Electro-optical detection has proven to be a valuable technique to study temporal profiles of THz pulses with pulse durations down to femtoseconds. As the Coulomb field around a relativistic electron bunch resembles the current profile, electro-optical detection can be exploited for non-invasive bunch length measurements at accelerators. We have developed a very compact and robust electro-optical detection system based on spectral decoding for single-shot longitudinal bunch profile monitoring at the European X-ray Free Electron Laser (XFEL) for electron bunch lengths down to 200 fs (rms). Apart from the GaP crystal and the corresponding laser optics at the electron beamline, all components are housed in 19 in. chassis for rack mount and remote operation inside the accelerator tunnel. An advanced laser synchronization scheme based on radio-frequency down-conversion has been developed for locking a custom-made Yb-fiber laser to the radio-frequency of the European XFEL accelerator. In order to cope with the high bunch repetition rate of the superconducting accelerator, a novel linear array detector has been employed for spectral measurements of the Yb-fiber laser pulses at frame rates of up to 2.26 MHz. In this paper, we describe all sub-systems of the electro-optical detection system as well as the measurement procedure in detail and discuss the first measurement results of longitudinal bunch profiles of around 400 fs (rms) with an arrival-time jitter of 35 fs (rms).

The longitudinal intra-train beam-based feedback has been recommissioned after major upgrades on the synchronization system of the FLASH facility. Those upgrades include: new bunch arrival time monitors (BAMs), the optical synchronization system accommodating the latest European XFEL design based on PM fibers, and installation of a small broadband normal conducting RF cavity. The cavity is located prior to the first bunch compressor at FLASH and allows energy modulation bunch-by-bunch (1 μs spacing) on the per mille range. Through the energy dependent path length of the succeeding magnetic chicane the cavity is used for ultimate bunch arrival time corrections. Recently the RF cavity operated 1 kW pulsed solid-state amplifier was successfully commissioned. First tests have been carried out incorporating the fast cavity as actuator together with SRF stations for larger corrections in our intra-train beam-based feedback pushing now arrival time stabilities towards 5 fs (rms). The latest results and observed residual instabilities are presented.

Synchrotrons and modern FEL light sources operate with bunch repetition rates in the MHz range. The profile of the electron beam inside the accelerator can be characterized with indirect experimental techniques where linear array detectors are employed to measure the emitted synchrotron radiation or the spectrum of a near-IR laser. To improve the performance of modern beam diagnostics we have developed KALYPSO, a detector system operating with a continuous frame rate of up to 2.7 MHz. To facilitate the integration in different experiments, a modular architecture has been adopted. Different semiconductor micro-strip sensors can be connected to front-end ASICs to optimize the quantum efficiency at different photon energies, ranging from visible light up to near-IR. The front-end electronics are integrated within an heterogeneous DAQ consisting of FPGAs and GPUs, which allows scientists to implement real-time data processing algorithms. The current version of the detector is in operation at the KARA synchrotron light source and at the European XFEL. In this contribution we present the detector architecture, the performance results and the on-going technical developments.

The novel linear array detector KALYPSO has been developed for beam diagnostics based on 1-dimensional profile measurements at high-repetition rate free-electron lasers (FEL) and synchrotron radiation facilities. The current version of KALYPSO has 256 pixels with a maximum frame rate of 2.7 MHz. The detector board, which comprises the radiation sensor, analog signal amplification, and analog-todigital signal conversion, has been designed as a mezzanine card that can be plugged onto application-specific carrier boards for data pre-processing and transmission. Either a Si or InGaAs sensor can be mounted for the detection of visible or near infrared radiation. Results obtained in several beam diagnostics applications at the European XFEL and FLASH are presented to demonstrate the powerful capabilities of the KALYPSO detector.

In this paper, we report on the first facility-wide evaluation of the optical synchronization system at the European XFEL, resulting in excellent arrival time stability of the electron bunches at the end of the 2 km long linac of the machine. It has been measured using two adjacent, individual singleshot femtosecond-resolution bunch arrival time monitors. While each of the monitors is independently connected by a stabilized optical fiber link to a master laser oscillator, with one being installed in the injector building and one in the experimental hall, these two reference lasers are tightly synchronized through another few-km long fiber link and by balanced optical cross-correlation. Thus, our results are not only benchmarking the accelerator performance, but at the same time the optical synchronization infrastructure itself. Femtosecond arrival time stability can only be achieved by also locking the RF reference for cavity field control to the stabilized optical reference and requires an unprecedented synchronization of the master laser oscillator to the accelerator’s master RF oscillator, enabled by a novel laser-to-RF phase detection scheme. Finally, with the seed oscillators of the experiment’s optical lasers tightly synchronized to the master laser oscillator, first pump-probe experiments at two independent scientific instruments proved a relative X-ray/optical timing jitter in the low tens of femtoseconds.

