Abstract
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.
Highlights
The self-interaction of a bunch with its own electric field can lead to potential well distortion and to a deformation in the longitudinal phase space [1]
The microbunching instability (MBI) threshold, it results in the formation of substructures on the longitudinal bunch profile and increased emission of coherent synchrotron radiation (CSR) at wavelengths corresponding to the size of the substructures [2]
The outbursts of THz radiation have been observed at various facilities, i.e., ALS [6], ANKA [7], Bessy II [2,8,9], CLS [10], Diamond [11,12]
Summary
The self-interaction of a bunch with its own electric field can lead to potential well distortion and to a deformation in the longitudinal phase space [1]. Observing the spectrum of the radiation emitted by the circulating electron bunches in the storage ring with single-shot detectors and turn-by-turn precision gives insights to the evolution of these structures. This is a complementary method to a direct observation of the electric field via electro-optical methods, providing a simpler experimental setup with faster repetition times [23,24,25,26,27]. Workarounds like a Kramers-Kronig dispersion relation give impressive results for known bunch shapes like a single Gaussian or cosine half wave [33] They fail for spikes and, especially, many small substructures as they happen for bunch charges above the MBI threshold being investigated here. Good agreement between simulations and measurements is demonstrated at two chosen bunch currents exemplarily
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