Measurements of intrinsic emittance dependence on rf field for copper photocathodes

Radio-frequency (rf) photoinjectors are used to generate high-brightness electron beams for a wide range of applications. Because of their outstanding beam quality, they are particularly well-suited as sources for X-ray free-electron lasers (FELs). The beam emittance, which is significantly influenced by the intrinsic emittance of the cathode, is fundamental for FELs, since it has a strong impact on the lasing performance and it defines the length and cost of the facility. In this paper we present measurements of the intrinsic emittance as a function of the rf field for a copper photocathode. Our measurements match with the theoretical expectations, showing that the intrinsic emittance can be reduced by decreasing the rf field at the cathode. We obtained normalized intrinsic emittances down to $350\mathrm{nm}/\mathrm{mm}$, the lowest values ever measured in a rf photoinjector.

Figure 1

Example of space-charge limit determination. The field at the cathode was at its nominal value ($49.9\mathrm{MV}/\mathrm{m}$) and the laser beam size was around $60\mu \mathrm{m}$. For bunch charges equal or below 1.0 pC the slice emittance is not affected by space charge.

Figure 2

Example of a Schottky scan: extracted charge (blue points) as a function of the rf phase. The effective work function ${\varphi }_{\text{eff}}$ is obtained by fitting the charge data according to Eqs. (2) and (3). The fit is indicated with the blue line. The red dashed line shows the field at the cathode for the different phases. For this example the cathode field at the extraction phase was at its nominal value ($49.9\mathrm{MV}/\mathrm{m}$) and the rms laser beam size was around $60\mu \mathrm{m}$.

Figure 3

Core slice emittance as a function of beam size for three different fields on the cathode. The nonlinear effect was reduced for lower fields and the normalized intrinsic emittance varied as expected from theory.

Figure 4

Normalized slice emittance (cyan, left axes) for a total bunch charge of 30 fC. The error bars are obtained by error propagation of the statistical beam-size errors. The longitudinal bunch charge profile is shown as gray bars (right axes).