Effects of energy chirp on bunch length measurement in linear accelerator beams

Bunch length measurements in electron LINear ACcelerator (LINAC) can be carried out by means of a disruptive optical technique based on a Radio Frequency Deflector (RFD). This device provides a vertical kick to the electrons in order to assess the bunch length through the vertical spot size measurement on a screen placed after the RFD. The measurement is based on some assumptions on the bunch proprieties that affect the accuracy. In particular, when the electron bunch at the RFD has a non-negligible energy chirp (i.e. the correlation between particle longitudinal positions and energies), the measurement is affected by a deterministic error. This error is directly related to the RFD phase offset. In this paper, a case study about the effects of the energy chirp on the bunch length measurements of the electron LINAC of the Gamma Beam Source at the Extreme Light Infrastructure–Nuclear Physics is reported. Relative error is estimated by using ELEctron Generation ANd Tracking (ELEGANT) code to define reference measurements of virtual bunch length.