Detailed characterization of electron sources yielding first demonstration of European X-ray Free-Electron Laser beam quality

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about $43\mathrm{MV}/\mathrm{m}$ at the photocathode and the other at about $60\mathrm{MV}/\mathrm{m}$. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be $1.26\pm 0.13\mathrm{mm}\mathrm{mrad}$ for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad.

Figure 1

Schematic diagram of the PITZ-1.6 diagnostics beam line including electron gun, booster cavity, dispersive arms (DISP), optical transition radiation and YAG screens (O/Y), and emittance measurement system (EMSY) stations.

Figure 2

(a) Transverse cross-sectional view of the gun cavity and (b) axial cross-section at the iris plane.

Figure 3

Field distribution on the axis of the booster cavity. Field amplitudes in cells measured after the final tuning shown with circles.

Figure 4

Schematic diagram of the vacuum technical arrangement at PITZ, including the location of titanium sublimation pumps (TSP) and ion getter pumps (IGP).

Figure 5

Schematic diagram of the aerogel detector housing.

Figure 6

Scheme of the laser and pulse trains recorded at several positions in the system.

Figure 7

(a) Image of the full-energy laser pulse using standard timing. (b) Magnified image of the full-energy laser pulse with fast readout. (c) Image obtained from the quartz-plate beam splitter. (d) Magnified image obtained using the Ce:YAG crystal. In each image, the relative intensity is highest in red, followed by green, blue, and either black or white to represent the zero level.

Figure 8

Typical temporal profile (red) of a micropulse within a pulse train measured during the summer 2007 run with a synchroscan streak camera with 2 ps resolution. A flattop fit (black) is overlaid in the picture. The pulse duration (FWHM) is 23.8 ps, rise and fall times are about 7.2 ps and the variation in the flattop from peak to peak is $\pm 10\%$. The profile used as input to the beam dynamics simulations (discussed in Sec. 3f3) is shown in blue.

Figure 9

Schematic diagram of the gun and booster cooling system (water flow is clockwise).

Figure 10

(a) Dark current measurements as a function of the main solenoid magnet current at various gun gradients for the gun cavity #3.1 (${\mathrm{Cs}}_2\mathrm{Te}$ cathode #58.1). (b) Comparison of the maximum dark current as a function of gun gradient for different Mo and ${\mathrm{Cs}}_2\mathrm{Te}$ cathode for the gun cavity #3.2.

Figure 11

Extracted charge as function of laser energy for cathode #90.1 measured at $60\mathrm{MV}/\mathrm{m}$. Blue circles represent the measured data and the red line shows the linear fit in the low-charge region used to estimate the cathode QE.

Figure 12

QE maps of ${\mathrm{Cs}}_2\mathrm{Te}$ photocathode #90.1 before (a) and after (b) operation in the linac (the color scale indicates the relative QE and is normalized separately for the two measurements).

Figure 13

Photographs of ${\mathrm{Cs}}_2\mathrm{Te}$ photocathode #90.1 before (a) and after (b) use in the photoinjector.

Figure 14

Survey spectra measured at photon energy of 900 eV for fresh cathodes (red: #42.3; magenta: #90.1) and used cathodes (blue: #34.6; black: #92.1).

Figure 15

ASTRA simulations for the influence of (a) space charge and (b) laser pulse length on the measured emittance ${\epsilon }_n$ over the thermal emittance ${\epsilon }_{\mathrm{th}}$ for a maximum accelerating gradient of $60\mathrm{MV}/\mathrm{m}$ (with rf phase adjusted for maximum mean momentum gain).

Figure 16

Measured thermal emittance as a function of rms laser spot size for cathode #90.1. Filled points represent data, unfilled points represent ASTRA simulations, and lines represent a linear fit to the data and simulation results with offsets.

Figure 17

Kinetic energies of emitted electrons for two ${\mathrm{Cs}}_2\mathrm{Te}$ cathodes as a function of the electric field during emission. The error bars represent only the statistical uncertainties of the analysis procedure.

Figure 18

Phase scan—electron bunch charge vs gun phase for different laser energies (laser transmission factors 33% and 40%). Zero phase ($\mathrm{rf}\mathrm{\text{gun phase}}={\Phi }_{\max }$) refers to the gun phase of the maximum mean momentum gain.

