Echo-enabled harmonic generation (EEHG) has been proposed as a seeding method for free-electron lasers but can also be employed to generate ultrashort radiation pulses at electron storage rings. With a twofold laser-electron interaction in two undulators ("modulators"), each followed by a magnetic chicane, an electron phase space structure with high harmonic content is produced, which gives rise to coherent emission of radiation at short wavelengths. The duration of the coherently emitted pulses in a third undulator ("radiator") is given by the laser pulse durations. Thus, EEHG pulses can be three orders of magnitude shorter but still more intense than conventional synchrotron light pulses. The worldwide first storage ring implementation of EEHG was undertaken at the 1.5-GeV synchrotron light source DELTA at TU Dortmund University by reconfiguring an electromagnetic undulator. With a total length of only 4.75 m, the setup is very compact and fits in a single straight section. The paper presents technical aspects of the EEHG implementation as well as first results.

Generating few- or sub-femtosecond radiation pulses in a free-electron laser (FEL) requires precise control of the longitudinal phase space density of the driving electron bunch, as the FEL process depends strongly on the bunch current and energy spread profile. In an experiment conducted at FLASH in Hamburg, Germany, an energy modulation with linearly changing amplitude is imprinted onto part of the bunch by a laser pulse in an undulator upstream of the first bunch compression chicane. In subsequent longitudinally dispersive sections, a short current spike is created, as the linearly modulated region is compressed more strongly than the rest of the bunch. Measurements with a transverse-deflecting X-band cavity verify the creation of a short current spike, whose duration falls below the temporal resolution of the measurement setup of approximately 7 fs.

DELTA, a 1.5-GeV electron storage ring facility operated by TU Dortmund University in Germany, celebrated its 30th anniversary in fall 2024. During its time in operation, the facility has been continuously developed to provide synchrotron radiation (SR) users with the most reliable and attractive radiation source possible. This includes continuous improvements of electron beam stability and lifetime, the installation of a new 7-T superconducting wiggler magnet with a specially adapted SR outlet chamber, as well as the integration of a second solid-state amplifier-driven radiofrequency system. In recent years, there have also been many exciting developments in the field of accelerator physics. These include the construction of a facility for generating ultrashort and coherent SR pulses, studies involving laser-induced terahertz radiation, and experiments conducted in single-electron mode that complemented ongoing research activities. Furthermore, projects focusing on intelligent system control using machine learning methods were successfully implemented. This report summarizes the most significant developments over the past years.