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The transverse coherence of the source is an important property for FEL experiments. Theory and simulations indicated different features for seeded and unseeded FELs but so far no direct comparison has been pursued experimentally on the same facility. At FERMI one has the unique possibility to test both configurations (SASE and seeding) within the same operating conditions. In this contribution we present the experimental results of the characterization of transverse coherence with special attention to the evolution of such property.

Improvement of the longitudinal coherence in the proposed Soft Xray FEL, the SXL, for the MAX IV Laboratory is an important design aspect to enhance the user case. One of the main considered methods is HBSASE. However the final compression in the MAX IV acceleratos is done at full energy, and thus leaving an energy chirp in the electron pulse. This chirp in longitudinal phase space has to be removed for an efficient implementation of HBSASE. In this paper we show in simulations how the phase space is improved by first overcompressing the pulse, and then correct it by a two-plate wake field de-chirper. The resulting pulse is then shown to have qualities such that, by HBSASE, a significant narrowing of the FEL bandwidth is achieved at 1 nm.

Accurate and high resolution detection of the Longitudinal Phase Space (LPS) of the electron beam is a great advantage for operating and setting up a FEL. In the case of the soft X-ray FEL being proposed at the MAX IV synchrotron facility in Lund, this information is mainly supplied by a Transverse Deflecting Cavity (TDC) which is currently being installed and scheduled for commissioning in the autumn. Performing the LPS measurement with the future TDC is limited in two regards: it is destructive and may be low in resolution as compared to the maximum compression possible in the MAX IV linac. In this project we propose using machine learning tools to implement a virtual diagnostic to retrieve the LPS information non-destructively using fast, non-invasive measurements and critical set-points in the linac as inputs for a neural network. In this paper we summarize the current progress of this project which has thus far focused on simulation studies of the TDC and the training of a virtual diagnostic using the TDC's simulated output.

The existing MAX IV 3 GeV linac could drive, with minor improvements, a soft X-ray Free Electron Laser and the aim of the SXL project has been so far to deliver a conceptual design of such a facility in the 1—5 nm wavelength range. The project was initiated by a group of Swedish users of FEL radiation and the design work was supported by the Knut and Alice Wallenberg foundation and by several Swedish universities and organizations (Stockholm, Uppsala, KTH Royal Institute of Technology, Stockholm-Uppsala FEL center, MAX IV laboratory and Lund University). In this paper we will focus on the baseline FEL performance based on two different accelerator operation modes (medium and short pulses) and give some hints of future developments after the first phase of the project such as 2 color/2pulses and HB-SASE.

Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand IPAC2022, IPAC, Bangkok,, Thailand, 12 Jun 2022 - 17 Jun 2022; [Geneva] : JACoW Publishing, Geneva, Switzerland 1463-1466 (2022). doi:10.18429/JACOW-IPAC2022-TUPOMS024, Currently, the Free electron laser user facility FLASH at DESY is undergoing a significant upgrade involving the complete transformation of one of its beamlines to allow external seeding. With the Echo-Enabled Harmonic Generation (EEHG) seeding method, we aim for the generation of fully coherent XUV and soft X-ray pulses at wavelengths down to 4 nm. The generated FEL radiation is sensitive to various electron beam properties, e.g., its energy profile imprinted either deliberately or by collective effects such as Coherent Synchrotron Radiation (CSR). In dedicated particle tracking simulations, one usually makes certain assumptions concerning the beam properties and the collective effects to simplify implementation and analysis. Here, we estimate the influence of some of the common assumptions made in EEHG simulations on the properties of the output FEL radiation, using the example of FLASH and its proposed seeding beamline. We conclude that the inherent properties of the FLASH1 beam, namely the negatively chirped energy profile, has dominant effect on the spectral intensity profile of the radiators output compare to that of the CSR induced chirp., Published by JACoW Publishing, Geneva, Switzerland, [Geneva]

