Measurements of wake-induced electron beam deflection in a dechirper at the Linac Coherent Light Source

The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat, metallic, corrugated plates, has been installed just upstream of the undulators in the Linac Coherent Light Source (LCLS). As a dechirper, with the beam passing between the plates on axis, longitudinal wakefields are induced that can remove unwanted energy chirp in the beam. However, with the beam passing off axis, strong transverse wakes are also induced. This mode of operation has already been used for the production of intense, multicolor photon beams using the fresh-slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as a function of offset, the strength of the transverse wakefields that are excited between the two plates, and also for the case of the beam passing near to a single plate. We compare with analytical formulas from the literature, and find good agreement. This report presents the first systematic measurements of the transverse wake strength in a dechirper, one that has been excited by a bunch with the short pulse duration and high energy found in an x-ray free electron laser.

Figure 1

Three corrugations of a horizontal dechirper. A rectangular coordinate system is centered on the symmetry axis of the chamber. The blue ellipse represents an electron beam propagating along the $z$ axis.

Figure 2

Sketch of the LCLS beam line, focusing on the RadiaBeam/SLAC dechirper and the diagnostics used in the measurements of this report. Note that the relative scale of distances is greatly distorted; e.g., in reality the linacs are much longer than the other objects in the figure. During the measurements, the first, vertical dechirper module is kept wide open and the second, horizontal one’s effect on the beam is investigated.

Figure 3

Deflection as function of beam offset from the axis in the horizontal dechirper module, showing data (plotting symbols) and calculations (curves), for half gap $a=1\mathrm{mm}$. Note that the error bars in the data are small and not visible. For the simulated curves, the gap parameter was reduced by 11% to fit the experimental data. The tilted straight line gives the slope of the curves at the origin. The bunch shape, with head to the left, is given in the inset figure.

Figure 4

Deflection as function of beam offset from the axis in one dechirper module, showing data (plotting symbols) and calculations (flat-top approximation, the curve), for half gap $a=1.55\mathrm{mm}$. Note that the error bars in the data are small and barely visible. The gap in the analytical calculation was reduced by 6% to fit the experimental data. The straight line gives the slope of the curve at the origin. The bunch shape, with head to the left, is given in the inset figure.

Figure 5

Single north jaw measurements of transverse (top) and longitudinal (bottom) wakefield kicks. The symbols give the data points, with their $b$ values shifted by $-161\mu \mathrm{m}$ (top) and $-138\mu \mathrm{m}$ (bottom); the curves give the analytical theory.

Figure 6

Single south jaw measurements of transverse (top) and longitudinal (bottom) wakefield kicks. The symbols give the data points, with their $b$ values shifted by $-58\mu \mathrm{m}$ (top) and $-73\mu \mathrm{m}$ (bottom); the curves give the analytical theory.