General and efficient dispersion-based measurement of beam slice parameters

Abstract

In free-electron lasers, it is essential to characterize the beam parameters as a function of the longitudinal position along the bunch, i.e., to determine the so-called slice parameters. Conventional measurements utilize transverse deflector structures to deflect the beam transversely as a function of time. Such structures are expensive and only allow the measurement of slice parameters in one transverse plane. Alternatively, the correlation between time and transverse position can be generated by introducing transverse dispersion to an energy-chirped beam. Dedicated dispersive sections can be used to measure the slice beam parameters in one transverse plane at one location. To extend the applicability of such measurements, we propose to actively generate the dispersion in the two transverse directions wherever required, thus enabling the measurement of beam slice parameters in both transverse planes at potentially any location of the accelerator. Our strategy does not incur any additional cost since it utilizes components already built into the accelerator for other purposes. The application of the method is illustrated with the SwissFEL injector test facility: we show simulations and measurements and we experimentally verify our proposal with the well-established transverse deflector approach.