The European Spallation Source, currently under construction in Lund, Sweden, will be the world���s most powerful neutron source. It is driven by a proton linac with a current of 62.5 mA, 2.86 ms long pulses at 14 Hz. The final section of its normal-conducting front-end consists of a 39 m long drift tube linac (DTL) divided into five tanks, designed to accelerate the proton beam from 3.6 MeV to 90 MeV. The high beam current and power impose challenges to the design and tuning of the machine and the RF amplitude and phase have to be set within 1% and 1 degrees of the design values. The usual method used to define the RF set-point is signature matching, which can be a challenging process, and new techniques to meet the growing complexity of accelerator facilities are highly desirable. In this paper we study the use of ML to determine the RF optimum amplitude and phase, using a single pass of the beam through the ESS DTL1 tank. This novel method is compared with the more established methods using scans over RF phase, providing similar results in terms of accuracy for simulated data with errors. We also discuss the results and future extension of the method to the whole ESS DTL., Proceedings of the 10th International Beam Instrumentation Conference, IBIC2021, Pohang, Rep. of Korea

Echo-Enabled Harmonic Generation (EEHG) is an external seeding technique for XUV and soft X-ray Free Electron Lasers (FEL). It has recently been experimentally demonstrated and currently many facilities worldwide intend to incorporate it in user operation. The EEHG process relies on very accurate and complex transformations of electron beam phase space by means of a series of undulators coupled to lasers and dispersive chicanes. As a result of the phase space manipulation, electrons are bunched at a high harmonic of the seed laser wavelength allowing coherent emission at few nm wavelength. Dispersion occurring in strong chicanes is imperative for implementation of this scheme and effective electron bunching generation. However, strong chicanes at the same time can be source of beam instability effects, such as Coherent Synchrotron Radiation (CSR), that can significantly grow in these conditions and suppress the bunching process. Therefore, there is a common need to investigate such effects in detail. Here, we discuss their treatment with simulation codes applied to a typical EEHG setup., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The European Spallation Source, currently under construction in Lund, Sweden, will be the world’s most powerful neutron source. It is driven by a proton linac with a current of 62.5 mA, 2.86 ms long pulses at 14 Hz. The final section of its normal-conducting front-end consists of a 39 m long drift tube linac (DTL) divided into five tanks, designed to accelerate the proton beam from 3.6 MeV to 90 MeV. The high beam current and power impose challenges to the design and tuning of the machine and the RF amplitude and phase have to be set within 1% and 1 degree of the design values. The usual method used to define the RF set-point is signature matching, which can be a time consuming and challenging process, and new techniques to meet the growing complexity of accelerator facilities are highly desirable. In this paper we study the usage of Machine Learning to determine the RF optimum amplitude and phase. The data from a simulated phase scan is fed into an artificial neural network in order to identify the needed changes to achieve the best tuning. Our test for the ESS DTL1 shows promising results, and further development of the method will be outlined., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

A Soft X-ray free electron laser (FEL) for the MAX IV Laboratory is currently in the design phase and it will use the existing 3 GeV linac. Present stability limits in the RF and the photocathode laser will affect the performance of the FEL. One of the critical elements for the design of a FEL is to have an estimation on jitter effects of the accelerator parameters on the X-ray radiation. In this regard, we implemented a start-to-end study using Astra, Elegant and Genesis in order to assess possible variations in pulse energy, photon pulse length and spectral width in the Soft X-ray Laser (SXL) radiation. This investigation provides insights on the final SXL performance variation due to RF and laser related jitter affecting the electron beam., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The SXL (Soft X-ray Laser) project developed a conceptual design for a soft X-ray Free Electron Laser in the 1–5 nm wavelength range, driven by the existing MAX IV 3 GeV linac. In this contribution we will focus on the FEL operation modes developed for the first phase of the project based on two different linac modes. The design work was supported by the Knut and Alice Wallenberg foundation and by several Swedish universities and organizations (Stockholm, Uppsala, KTH Royal Institute of Technology, Stockholm-Uppsala FEL center, MAX IV laboratory and Lund University)., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

