Longitudinal density modulation of unstable bunches emitting coherent IR

Observations of coherent infrared (IR) light emissions from /spl sim/30 cm long electron bunches at the NSLS VUV ring have been reported. The emissions occurred above some current threshold and had a maximum spectral content near 42 GHz, leading to speculations that they were due to a longitudinal instability creating a /spl sim/7 mm modulation of the bunch density. To check this experimentally we performed streak camera measurements recording a large number of beam profiles taken both during and between the coherent emission bursts. In the course of our studies we have directly proven that the longitudinal beam density indeed experiences a significant modulation when coherent emission bursts occur.