Novel seeding techniques for generation of high repetition rate coherent nanometer FEL radiation

igh gain free electron lasers (FELs) generate radiation of unprecedented brightness and unique properties and have proven to be a useful tool for applications in a wide range of studies in physics, biology, medicine and chemistry. External seeding techniques have been experimentally demonstrated and aim to improve the intrinsically poor temporal coherence of a high-gain FEL starting from shot noise. Seeded schemes, like the high gain harmonic generation (HGHG), are based on frequency up-conversion and generate fully coherent radiation thanks to the external seed laser that initiates the process in the FEL. However, the dependence of the properties of the seeded radiation on those of the seed laser is at the same time a limiting factor. The repetition rate of the state-of-the-art seed lasers used in external seeding is incompatible with the repetition rates of modern high-gain FELs based on superconducting technology. In addition, seeded radiation is a harmonic of the seed laser wavelength, a feature that limits the output wavelength to above a few nanometers and restricts its tunability. To overcome these limitations it is necessary to search for new possibilities in FELs.The scope of this cumulative thesis is to introduce novel ideas that allow us to achieve high repetition rate and fully coherent radiation at an extended and tunable wavelength range. The three proposals of this thesis aim to reduce the dependence of external seeding schemes on the seed laser source and at the same time, maintain the full coherence of seeded radiation. The first proposal is an optical klystron-based HGHG, which modifies the conventional HGHG beamline in a way that relaxes the stringent requirements on the seed laser power by several orders of magnitude. This way, the repetition rate of the seed laser source can be increased, or seed laser sources of shorter wavelengths can be used instead. The second proposal is an HGHG seeded oscillator-amplifier setup: an optical cavity captures a conventional low repetition rate seed laser pulse and stores it to seed electron bunches at a high repetition rate. The third proposal is an HGHG oscillator-amplifier that eliminates the dependence on external seed lasers. Instead of the external laser, the electrons generate the light pulse themselves, starting from shot noise, and the radiation is stored in the optical cavity to seed electron bunches at high repetition rates. In addition to the high repetition rate, this scheme allows shorter and tunable seeded radiation. This type of radiation has never been possible in the past and can greatly benefit already existing experiments and support new experiments and more discoveries by accelerating the ongoing science at FELs

Advanced Scheme to Generate MHz, Fully Coherent FEL Pulses at nm Wavelength

Current FEL development efforts aim at improving the control of coherence at high repetition rate while keeping the wavelength tunability. Seeding schemes, like HGHG and EEHG, allow for the generation of fully coherent FEL pulses, but the powerful external seed laser required limits the repetition rate that can be achieved. In turn, this impacts the average brightness and the amount of statistics that experiments can do. In order to solve this issue, here we take a unique approach and discuss the use of one or more optical cavities to seed the electron bunches accelerated in a superconducting linac to modulate their energy. Like standard seeding schemes, the cavity is followed by a dispersive section, which manipulates the longitudinal phase space of the electron bunches, inducing longitudinal density modulations with high harmonic content that undergo the FEL process in an amplifier placed downstream. We will discuss technical requirements for implementing these setups and their operation range based on numerical simulations

Effect of Insertion Devices Tapering Mode of Operation on the MAX IV Storage Rings

