Preservation of low slice emittance in bunch compressors

Minimizing the dilution of the electron beam emittance is crucial for the performance of accelerators, in particular for free electron laser facilities, where the length of the machine and the efficiency of the lasing process depend on it. Measurements performed at the SwissFEL Injector Test Facility revealed an increase in slice emittance after compressing the bunch even for moderate compression factors. The phenomenon was experimentally studied by characterizing the dependence of the effect on beam and machine parameters relevant for the bunch compression. The reproduction of these measurements in simulation required the use of a 3D beam dynamics model along the bunch compressor that includes coherent synchrotron radiation. Our investigations identified transverse effects, such as coherent synchrotron radiation and transverse space charge as the sources of the observed emittance dilution, excluding other effects, such as chromatic effects on single slices or spurious dispersion. We also present studies, both experimental and simulation based, on the effect of the optics mismatch of the slices on the variation of the slice emittance along the bunch. After a corresponding reoptimization of the beam optics in the test facility we reached slice emittances below 200 nm for the central slices along the longitudinal dimension with a moderate increase up to 300 nm in the head and tail for a compression factor of 7.5 and a bunch charge of 200 pC, equivalent to a final current of 150 A, at about 230 MeV energy.

Figure 1

Schematic layout of the SwissFEL Injector Test Facility.

Figure 2

Measured horizontal slice emittance along the bunch for different bunch compression factors C (top plot). The core slice is the one corresponding to the 0 slice number. The streaked beam is imaged on a screen and corresponding current profile (bottom plot). The beam energy is 200 MeV, the dipole bending angle 4.07°. The length of the single slice is scaled by the compression factor to ease the visual comparison of the different measurements.

Figure 3

Measured relative core emittance increase versus the bunch compression factor extracted from the data shown in Fig. 2.

Figure 4

Measured relative core emittance increase versus the beam energy for a constant compression factor of 5 (dipole bending angle at 4.07°.)

Figure 5

Measured relative core emittance increase versus the dipole bending angle for a constant compression factor of 5.

Figure 6

Measured horizontal slice emittance starting from about 500 nm (top) and 250 nm (bottom) along the bunch after the compression. The maximum compression factor for both cases is 11.

Figure 7

Measured horizontal slice emittance (top) and mismatch with respect to the core (bottom) along the bunch for different beam optics settings along the bunch compressor. The compression factor is 8. The inset in the lower plot lists the horizontal $\alpha$ at the entrance of the bunch compressor. These optics are expected to be similar to the ones shown in Fig. 15, but not the same because of some other small adjustments during the optimization.

Figure 8

Observed horizontal slice emittance along the bunch for different deflecting voltages ($V$) of the transverse deflecting cavity. The compression factor is fixed to 5 (dipole bending angle 4.07°). Two measurements performed on the uncompressed beam under equal conditions are shown for reference.

Figure 9

Measured horizontal slice emittance for the uncompressed, compressed and decompressed beam. (For this measurement the bunch charge was limited to 70 pC because of a temporary depletion of the photocathode quantum efficiency.)

Figure 10

Simulated horizontal slice emittance along the bunch for a compression factor of 15 obtained by tracking the beam in the bunch compressor using the 1D model of CSRTrack. The horizontal slice emittance along the bunch before compression and the vertical after compression are also shown for comparison.

Figure 11

Simulated horizontal slice emittance along the bunch for two different compression factors using the 3D csr_g_to_p model of CSRTrack. The horizontal slice emittance along the bunch before compression is also shown for comparison.

Figure 12

Simulated horizontal slice emittance along the bunch for different energies ($E$) of the incoming beam. The compression factor is fixed to 15.

Figure 13

Simulated horizontal slice emittance along the bunch for different bending angles of the dipoles in the magnetic chicane (BC angle). The compression factor is fixed to 15.

Figure 14

Simulated horizontal core emittance after compression (left axis) versus initial emittance and corresponding relative increase (right axis). The extrapolation of the core emittance after compression for the limit of zero initial emittance is also shown. The compression factor is fixed to 15.

Figure 15

Simulated beam optics along the bunch compressor (top) for the scan shown in Fig. 16 and schematic layout of the SITF bunch compressor lattice (bottom).

Figure 16

Simulated horizontal slice emittance along the bunch for different beam optics along the bunch compressor. The compression factor is 15. The inset lists the horizontal $\alpha$ at the entrance of the bunch compressor.

Figure 17

Measured horizontal slice emittance (top) and mismatch with respect to the core (bottom) along the bunch. The compression factor is fixed to 8. The measurements with compressed beam are repetitions under the same conditions, performed to assess the reproducibility of the result.

Figure 18

Simulated horizontal slice emittance along the bunch for a compression factor of 15 after matching the bunch with low and high $k$-values in the matching quadrupoles. The vertical slice emittance of the compressed beam is shown for comparison.

Figure 19

Generated mismatches along the bunch with respect to the central slice of the beam assumed as input distributions in the simulations. The large-mismatch beam (similar to the measurement shown in Fig. 17) is obtained by the rotation of the horizontal transverse phase space of the small-mismatch beam (corresponding to the measurement shown in Fig. 7).

Figure 20

Simulated horizontal slice emittance along the bunch for a compression factor of 15 using the large-mismatch bunch as input distribution at the entrance of the magnetic chicane. The vertical slice emittance is also shown for comparison.

Figure 21

Simulated horizontal slice emittance along the bunch for different beam optics settings in the bunch compressor for a compression factor of 15, obtained with the large-mismatch bunch as input distribution. The vertical slice emittance is also shown for comparison.