Electron Laser In Hamburg Research Articles

Flexible ptychography platform to expand the potential of imaging at free electron lasers.

Ptychography, a scanning coherent diffraction imaging method, can produce a high-resolution reconstruction of a sample and, at the same time, of the illuminating beam. The emergence of vacuum ultraviolet and X-ray free electron lasers (FELs) has brought sources with unprecedented characteristics that enable X-ray ptychography with highly intense and ultra-fast short-wavelength pulses. However, the shot-to-shot pulse fluctuations typical for FEL pulses and particularly the partial spatial coherence of self-amplified spontaneous emission (SASE) FELs lead to numerical complexities in the ptychographic algorithms and ultimately restrict the application of ptychography at FELs. We present a general adaptive forward model for ptychography based on automatic differentiation, which is able to perform reconstructions even under these conditions. We applied this model to the first ptychography experiment at FLASH, the Free electron LASer in Hamburg, and obtained a high-resolution reconstruction of the sample as well as the complex wavefronts of individual FLASH pulses together with their coherence properties. This is not possible with more common ptychography algorithms.

Cavity tilt measurement using beam excited higher order mode signals at the Free Electron Laser FLASH

At the Free Electron Laser in Hamburg (FLASH), TESLA-type superconducting (SC) cavities are used for the acceleration of electron bunches. Higher Order Modes (HOMs) excited by electron bunches may cause emittance growth and decrease the FEL performance. The misalignment of the SC cavities housed inside cryo-modules is one of the main factors which enhance the coupling of HOMs to the bunches. We consider the cavity offset and tilt as the two most relevant misalignments. These can be measured by utilizing the beam excited HOM signals. The cavity offset has been measured before in several modules at FLASH, while the tilt has so far proved to be much more difficult to be measured. This is because the beam trajectory tilt excited modes are much weaker than the ones excited by offset. We used a method for measuring cavity tilt by carefully targeting the beam through the middle of a cavity. An efficient data processing routine based on signal fitting was applied. For the first time, the tilts of all cavities inside a cryo-module are measured. The ratio between the offset and angle dependence of the HOMs agrees well among the different cavities.

Parallel Broadband Femtosecond Reflection Spectroscopy at a Soft X-Ray Free-Electron Laser

X-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.

High resolution and low charge button and strip-line beam position monitor electronics upgrade at FLASH

Historically the FLASH (Free Electron Laser in Hamburg) facility at DESY (Deutsches Elektronen-Synchrotron) in Germany has foreseen operation in a charge range from 1 nC-3 nC for which a VME based BPM (Beam Position Monitor) system has been in operation since 2005 with a later upgrade to lower charges. In the past few years the standard machine operation settled at a few hundreds of pC with a tendency to smaller charges down to 100 pC and less. The performance of the BPM system at charges below 300 pC was in many locations along the machine unsatisfactory, making the operation of the linac more unstable and less reliable. Therefore a new BPM electronic system based on the utca (Micro Telecommunication Computing Architecture) for physics MTCA.4 standard has been developed to overcome the limitations of the old electronics and has already been successfully in operation in FLASH 2. A substantially improved version of the RF (radio frequency) front-end and digital electronics/firmware has been developed in 2016 and tested successfully. The peak detector electronics have been extended to a double peak detection in four channels and the fully customized Firmware is working machine synchronous. In summer 2017 all old button and strip-line BPM electronics have been replaced with the new type. This paper summarizes the features and performance of the new BPM system, compares the beam jitter free resolution of the old and new BPM system and highlights its high single shot resolution of better than 10 μm rms at a charge of 15 pC.

Coherence properties of the radiation from FLASH

Free electron LASer in Hamburg is the first free electron laser user facility operating in the vacuum ultraviolet and soft X-ray wavelength range. Many user experiments require knowledge of the spatial and temporal coherence properties of the radiation. In this paper, we present a theoretical analysis of the coherence properties of the radiation for the fundamental and for the higher odd frequency harmonics. We show that temporal and spatial coherence reach their maxima close to the free electron laser (FEL) saturation but may degrade significantly in the post-saturation regime. We also find that the pointing stability of short FEL pulses is limited due to the fact that nonazimuthal FEL eigenmodes are not sufficiently suppressed. We discuss possible ways for improving the degree of transverse coherence and the pointing stability.

Ultrafast reduction of the total magnetization in iron

Surprisingly, if a ferromagnet is exposed to an ultrafast laser pulse, its apparent magnetization is reduced within less than a picosecond. Up to now, the total magnetization, i.e., the average spin polarization of the whole valence band, was not detectable on a sub-picosecond time scale. Here, we present experimental data, confirming the ultrafast reduction of the total magnetization. Soft x-ray pulses from the free electron laser in Hamburg (FLASH) extract polarized cascade photoelectrons from an iron layer excited by a femtosecond laser pulse. The spin polarization of the emitted electrons is detected by a Mott spin polarimeter.

