A circularly polarized beam deflector for direct measurement of ultra short electron bunches

Described herein is an accurate means of directly measuring the longitudinal phase space of ultra short electron bunches typically produced by advanced design short wavelength accelerators and photoinjectors. A circularly polarized RF beam deflector provides a highly amplified image display having electron bunch phase and energy distributions projected in orthogonally separated azimuthal and radial directions, respectively. Combining such a microwave deflector with an emittance limited focused beam offers a simple on-line diagnostic that avoids the complexity, sensitivity and a priori assumptions associated with electro-optic frequency domain methods, and also avoids RF kicker bunch length measurement ambiguities caused by longitudinal mixing of the particle energy and phase distributions. Accurate self calibration, and the avoidance of pulse jitter by directly coupling the deflector power from the linac RF source, are additional attractive features of this technique. Design parameters and phase orbit characteristics are presented for an on-line diagnostic system having a 1/4% momentum resolution, a 56 femtosecond time resolution and a 16 femtosecond sampling gate, for evaluating the short bunch longitudinal phase space performance of the 17 GHz linac at the MIT Plasma Science and Fusion Center