OPTICAL SYNCHRONIZATION AND ELECTRON BUNCH DIAGNOSTIC
Michael KuntzschU. LehnertF. RoeserR. SchurigMichael BousonvilleMarie Kristin CzwalinnaH. SchlarbSebastian SchulzS. Vilcins
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The commissioning of the extended linear electron accelerator ELBE at Helmholtz-Zentrum DresdenRossendorf started in autumn 2012. The new beamlines will deliver short electron pulses with 150 fs duration and bunch charge up to 1 nC. This will drive two THzsources, one broadband CTR/CDR and a narrowband undulator source. To enable highly resolved pump-probe experiments with table top laser sources pump and probe beams have to be synchronized on a 100 fs time scale. This paper describes how the synchronization at ELBE is done for the upcoming experiments and focus on first measurement results of the ELBE bunch arrival time monitor (BAM) based on that system.Keywords:
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The construction of a beam-monitor system for XFEL/SPring 8 “SACLA” was completed. The system was developed to realize a spatial resolution of less than 3 μm to align the beam orbit for an undulator section of about 100 m long and a temporal resolution to measure bunch lengths from 1 ns to 30 fs to maintain a constant peak beam current conducting stable SASE lasing. The system principally comprises cavity-type beam-position monitors, current monitors, screen monitors and bunchlength measurement instruments, such as an rf deflector and CSR detectors. Commissioning of SACLA started from March 2011, and the monitors performed sufficient roles to tune the beams for lasing. The achieved over-all performances of the system including DAQ are: the beam position monitor has a spatial resolution of 600 nm; the bunch-length monitors observe bunch lengths from 1ns in an injector with velocity bunching to less than 30 fs after three-stage bunch compressors. The less than a 3 μm spatial resolution of the screen monitor was also confirmed in practical beam operation. By these fulfilled performances, stable lasing of SACLA is achieved.
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Superconducting radiofrequency (SRF) accelerator technology can provide relativistic electron bunches with quasi-cw repetition rates scaling from the kHz to GHz regime [1, 2, 3, 4]. The SRF accelerator ELBE [1] is up to now Europe’s only quasi-cw linear electron accelerator (LINAC) that has been operating for more than 10 years as a driver for several secondary radiation sources. In the past four years the accelerator has been upgraded to eventually also allow for electron bunch compression down to the sub ps regime. Although recent combined electro-optic and interferometric measurements show that the sub ps regime has not yet been reached, exhibiting a shortest bunch duration of 1.2 ps (FWHM). The ELBE accelerator in combination with the super-radiant THz sources represents a novel test bed for diagnostics on quasi-cw electron beams. This has been recognized by the Helmholtz-Research Association by defining the THz sources at ELBE as a test facility for diagnostics on quasicw electron beams. The development of suitable diagnostic techniques is of great importance for various energy recovery linac accelerators (ERL) coming online in the next few years [2, 4] as well as facilities like the European X-FEL [5] and FLASH [6] when operated in long bunch train mode of operation.
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The aim of the Femto-Slicing project at SOLEIL is to generate 100 fs X-rays pulses on two beamlines, CRISTAL and TEMPO, for pump-probe experiments in the hard and soft X-rays regions. Two fs lasers are currently in operation on TEMPO and CRISTAL for pump-probe experiments on the ps time scale enabling time resolved photoemission and photodiffraction studies. The Femto-Slicing project is based on the fs laser of the CRISTAL beamline, which can be adjusted to deliver 3 mJ pulses of 30 fs duration at 2.5 kHz. The laser beam will be separated in three branches: one delivering about 2 mJ to the modulator Wiggler and the other ones delivering the remaining energy to the TEMPO and CRISTAL experiments. This layout will yield natural synchronization between IR laser pump and X-ray probe pulses, only affected by jitter associated with beam transport. In this paper, we present the current status of the Femto-Slicing project at SOLEIL, with particular emphasis on the expected performance, and the design and construction of the laser beam transport and the diagnostics implementation.
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The upgrade of the BESSY II light source in Berlin towards the Variable pulse-length Storage-Ring BESSY VSR will lead to a complex fill pattern. This involves co-existing electron bunches with significant variations of bunch-length, bunch charge as well as charge density. Among many other boundary conditions, this calls for bunch resolved measurements with sub-ps time resolution and micrometer spatial resolution. Currently, we are constructing a diagnostic platform connected to three new dipole beamlines for visible light as well as THz measurements. The mid-term aim is a 24/7 use of beam-diagnostic tools and the development of advanced methods for specific purposes. Recently, we have set-up a sub-ps streak camera* and we are investigating other innovative methods for bunch-length** as well as lateral size determination using visible light*** at the first of our new diagnostic dipole beamlines. Preliminary results as well as our concepts for achieving high sensitivity, good signal-to-noise ratio and time resolution will be presented and discussed at the conference.
