Longitudinal electron bunch measurements at the Stanford superconducting accelerator

Measurement of the longitudinal density profile or bunch shape of picosecond and sub-picosecond relativistic particle bunches is an active area of research at many particle accelerator facilities. This thesis evaluates and improves upon two well-known methods of picosecond electron bunch shape measurement: spectral analysis of coherent radiation from the beam, and off-phase acceleration followed by bunch shape analysis in an energy spectrometer. The spectra of coherent transition radiation (CTR) and coherent synchrotron radiation from a 20 MeV picosecond bunched beam at the Stanford Superconducting Accelerator (SCA) are analyzed with bolometric detectors and a Michelson interferometer to infer the bunch length and approximate shape with sub-picosecond resolution. The effects of imperfections in the optical measurement system are discussed and evaluated. These results are compared with measurements of the same beam using a new variation of the off-phase acceleration technique. In this improved technique, the addition of longitudinal beam dispersion from a magnetic chicane located before the off-phase accelerator and energy spectrometer allows measurement of beams with significant energy spread at resolutions approaching 100 femtoseconds for the first time. The improved off-phase acceleration measurements are consistent with the CTR results but yield much better longitudinal information, including the exact bunch shape. The usefulness of the new off-phase acceleration technique is demonstrated by observing the shapes of sharply-peaked, magnetically compressed electron bunches. These measurements help to explain the anomalously bright coherent undulator radiation previously observed as a diagnostic for operation of the FIREFLY free electron laser (FEL) at the SCA. The new off-phase acceleration technique is also used to make the first direct time-domain observations of FEL microbunching of an electron beam, confirming FEL theory and conclusively demonstrating the femtosecond resolution and fidelity of the measurement technique. The potential improvement of this technique and limitations of its resolution to the 10–30 femtosecond range at newer accelerator facilities are discussed. We conclude that, although spectral techniques will remain useful as operational picosecond beam diagnostics, time-domain measurements are necessary to calibrate and validate the optical instruments. The dispersion improved off-phase acceleration technique is an excellent option for this purpose and for future femtosecond beam diagnostics.