Editorial

Beam halo measurements and collimations are of great importance at the European XFEL, especially for the operation at high repetition rates (27000 pulses/s). First beam halo measurements have been performed during the commissioning using the wire scanners installed before and after the ~200 m long post-linac collimation section. We present the measurement results and the comparison of beam halo distributions before and after the collimation section

Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL

X-ray pump/X-ray probe applications are made possible at X-ray Free Electron Laser (XFEL) facilities by generating two X-ray pulses with different wavelengths and controllable temporal delay. In order to enable this capability at the European XFEL, an upgrade project to equip the soft X-ray SASE3 beamline with a magnetic chicane is underway. In the present paper we describe the status of the project, its scientific focus and expected performance, including start-to-end simulations of the photon beam transport up to the sample, as well as recent experimental results demonstrating two-color lasing at photon energies of 805 eV + 835 eV and 910 eV + 950 eV. Additionally, we discuss methods to analyze the spectral properties and the intensity of the generated radiation to provide on-line diagnostics for future user experiments

Design and validation of a multi-task, multi-context protocol for real-world gait simulation

Background: Measuring mobility in daily life entails dealing with confounding factors arising from multiple sources, including pathological characteristics, patient specific walking strategies, environment/context, and purpose of the task. The primary aim of this study is to propose and validate a protocol for simulating real-world gait accounting for all these factors within a single set of observations, while ensuring minimisation of participant burden and safety. Methods: The protocol included eight motor tasks at varying speed, incline/steps, surface, path shape, cognitive demand, and included postures that may abruptly alter the participants’ strategy of walking. It was deployed in a convenience sample of 108 participants recruited from six cohorts that included older healthy adults (HA) and participants with potentially altered mobility due to Parkinson’s disease (PD), multiple sclerosis (MS), proximal femoral fracture (PFF), chronic obstructive pulmonary disease (COPD) or congestive heart failure (CHF). A novelty introduced in the protocol was the tiered approach to increase difficulty both within the same task (e.g., by allowing use of aids or armrests) and across tasks. Results: The protocol proved to be safe and feasible (all participants could complete it and no adverse events were recorded) and the addition of the more complex tasks allowed a much greater spread in walking speeds to be achieved compared to standard straight walking trials. Furthermore, it allowed a representation of a variety of daily life relevant mobility aspects and can therefore be used for the validation of monitoring devices used in real life. Conclusions: The protocol allowed for measuring gait in a variety of pathological conditions suggests that it can also be used to detect changes in gait due to, for example, the onset or progression of a disease, or due to therapy. Trial registration: ISRCTN—12246987

Machine Protection for FLASH and the European XFEL

The Free-Electron Laser in Hamburg (FLASH) and the future European X-Ray Free-Electron Laser (XFEL) are sources of brilliant extreme-ultraviolet and X-ray radiation pulses. Both facilities are based on superconducting linear accelerators (linacs) that can produce and transport electron beams of high average power. With up to 90 kW or up to 600 kW of power, respectively, these beams hold a serious potential to damage accelerator components. This thesis discusses several passive and active machine protection measures needed to ensure safe operation. At FLASH, dark current from the rf gun electron source has activated several accelerator components to unacceptable radiation levels. Its transport through the linac is investigated with detailed tracking simulations using a parallelized and enhanced version of the tracking code Astra; possible remedies are eval-uated. Beam losses can lead to the demagnetization of permanent magnet insertion devices. A number of beam loss scenarios typical for FLASH are investigated with shower simulations. A shielding setup is designed and its efficiency is evaluated. For the design parameters of FLASH, it is concluded that the average relative beam loss in the undulators must be controlled to a level of about $10^{−8}$. FLASH is equipped with an active machine protection system (MPS) comprising more than 80 hotomultiplier-based beam loss monitors and several subsystems. The maximum response time to beam losses is less than 4μs. Setup procedures and calibration algorithms for MPS subsystems and components are introduced and operational problems are addressed. Finally, an architecture for a fully programmable machine protection system for theXFEL is presented. Several options for the topology of this system are reviewed, with the result that an availability goal of at least 0.999 for the MPS is achievable with moderate hardware requirements

Machine Protection

Conventional linacs used for modern free-electron lasers carry electron beams of unprecedented brightness with average powers ranging from a few watts to hundreds of kilowatts. Energy recovery linacs are already operated as radiation sources with nominal electron beam powers beyond 1 MW, and this figure can only be expected to increase in the future. This lecture discusses the scope of machine protection for these accelerators, reviews the parameters of existing and planned facilities, and gives an overview of typical hazards and damage scenarios. A brief introduction to the interaction of electron beams with matter is given, including a simple model for estimating some properties of electromagnetic cascades. A special problem common to most light sources—the field loss of permanent magnet undulators and its consequences for the emission of radiation—is discussed in the final section

Beam Loss Simulations for the Implementation of the Hard X-ray Self-Seeding System at European XFEL

The European XFEL is designed to be operated with a nominal beam energy of 17.5 GeV at a maximum repetition rate of 27000 bunches/second. The high repetition rate together with the high loss sensitivity of the undulators raises serious radiation damage concern, especially for the implementation of the Hard X-ray Self-Seeding (HXRSS) system, where a 100 µm thick diamond crystal will be inserted close to the beam in the undulator section. Since the seeding power level highly depends on the delay of the electron beam with respect to the photon beam, it is crucial to define the minimum electron beam offset to the edge of the crystal in the HXRSS chicane. At European XFEL a ~200 m long post-linac collimation section has been designed to protect the undulators. In the HXRSS scheme, however, beam halo particles hitting the crystal can generate additional radiation. Particle tracking simulations have been performed using GEANT4 and BDSIM for the undulator and the collimation section, respectively. The critical number of electrons allowed to hit the crystal is estimated for a certain operation mode and the efficiency of beam halo collimation is investigated to predict the minimum HXRSS chicane delay

Magnet Server and Control System Database Infrastructure for the European XFEL

The linear accelerator of the European XFEL will use more than 1400 individually powered electromagnets for beam guidance and focusing. Front-end servers establish the low-level interface to several types of power supplies, and a middle layer server provides control over physical parameters like field or deflection angle in consideration of the hysteresis curve of the magnet. A relational database system with stringent consistency checks is used to store configuration data. The paper focuses on the functionality and architecture of the middle layer server and gives an overview of the database infrastructure

Radiological safety studies for the TeraFERMI beamline at FERMI@elettra

The TeraFERMI beamline, currently under construction at the FERMI seeded free electron laser (FEL) facility in Trieste, Italy, will provide terahertz (THz) pulses during normal FEL operation. The THz radiation will be produced in the beam dump section of FERMI Undulator Hall by the interaction of the electron beam with a thin metallic target. It will then be transported to the Experimental Hall inside a large diameter (about 22 cm) vacuum chamber, crossing two shielding walls. The radiological safety implications related to the beamline construction, which was not foreseen in the original accelerator layout, have been investigated using FLUKA Monte Carlo code. Radiation measurements have been carried out to validate simulation results