Feedback Control of Loaded Q Values of the Superconducting Cavities at FLASH

The well-established technology of superconducting niobium cavities (TESLA - TeV-Energy Superconducting Linear Accelerator) finds an increasing number of applications for linear accelerators in high-energy physics experiments. Together with cavity design and manufacturing, control systems for accelerating field parameters were developed. The digital control system developed for cryomodule operation at the Free Electron Laser in Hamburg (FLASH) is able to perform field amplitude and phase regulation with the precision requested for the multiple user experiments carried at FLASH and according to the required laser light parameters. While new experiments (like the International Linear Collider or the European X-FEL) are based on the same technology (and similar control systems), they require additional levels of controlling superconducting structures parameters in order to comply with tighter power budget overheads and finer field regulation requirements. Consequently, the possibility of controlling such parameters as cavity loaded quality factor or resonance frequency is becoming more attractive. The necessity and the benefits of regulating these parameters have been described in and . In this paper, we describe the algorithm for feedback control of the loaded Q in TESLA cavities by means of motor control of the fundamental power coupler.

Vector Modulator Card for MTCA-Based LLRF Control System for Linear Accelerators

Modern low-level radio frequency (LLRF) control systems of linear accelerators are designed to achieve extremely precise field amplitude and phase regulation inside superconducting cavities. One of the crucial components of the feedback loop is a vector modulator used to drive the high-power RF chain supplying the accelerating cavities. The LLRF control systems for the Free Electron Laser in Hamburg (FLASH) and European X-ray Free Electron Laser (XFEL) are based on the emerging Micro-Telecommunications Computing Architecture (MTCA.4) platform offering numerous advantages for high-performance control systems. This paper describes the concept, design, and performance of the first vector modulator (uVM) module dedicated for RF controls compliant with the MTCA.4 specification developed by PCI Industrial Computer Manufacturers Group (PICMG). The uVM module has been built as a double-width, mid-size rear transition module (RTM) that is accessed by an advanced mezzanine card (AMC). The uVM module incorporates digital, analog, and diagnostic subsystems. The digital part is based on Xilinx Spartan 6 family field-programmable gate array (FPGA), with several fast gigalink connections to a control module. The uVM module is equipped with an intelligent platform management interface (IPMI) circuit required by the MTCA.4 standard. The FPGA controls the analog part, which includes fast, high-precision digital-to-analog converters (DACs), In-phase and quadrature modulator chips, programmable attenuators, power amplifiers, and fast RF gates for an external interlock system. The RF chain can be adopted to different carrier frequencies covering a frequency range from 50 MHz to 6 GHz. The design has been carefully optimized for high linearity and low-output signal phase noise. The diagnostic system of the RF chain allows monitoring of input and output power levels and failure detection in the RF circuits. A low-noise and high-performance clocking system makes the uVM a universal device, extending the scope of applications beyond the LLRF control systems. Extensive tests of the board were performed, and the measurement results are presented and discussed in this paper.

Towards imaging of ultrafast molecular dynamics using FELs

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br2 molecules were transiently ‘fixed in space’ using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.

The XUV split-and-delay unit at beamline BL2 at FLASH

For time-resolved extreme ultraviolet (XUV) pump–XUV probe experiments at the Free electron LASer in Hamburg (FLASH), a split-and-delay unit (SDU) has been built. It is implemented in beamline BL2 which provides a focal spot size of about 20 μm diameter in the experiment. The beam is divided geometrically into two paths which can be delayed from −3 to +15 ps with respect to each other. The transmission up to 200 eV photon energy is above 35% in one beam path and 74% in the other. The latter transmits the XUV beam again from 305 to 570 eV (>1% transmission). Thus almost the whole spectral range at FLASH is covered by the SDU with reasonable transmission, including the option to transport high-energy third harmonic radiation in one of the beam paths. Both beam paths are realigned into the original direction of the radiation at the end of the SDU. Thus the utilization of the divided as well as the original beam is enabled by simply moving the optical elements of the SDU into or out of the beam. Using the SDU, the coherence length and the average pulse duration at FLASH was determined to be 0.9–1.8 μm, depending on the wavelength, and about 30 fs, respectively, for the specific electron bunch parameters.

Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+–N5+ channel for nitrogen and up to the I8+–I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.

Above-threshold two-colour ionization signal of singly charged neon

In this work we report the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. The photoelectron spectrum dependence of the electrons ejected from singly-charged neon (Ne+) on the relative polarisation of the XUV and optical laser fields was investigated both experimentally and theoretically.

Angular distributions and correlations in sequential three-photon triple atomic ionization

A theoretical description of the sequential three-photon triple atomic ionization is given within a model of stepwise ionization with emphasis on the angular distributions and angular correlations of the emitted electrons. The theory is applied to the photoionization of neon atom at 90.5 eV, corresponding to the experiment performed at the Free electron LASer in Hamburg (FLASH) by A. Rouzée et al (Phys. Rev. A 83 031401(R) (2011)).

