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

We report the experimental demonstration of bunch length measurements using a corrugated metallic beam pipe as a passive deflector. The corrugated beam pipe has been adopted for reducing longitudinal chirping after the bunch compressors in several XFEL facilities worldwide. In the meantime, there have been attempts to measure the electron bunch’s longitudinal current profile using the dipole wakefields generated in the corrugated pipe. Nevertheless, the bunch shape reconstructed from the nonlinearly deflected beam suffers from significant distortion, particularly near the head of the bunch. In this paper, we introduce an iterative process to improve the resolution of the bunch shape reconstruction. The astra and elegant simulations have been performed for pencil beam and cigar beam cases, in order to verify the effectiveness of the reconstruction process. To overcome the undesirable effects of transverse beam spreads, a measurement scheme involving both the corrugated beam pipe and the spectrometer magnet has been employed, both of which do not require a dedicated (and likely very expensive) rf system. A proof-of-principle experiment was carried out at Pohang Accelerator Laboratory (PAL) Injector Test Facility (ITF), and its results are discussed together with a comparison with the rf deflector measurement.

Figure 1

Geometry of the corrugated-wall dechirper [1, 2]. The blue ellipsoid indicates a beam bunch moving parallel to the $z$-axis with a transverse offset.

Figure 2

The dipole wakefields calculated with the analytical model.

Figure 3

Layout of the PAL-ITF beam line [2, 24]. Two accelerating cavities L0a and L0b are available in the beam line. For this passive deflector experiment, however, L0b was turned off. The corrugated-wall chamber is located between the positions of screen-3 and screen-4, and the RF deflecting cavity is installed after screen-4. Before the spectrometer magnet, screen-5 is inserted, and screen-6 measures the spectrometer output.

Figure 4

(a) Simulation results of the two-dimensional particle distribution and intensity distribution $f(y)$ of the pencil beam on screen-6, and (b) corresponding current distributions. The blue line in (b) represents the input current distribution at the dechirper and the black line the reconstructed current distribution.

Figure 5

(a) Simulation results of the two-dimensional particle distribution projected on screen-6, and (b) the corresponding pencil beam’s intensity distribution $f(y)$. The black diamonds with red curve in (a) represent the projection of the pencil beam.

Figure 6

Simulation results of the current distributions for a cigar (fat) beam. The blue line represents the input current distribution at the dechirper and the black line the reconstructed current distribution.

Figure 7

(a) Screen-6 image taken with dechirper offset of 2 mm and spectrometer magnet on, and (b) the projected image of the corresponding pencil beam. The bunch head is at the upper-left corner of the screen-6 image.

Figure 8

Intensity distribution $f(y)$ of the pencil beam extracted from the screen-6 image.

Figure 9

Longitudinal current profiles $I(\tau )$ are measured either by the dechirper (black line) or by the TCAV (blue line). The measured FWHMs are $3.895\pm 0.058\mathrm{ps}$ (black line) and $4.265\pm 0.021\mathrm{ps}$ (blue line), respectively. The negative time ($\tau <0$) corresponds to the front part of the bunch.

Figure 10

Screen-5 image taken with transverse deflecting cavity on, and the corresponding intensity distribution $F(y)$ along the vertical direction.