As a part of the efforts to improve the longitudinal coherence in the design of the Soft X-ray FEL (the SXL) at MAX IV, we present a possible implementation of the EEHG harmonic seeding scheme partly integrated into the second bunch compressor of the existing LINAC. A special focus is given to the effect of CSR on the resulting EEHG bunching and on how this unwanted effect might be controlled., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

A Soft X-ray FEL (the SXL) using the existing 3 GeV linac at the MAX IV Laboratory is currently in the design phase. In this contribution, start-to-end simulations, including the photo-injector simulations using ASTRA, the linac simulations using ELEGANT and the FEL simulations using GENESIS, are presented for 100 pC and 10 pC operation modes. The features of the electron beam from the MAX IV linac and their impact on the FEL performance are discussed., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

The design of the pre-injector, including the new gun, for the SXL project is being finalised for the desired modes of operation, 100 pC and 10 pC with short bunches. The photocathode gun is currently being manufactured and experiments in the MAX IV guntest facility are under preparation to verify the design. In this paper we present the design of the gun and the pre-injector and show some results from simulations using MOGA indicating an emittance below 0.3 mm mrad., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

As the ECHO enabled harmonic generation (EEHG) scheme draws such intense focus from the FEL community, we conduct simulations to evaluate the challenges of implementing said scheme in different FEL layouts. Nonlinear processes such as this require extensive simulations to harmonize all system specific properties like seed lasers and electron beam properties. Along with optimizing the original EEHG scheme* one can consider, for example, altering the seed laser pulse to optimize the bunching for a machine specific chirp. We study the EEHG as a possible seeding method aimed at increasing coherence of the photon beam for the prospective SXL FEL beamline at MAXIV. The particular chirp of the electron beam through the MAXIV LINAC generates some specific requirements in implementing EEHG but may also offer an opportunity for exotic operation modes of this FEL., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The ability to generate two-color free-electron laser (FEL) radiation enables a wider range of user experiments than just single-color FEL radiation. There are different schemes for generating the two colors, the original being to use a single bunch and two sets of undulators with different K-parameters. A development of the scheme has recently been shown, where two separate bunches in the same RF bucket are used for lasing at different wavelengths. We here investigate the feasibility of accelerating and compressing a double-bunch time structure generated in the photocathode electron gun for subsequent use in a soft X-ray FEL at the MAX IV Laboratory., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

We present a setup for producing and characterizing picosecond ultraviolet laser pulses for use in the MAX IV photocathode electron gun preinjector. Frequency-tripled laser pulses from a commercial laser system are shaped directly in the ultraviolet domain using a Fourier-domain pulse shaper. The pulses were characterized using a transitent grating FROG. We discuss a proposed upgrade of the pulse shaper, as well as its limitations., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

The MAX IV LINAC operates both as a full-energy injector for two electron storage rings, and as a driver for a Short Pulse Facility (SPF). A soft X-ray Laser (SXL) beamline will also be installed in the end of the existing LINAC. For SPF and SXL operation, it is important to characterize beam parameters such as bunch profile, slice energy spread and slice emittance. For these measurements, two 3 m long transverse deflecting RF structures with a matching section are being developed. The structures are operating at S-band and have variable polarizations. When fed via a SLED pulse compressor, the two structures can generate a total integrated deflecting voltage higher than 100 MV which is sufficient for measurements with temporal resolutions down to 1 fs. This paper describes the initial RF design of the deflecting structures., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

The MAX IV Linac is now in routine operation for injection into two storage rings, and as a high-brightness driver for a Short Pulse Facility (SPF). In short-pulse mode the electron bunch is created in a photo cathode gun and compressed in two double achromat bunch compressors that also linearize longitudinal phase space with the second order transfer matrix element T566. T566 in the compressors can be tweaked with weak sextupoles located at high dispersion. In this paper we present the current experience from operating the bunch compressors at MAX IV and results from initial measurements of longitudinal phase space using the zero-crossing method., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

