Free-electron Laser Experiments Research Articles

Novel active signal compression in low-noise analog readout at future X-ray FEL facilities

This work presents the design of a low-noise front-end implementing a novel active signal compression technique. This feature can be exploited in the design of analog readout channels for application to the next generation free electron laser (FEL) experiments. The readout architecture includes the low-noise charge sensitive amplifier (CSA) with dynamic signal compression, a time variant shaper used to process the signal at the preamplifier output and a 10-bit successive approximation register (SAR) analog-to-digital converter (ADC). The channel will be operated in such a way to cope with the high frame rate (exceeding 1 MHz) foreseen for future XFEL machines. The choice of a 65 nm CMOS technology has been made in order to include all the building blocks in the target pixel pitch of 100 μm. This work has been carried out in the frame of the PixFEL Project funded by the Istituto Nazionale di Fisica Nucleare (INFN), Italy.

Adaptation of SPSIM for Simulation of Diffraction Images in XFEL Experiments

Abstract The new free-electron lasers (FELs) open up new possibilities for the study of biological particles such as proteins and viruses. Development of new data analysis algorithms requires large datasets of diffraction images produced by simulation. Existing programs are not capable of effective use of modern computational resources, and especially not adapted to the hybrid architecture with GPU. We explored the possibility of performance increase in the SPSIM program for simulation of large datasets of diffraction images, using CUDA technology. Also, we have implemented new capabilities in simulation of FEL experiments special features.

Andrew Marienhoff Sessler

Andrew Marienhoff Sessler

Characterization of spatially resolved high resolution x-ray spectrometers for high energy density physics and light source experiments.

A high resolution 1D imaging x-ray spectrometer concept comprising a spherically bent crystal and a 2D pixelated detector is being optimized for diagnostics of small sources such as high energy density physics (HEDP) and synchrotron radiation or x-ray free electron laser experiments. This instrument is used on tokamak experiments for Doppler measurements of ion temperature and plasma flow velocity profiles. Laboratory measurements demonstrate a resolving power, E/ΔE of order 10,000 and spatial resolution better than 10 μm. Initial tests of the high resolution instrument on HEDP plasmas are being performed.

Design and optimization of an electromagnet undulator

In this paper we discuss the design and optimization of an electromagnet undulator. A short period electromagnet undulator has been reported and used for free electron laser experiment under the approximation of infinite permeability of the lamination core. Here we remove this approximation and present analytical expressions for an alternate design concept on the electromagnet undulator.

Development of an X-ray pixel detector with multi-port charge-coupled device for X-ray free-electron laser experiments

This paper presents development of an X-ray pixel detector with a multi-port charge-coupled device (MPCCD) for X-ray Free-Electron laser experiments. The fabrication process of the CCD was selected based on the X-ray radiation hardness against the estimated annual dose of 1.6 × 10(14) photon/mm(2). The sensor device was optimized by maximizing the full well capacity as high as 5 Me- within 50 μm square pixels while keeping the single photon detection capability for X-ray photons higher than 6 keV and a readout speed of 60 frames/s. The system development also included a detector system for the MPCCD sensor. This paper summarizes the performance, calibration methods, and operation status.

Dynamics of Hollow Atom Formation in Intense X-Ray Pulses Probed by Partial Covariance Mapping

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping" to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

The first data from the Orion laser; measurements of the spectrum of hot, dense aluminium

The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10 g/cc by using aluminium samples buried in plastic foils or diamond sheets. The aluminium was heated to electron temperatures between 500 eV and 700 eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the effect of the ionization potential depression as a function of density via the delocalization of n = 3 levels and disappearance of n = 3 transitions in He-like and H-like aluminium. The data are compared to simulated spectra, which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription; a simple ion sphere model and an alternative due to Ecker and Kröll suggested by recent X-ray free-electron laser experiments. The experimental data are in reasonable agreement with the model of Stewart and Pyatt, but are in better agreement with a simple ion sphere model. The data indicate that the Ecker and Kröll model overestimates substantially the ionization potential depression in this regime.

Observations of the Effect of Ionization-Potential Depression in Hot Dense Plasma

The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10 g/cc by using aluminum samples buried in plastic foils or diamond sheets. The aluminum was heated to electron temperatures between 500 and 700eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the effect of the ionization potential depression as a function of density. The data are compared to simulated spectra which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription and an alternative due to Ecker and Kröll suggested by recent x-ray free-electron laser experiments. The experimental data are in closer agreement with simulations using the model of Stewart and Pyatt.

