We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam

The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and testbed for the development and realization of SwissFEL, the X-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultra-low-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range.

Commissioning experience and beam physics measurements at the SwissFEL Injector Test Facility

At SwissFEL, a fully noninvasive characterization of the beam energy distribution can be performed by means of a synchrotron radiation monitor (SRM) imaging in the visible the transverse profile of the electron beam in a magnetic chicane. Under conditions of off-crest acceleration in the SwissFEL injector, according to a first order Taylor series expansion, the distribution of the relative energy of the electron beam can be linearly expressed as a function of the distribution of the electron longitudinal coordinates via the coefficient of the rf induced beam energy chirp. It is hence possible to express the distribution of the electron longitudinal coordinates at the entrance of the magnetic chicane of SwissFEL as a function of the distribution of the dispersed electron trajectories in the horizontal plane of the magnetic chicane. Machine parameters and instrument data at SwissFEL can be beam-synchronously acquired. A shot-by-shot correlation between the analysis results of the SRM camera images and the linear coefficient of the beam energy chirp resulting from the rf parameters of the injector can be hence established. Finally, taking into account the compression factor of the magnetic chicane, the electron bunch length at the exit of the magnetic chicane can be expressed as a function of the horizontal size of the electron beam imaged by the SRM. Bunch length measurements performed by means of the SRM in the first magnetic chicane of SwissFEL will be presented.

/pdf/bunch-length-and-energy-measurements-in-the-bunch-compressor-44jme8ajo4.pdf

Bunch length and energy measurements in the bunch compressor of a free-electron laser

The longitudinal current profile and slice emittance are important bunch parameters for the operation of an X-ray free-electron laser. At the European XFEL, dedicated diagnostic sections equipped with transverse deflecting RF structures (TDS) have been installed for the control and optimisation of these parameters. Travelling-wave TDS in combination with fast kicker magnets and off-axis screens allow for the study of individual bunches without affecting the other bunches in the super-conducting linear accelerator which can generate bunch trains of up to 2700 bunches at 4.5 MHz within 600 microsecond RF pulses at a repetition rate of 10 Hz. The measurement of the slice emittance is realised in a static F0D0 lattice equipped with four individual screen stations. Variations of the longitudinal bunch profile or slice emittance along the bunch train may lead to degraded FEL performance for parts of the train which reduces the effective available number of bunches for FEL operation. By gradually adjusting the timing, individual bunches along the bunch train can be measured in order to optimise the overall beam parameters for all bunches in the train. In this paper, we describe in detail the diagnostic concept and present first measurement results of the projected and slice emittance along the bunch train.

/pdf/online-longitudinal-bunch-profile-and-slice-emittance-yd3sqc9t22.pdf

Online Longitudinal Bunch Profile and Slice Emittance Diagnostics at the European XFEL

For SwissFEL electron beam diagnostics, we combine application-specific detectors and front-end electronics with a common solution for digitization, interfacing and FPGA-based digital signal processing. Many key components and standards we use were initially developed by PSI for the European XFEL BPM system, but are equally suited for a broad range of SwissFEL diagnostics systems with little or no modifications. Examples are e.g. the FPGA signal processing hardware and firmware/software, ADC and DAC boards, interface boards or peak detection front-end electronics. By following a modular generic hardware and firmware/software design approach, we can cover a larger number of different monitor types with moderate development effort. Applications of our generic platform include BPMs, bunch length monitors, beam arrival time monitors, beam loss monitors, and digital laser phase locked loops (PLLs). This paper gives an overview of the design, present and future applications of our generic platform, discussing the synergies and differences of the required hardware, firmware and embedded software solutions.

Overview of Applications and Synergies of a Generic FPGA-Based Beam Diagnostics Electronics Platform at SwissFEL

SwissFEL will be a hard X-ray fourth generation light source to be built at the Paul Scherrer Institute (PSI), Switzerland. In SwissFEL the electron bunches will be produced with a length of 1-3ps rms and will then be compressed by a factor of more than 1000 down to a few fs in order to generate ultra short x-ray pulses. Therefore reliable, accurate and noninvasive longitudinal diagnostics is essential after each compressing stage. In order to meet the requirements of this machine, new monitors have to be developed, in particular for low-charge operation at 10pC. We will present recent results of setups that measure electro-magnetic radiation, namely edge, synchrotron and diffraction radiation, emitted by the electron bunches (far field, spectral domain). These monitors are tested in the SwissFEL Injector Test Facility, together with a state of the art S-band Transverse Deflecting Cavity.

DEVELOPMENT OF ELECTRON BUNCH COMPRESSION MONITORS FOR SwissFEL

High-brightness electron bunches with small transverse emittance are required to drive X-ray free-electron lasers (FELs). For the measurement of the transverse emittance, the quadrupole scan and multi-screen methods are the two most common procedures. By employing a transverse deflecting structure, the measurement of the slice emittance becomes feasible. The quadrupole scan is more flexible in freely choosing the data points during the scan, while the multi-screen method allows on-line emittance measurements utilising off-axis screens in combination with fast kicker magnets. The latter is especially the case for highrepetition multi-bunch FELs, such as the European X-ray Free-Electron Laser (XFEL), which offer the possibility of on-line diagnostics. In this paper, we present comparative measurements of projected and slice emittance applying these two methods at the SwissFEL Injector Test Facility and discuss the implementation of on-line diagnostics at the European XFEL.

COMPARISON OF QUADRUPOLE SCAN AND MULTI-SCREEN METHOD FOR THE MEASUREMENT OF PROJECTED AND SLICE EMITTANCE AT THE SwissFEL INJECTOR TEST FACILITY

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DEVELOPMENT OF ELECTRON BUNCH COMPRESSION MONITORS FOR SwissFEL

COMPARISON OF QUADRUPOLE SCAN AND MULTI-SCREEN METHOD FOR THE MEASUREMENT OF PROJECTED AND SLICE EMITTANCE AT THE SwissFEL INJECTOR TEST FACILITY