Time-resolved XUV-induced isomerization and H3 formation in C2H4 cation

Using a split-mirror setup attached to a Reaction Microscope at the Free electron LASer in Hamburg (FLASH) we traced as function of time the migration of a hydrogen atom in C2H4+ from one end of the molecule to the other by coincident CH+ + CH3+ fragment detection. In addition, the observed H3++C2H+ channel provides for the first time evidence for an isomerization-induced formation mechanism of H3+ molecules.

Two-Photon Double Ionization of Lithium at FLASH

Two- and three-photon double ionization of atomic lithium by intense (I ≈ 1014 W/cm2) VUV light was investigated at the free electron laser in Hamburg (FLASH). Using a reaction microscope the recoil ion momentum distributions were recorded for two different photon energies of 50 and 59 eV. The former covers the case of non-resonant double ionization, whereas the latter gives rise to resonant excitation followed by double ionization. These reactions are of special interest as theoretical calculations for few-photon - few-electron reactions just become feasible for simple atomic targets.

Potential of the FLASH free electron laser technology for the construction of a kW-scale light source for next-generation lithography

The driving engine of the free electron laser in Hamburg (FLASH) is an L-band superconducting accelerator. It is designed to operate in burst mode with an 800-ms pulse duration at a repetition rate of 10 Hz. The maximum accelerated beam current during the macropulse is 9 mA. Our analysis shows that the FLASH technology has great potential since it is possible to construct a FLASH-like free electron laser operating at the wavelength of 13.5 and 6.8 nm with an average power of up to 2.6 kW. Such a source meets the physical requirements for the light source for next-generation lithography.

Dichroism in the above-threshold two-colour photoionization of singly charged neon

In this work we report on the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. Applying energy-resolved photoelectron spectroscopy, the dependence of the electrons ejected from singly charged neon (Ne+) on the relative polarization of the XUV and optical laser fields was investigated. The electron spectra exhibit a strong dependence on the degree of the relative orientation of the linear polarization vectors. A quantitative analysis was undertaken using multichannel time-dependent perturbation theory in order to reproduce the magnitude of the ejected low kinetic energy electrons as a function of the relative polarization directions and explicitly taking into account the influence of the residual Ne2 + cores (1D,3P). It is concluded that the variation of the photoelectron spectrum with the deviation of XUV and optical field directions is mainly related to the different responses of the magnetic substates of the ejected electrons in their interaction with the laser field.

Low-charge, hard x-ray free electron laser driven with anX-band injector and accelerator

After the successful operation of the Free Electron Laser in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS), soft and hard x-ray free electron lasers (FELs) are being built, designed, or proposed at many accelerator laboratories. Acceleration employing lower frequency rf cavities, ranging from $L$-band to $C$-band, is usually adopted in these designs. In the first stage bunch compression, higher-frequency harmonic rf system is employed to linearize the beam's longitudinal phase space, which is nonlinearly chirped during the lower frequency rf acceleration process. In this paper, a hard x-ray FEL design using an all $X$-band accelerator at 11.424 GHz (from photocathode rf gun to linac end) is presented, without the assistance of any harmonic rf linearization. It achieves LCLS-like performance at low charge using $X$-band linac drivers, which is more versatile, efficient, and compact than ones using $S$-band or $C$-band rf technology. It employs initially 42 microns long (rms), low-charge (10 pC) electron bunches from an $X$-band photoinjector. An overall bunch compression ratio of roughly 100 times is proposed in a two stage bunch compressor system. The start-to-end macroparticle 3D simulation employing several computer codes is presented in this paper, where space charge, wakefields, and incoherent and coherent synchrotron radiation effects are included. Employing an undulator with a short period of 1.5 cm, a Genesis FEL simulation shows successful lasing at a wavelength of 0.15 nm with a pulse length of 2 fs and a power saturation length as short as 20 meters, which is equivalent to LCLS low-charge mode. Its overall length of both accelerators and undulators is 180 meters (much shorter than the effective LCLS overall length of 1230 meters, including an accelerator length of 1100 meters and an undulator length of 130 meters), which makes it possible to be built in places where only limited space is available.

High-order harmonic generation enhanced by XUV light

The combination of high-order harmonic generation (HHG) with resonant XUV excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process, whereas the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d→4p resonance in the ion. We assume an 800 nm laser with an intensity of about 10(14) W/cm2 and XUV radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10(13)-10(16)W cm2. Our prediction opens perspectives for nonlinear XUV physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals.

Angle-resolved photoelectron spectroscopy of sequential three-photon triple ionization of neon at 90.5 eV photon energy

Multiple photoionization of neon atoms by a strong 13.7 nm (90.5 eV) laser pulse has been studied at the FLASH free electron laser in Hamburg. A velocity map imaging spectrometer was used to record angle-resolved photoelectron spectra on a single-shot basis. Analysis of the evolution of the spectra with the FEL pulse energy in combination with extensive theoretical calculations allows the ionization pathways that contribute to be assigned, revealing the occurrence of sequential three-photon triple ionization.