Strong evidence of self-excited emission of coherent synchrotron radiation in the microwave spectral region was observed at the Synchrotron Ultraviolet Radiation Facility (SURF III), the electron storage ring at the National Institute of Standards and Technology (NIST). The microwave emission - between 25 mm and 35 mm wavelength (about 1/10 the rms bunch length) - was dominated by intense bursts of radiation. The bursts occur only when the beam is unstable; namely, during bunch length relaxation oscillations. Furthermore, the shape, width, and period of these bursts depend strongly on the operational parameters of the storage ring. In this study, simulations were performed to address the main features of the longitudinal instability and to explain the detailed dynamics in the phase space during the bunch blowup phase. We show that, driven by the rf cavity higher-order modes (HOMs), the beam shows development of high-frequency features in the nonlinear time evolution of the bunch mode oscillations. These features suggest the possibility of spontaneous, self-induced microbunching of the electrons in the bunch, leading to the observed coherent enhancement of the emitted radiation.
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