Evaluation of data-acquisition front ends for handling high-bandwidth data from X-ray 2D detectors: A feasibility study

Two-dimensional (2D) X-ray detectors are indispensable for synchrotron radiation and X-ray free-electron laser experiments, such as coherent X-ray imaging, spectroscopy, and time-resolved experiments. In these experiments, special, temporal, or photon-energy information is projected onto the surface of a 2D X-ray detector, and it is generally accepted that detectors with a larger number of pixels and a higher dynamic range will provide better information on the sample. An example of a high-performance application in this area is SOPHIAS (Silicon-On-Insulator Photon Imaging Array Sensor), which is a next generation detector under development at the SPring-8 facility, Japan. Since such systems demand a high-bandwidth front end for data acquisition (DAQ), a prototype front end for SOPHIAS is also under development. Here, we have performed a feasibility study of the prototype front end using an evaluation board, which consists of an FPGA (field-programmable gate array) with an FMC (FPGA mezzanine card) interface to support various physical layers of sensor readout modules and back-end DAQ. The bandwidths were measured for various combinations of protocols and physical layers. In many photon science applications, scalability from a single module to many modules is important, so a compact desktop-type DAQ system was also evaluated. Measurements of the bandwidth using the evaluation board indicated that an effective bandwidth of 9 Gbps and 16 Gbps was achieved using SFP+ (Small Form-factor Pluggable Plus) with XAUI (X (ten) Attached Unit Interface) and PCI Express (Peripheral Component Interconnect Express), respectively.

The Vacuum System for SDUV FEL Experiment Facility

Abstract Among different approaches towards next generation light source, Free Electron Laser (FEL) is a promising candidate for its excellent light performance and engineering feasibility. SDUV FEL (Shanghai Deep Ultraviolet Free Electron Laser) is a test facility in the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. First FEL commissioning was conducted in 2009, and preliminary experiment was conducted. In this paper, the vacuum system of facility is described, including overview layout, subsystem and component design. Some installation and running experience is also presented.

TOF-OFF: A method for determining focal positions in tightly focused free-electron laser experiments by measurement of ejected ions

Pulse intensities greater than 10 17 Watt/cm 2 were reached at the FLASH soft X-ray laser in Hamburg, Germany, using an off-axis parabolic mirror to focus 15 fs pulses of 5–70 μJ energy at 13.5 nm wavelength to a micron-sized spot. We describe the interaction of such pulses with niobium and vanadium targets and their deuterides. The beam produced craters in the solid targets, and we measured the kinetic energy of ions ejected from these craters. Ions with several keV kinetic energy were observed from craters approaching 5 μm in depth when the sample was at best focus. We also observed the onset of saturation in both ion acceleration and ablation with pulse intensities exceeding 10 16 W/cm 2, when the highest detected ion energies and the crater depths tend to saturate with increasing intensity. A general difficulty in working with micron and sub-micron focusing optics is finding the exact focus of the beam inside a vacuum chamber. Here we propose a direct method to measure the focal position to a resolution better than the Rayleigh length. The method is based on the correlation between the energies of ejected ions and the physical dimensions of the craters. We find that the focus position can be quickly determined from the ion time-of-flight (TOF) data as the target is scanned through the expected focal region. The method does not require external access to the sample or venting the vacuum chamber. Profile fitting employed to analyze the TOF data can extend resolution beyond the actual scanning step size.

Full Field X-Ray Imaging - Microanalysis between Table Top and Free Electron Laser Experiments

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

X-ray holographic microscopy with zone plates applied to biological samples in the water window using 3rd harmonic radiation from the free-electron laser FLASH

The imaging of hydrated biological samples - especially in the energy window of 284-540 eV, where water does not obscure the signal of soft organic matter and biologically relevant elements - is of tremendous interest for life sciences. Free-electron lasers can provide highly intense and coherent pulses, which allow single pulse imaging to overcome resolution limits set by radiation damage. One current challenge is to match both the desired energy and the intensity of the light source. We present the first images of dehydrated biological material acquired with 3rd harmonic radiation from FLASH by digital in-line zone plate holography as one step towards the vision of imaging hydrated biological material with photons in the water window. We also demonstrate the first application of ultrathin molecular sheets as suitable substrates for future free-electron laser experiments with biological samples in the form of a rat fibroblast cell and marine biofouling bacteria Cobetia marina.

