Measurements and analysis of a high-brightness electron beam collimated in a magnetic bunch compressor

A collimator located in a magnetic bunch compressor of a linear accelerator driven x-ray free electron laser has many potential applications, such as the removal of horns in the current distribution, the generation of ultrashort beams, and as a diagnostic of the beam slice emittance. Collective effects, however, are a major concern in applying the technique. Systematic measurements of emittance and analysis were performed using a collimator in the first bunch compressor of the Linac Coherent Light Source (LCLS). In the nominal, undercompressed configuration using the collimator we find that the $y$ emittance (nonbending plane) is not increased, and the $x$ emittance (in the bending plane) is increased by about 25%, in comparison to the injector emittance. From the analysis we conclude that the parasitic effects associated with this method are dominated by coherent synchrotron radiation (CSR), which causes a “systematic error” for measuring slice emittance at the bending plane using the collimation method. In general, we find good agreement between the measurements and simulations including CSR. However, for overcompressed beams at smaller collimator gaps, an extra emittance increase is found that does not agree with 1D simulations and is not understood.

Figure 1

Simulated electron beam phase space and current profile at the entrance of the LCLS undulator. (a) and (c) are for regular compression setup with full charge of 250 pC without collimation; (b) and (d) show the collimated beam cutting down to about 150 pC using the BC1 collimator. We see clearly the reduced current horns and flat phase space with the truncation mode.

Figure 2

Schematic layout of the LCLS injector and BC1. The horizontal collimator is located in the center of the BC1 chicane.

Figure 3

Projected emittance measured at the 135 MeV point of the LCLS injector using the OTR screen and the wire scanner, for cases LH chicane on and off. The measured emittance ${ϵ}_{xn}$ is very close to ${ϵ}_{yn}$. The beam charge is 150 pC and the peak current is about 50 A for the measurements.

Figure 4

Horizontal slice emittance, comparing the cases LH chicane on and off, as measured in the LCLS injector at 135 MeV, for $Q=150\mathrm{pC}$. The bunch shape is given in green. Note that emittance values measured in the beam tails are inaccurate and should be ignored.

Figure 5

Sketch of the LCLS first chicane, BC1 (not scaled). The linac L1S is used to introduce an energy-time correlation needed for bunch compression at BC1, while the $x$-band linac L1X is used to linearize longitudinal phase space. The wire scanner for emittance measurements is located just downstream of BC1. Each bend length is 0.2 m and bending radius is 2 m.

Figure 6

Examples of measured beam images at the OTR screen just downstream of the collimator: the full, 150 pC chirped beam (top) and collimated beams with different collimator gaps (middle and bottom).

Figure 7

Measured $x/y$-emittances and charge vs collimator gap. The collimator gap $g=10\mathrm{cm}$ for the full open case.

Figure 8

Measured $x$- and $y$-emittance of collimated slice beam along the long bunch with $g=0.5\mathrm{mm}$ of collimator gap.

Figure 9

Measured (left) and simulated (right) emittance vs collimator gap for different amounts of bunch compression. The L1S phase of $-27°$, $-30°$ and $-32°$ corresponds to under compression , full compression and overcompression, respectively.