Overview of the Micro-bunching Instability in Electron Storage Rings and Evolving Diagnostics

The micro-bunching instability is a longitudinal instability that leads to dynamical deformations of the charge distribution in the longitudinal phase space. It affects the longitudinal charge distribution, and thus the emitted coherent synchrotron radiation spectra, as well as the energy distribution of the electron bunch. Not only the threshold in the bunch current above which the instability occurs, but also the dynamics above the instability threshold strongly depends on machine parameters, e.g. accelerating voltage, momentum compaction factor, and beam energy. All this makes the understanding and potential mitigation or control of the micro-bunching instability an important topic for the next generation of light sources and circular e+e− colliders. This contribution will give an overview of the micro-bunching instability and discuss how technological advances in the turn-by-turn and bunch-by-bunch diagnostics are leading to a deeper understanding of this intriguing phenomenon

Influence of Different Beam Energies on the Micro-Bunching Instability

During the operation of an electron synchrotron with short electron bunches, the beam dynamics are influenced by the occurrence of the micro-bunching instability. This collective instability is caused by the self-interaction of a short electron bunch with its own emitted coherent synchrotron radiation (CSR). Above a certain threshold bunch current dynamic micro-structures start to occur on the longitudinal phase space density. The resulting dynamics depend on various parameters and were previously investigated in relation to, amongst others, the momentum compaction factor and the acceleration voltage. In this contribution, the influence of the energy of the electrons on the dynamics of the micro-bunching instability is studied based on measurements at the KIT storage ring KARA (Karlsruhe Research Accelerator)

Impedance studies of a corrugated pipe for KARA

It is planned to install an impedance manipulation struc-ture in a versatile chamber at the KIT storage ring KARA(KArlsruhe Research Accelerator) to study and eventuallycontrol the influence of an additional impedance on the beamdynamics and the emitted coherent synchrotron radiation.For this purpose the impedance of a corrugated pipe is underinvestigation. In this contribution we present first results ofsimulations showing the impact of different structure param-eters on its impedance and wake potential

Continuous bunch-by-bunch spectroscopic investigation of the micro-bunching instability

Electron accelerators and synchrotrons can be operated to provide short emission pulses due to longitudinally compressed or sub-structured electron bunches. Above a threshold current, the high charge density leads to the micro-bunching instability and the formation of sub-structures on the bunch shape. These time-varying sub-structures on bunches of picoseconds-long duration lead to bursts of coherent synchrotron radiation in the terahertz frequency range. Therefore, the spectral information in this range contains valuable information about the bunch length, shape and sub-structures. Based on the KAPTURE readout system, a 4-channel single-shot THz spectrometer capable of recording 500 million spectra per second and streaming readout is presented. First measurements of time-resolved spectra are compared to simulation results of the Inovesa Vlasov-Fokker-Planck solver. The presented results lead to a better understanding of the bursting dynamics especially above the micro-bunching instability threshold.Comment: 12 pages, 11 figure

On the Perturbation of Synchrotron Motion in the Micro-Bunching Instability

The self-interaction of short electron bunches with their own radiation field can have a significant impact on the longitudinal beam dynamics in a storage ring. While higher bunch currents increase the power of the emitted CSR which can be provided to dedicated experiments, it simultaneously amplifies the strength of the self-interaction. Eventually, this leads to the formation of dynamically changing micro-structures within the bunch and thus fluctuating CSR emission, a phenomenon that is generally known as micro-bunching or micro-wave instability. The underlying longitudinal dynamics can be simulated by solving the VFP equation, where the CSR self-interaction can be added as a perturbation to the Hamiltonian. In this contribution, we focus on the perturbation of the synchrotron motion that is caused by introducing this additional wake field. Therefore, we adopt the perspective of a single particle and eventually comment on its implications for collective motion. We explicitly show how the shape of the parallel plates CSR wake potential breaks homogeneity in the longitudinal phase space and propose a quadrupole-like mode as potential seeding mechanism of the micro-bunching instability. Moreover, we consider synchrotron motion above the instability threshold and thereby motivate an approach to control of the occurring micro-bunching dynamics. Using dynamically adjusted RF amplitude modulations we can directly address the continuous CSR-induced perturbation at the timescale of its occurrence, which allows for substantial control over the longitudinal charge distribution. While the approach is not limited to this particular application, we demonstrate how this can significantly mitigate the micro-bunching dynamics directly above the instability threshold. The gained insights are supported and verified using the VFP solver Inovesa and put into context with measurements at KARA

Status of Operation With Negative Momentum Compaction at KARA

For fu­ture syn­chro­tron light source de­vel­op­ment novel op­er­a­tion modes are under in­ves­ti­ga­tion. At the Karl­sruhe Re­search Ac­cel­er­a­tor (KARA) an op­tics with neg­a­tive mo­men­tum com­paction has been pro­posed, which is cur­rently under com­mis­sion­ing. In this con­text, the col­lec­tive ef­fects ex­pected in this regime are stud­ied with an ini­tial focus on the head-tail in­sta­bil­ity and the mi­cro-bunch­ing in­sta­bil­ity re­sult­ing from CSR self-in­ter­ac­tion. In this con­tri­bu­tion, we will pre­sent the pro­posed op­tics and the sta­tus of im­ple­men­ta­tion for op­er­a­tion in the neg­a­tive mo­men­tum com­paction regime as well as a pre­lim­i­nary dis­cus­sion of ex­pected col­lec­tive ef­fects

Increasing the Single-Bunch Instability Threshold by Bunch Splitting Due to RF Phase Modulation

RF phase modulation at twice the synchrotron frequency can be used to split a stored electron bunch into two or more bunchlets orbiting each other. We report on time-resolved measurements at the KArlsruhe Research Accelerator (KARA), where this bunch splitting was used to increase the threshold current of the microbunching instability, happening in the short-bunch operation mode. Switching the modulation on and off, reproducibly influences the sawtooth behaviour of the emitted coherent synchrotron radiation