FLASH

FLASH. The Free-electron LASer in Hamburg is a pioneering user facility that generates extremely intense, ultrashort pulses of ultraviolet and soft X-ray light. Operated by the Deutsches Elektronen-Synchrotron research center, it was the world's first free-electron laser to produce laser-like radiation in this spectral range, enabling groundbreaking experiments in physics, chemistry, and structural biology. Its success directly paved the way for subsequent hard X-ray facilities like the European XFEL and the Linac Coherent Light Source.

Overview

FLASH represents a revolutionary type of light source known as a free-electron laser, where high-energy electrons accelerated in a linear accelerator are passed through a series of powerful undulator magnets to produce coherent, laser-like flashes of light. Unlike conventional synchrotron radiation sources, it provides pulses of unprecedented peak brightness and ultrashort duration, down to the femtosecond timescale. This unique capability allows scientists to conduct "pump-probe" experiments, capturing molecular movies of chemical reactions and phase transitions. The facility has been a critical testbed for accelerator and FEL technologies, influencing major projects worldwide including the SwissFEL and LCLS at SLAC National Accelerator Laboratory.

Technical specifications

The heart of FLASH is a superconducting linear accelerator based on TESLA technology, which can accelerate electron bunches to energies up to 1.25 gigaelectronvolts. These electron bunches are then compressed to high peak currents before entering the undulator hall, which houses multiple magnetic undulator sections. The facility can tune its output wavelength across a broad range from the extreme ultraviolet at 4.2 nanometers down to the soft X-ray region at 0.4 nanometers. Each light pulse can deliver over 10¹² photons with durations as short as 10-50 femtoseconds, enabling studies of processes like Auger electron emission and valence electron dynamics. Key subsystems include a sophisticated laser heater for controlling instabilities and a state-of-the-art photon diagnostics suite.

Scientific applications

Research at FLASH has led to seminal discoveries across multiple disciplines, particularly in probing the structure and dynamics of matter at atomic scales. In atomic physics, experiments have precisely clocked the decay of highly excited inner-shell electron states in atoms like neon and krypton. In molecular biology, the intense pulses have been used for single-shot imaging of non-crystalline samples like viruses and cellular structures, a technique further advanced at the European XFEL. Chemistry studies have successfully filmed the breaking of molecular bonds in processes such as the photodissociation of iodine. Furthermore, investigations into warm dense matter, a state relevant to planetary science, have been conducted by rapidly heating materials like aluminum with the FEL pulse.

Development and history

The development of FLASH originated from the TESLA Test Facility at DESY, which was established to develop technology for a proposed TeV-energy superconducting linear collider. A major milestone was achieved in 2000 when the TTF produced its first spontaneous undulator radiation in the vacuum ultraviolet. The first lasing at a wavelength of 109 nm in 2001, followed by rapid progression to shorter wavelengths, marked the birth of the facility, which was renamed FLASH in 2005. Its success demonstrated the feasibility of the SASE principle for hard X-rays, providing the blueprint for the international European XFEL project. Continuous upgrades, such as the FLASH2020+ project, have expanded its capabilities, adding a second undulator beamline for increased user capacity.

Operational facilities

FLASH operates as an international user facility, with beam time allocated through a competitive proposal review process managed by an independent scientific advisory committee. The experimental hall houses several permanent end-stations, such as the PG2 beamline for gas-phase experiments and the BL3 beamline for solid-state and imaging studies, each equipped with specialized instrumentation like reaction microscopes and velocity map imaging spectrometers. The facility is integrated within the broader campus of DESY in the Bahrenfeld district of Hamburg, sharing infrastructure and expertise with other major instruments like PETRA III. User support is provided by teams from DESY, the University of Hamburg, and numerous international collaborating institutions. Category:Free-electron lasers Category:Research facilities in Germany Category:DESY