Stephan Wesch

Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source

Terahertz (THz) pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena.

A multi-channel THz and infrared spectrometer for femtosecond electron bunch diagnostics by single-shot spectroscopy of coherent radiation

Abstract The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. In this paper, a frequency-domain diagnostic method is described that is capable of resolving structures in the femtosecond regime. A novel in-vacuum spectrometer has been developed for spectroscopy of coherent radiation in the THz and infrared range. The spectrometer is equipped with five consecutive dispersion gratings and 120 parallel readout channels; it can be operated either in short (5– 44 μ m ) or in long wavelength mode (45– 430 μ m ). Fast parallel readout permits the spectroscopy of coherent radiation from single electron bunches. Test measurements at the soft X-ray free-electron laser FLASH, using coherent transition radiation, demonstrate excellent performance of the spectrometer. The device is planned for use as an online bunch profile monitor during regular FEL operation.

Electron beam profile imaging in the presence of coherent optical radiation effects

High-brightness electron beams with low energy spread at existing and future x-ray free-electron lasers are affected by various collective beam self-interactions and microbunching instabilities. The corresponding coherent optical radiation effects, e.g., coherent optical transition radiation, render electron beam profile imaging impossible and become a serious issue for all kinds of electron beam diagnostics using imaging screens. Furthermore, coherent optical radiation effects can also be related to intrinsically ultrashort electron bunches or the existence of ultrashort spikes inside the electron bunches. In this paper, we discuss methods to suppress coherent optical radiation effects both by electron beam profile imaging in dispersive beamlines and by using scintillation imaging screens in combination with separation techniques. The suppression of coherent optical emission in dispersive beamlines is shown by analytical calculations, numerical simulations, and measurements. Transverse and longitudinal electron beam profile measurements in the presence of coherent optical radiation effects in non-dispersive beamlines are demonstrated by applying a temporal separation technique.

FLASHForward: DOOCS Control System for a Beam-Driven Plasma-Wakefield Acceleration Experiment

The FLASHForward project at DESY is an innovative beam-driven plasma-wakefield acceleration experiment integrated in the FLASH facility, aiming to accelerate electron beams to GeV energies over a few centimeters of ionised gas. These accelerated beams are tested for their capability to demonstrate exponential free-electron laser gain; achievable only through rigorous analysis of both the driver and witness beams phase space. The thematic priority covered in here the control system part of FLASHForward. To be able to control, read out and save data from the diagnostics into DAQ, the DOOCS control system has been integrated into FLASH Forward. Laser beam control, over 70 cameras, ADCs, timing system and motorised stages are combined into the one DOOCS control system as well as vacuum and magnet controls. Micro TCA for Physics (MTCA.4) is the solid basic computing system, supported from high power workstations for camera readout and normal Linux computers. FLASH FLASH [1], a soft X-ray free-electron laser, is available to the photon science user community for experiments since 2005. Ultra-short X-ray pulses, shorter than 30 femtoseconds, are produced using the SASE process. The FLASH facility operates two SASE beamlines in parallel: FLASH1 & FLASH2 and as third beamline FLASHForward (see Fig. 1). Pulses of FLASH come in bursts of several hundred pulses with a repetition rate of 10 Hz. Figure 1: FLASH layout. DOOCS INSIDE FLASHFORWARD As FLASHForward is part of FLASH, it is obvious to us also the FLASH control system infrastructure DOOCS. The whole server architecture as existing hardware and software is a big advantage to integrate the needed detector hardware for FLASHForward. As most of the FLASHForward part is a laser system, diagnostic is slightly different as in FLASH. Laser alignment can be detected with chip cameras. Hence the amount of cameras is increasing easily to more than 70. Other components, like the timing system must be adapted in FLASHForward to get a synchronization to FLASH, which is necessary for all data taking. In the following chapters a short description of the most important components and systems are made. DOOCS CONTROL SYSTEM DOOCS, the Distributed Object Oriented Control System [2] was designed for FLASH. Currently it is extended to control the European XFEL accelerator (see Fig. 2). Recent developments for the client side applications are written in JAVA to allow them to be used on many computer platforms. This object oriented abstraction model helps for clean programming interfaces and in the overall system design including the hardware for a machine and is a significant step forward in the goal to improve software productivity and quality. Figure 2: DOOCS structure. COMPUTING HARDWARE MicroTCA.4, is a new standard form the PICMG to extend the applications of the existing μTCA crate system. This extension was developed in an international collaboration of High Energy Physics laboratories and many industrial partners within the PICMG organization. It is fully ___________________________________________ † sven.karstensen@desy.de Proceedings of IPAC2018, Vancouver, BC, Canada Pre-Release Snapshot 27-May-2018 12:00 UTC

Echtzeitspektroskopie von Ferninfrarotstrahlung zur Strahldiagnose an Linearbeschleunigern

Zusammenfassung Freie-Elektronen-Laser für den Röntgenbereich, in denen ein hochenergetischer Elektronenstrahl als Lasermedium dient, erfordern Strahlparameter weit jenseits denen herkömmlicher Beschleunigeranlagen. Von zentraler Bedeutung ist hierbei die Länge der relativistisch beschleunigten Elektronenpakete („bunche“), die in speziellen Magnetanordnungen auf deutlich unter 100 μm komprimiert werden. Dabei werden für einige 100 fs Spitzenströme im kA-Bereich erzeugt. Durch diverse Prozesse kann die Abstrahlung von breitbandiger Infrarotstrahlung im Millimeter- bis Mikrometerbereich erzwungen werden. Die Intensität und die spektrale Zusammensetzung dieser Strahlung geben Auskunft über das Stromprofil der Elektronenpakete. In den letzten Jahren wurden erhebliche Fortschritte erzielt, diese Strahlung spektral aufgelöst zu messen und somit leistungsfähige Methoden zur Überwachung des Beschleunigungs- und Kompressionsprozesses bereitzustellen.

Longitudinal bunch profile diagnostics with coherent radiation at FLASH

The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. A novel in-vacuum polychromator (CRISP4) has been developed for measuring coherent radiation in the THz and infrared range. The polychromator is equipped with five consecutive dispersion gratings and 120 parallel readout channels. It can be operated either in short (5-44 μm) or in long wavelength mode (45-430 μm). Fast parallel readout permits the monitoring of coherent radiation from single electron bunches. Due to the large wavelength range covered and the absolute calibration of the device, Kramers-Kronig based phase retrieval allows to online reconstruct a longitudinal bunch profile from the measured coherent radiation spectrum. The device is used as a bunch length monitoring and tuning tool during routine operation at the Free-electron Laser in Hamburg (FLASH). Comparative measurements with the transverse deflecting structure show excellent agreement of both methods.