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FLASH at DESY, Hamburg, Germany is the first free-electron laser (FEL) operating in the extreme ultraviolet (EUV) and soft x-ray wavelength range. FLASH is a user facility providing femtosecond short pulses with an unprecedented peak and average brilliance, opening new scientific opportunities in many disciplines. The first call for user experiments has been launched in 2005. The FLASH linear accelerator is based on TESLA superconducting technology, providing several thousands of photon pulses per second to user experiments. Probing femtosecond-scale dynamics in atomic and molecular reactions using, for instance, a combination of x-ray and optical pulses in a pump and probe arrangement, as well as single-shot diffraction imaging of biological objects and molecules, are typical experiments performed at the facility. We give an overview of the FLASH facility, and describe the basic principles of the accelerator. Recently, FLASH has been extended by a second undulator beamline (FLASH2) operated in parallel to the first beamline, extending the capacity of the facility by a factor of two.
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We describe a future possible upgrade of the European XFEL consisting in the construction of an undulator beamline dedicated to life science experiments. The availability of free undulator tunnels at the European XFEL facility offers a unique opportunity to build a beamline optimized for coherent diffraction imaging of complex molecules, like proteins and other biologically interesting structures. Crucial parameters for such bio-imaging beamline are photon energy range, peak power, and pulse duration. Key component of the setup is the undulator source. The peak power is maximized in the photon energy range between 3 keV and 13 keV by the use of a very efficient combination of self-seeding, fresh bunch and tapered undulator techniques. The unique combination of ultra-high peak power of 1 TW in the entire energy range, and ultrashort pulse duration tunable from 2 fs to 10 fs, would allow for single shot coherent imaging of protein molecules with size larger than 10 nm. Also, the new beamline would enable imaging of large biological structures in the water window, between 0.3 keV and 0.4 keV. In order to make use of standardized components, at present we favor the use of SASE3-type undulator segments. The number segments, 40, is determined by the tapered length for the design output power of 1 TW. The present plan assumes the use of a nominal electron bunch with charge of 0.1 nC. Experiments will be performed without interference with the other three undulator beamlines. Therefore, the total amount of scheduled beam time per year is expected to be up to 4000 hours.
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We report the design, performance, and installation of the beam diagnostic system of XFEL/SPring-8. The electron beam bunches of the XFEL accelerator are compressed from 1 ns to 30 fs by bunch compressors without emittance growth and the peak-current fluctuation of the accelerator directly causes SASE fluctuation. To maintain stable bunch compression process, the accelerator requires rf cavity beam-position monitors (BPM) with 100 nm resolution, OTR screen monitors (SCM) with a few micro-meter resolution, fast beam current monitors (CT), and temporal structure measurement systems with resolution of under a picosecond. The performance of each instrument was tested at the SCSS test accelerator and was found to be sufficient for our requirement. To measure the temporal structure of the electron bunch, three types of measurement systems a streak camera, an EO sampling measurement, and a transverse deflecting cavity with a resolution of few-tens femtoseconds have been prepared. The streak camera and EO sampling show sub-picosecond resolution. The installation of these beam diagnostic systems to the XFEL accelerator is well under way.
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SPring-8 Angstrom Compact free electron Laser (SACLA) is now under commissioning operation, aimed at the generation of a sub-angstrom free electron laser (FEL). In order to ensure the stable FEL generation, nondestructive bunch length monitors utilizing coherent synchrotron radiation (CSR), which were proposed to indirectly observe bunch lengths from 10 ps to 30 fs, were installed at each of three bunch compressors (BC1, BC2, BC3). The CSR is detected by a pyroelectric detector with a simple organic lens optical system. We examined the CSR monitor at BC2, and measured the bunch lengths using it combined with that of an rf deflector cavity. The results indicated that the monitor enables us to measure the sub-picosecond bunch length with a precision of less than 10%.
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Longitudinal bunch compression in magnetic chicanes is used at the Free-electron LASer in Hamburg FLASH for the generation of ultra-short electron bunches. A Synchrotron Radiation monitor (SRM) has been installed behind the third dipole of the first bunch compressor to measure the energyandenergyprofile ofthe dispersedbunches. An intensified CCD camera records the emitted SR in the visible and enables one to select single bunches out of a bunch train. The performanceof the system has been tested for different accelerator settings. The setup serves as a test bed for the European X-ray Free Electron Laser.
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