Pilot Study of Synchronization on a Femtosecond Scale between the Electron Gun REGAE and a Laser-Plasma Accelerator

Laser wakefield acceleration (LWFA) is a novel technique to accelerate charged particles. Acceleration is achieved by a high-power laser pulse transmitting a gas target where electrons and ions form a strong wakefield with gradients up to 100 GV/m. Hence, the size of the laser-plasma accelerator (LPA) is significantly smaller compared to conventional radio frequency (RF) accelerators, because its accelerating gradients are 3 orders of magnitude higher. At present, electron beams generated by LWFA do not satisfy all requirements to make them directly usable for applications such as LPA driven free-electron laser (FEL). Pointing stability and relatively high energy spread are the major limiting factors. Typically, plasma electrons are self-injected in the plasma wake which is created by a high-power laser. There is a lack of control for the injection process and there is no direct access for diagnostics. In order to overcome these challenges and better understand the overall LWFA process, external injection experiments are planned at Deutsches Elektronen-Synchrotron (DESY) in the framework of the Laboratory for Laser and beam-driven plasma Acceleration (LAOLA) collaboration. Thus, well characterized and ultrashort (< 10 fs) electron bunches from the conventional RF accelerator Relativistic Electron Gun for Atomic Exploration (REGAE) will be injected into the laser driven plasma wake. This approach allows to reconstruct and map the plasma wakefield by post diagnosing the injected electron bunches by measuring the energy spectra of it for different injection times. To conduct such a pump-probe type of experiment, synchronization with femtosecond accuracy is required between the electron bunches from REGAE and the high-power driver laser. Two main aspects of the laser synchronization are presented in this thesis. First, a detailed experimental investigation of the conventional, fast photodiode based direct conversion laser-to-RF synchronization setup and its limitations are given. Second, an advanced Mach-Zehnder modulator (MZM) based laser-to-RF synchronization setup has been successfully developed and tested. The conceptual design, a mathematical analysis, tolerance studies and experimental evaluation is presented. Electron beam-based measurements have been performed at REGAE where MZM based laser synchronization achieved a factor of 10 performance improvement in terms of amplitude-to-phase modulation (AM-PM) conversion compared to the previously used conventional photodiode based laser synchronization setup. This setup has been employed to phase lock the REGAE photo-injector laser with excellent long term timing drift performance of 31 fs peak-to-peak over 43 h and a short term timing jitter of 11 fs rms

FS Level Laser-to-RF Synchronization at REGAE

The Relativistic Electron Gun for Atomic Exploration (REGAE) is a unique linear accelerator capable of producing ultrashort (~ 10 fs) electron bunches for studying fast processes in matter by means of ultrafast electron diffraction (UED) experiments. Additionally, REGAE is suitable for upcoming external injection experiments for laser wakefield acceleration (LWFA). In order to carry out both mentioned experiments, it is crucial to achieve fs level stability in terms of Laser-to-RF synchronization. In this paper we present an advanced laser-to-RF synchronization scheme based on integrated Mach-Zehnder modulator. The setup demonstrated the Titanium Sapphire photo-injector laser synchronization with 11 fs (rms) precision in the bandwidth up to 100 kHz. Long term timing drift measurements showed unprecedented peak-to-peak stability of 31 fs (7 fs rms) over 43 hours of measurement time. In addition, AM-PM coefficient of the MZM based laser-to-RF synchronization setup has been evaluated and showed a factor of 10 improved performance compared to conventional direct conversion based laser synchronization setup

Precise Laser-to-RF Synchronization of Photocathode Lasers

RF photo-injectors are used in various large, mid and small-scale accelerator facilities such as X-ray Free Electron Lasers (XFELs), external injection-based laser-driven plasma accelerators (LPAs) and ultrafast electron diffraction (UED) sources. Many of these facilities require a high precision synchronization of the photo-injector laser system, either because of beam dynamics reasons or the photo-injector directly impacting pump-probe experiments carried out to study physical processes on femtosecond timescales. It is thus crucial to achieve synchronization in the order of 10 fs rms or below between the photocathode laser and the RF source driving the RF gun. In this paper, we present the laser-to-RF synchronization setup employed to lock a commercial near-infrared (NIR) photocathode laser oscillator to a 2.998 GHz RF source. Together with the first results achieving ~ 10 fs rms timing jitter in the measurement bandwidth from 10 Hz up to 1 MHz, we describe an advanced synchronization setup as a future upgrade, promising even lower timing jitter and most importantly long-term timing drift stability

First Results on Femtosecond Level Photocathode Laser Synchronization at the SINBAD Facility