Figure 19

Determination of the zero phase. The low-charge (70 pC maximum) phase scan is shifted to fit its left edge to the integrated laser temporal profile. Usual operation is at around 40 degrees rf-gun phase.

Figure 20

Electromagnetic fields inside the PITZ rf gun.

Figure 21

Cathode laser BBA: movement of the electron beam center position ($x$, $y$) on a screen 0.78 downstream of the cathode with a variation of the laser launch phase. The corresponding displacement of the laser at the cathode is calculated from its relative positions on the cathode imaging screen and plotted as well.

Figure 22

Main solenoid BBA: movement of the electron beam center position ($x$, $y$) with a variation of the main solenoid current. Error bars on the plot correspond to the electron beam position jitter at the screen.

Figure 23

Measured and simulated (a) mean momentum and (b) momentum spread of the electron beam from gun prototype 3.1 with a peak field at the cathode of about $40\mathrm{MV}/\mathrm{m}$.

Figure 24

Measured and simulated (a) mean momentum and (b) momentum spread of the electron beam from gun prototype 3.2 with increased peak field at the cathode.

Figure 25

(a) Mean momentum and (b) momentum spread after the booster measured for different booster phases at a gun phase with maximum energy gain. The solid line presents the results from ASTRA simulations.

Figure 26

Measurements and simulations of the electron bunch length as a function of the gun (a) and booster phase (b).

Figure 27

Current profile measurements taken for gun and booster phase with (a) maximum energy gain and (b) booster phase 60 degrees off crest (three consecutive measurements). The bunch charge is 1 nC and the laser beam had a flattop distribution in longitudinal (FWHM about 20 ps) and transverse direction (diameter about 1.5 mm).

Figure 28

Measured (a) and simulated (b) longitudinal phase space and projections: longitudinal (c) and momentum (d) distribution for 1.1 nC bunch charge, phase of maximum mean momentum gain plus 10 degrees and a longitudinal laser distribution with about 20 ps FWHM pulse length.

Figure 29

Transverse projected emittance as a function of the current in the main solenoid. The gun gradient was $\sim 43\mathrm{MV}/\mathrm{m}$ and the gun phase chosen $-2\mathrm{deg}$ with respect to the maximum mean momentum gain. The booster phase with respect to the maximum acceleration phase was set to $-10\mathrm{deg}$.

Figure 30

The best emittance result from gun 3.1 in comparison with a corresponding ASTRA simulation and measurement from gun 3.2 for cathode gradients of around $40\mathrm{MV}/\mathrm{m}$. The applied machine parameters for each case are described in the text.

Figure 31

Horizontal phase-space distribution at 4.3 m from the cathode, gun prototype 3.1 at $\sim 43\mathrm{MV}/\mathrm{m}$, total beam momentum $12.99\mathrm{MeV}/c$, main solenoid current of 282 A, rf phases with respect to maximum mean momentum gain of gun, $-2\mathrm{deg}$, and booster, $-5\mathrm{deg}$.

Figure 32

Simulated normalized beam emittance and transverse rms size along the beam line for the optimized cases 42 and $60\mathrm{MV}/\mathrm{m}$ peak field at the cathode.

Figure 33

Simulated bunch current profiles and slice emittance 4.3 m downstream of the cathode for the optimized cases 42 and $60\mathrm{MV}/\mathrm{m}$ peak field at the cathode.

Figure 34

Simulated normalized beam emittance vs beam momentum gain in the booster for two gun gradients.

Figure 35

Measured normalized emittance as a function of the main solenoid current (laser rms spot size in the transverse directions of 0.353 and 0.376 mm). The mean beam momentum after the booster was measured to be $14.45\mathrm{MeV}/c$.

Figure 36

Minimum of the emittance as a function of the momentum gain in the booster.

Figure 37

Phase-space distribution at an electron beam mean momentum of $14.45\mathrm{MeV}/c$, main solenoid current 373 A, and with the phases of gun and booster at maximum mean momentum gain. The white contour surrounds the phase-space area that remains after a 10% charge cut has been applied (see text).

Figure 38

Normalized projected rms emittance as a function of the charge cut fraction starting from the lowest density regions in the phase space.