Beam-driven plasma-wakefield acceleration is an acceleration scheme promising accelerating fields of at least two to three orders of magnitude higher than in conventional radiofrequency accelerating structures. The scheme relies on using a charged particle bunch (driver) to drive a non-linear plasma wake, into which a second bunch (witness) can be injected at an appropriate distance behind the first, yielding a substantial energy gain of the witness bunch particles. This puts very special demands on the machine providing the particle beam. In this article, we use simulations to show that, if driver-witness-bunches can be generated in the photo-cathode electron gun, the MAX IV Linear Accelerator could be used for plasma-wakefield acceleration., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The MAX IV linac is used both for full energy injection into two storage rings at 3 GeV and 1.5 GeV, and as a high brightness driver for a Short Pulse Facility (SPF). The linac has also been designed to handle the high demands of an FEL injector. The linac is now routinely injecting into the two storage rings, and commissioning work is focused towards delivering high brightness pulses to the SPF. In this paper we present results from characterisation of the linac in ring injection mode, as well as results from measurements of key parameters for the SPF such as bunch length and emittance., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The MAXIV injector has two guns - a thermionic used for ring injections, and a photocathode used for short pulse facility operation. A commercial Ti:sapphire laser from KMLabs drives the copper based photocathode gun. It has been running without major issues for more than 3 years. The laser delivers up to §I{500}{\textmu J} on the cathode at the third harmonic, §I{263}{nm}, via a vacuum laser transport system. To achieve the desired pulse duration of 2–§I10{ps} the laser pulses, originally ~§I{100}{fs} long, are stretched with a prism pair and the resulting §I{1.5}{ps} pulses stacked by a series of birefringent \textalpha -BBO crystals. Diagnostics consist of photodiodes, spectrometers, and cameras. Longitudinal pulse characterization is done with a cross correlator and a UV FROG., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

Free Electron Lasers (FEL) are one of the most brilliant light sources in the world and their unique properties are driving worldwide research in understanding and improving them. Numerous papers have already been published describing the output of the FEL in terms of coherence and bandwidth. In this contribution, however, we focus on how the coherence evolves along the FEL undulator and on what factors influence it the most. Using Genesis^{−1}.3* we have been able to follow and record the light field as it is being produced in the undulator. Our analysis method takes advantage of the extensively studied double pinhole experiment and uses the principles behind it to create a tool for extracting coherence information from the radiation field. We will present the scope, limitations and advantages of these virtual experiments as well as an application on an example FEL, to showcase what kind of information can be extracted using this method., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The gun test stand from MAX-Lab has been upgraded and moved to a new facility at the MAX IV Laboratory. The new test stand will reuse parts of the equipment from the old test stand but a number of improvements to the setup are be made. In this paper we report on the design of the new gun test stand, research plans in the near future as well as planned and possible future research topics., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

A motivation for the development of a versatile, programmable source of shaped picosecond pulses for use in photocathode electron gun preinjectors is presented. We present the experimental setup for arbitrary longitudinal pusle shaping of the MAX IV photocathode gun laser. The setup consists of a grating-based Fourier-domain shaper capable of stretching the pulses directly in the UV domain. Preliminary results are presented and discussed., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The MAX IV injector design predicts a beam with 100 pC of charge and an emittance lower than 1 mm mrad. The photocathode pre-injector is based on the now close to standard 1.6-cell gun adapted to 2.9985 GHz, in combination with a Ti:Sapphire laser system. This system reaches the requirements of the injector operation for the SPF, but can be tuned beyond specifications to open up new operation modes. During 2016 and 2017 several aspects where investigated to improve the emittance from the current gun, the goal was to meet the SPF specifications. In this paper we report on the progress, discuss the steps taken leading to a final emittance of ~ 1 mm mrad and beyond., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

In a free-electron laser equipped with variable-gap undulator modules, the technique of undulator tapering opens up the possibility to increase the radiation power beyond the initial saturation point, thus enhancing the efficiency of the laser. The effectiveness of the enhancement relies on the proper optimization of the taper profile. In this work, a multidimensional optimization approach is implemented empirically in the x-ray free-electron laser FLASH2. The empirical results are compared with numerical simulations.