A method for estimating the temperature in high energy density free electron laser experiments

Present and forthcoming free electron laser (FEL) large scale facilities deliver high fluence ultrafast soft and hard X-ray pulses able to create and probe warm dense matter (WDM). Proper diagnostic for basic physical quantities, like temperature and density, is necessary, but the short lifetime of the WDM state (few ps) makes their measurements a challenging task. In this work we propose a method to estimate the WDM temperature using the experimental information from a slow temperature pyrometric probe exploiting the properties of the heat diffusion equation. Numerical simulations show that for typical thin foil samples, a temperature measurement with 1–10μs temporal resolution at the distance of about 300–500μm from the beam center contains sufficient information to retrieve the initial spatial temperature distribution with sufficient accuracy providing information on the temperature reached in the WDM regime. The inversion of the experimental information is obtained by means of a Bayesian approach exploiting a Metropolis Monte Carlo numerical procedure. The model and calculations presented in this work provide the theoretical background for the development of a device for temperature diagnostics of the TIMEX end-station at the Fermi@Elettra FEL facility.

Emittance control and RF bunch compression in the NSRRC photoinjector

The high-brightness photoinjector being constructed at the National Synchrotron Radiation Research Center is for testing new accelerator and light-source concepts. It is the so-called split photoinjector configuration in which a short solenoid magnet is used for emittance compensation. The UV-drive laser pulses are also shaped to produce uniform cylindrical bunches for further reduction of beam emittance. However, limited by the available power from our microwave power system, the nominal accelerating gradient in the S-band booster linac is set at 18 MV/m. A simulation study with PARMELA shows that the linac operating at this gradient fails to freeze the electron beam emittance at low value. A background solenoid magnetic field is applied for beam emittance control in the linac during acceleration. A satisfactory result that meets our preliminary goal has been achieved with the solenoid magnetic field strength at 0.1 T. RF bunch compression as a means to achieve the required beam brightness for high-gain free-electron laser experiments is also examined. The reduction of bunch length to a few hundred femtoseconds can be obtained.

Laser comb with velocity bunching: Preliminary results at SPARC

A new technique, named “laser comb”, was tested during the last SPARC run. It is able to produce electron pulse trains with a charge of some hundreds pC, a repetition rate of some terahertz, and a sub-picosecond length. This technique is based on the velocity bunching configuration of the SPARC injector. It can be useful to drive pump and probe free-electron laser experiments, to generate coherent excitation of plasma waves in plasma accelerators, and to produce narrow band terahertz radiation. In this paper, we describe the experimental results achieved so far and provide a comparison with simulations.

CAMP – A new endstation for simultaneous detection of photons and charged particles in free electron lasers experiments

We have designed a multi-purpose experimental chamber especially adapted to accommodate unique large-area, single-photon counting pnCCD detectors, developed by the Max Planck Institute Semiconductor Laboratory, together with advanced many-particle ion and electron imaging spectrometers (reaction microscope, REMI; velocity map imaging, VMI) for simultaneous detection of scattered and fluorescent photons and charged particles in experiments at Free Electron Lasers.

Two electron response to an intense x-ray free electron laser pulse

New x-ray free electron lasers (FELs) promise an ultra-fast ultra-intense regime in which new physical phenomena, such as double core hole formation in at atom, should become directly observable. Ahead of x-ray FEL experiments, an initial key task is to theoretically explore such fundamental laser-atom interactions and processes. To study the response of a two-electron positive ion to an intense x-ray FEL pulse, our theoretical approach is a direct numerical integration, incorporating non-dipole Hamiltonian terms, of the full six-dimensional time-dependent Schroedinger equation. We present probabilities of double K-shell ionization in the two-electron positive ions Ne8+ and Ar16+ exposed to x-ray FEL pulses with frequencies in the range 50 au to 300 au and intensities in the range 1017 to 1022 W/cm2.

Photodissociation of the HeH+ molecular ion

The photodissociation cross section of the molecular ion HeH+ was calculated within the Born-Oppenheimer approximation for a parallel, a perpendicular, and an isotropic orientation of the molecular axis with respect to the field, considering also different initial vibrational and rotational states. The results were compared to recent data from a free-electron laser experiment performed at the FLASH facility. Within the experimental uncertainties theoretical and experimental results are compatible with each other.