Pardis Niknejadi

Coherent synchrotron radiation (CSR) is known for its detrimental effects on the performance of free electron lasers (FEL). While it is commonly considered a source of beam quality degradation in linacs, in externally seeded FEL schemes, CSR is known to spoil the bandwidth of the FEL radiation. Investigating CSR’s effect on seeded FELs requires a realistic estimation of CSR-induced energy modulations, known as wakes. Several models exist for calculating CSR wakes, differing in their treatment of transient and shielding effects. This work applies various models to compute CSR wakes in a magnetic chicane. In particular, it highlights the importance of the chicane chamber shielding for relatively long electron bunches used in seeded FELs. We use the practical example of the echo-enabled harmonic generation (EEHG) seeding technique to establish that proper consideration of transient and shielding effects is essential for estimating the impact of CSR on seeded FEL performance. This corrected estimate is vital for reaching the near-Fourier limit bandwidth of EEHG by the currently available methods. Published by the American Physical Society 2025

As the climate crisis intensifies, understanding the environmental impact of professional activities is paramount, especially in sectors with historically significant resource utilisation. This includes High Energy Physics (HEP) and related fields, which investigate the fundamental laws of our universe. As members of the young High Energy Physicists (yHEP) association, we investigate the CO₂-equivalent emissions generated by HEP-related research on a personalised per-researcher level, for four distinct categories: Experiments, tied to collaborations with substantial infrastructure; Institutional, representing the resource consumption of research institutes and universities; Computing, focussing on simulations and data analysis; and Travel, covering professional trips to conferences, etc. The findings are integrated into a tool for self-evaluation, the Know-your-footprint (Kyf) calculator, allowing the assessment of the personal and professional footprint and optionally sharing the data with the yHEP association. The study aims to heighten awareness, foster sustainability, and inspire the community to adopt more environmentally responsible research practices urgently.

Given the urgency to reduce fossil fuel energy production to make climate tipping points less likely, we call for resource-aware knowledge gain in the research areas on Universe and Matter with emphasis on the digital transformation. A portfolio of measures is described in detail and then summarized according to the timescales required for their implementation. The measures will both contribute to sustainable research and accelerate scientific progress through increased awareness of resource usage.

The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and has started a virtuous collaboration between machine experts and photon users to improve further and optimize the generated X-ray pulses. Over the recent years, this has led to the development of several advanced free-electron laser (FEL) schemes to tailor the photon properties to specific experimental demands. Presently, tunable wavelength X-ray pulses with extremely high brilliance and short pulse characteristics are a few of the many options available at FELs. Few facilities can offer options such as narrowband or extremely short pulses below one fs duration and simultaneous pulses of multiple colors enabling resonant X-ray pump—X-ray probe experiments with sub fs resolution. Fully coherent X-ray radiation (both spatial and temporal) can also be provided. This new option has stimulated the application of coherent control techniques to the X-ray world, allowing for experiments with few attoseconds resolution. FELs often operate at a relatively low repetition rate, typically on the order of tens of Hz. At FLASH and the European XFEL, however, the superconducting accelerators allow generating thousands of pulses per second. With the implementation of a new seeded FEL line and with an upgrade at FLASH linac, all the new features will become available in the soft X-ray spectral range down to the oxygen K edge with unprecedented average photon flux due to the high repetition rate of pulses.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-20676-1

Plasma-wakefield accelerators driven by intense particle beams promise to significantly reduce the size of future high-energy facilities. Such applications require particle beams with a well-controlled energy spectrum, which necessitates detailed tailoring of the plasma wakefield. Precise measurements of the effective wakefield structure are therefore essential for optimising the acceleration process. Here we propose and demonstrate such a measurement technique that enables femtosecond-level (15 fs) sampling of longitudinal electric fields of order gigavolts-per-meter (0.8 GV m⁻¹). This method—based on energy collimation of the incoming bunch—made it possible to investigate the effect of beam and plasma parameters on the beam-loaded longitudinally integrated plasma wakefield, showing good agreement with particle-in-cell simulations. These results open the door to high-quality operation of future plasma accelerators through precise control of the acceleration process.

Density modulations in plasma caused by a high-intensity laser or a high charge density electron pulse can generate extreme acceleration fields. Acceleration of electrons in such fields may produce ultra-relativistic, quasi-monoenergetic, ultra-short electron bunches over distances orders of magnitudes shorter than in state-of-the-art radio-frequency accelerators. FLASHForward is a beam-driven plasma wakefield accelerator (PWFA) project at DESY with the goal of producing, characterizing, and utilizing such beams. Temporal characterization of the acceleration process is of crucial importance for improving the stability and control in PWFA beams. While measurement of the transient field of the femtosecond bunch in a single shot is challenging, in recent years novel techniques with great promise have been developed [1, 2]. This work discusses the plans and status of the transverse diagnostics at FLASHForward.