Analysis of photoinjector transverse phase space in action and phase coordinates

Photoinjectors are the main high brightness electron sources for x-ray free electron lasers (XFELs). Photoinjector emittance reduction is one of the key knobs for improving XFEL lasing, so precise emittance measurement is critical. It is well known that rms emittance is very sensitive to low intensity tails of particle distributions in the phase space, whose measurement depends on the signal to noise ratio and image processing procedures. Such sensitivities make the interpretations of beam transverse brightness challenging, leading to different emittance definitions to reduce the impact of tail particles. In this paper, transverse phase space is analyzed in action and phase coordinates for both analytical models and experiments, which gives a more intuitive way to calculate the beam core brightness.

Figure 1

rms emittance and beam brightness vs 5% charge emittance degradation according to Eq. (3).

Figure 2

Phase space density and charge integration along the action axis according to Eq. (11).

Figure 3

Schematic plot of the projected emittance measurement beamline at PITZ, High1.scr1, and High1.scr4 are two screen stations for slit-scan emittance measurements. The slit is installed at High1.scr1 and beamlet images are measured at High1.scr4. A dipole magnet is for beam energy measurement.

Figure 4

Analytical estimation of lowest phase space density (normalized by the peak density) that can be reliably measured in slit scan vs SNR, assuming a Gaussian phase space. The measured charge fraction by slit scan is estimated by Eq. (17).

Figure 5

Slit scan measurement of the 1-nC beam (with flattop temporal laser) vs measurement camera gain, maximum beam signal from camera CCD reading is kept the same during gain adjustments.

Figure 6

1-nC beam measured with two camera gains, unscaled rms emittances are $0.81\mu \mathrm{m}$ at 4 dB and $0.53\mu \mathrm{m}$ at 20 dB, respectively. The lower emittance value of the 20 dB case is a result of missing low density tails due to a higher noise floor.

Figure 7

Gaussian core analysis of 1 nC phase space measured at 4-dB camera gain, (a) scattering plot, (b) binning plot. The absolute phase space density is normalized to the max value at action $J=0$. The Gaussian fitting is based on Eq. (10). The emittance integration is based on Eq. (9).

Figure 8

Virtual cathode laser profiles with three diameters.

Figure 9

Gaussian core analysis of 250-pC beam phase space, (a) scattering plot, (b) binning plot. The absolute phase space density is normalized to the max value at action $J=0$. The Gaussian fitting is based on Eq. (10). The emittance integration is based on Eq. (9).

Figure 10

Time vs action for 1-nC beam with a 17-ps flattop laser from astrasimulations.

Figure 11

Time vs action for 250 pC beam with a 7 ps Gaussian laser from astra simulations.