorption edge at 284eV. We describe the operation and performance of the LCLS in the newly established regime between 250 and 500eV.
[1] Emma, P. et al., “First lasing and operation of an ˚angstrom-wavelength free-electron laser,” Nature Pho-
ton. 4(9), 641–647 (2010).
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
Cockcroft Institute, Warrington, Cheshire, United Kingdom
DESY, Hamburg, Germany