Abstract
Coherent Smith-Purcell (SP) radiation originating from three different gratings has been measured at End Station A, SLAC, and has been used to reconstruct the time profile of the electron bunches. The beam energy during these experiments was 28.5 GeV ($\ensuremath{\gamma}\ensuremath{\cong}55\text{ }773$) and the number of electrons in the bunch was $0.9--1.4\ifmmode\times\else\texttimes\fi{}{10}^{10}$. The spectral distribution of the radiated energy was measured by means of an array of 11 pyroelectric detectors. Typical values of the FWHM of the bunch length are about 2.5 ps, but sharper peaks with FWHM less than 2.0 ps have also been observed. The longitudinal profile also varies with accelerator conditions and can best be approximated by a superposition of 3--4 Gaussian curves. Some typical profiles are presented, together with a discussion of the limitations and strengths of coherent SP radiation as a diagnostic tool. It is concluded that SP radiation offers excellent prospects in this respect, not only in the picosecond range, but potentially in the femtosecond range as well.
Highlights
The longitudinal profile of two colliding bunches is a significant parameter that enters into the calculation of a number of beam-beam effects at the collision point and, it plays a crucial role in the optimization of the collider luminosity [1,2]
Three different gratings and an array of 11 pyroelectric detectors were used for this purpose
The main conclusions of this work are as follows: (i) The radiated energy is in line with the predictions of the theory that describes SP radiation in terms of currents induced on the grating surface [Figs. 6(a) and 6(b)]
Summary
The longitudinal (time) profile of two colliding bunches is a significant parameter that enters into the calculation of a number of beam-beam effects (e.g. beamstrahlung, beam deflection) at the collision point and, it plays a crucial role in the optimization of the collider luminosity [1,2] It is of great interest in the context of x-ray free electron lasers (FELs). The present paper deals with experiments carried out using another, related, radiative process, Smith-Purcell (SP) radiation [3] This is the term used to describe the radiation emitted through the interaction of a charged particle beam (electrons, in our case) with a nearby periodic metallic structure, the grating. The details of this process can be found in numerous publications [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] and in the references contained therein, so we restrict ourselves to a brief summary of its essential features
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