The European X-ray Free-Electron Laser generates most powerful and brilliant X-ray laser pulses. Exact knowledge about the longitudinal electron bunch profile is crucial for the operation of the linear accelerator as well as for photon science experiments. The only longitudinal diagnostic downstream of the main linac is based on spectroscopy of diffraction radiation (DR). The spectral intensity of the DR in the THz and infrared regime is monitored by a four-staged grating spectrometer and allows non-invasive bunch length characterization based on form factor measurements in the range 0.7 – 60 THz. As the readout and signal shaping electronics of the spectrometer allow MHz readout rates, the longitudinal bunch profile of all bunches inside the bunch train can be characterized non-invasively and simultaneously to FEL operation. In this paper, form factor measurements along the bunch train will be described and presented as well as the resulting reconstructed current profiles.

Three electro-optical bunch length detection systems based on spectral decoding have been installed and are being commissioned at the European XFEL. The systems are capable of recording individual longitudinal bunch profiles with sub-picosecond resolution at a bunch repetition rate 1.13 MHz. Bunch lengths and arrival times of entire bunch trains with single-bunch resolution have been measured as well as jitter and drifts for consecutive bunch trains. In this paper, we present first measurement results for the electrooptical detection system located after the second bunch compressor. A preliminary comparison with data from the bunch arrival-time monitor shows good agreement.

In modern free electron laser facilities like FLASH I/II and European XFEL at DESY a high resolution intra bunch train arrival time measurement is mandatory, providing a crucial information for the beam based feedback system. For this purpose a Bunch Arrival Time Monitor (BAM) was developed, based on an electro-optical scheme where an ultra-short pulsed laser is employed. A BAM is composed of several subsystems, including stepper motors, power management, dedicated readout board, management board for voltage settings, temperature sensors and temperature controller and optical amplifier. Part of the electronics is developed using the MicroTCA standard. We will present in this poster the basic requirements for the BAM, software design and implementation developed to manage the subsystems and their interactions.

At today’s free-electron lasers, high-resolution electron bunch arrival time measurements have become increasingly more important in fast feedback systems for a timing jitter reduction down to the femtosecond level as well as for time-resolved pump-probe experiments. This is fulfilled by arrival time monitors which employ an electro-optical detectionschemebymeansofsynchronisedultrashortlaserpulses. Even more, at FLASH and the European XFEL the measurement has to cover a wide range of bunch charges from 1nC down to 20pC with equally sub-10fs resolution. To meet these requirements, recently a high bandwidth pickup electrode with a cut-off frequency above 40GHz has been developed. These pickups are installed at the macro-pulsed SRF accelerator of the free-electron laser FLASH and at the macro-pulsed continuous wave SRF accelerator ELBE. In this paper we present an evaluation of the pickup performance by direct signal measurements with high bandwidth oscilloscopes and by use of the electro-optical arrival time monitor.

European XFEL comprises a 17.5 GeV linear accelerator for the generation of hard X-rays. Electron bunches from 20 pC to 1 nC will be produced with a length of a few ps in the RF gun and compressed by three orders of magnitude in three bunch compressor (BC) stages. European XFEL is designed to operate at 10 Hz delivering bunch trains with up to 2700 bunches separated by 222 ns. The high intra-bunch train repetition rate offers the unique possibility of stabilizing the machine with an intra-bunch train feedback, which puts in turn very high demand on fast longitudinal diagnostics. Two different systems will be installed in several positions of the machine. Five bunch compression monitors (BCM) will monitor the compression factor of each BC stage and be used for intra-bunch train feedbacks. A THz spectrometer will be used to measure parasitically the longitudinal bunch profile after the energy collimator at 17.5 GeV beam energy. We will present concepts for fast longitudinal diagnostics for European XFEL based on coherent radiation, newest developments for high repetition rate measurements and simulations for the feedback capability of the system.

FLASH is a high-gain free-electron laser (FEL) in the soft x-ray range. This paper discusses the generation of very short FEL pulses in the Self-Amplified Spontaneous Emission (SASE) - mode without an external seeding signal. In the optimal case a SASE-FEL can be operated in the so-called single-spike mode. At FLASH a new photoinjector laser has been commissioned, which allows the generation of shorter bunches with low bunch charge directly at the photo-cathode. This shorter injector laser reduces the required bunch compression for short pulses and thus allows a stable SASE performance with shorter pulses. First SASE performance using the new injector laser has been demonstrated and electron bunch and FEL radiation properties have been measured. These measurements are presented and next steps towards single spike operation are discussed.

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.

Dedicated longitudinal electron beam diagnostics is highly demanded for the control and optimization of modern X-ray free-electron lasers (XFEL). At the European XFEL (E-XFEL), 3 transverse deflecting structures (TDS) will be installed at different locations of the accelerator for measurements of slice emittance and longitudinal profile. Operation of a TDS in combined use with an energy spectrometer, e.g. a dispersive section after a single dipole magnet, allows additionally for longitudinal phase space measurements. However, utilization of a dipole magnet is not compatible with single-bunch measurements in multibunch operation mode, which will be the standard operation mode of the E-XFEL. In this paper, we propose a longitudinal phase space diagnostic beamline consisting of a TDS, fast kicker and septum magnet for the E-XFEL. The layout of the accelerator lattice with optimized optics for longitudinal phase space measurements and numerical simulation studying the performance of the beamline will be presented.

Received 13 June 2012DOI:https://doi.org/10.1103/PhysRevSTAB.15.069902This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2012 American Physical Society