SINBAD, the "short-innovative bunches and accelerators at DESY" is an accelerator research and development facility which will host various experiments. SINBAD-ARES linac is a conventional S-band linear accelerator which will be capable of producing ultra-short electron bunches with duration of few femtoseconds and energy of up to 100 MeV. In order to fully utilize the potential of ultra-short electron bunches while probing the novel acceleration techniques (e.g. external injection in LWFA), it is crucial to achieve femtosecond level synchronization between photocathode laser and RF source driving the RF gun of the ARES linac. In this paper we present the first results on the synchronization of the near-infrared photocathode laser to the RF source with the residual timing jitter performance of ~10 fs rms. These results were obtained using a conventional laser-to-RF synchronization setup employing heterodyne detection of an RF signal generated by impinging the laser pulses to a fast photodetector. In addition, we describe an advanced laser-to-RF phase detection scheme as a future upgrade; promising even lower timing jitter and most importantly the long-term timing drift stability

LLRF Control and Synchronization System of the ARES Facility

The linear accelerator ARES (Accelerator Research Experiment at SINBAD) is a new research facility at DESY. Electron bunches with a maximum repetition rate of 50 Hz are accelerated up to 155 MeV. The facility aims for ultra-stable sub-femtosecond arrival-times and high peak-currents at the experiment, placing high demands on the reference distribution and field regulation of the S-band RF structures. In this paper, we report on the current status of the RF reference generation, facility-wide distribution, and the LLRF systems of the RF structures

Novel Femtosecond Level Synchronization of Titanium Sapphire Laser and Relativistic Electron Beams

Laser driven plasma accelerators are offering high gradient (∼10-100 GV/m), high quality (low emittance, short bunch length) electron beams, which can be suitable for future compact, bright and tunable light sources. In the framework of the Laboratory for Laser-and beam-drivenplasma Acceleration (LAOLA) collaboration at Deutsches Elektronen-Synchrotron (DESY) the external injection experiment for injecting electron bunches from a conventional RF accelerator into the linear plasma wave is in progress. External injection experiments at REGAE (Relativistic Elec-tron gun for Atomic Exploration) require sub-20fs precision synchronization of laser and electron beams in order to perform a beam scan into the plasma wave by varying the delay between electron beam and laser pulses. In this paper we present a novel optical to microwave synchronizationscheme, based on a balanced single output integrated Mach-Zehnder Modulator (MZM). The scheme offers a highly sensitive phase detector between a pulsed 800 nm Ti:Sa. laser and a 3 GHz microwave reference source. It is virtually independent of input laser power fluctuations and it offers femtosecond long-term precision. Together with the principal of operation of this setup, we will present promising preliminary experimental measurements of the new detector stability

Large-Scale Optical Synchronization System of the European XFEL with Femtosecond Precision

Femtosecond pulsed optical synchronization systems have evolved over the last few years and are now a mature technique to synchronize FELs. A large-scale femtosecond-precision synchronization system with up to 44 end-stations has been constructed at the European XFEL to meet the FEL synchronization stability requirements. The synchronization system is used to phase-lock various laser systems with femtosecond accuracy, to precisely measure the electron bunch arrival time along the accelerator for fast arrival time feedbacks and to locally phase stabilize the phase of the RF reference signals for the accelerator RF controls on a femtosecond level. The architecture of the large-scale synchronization system and design choices made to achieve the reliability, maintainability and performance requirements are presented together with measurement results from the past year of operation

Femtosecond Stable Laser-to-RF Phase Detection for Optical Synchronization Systems

Optical reference distributions have become an indispensable asset for femtosecond precision synchronization of free-electron lasers. At FLASH and for the future European XFEL, laser pulses are distributed over large distances in round-trip time stabilized fibers to all critical facility sub-systems. Novel Laser-to-RF phase detectors will be used to provide ultra phase stable and long-term drift free microwave signals for the accelerator RF controls. In this paper, we present the recent progress on the design of a fully integrated and engineered version of the L2RF phase detector, together with first experimental results demonstrating so-far unrivaled performance

Design and Operation of the Integrated 1.3 GHz Optical Reference Module with Femtosecond Precision

In modern Free-Electron Lasers like FLASH or the European XFEL, the short and long-term stability of RF reference signals gains in importance. The requirements are driven by the demand for short FEL pulses and low-jitter FEL operation. In previous publications, a novel, integrated Mach-Zehnder Interferometer based scheme for a phase detector between the optical and the electrical domain was presented and evaluated. This Laser-to-RF phase detector is the key component of the integrated 1.3 GHz Optical Reference Module (REFM-OPT) for FLASH and the European XFEL. The REFM-OPT will phase-stabilize 1.3 GHz RF reference signals to the pulsed optical synchronization systems in these accelerators. Design choices in the final hardware configuration are presented together with measurement results and a performance evaluation from the first operation period in the European XFEL