Emittance measurements and minimization at the SwissFEL Injector Test Facility

The emittance of the electron beam is crucial for Free-Electron Laser facilities: it has a strong influence on the lasing performance and on the total length of the accelerator. We present our procedure to measure and minimize the projected and slice emittance at the SwissFEL Injector Test Facility. The normalized slice emittance resolution achieved is about 3 nm and the longitudinal resolution is about 13 fs, with measurement errors estimated to be below 5%. After performing a full optimization we have obtained, for uncompressed beams, a slice emittance of about 200 nm for a beam charge of 200 pC, and a slice emittance of about 100 nm for 10 pC. These values are consistent with our simulations and are well below the requirements of the SwissFEL under construction at the Paul Scherrer Institute. At these bunch charges our measured slice emittances are, to our knowledge, the lowest reported so far for an electron linear accelerator.

Figure 1

Relative FEL power as a function of the normalized emittance. The reference case (100% relative FEL power) corresponding to the nominal emittance of $0.43\mu \mathrm{m}$ is indicated by a red dot. The calculations are done with Genesis [3] after 12 undulator modules for the SwissFEL design parameters at 200 pC [1]. We note that the FEL process does not reach saturation within 12 modules for normalized emittances larger than 550 nm.

Figure 2

Schematic layout of the SwissFEL Injector Test Facility (not to scale). The total length is about 60 m.

Figure 3

Schematic overview of the setup for slice emittance measurements at the SwissFEL Injector Test Facility.

Figure 4

Optics for the slice emittance measurements. The $\beta$-functions at the measurement screen (top), phase advance between deflector and screen (middle), and the required $k$-values for the measurement quadrupoles during the scan (bottom), plotted for each scan index $i$.

Figure 5

An example of matching iterations of the longitudinal beam core. The emittance measurements (top) are compared with the mismatch parameter $\xi$ along the bunch (bottom). The beam charge is 200 pC.

Figure 6

Typical image of the deflected beam at the transverse profile monitor. The streak correlates the vertical with the longitudinal direction, and the slice emittance is reconstructed in the horizontal plane. Around the centroid (magenta cross) a region-of-interest is defined (green box), which is further divided into slices (dashed green lines). Centroid positions within each slice are calculated (red diamonds) and compared with the center of a Gaussian fit (black crosses).

Figure 7

Six out of the sixteen transverse beam images of a projected emittance measurement at 200 pC. The horizontal and vertical phase advances between each of the distributions are 30 degrees. The yellow lines are the projections to the horizontal and vertical axes and the magenta lines correspond to the Gaussian fits.

Figure 8

Coupling correlations before and after the correction. The coupling contribution to the emittance was reduced from 11.3% to 0.6%.

Figure 9

Normalized slice emittance in the horizontal plane for a bunch charge of 10 pC (blue). The error bars are obtained by error propagation of the statistical beam-size errors. The longitudinal bunch charge profile is shown in gray bars.

Figure 10

Same as Fig. 9 for a bunch charge of 200 pC.

Figure 11

Left: typical temporal laser profile for a bunch charge of 200 pC. Right: spatial laser profile at the cathode for the measurements done at 200 pC (the white lines are the projections onto the horizontal and vertical axes).

Figure 12

Same as Fig. 9 for a bunch charge of about 30 fC.