In a storage ring with an extremely small global phase slippage, the bunch length can vary significantly within one turn due to the partial phase slippage and transverse-longitudinal coupling, which means the adiabatic approximation usually adopted for longitudinal dynamics breaks down. The impact of such a bunch length variation and exchange of the head and tail part of the beam arising from the partial phase slippage on the coherent synchrotron radiation (CSR) induced longitudinal microwave instability (MWI) threshold has recently been theoretically investigated by some of the authors*. In this work, we have extended the study to consider the influence of transverse-longitudinal coupling.

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 the interaction of electrons with femtosecond laser pulses in two undulators (”modulators”), each followed by a magnetic chicane, a longitudinal phase space structure with high harmonic content is produced, which gives rise to coherent emission of radiation at harmonics of the laser wavelength. 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. After reviewing proposals of EEHG at different storage rings, the latest results of a first demonstration experiment at the 1.5-GeV synchrotron light source DELTA at TU Dortmund University are presented.

A low-intensity double-slit experiment in the time domain has been undertaken by measuring the spectral distribution of synchrotron light from a single relativistic electron in a storage ring. In two consecutive undulators with a dispersive section between them (known as optical klystron), an electron beam emits two temporally separated light pulses leading to a spectrum with interference fringes - in close analogy to the angular distribution of light behind two spatially separated slits. Experiments at the electron storage rings DELTA in Dortmund, Germany, and UVSOR-III in Okazaki, Japan, show directly that the spectral distribution of accumulated synchrotron light from a single electron is essentially the same as the spectrum from a beam of many electrons. While the latter is usually explained by interference between simultaneous the light waves from the two undulators, the single-electron experiments demonstrate the uncertainty of the photon source point over several meters.

Steady-state microbunching (SSMB) has been proposed as a scheme to generate high average power coherent synchrotron radiation at short wavelengths from an electron storage ring. To evaluate the feasibility of this scheme, a proof-of-principle experiment is conducted at the Metrology Light Source (MLS) in Berlin, the first phase of which (PoP I) was concluded in 2024. PoP I utilized a single-shot laser system to provide an energy modulation to the electron beam and investigated the fundamental requirements on storage ring dynamics to enable SSMB. Recently, the second phase of the SSMB proof-of-principle experiment (PoP II) has commenced at the MLS, in which a high repetition rate phase-locked laser system provides turn-by-turn energy modulation of the electron beam on consecutive revolutions around the storage ring. The main goal of SSMB PoP II is to show electrons can be confined to individual microbuckets defined by this laser interaction, reaching a quasi-steady state. This paper presents the first results obtained in SSMB PoP II, where coherent synchrotron radiation has been detected following multi-turn laser modulation, and the ongoing systematic studies of the underlying microbunching process.

In a storage ring with an extremely small global phase slippage, the bunch length can vary significantly within one turn due to the partial phase slippage and transverse-longitudinal coupling, which means the adiabatic approximation usually adopted for longitudinal dynamics breaks down. The impact of such a bunch length variation and exchange of the head and tail part of the beam arising from the partial phase slippage on the coherent synchrotron radiation (CSR) induced longitudinal microwave instability (MWI) threshold has recently been theoretically investigated by some of the authors*. In this work, we have extended the study to consider the influence of transverse-longitudinal coupling.