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The reaction microscope (REMI) endstation for atomic and molecular science at the free-electron laser FLASH2 at DESY in Hamburg is presented together with a brief overview of results recently obtained. The REMI allows coincident detection of electrons and ions that emerge from atomic or molecular fragmentation reactions in the focus of the extreme-ultraviolet (XUV) free-electron laser (FEL) beam. A large variety of target species ranging from atoms and molecules to small clusters can be injected with a supersonic gas-jet into the FEL focus. Their ionization and fragmentation dynamics can be studied either under single pulse conditions, or for double pulses as a function of their time delay by means of FEL-pump–FEL-probe schemes and also in combination with a femtosecond infrared (IR) laser. In a recent upgrade, the endstation was further extended by a light source based on high harmonic generation (HHG), which is now available for upcoming FEL/HHG pump–probe experiments. [ABSTRACT FROM AUTHOR]

Abstract: Pulse intensities greater than 1017 Watt/cm2 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 1016 W/cm2, 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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]