Tapering is a mode of operation of insertion devices that allows the users to perform scanning in a range of photon energies. NanoMAX, BioMAX and BALDER are all beamlines of the 3 GeV MAX IV storage ring and will provide this special mode of operation for their users. In this thesis, the spectra of NanoMAX and BioMAX while operating with tapering were studied and feed forward tables that cancel out the closed orbit distortion caused by the insertion devices were generated. Moreover, a study of the nature of the closed orbit distortion was performed, aiming at simplifying the feed forward table measurements that can currently be quite time consuming. In addition, the effect of BALDER, which is a wiggler and currently the strongest insertion device in the storage ring, on the electron beam was studied. Apart from the feed forward table that corrects for the closed orbit distortion, BALDER induces a beta beat and tune shift which has to be eliminated in order to make the insertion device transparent to the electron beam. This correction is needed in order to keep the beam life time unaffected and ensure a stable operation. For this reason, a two-stage correction scheme was proposed. First, a local correction with the quadrupoles adjacent to BALDER was performed in order to eliminate the beta beat induced by the wiggler. As a second step a global correction was applied. The aim of the global correction is to bring the tune back to the design values.Accelerators are huge machines (up to tens of kilometers long) dedicated to accelerate particles, such as protons and electrons, up to very high energies which makes them travel almost with the speed of light. Worldwide there are many different types of accelerators used for a wide range of applications and research. CERN is probably the most well-known research center related to accelerators in the world. At CERN, there are mostly high energy physics experiments running with accelerators called colliders, named after the particle collisions that occur in this case. This means that the purpose of those collisions is to seek answers to fundamental questions, such as which are the blocks that the universe is made of. On the other hand, MAX IV uses accelerators for different reasons. Electrons are accelerated in order to produce light which makes MAX IV a big photon factory. This type of accelerator is called a light source and it is more common than colliders. The photons are produced in magnetic devices called insertion devices and they are X-rays, like the rays used in the airport security. However, the intensity of the X-rays produced in this case is huge and hence, they can be used for experiments of a great variety. For example, by using this light we can study the structure of proteins. This information can explain their functions and therefore, it will offer an insight into how human body works. In addition, the light produced can be used for research in many other disciplines such as material science, environmental science and archeology. Some experiments running at MAX IV require light of very specific prop- erties. Normally, the insertion devices consist of a large number of pairs of magnetic poles, like earth has the north and south pole, which are placed in a constant distance from each other. This leads to two girders with magnetic poles parallel to each other. However, in order to produce this different type of light the two girders of the insertion device must not be parallel to each other but open like a fan. This mode of operation is called tapering mode. The spectrum of the light produced in this case consists of high intensity light that consists of a wide range of energies. Therefore, this light is not as monoenergetic as before and it can be used for scanning within a range of energies when the optimum energy for the experiment is unknown. However, using the insertion devices in both normal and tapering mode produces a magnetic field that disturbs the electron beam and hence all the experiments running in the laboratory. For example, using this mode without any correction applied can possibly ”kick” the electrons outside of the aperture of the accelerator in which electrons can move. In this case the electrons would be lost and no experiment would be able to run. The first goal is to test and offer the tapering mode to the experiments at MAX IV. Afterwards, the orbit of the electrons is corrected back to the desired one when the insertion devices operate both with tapering and without. This correction can establish that the electrons are in the right design orbit along the accelerator and therefore the experiments at MAX IV are running without problems

Pseudo-single-bunch mode for a 100 MHz storage ring serving soft X-ray timing experiments

At many storage rings for synchrotron light production there is demand for serving both high-flux and timing users simultaneously. Today this is most commonly achieved by operating inhomogeneous fill patterns, but this is not preferable for rings that employ passive harmonic cavities to damp instabilities and increase Touschek lifetime. For these rings, inhomogeneous fill patterns could severely reduce the effect of the harmonic cavities. It is therefore of interest to develop methods to serve high-flux and timing users simultaneously without requiring gaps in the fill pattern. One such method is pseudo-single-bunch (PSB), where one bunch in the bunch train is kicked onto another orbit by a fast stripline kicker. The light emitted from the kicked bunch can then be separated by an aperture in the beamline. Due to recent developments in fast kicker design, PSB operation in multibunch mode is within reach for rings that operate with a 100 MHz RF system, such as the MAX IV and Solaris storage rings. This paper describes machine requirements and resulting performance for such a mode at the MAX IV 1.5 GeV storage ring. A solution for serving all beamlines is discussed as well as the consequences of beamline design and operation in the soft X-ray energy range

High repetition rate seeded FEL with an optical klystron in high-gain harmonic generation

Many high-gain FELs worldwide are planning to incorporate seeding setups into their day-to-day operation. These techniques provide both longitudinal and transverse coherence and extendedcontrol of the output FEL spectral properties. However, the output wavelength and repetition ratestrongly depend on the properties of the seed laser system. With the laser peak power requiredfor successful seeded operation, it is currently not possible to increase their repetition rate to anextent that it matches the electron bunch repetition rate of superconducting accelerators. Here, weinvestigate the advantages of a modification of standard seeding setups, by combining the seedingwith the so-called optical klystron. With this new seeding setup it is possible to decrease the seedlaser power requirements and therefore, seed laser systems can increase their repetition rate at thesame wavelength. We show simulation results in a high-gain harmonic generation (HGHG) setupfor a range of harmonics (8th to 15th) and we verify the reduction of seed laser power required withan Optical Klystron (OK) HGHG. Finally, we comment on the stability of the proposed setup tojitter sources and to shot to shot fluctuations and compare to the standard HGHG scheme

