A Transverse RF Deflecting Structure for Bunch Length and Phase Space Diagnostics

A traveling wave transverse rf deflecting structure is discussed as a tool for bunch length measurement and as an aid to detailed phase space diagnostics in future FEL projects. The high frequency time variation of the deflecting fields is used to ‘pitch’ or ‘yaw’ the electron bunch where the resulting transverse beam width measured on a simple profile monitor represents a reliable, single-shot measure of the absolute bunch length. A small rf phase shift, from the zero-crossing, adds a net centroid kick to the bunch so that it intercepts an off-axis screen. The deflecting rf is then pulsed at a much lower rate than the machine rate, and a pulse-stealing process is used to monitor the bunch length continuously. Multiple screens at appropriate phase advance locations might also be used to monitor the ‘slice’ (i.e., time-correlated) transverse emittance. A screen placed at a point of significant momentum dispersion can also be used to reveal the time-dependent energy spread. Suitable S-band structures were built and tested in the 1960's at SLAC and one has now been installed in the SLAC linac and configured to measure bunch lengths of test beams. Simulations of these schemes are presented for the Linac Coherent Light Source [1] and the existing SLAC linac.