Investigation of coherent emission from the NSLS VUV ring

Bursts of coherent radiation are observed from the NSLS VUV ring near a wavelength of 7 mm. The bursts occur when the electron beam current (I) exceeds a threshold value (I/sub th/), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-I/sub th/)/sup 2/. With other parameters held nearly constant, the threshold current value is found to increase quadratically with synchrotron frequency, indicating a linear dependence on momentum compaction. It is believed that the coherent emission is a consequence of micro-bunching of the electron beam due to the microwave instability.