A transverse rf deflecting structure for bunch length and phase space diagnostics

Abstract

*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.

LCLS Injector Straight-Ahead Spectrometer

“…They demonstrate that a direct measurement of the longitudinal phase space at the end of the injector beamline will be available when the beam is vertically deflected at TCAV1 transvserse deflecting cavity. Using a cavity voltage of 0.8 MV/m, one estimates that the kick along the 3mm (10ps) electron beam will be of +/-0.5 mrad, by applying formula (2) of [1]. This provides a vertical beam size at the screen of 12 mm.…”

Generation and measurement of velocity bunched ultrashort bunch of pC charge

“…For the low energy regime, the most widely employed method for bunch length measurement is using a time-varying transverse electrical field provided by a rf deflecting cavity to convert the longitudinal distribution into transverse coordinate [18,19]. This method is straightforward and robust but is not capable of capturing the locally compressed ultrashort bunch length in situations where bunch length changes significantly over distances shorter than the length of the cavity.…”

Design and application of multimegawattX-band deflectors for femtosecond electron beam diagnostics

“…The assumption here is that the beam size on the screen doubles with application of the defecting voltage. For details on the derivation see [4,5]. The 3 GHz deflecting structures built and tested for the SLAC linac in the 1960s are now used as indispensable diagnostics for LCLS operation [1,6].…”