Expected Radiation Properties of the Harmonic Afterburner at FLASH2

We discuss the afterburner option to upgrade the FLASH2 undulator line, at the FLASH facility in the Hamburg area, for delivering short wavelengths down to approximately 1.5 nm with variable polarization. This relatively straightforward upgrade enables us the study of the scientific cases in L- absorption edges of rare earth metals. The proposed afterburner setting with an energy upgrade to 1.35 GeV would potentially cover many of the community’s requests for the short wavelengths radiation and circular polarization. We also study the influence of reverse tapering on the radiation output. This contribution presents a series of simulations for the afterburner scheme and some of the technical choices made for implementation

Seeding with a Harmonic Optical Klystron Resonator Configuration in a High Repetition Rate Free Electron Laser

The generation of highly coherent radiation in a high repetition rate free electron laser driven by a superconducting linear accelerator has become a topic of growing interest. External seeding schemes like high-gain harmonic generation (HGHG) and echo-enabled harmonic generation are proven to be able to generate coherent radiation in the extreme ultraviolet and x-ray range. However, the repetition rate of current laser systems with sufficient power to modulate the electron beam is limited to the kilohertz range. Recently, to achieve seeding at a high repetition rate, an optical resonator scheme has been introduced to recirculate the radiation in the modulator to seed the electron bunches. In this paper, two harmonic optical klystron resonator configurations either starting with a seed laser or starting from shot noise are studied. With the harmonic optical klystron as the seeding source, the efficiency of harmonic radiation generation at a comparatively high harmonic can be significantly enhanced when compared with a standard single-stage HGHG. Simulation results show that highly coherent, stable, and high repetition rate pulses in the water window range could be generated by the proposed seeding schemes. Some practical considerations including beam energy chirp effects and the power density effects on mirrors are discussed

Optimization and stability of a high-gain harmonic generation seeded oscillator amplifier

The free-electron laser (FEL) community is interested in taking full advantage of the high-repetition-rates of FELs run by superconducting machines while maintaining the spectral properties achieved with external seeding techniques. Since the feasibility of seed lasers operating at a repetition-rate of MHz and with sufficient energy in a useful wavelength range, such as the ultraviolet (UV) range is challenging, a seeded oscillator-amplifier scheme is proposed instead for generation of fully coherent and high-repetition-rate radiation. The process is triggered by an external seed laser while an optical feedback system feeds the radiation back to the entrance of the modulator where it overlaps with the next electron bunch. Downstream from the feedback system, the electron bunches are then used for harmonic generation. We discuss the optimization of dedicated simulations and we investigate the stability of this scheme with numerical simulations. As a result, we address the control of the reflectivity of the resonator as a key parameter to achieve a stable HGHG seeded radiation. Finally, we show the impact of the power fluctuations in the oscillator on the bunching amplitude with analytical and simulated results. The output FEL radiation wavelengths considered are 4.167 nm and 60 nm

Impact of Electron Beam Energy Chirp on Seeded FELs

Seeded FELs enable the generation of fully coherent, transform-limited and high brightness FEL pulses, as the start-up process is driven by an external coherent light pulse. During the design process of such FELs, it is important to choose carefully the electron beam parameters to guarantee high performance. One of those parameters is the electron beam energy chirp. In this contribution, we show simulation results and we discuss how the electron beam energy chirp affects the final spectrum

Advanced Scheme to Generate MHz, Fully Coherent FEL Pulses at nm Wavelength

Current FEL development efforts aim at improving the control of coherence at high repetition-rate while keeping the wavelength tunability. Seeding schemes, like HGHG and EEHG, allow for the generation of fully coherent FEL pulses, but the powerful external seed laser required limits the repetition-rate that can be achieved. In turns, this impacts the average brightness, and the amount of statistics that experiments can do. In order to solve this issue, here we we take a unique approach and discuss the use of one or more optical cavities to seed the electron bunches accelerated in a superconducting linac to modulate their energy. Like standard seeding schemes, the cavity is followed by a dispersive section, which manipulates the longitudinal phase space of the electron bunches, inducing longitudinal density modulations with high harmonic content that undergo the FEL process in an amplifier placed downstream. We will discuss technical requirements for implementing these setups and their operation range based on numerical simulations