Abstract
Strong evidence for self-excited emission of coherent synchrotron radiation in the microwave spectral region was observed at the Synchrotron Ultraviolet Radiation Facility (SURF III) electron storage ring at the NIST. The microwave emission between 25 and 35 mm was dominated by intense bursts of radiation. The intensity enhancement during these bursts was on the order of 10 000 to 50 000 over the incoherent value. The shape, width, and period of the bursts depend strongly on the operational parameters of the storage ring. Coherent microwave emission was observed only when the beam was unstable, namely, during bunch-length relaxation oscillations. We report on the measurements of the microwave bursts, and correlate the data with signals from a beam monitor electrode and photodiode detector. The coherent enhancement of the radiation intensity is ascribed to spontaneous self-induced microbunching of the electrons within the bunch.
Highlights
Self-excited coherent microwave synchrotron radiation was first observed in an electron storage ring at the Synchrotron Ultraviolet Radiation Facility SURF II [1,2]
We report on detailed measurements of the microwave intensity carried out while simultaneously collecting signals from a capacitive beam monitor electrode (BME) and from a photodiode system after SURF was upgraded to version III [6]
The coherent enhancement of microwave intensity at SURF III appears to be attributed to wakefield-induced, 054401-8
Summary
Self-excited coherent microwave synchrotron radiation was first observed in an electron storage ring at the Synchrotron Ultraviolet Radiation Facility SURF II [1,2]. Given the observed microwave emission between 25 and 35 mm, CSR is not ruled out if there is some form of microbunching, e.g., if there is a density modulation in this case of the order of 1͞10 the bunch length. In this case, the inequality (4) can be satisfied by replacing lb by lm, the length of the microbunch. Relaxation oscillations of the bunch length have long been observed in the SURF ring, first reported by Rakowsky [4,5] The period of these oscillations is observed to coincide with the period of the microwave bursts [1,2] both in broad and fine temporal detail, strongly suggesting a correlation. We discuss a model that reproduces most of the features of the experimental results and offers an explanation of the origin of the microwave bursts
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