The linear accelerator at the MAX IV facility in Lund, Sweden, was constructed for injection and top up of the two storage rings and as a high brightness driver for the Short Pulse Facility. It is also prepared to be used as an injector for a possible future Free Electron Laser. Installation of the linac was completed and beam commissioning started in the early fall of 2014. In this paper we present the progress during the first phase of commissioning along with results from initial measurements of optics, emittance, beam energy and charge., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

The Short Pulse Facility (SPF) of the MAX IV Laboratory in Lund, Sweden features the production of ultrashort, incoherent x-ray pulses. It is driven by a 3-GeV linac and comprises two 5-metre undulator modules. While the SPF is designed for spontaneous radiation, we explore alternative operation modes in which the SPF functions as a simple free-electron laser (FEL). In this article, we characterize two of them in time-dependent numerical simulations. We perform a sensitivity study on the electron beam parameters and examine the technique of single-step tapering., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

The MAX IV facility in Lund, Sweden is currently under commissioning. There are two guns in the current MAX IV injector, one thermionic gun for storage ring injection and one photocathode gun for the Short Pulse Facility. There is a possibility of extending the facility to include a Free Electron Laser. To investigate how the beam from the injector can be improved and how to match it to the future requirements for a FEL, the emittance meter from SPARC has been recommissioned at the MAX IV gun test stand. In this paper we report on the progress of this work and results from the first measurements., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

The MAX IV facility in Lund, Sweden is currently under commissioning. In the MAX IV injector there are two guns, one thermionic gun for storage ring injection and one photocathode gun for the Short Pulse Facility. The commissioning of the injector and the LINAC has been ongoing for the last year and ring commissioning is due to start shortly. In this paper we will present the results from beam performance experiments for the injector at the current stage of commissioning., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

Knowing the longitudinal phase space of an electron beam is one of the most important and crucial issues in short-pulses linacs. To achieve this task expensive and rather complicated setups (like transverse deflecting cavities) are usually implemented. The MAX IV 3 GeV linac will be used to inject into two rings and to drive a short pulse facility. Nevertheless, a more deep understanding of the beam quality would be useful especially in view of an upgrade as FEL driver. Another interesting aspect is to evaluate how the double-achromat bunch compressors are performing. We are studying how to implement off-phase acceleration: last part of the linac will be set at zero-crossing phase and the transfer line to the 3 GeV ring could be used as energy spectrometer to retrieve the bunch profile. In the present configuration of the MAX IV linac this procedure will allow to check the bunch length after the first bunch compressor. Since it is work in progress, in this contribution we present a sketch of the measurement and the feasibility of the method will be explored by means of simulations., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

In a free-electron laser (FEL), the technique of wiggler tapering enables the sustained growth of radiation power beyond the initial saturation. With the goal to develop an X-ray FEL in the terawatt power regime, it is important to utilize this technique and optimize the taper profile, giving the wiggler parameter as a function of the distance along the wiggler line. This work examines two methods of optimization, which are based on the theoretical analysis by Kroll, Morton and Rosenbluth (KMR). Using the numerical simulation code GENESIS, the methods are applied to a case for the possible future FEL at the MAX IV Laboratory in Lund, Sweden, as well as a case for the LCLS-II., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

In the second part of 2014, the 3GeV linac at the MAX IV laboratory will enter its commissioning stage. Equipped with two guns, the linac will act as a full energy injector for the two storage rings and at the same time provide high brightness pulses to a Short Pulse Facility (SPF). Compression in the linac is done in two double achromats with fixed R56 that relies upon the RF phase introduced energy chirp, which in this case is strongly enhanced by the longitudinal wakefields. Since the longitudinal wakefields plays a major role in the compression and bunch shaping they need to be carefully investigated during the commissioning. In this proceeding we will discuss a measurement technique that will be used during commissioning to characterize the longitudinal wakefields and their precise effects on e.g. the bunch shape and the energy spread. Predictions obtained from particle tracking will be presented., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany