A non-interrupting electron beam diagnostic using coherent off-axis undulator radiation

We propose a technique for measuring the length of an electron bunch from a linear accelerator without interrupting the electron beam. Bunch lengths are measured by observing off-axis undulator radiation. The wavelength of undulator radiation increases with the angle of emission. At angles as large as 10 degrees, the wavelength may be longer than the electron bunch, and as a result coherence effects emerge. As the angle of observation increases and the radiation becomes coherent, the intensity may change by up to a factor of 10⁹. The angle at which this change occurs is related to the bunch length. Thus the bunch length may be obtained by observing the change in intensity of the radiation with respect to the angle of observation. This electron beam diagnostic does not interrupt the electron beam and would be useful for single-pass FELs. Electron bunches could be characterized while the FEL is operating, and thus the FEL could be optimized in real time. In this paper, we develop the theory of coherent off-axis undulator radiation (COUR), including near-field effects, and we discuss future experimental efforts to observe this radiation and to use it to characterize electron bunches