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

Abstract

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, multi-color photon beams using the Fresh-Slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as 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.

Measurements of electron beam deflection and rf breakdown rate from a surface wave guided in metallic mm-wave accelerating structures

“…The beam deflection is calculated from the R 34 coefficient which is determined with finite tolerances from beam optics parameters which may introduce additional errors. Our studies suggest possible application of the surface wave for beam manipulations, such as beam dechirpers [28][29][30][31][32], or beam diagnostic, such as passive deflectors [33,34]. The deflecting wakefields create a ramp of deflecting forces along the length of the bunch, which could be used for bunch measurements at sub-fs resolution [28,33,34].…”

Wake measurements of a dechirper jaw with nonzero tilt angle

“…The bunch at the end of the LCLS linacs is short with an approximately uniform distribution. The kick factor for a short, uniform bunch of full length , passing by a single dechirper plate at offset b (with no tilt), is [3][4][5]…”

“…For each data set one jaw was moved near the beam trajectory (while the three others were moved far away from it), following the sequence the horizontal jaws-North then South-followed by the vertical jaws-Top then Bottom. (It was later discovered that the [3], these longitudinal results are extremely noisy, are not much help in the analysis of the measurements, and will not be discussed further. )…”

“…From the foregoing results (leaving out those of the South jaw, with their non-physical features), we see that, by performing single plate wake measurements while sweeping the tilt parameter d, we manage to obtain: the tilt setting that corresponds to zero plate tilt d 0 , the shift in average offset parameter b 0 , and the downstream BPM 590 reading that corresponds to zero wake kick, x b0 (or y b0 ). In the previous single plate wake measurements at the LCLS, described in Ref [3], the plate was scanned in b while the wake effect was In addition, we derive an explicit analytical formula for the transverse wake kick of a single dechirper plate, as function of plate offset and tilt angle with respect to the beam orbit. We present wake measurements with the different LCLS dechirper jaws and show that, for the kick factors, agreement with theory is excellent.…”

Use of a corrugated beam pipe as a passive deflector for bunch length measurements

“…[14]. Moreover, recent measurements provide very detailed information on the wakefields and beam deflections in the corrugated dechirper [15][16][17][18]. In this paper, we further investigate this novel idea of measuring the bunch length using a dechirper of rectangular geometry (which would provide more operational flexibility [2]), and introduce a method to reconstruct the bunch shape even in the presence of nonlinear transverse kicks.…”