Evidence for millimeter-wave coherent emission from the NSLS VUV ring

Coherent synchrotron radiation from the NSLS VUV ring has been detected and partially characterized. The observations have been performed at the new far infrared beamline U12IR. The coherent radiation is peaked near a wavelength of 7 mm and occurs in short duration bursts. The bursts occur only when the electron beam current (I) exceeds a threshold value (Ith), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-Ith)2. The coherent emission implies micro-bunching of the electron beam due to a longitudinal instability.