Wavefront Analysis of Nonlinear Self-Amplified Spontaneous-Emission Free-Electron Laser Harmonics in the Single-Shot Regime

The single-shot spatial characteristics of the vacuum ultraviolet self-amplified spontaneous emission of a free electron laser (FEL) is measured at different stages of amplification up to saturation with a Hartmann wavefront sensor. We show that the fundamental radiation at 61.5 nm tends towards a single-mode behavior as getting closer to saturation. The measurements are found in good agreement with simulations and theory. A near diffraction limited wavefront was measured. The analysis of Fresnel diffraction through the Hartmann wavefront sensor hole array also provides some further insight for the evaluation of the FEL transverse coherence, of high importance for various applications.

Figure 1

SCSS test accelerator layout: a 11 m-long 250 MeV accelerator, two in-vacuum undulators (each 4.5 m long, 300 periods of ${\lambda }_u=15\mathrm{mm}$) [43], quadrupoles (Q-poles), beam position monitors (BPM) and optical transition radiation (OTR) screens. HWS located at 17.5 m from the end of the second undulator, behind two 5° grazing incidence Au-coated flat mirrors; uniform grid array made out of a $50\mu \mathrm{m}$ thick nickel plate and composed of $35\times 35$ square holes ($150\mu \mathrm{m}$ size squares, rotated by 25° to avoid cross-talking between adjacent spots in the measurement plane, and spaced by $450\mu \mathrm{m}$ over a $15.75\times 15.75{\mathrm{mm}}^2$ effective area); back-illuminated thinned 16-bit XUV detection CCD camera ($1024\times 1024$ pixels of $13\mu \mathrm{m}$), coupled to a 10 Hz operating fast shutter and synchronized with the FEL pulse train, allowing for single-shot wavefront measurements.

Figure 2

(a),(d): Hartmann patterns of a single-pulse of the saturated fundamental SASE radiation at 61.5 nm. (b), (e): Respective normalized intensity profiles. (c),(f): Respective phase aberrations given in terms of optical path difference. (a),(b),(c): Experimental data. (d),(e),(f): Simulations performed with GENESIS and SRW (1.43 nm.rad emittance, 0.02% energy spread, 330 A peak current, 0.18 mm-rms bunch length). Because of shot-to-shot beam intensity and size instabilities at the HWS position and for relevant comparison of the wavefront measurements, analysis pupils are limited to $1/e^2$ of the maximum intensity, corresponding to $4\sigma$ and 86% of a Gaussian incident flux.

Figure 3

Measured evolution of the SASE radiation characteristics as a function of $\rho N$: (○) $\delta I/I$ intensity fluctuation measured with a photodiode, (▴) energy ($\mu \mathrm{J}$), radiation normalized rms size $\sigma /{\sigma }_0$ in (□) $x$ and (■) $y$. Line: calculated normalized energy [26].

Figure 4

HWS diffraction pattern of a single-pulse of the SASE radiation third harmonic at 20.5 nm: (a) experimental measurement, (b) SRW-GENESIS simulation using the experimental spatial resolution ($13\mu \mathrm{m}$), (c) analytical calculation considering a fully coherent wave (point source) and using a $1\mu \mathrm{m}$ spatial resolution. (d),(e) and (f) Respective diagonal cross-sections (along the white lines). (e) continuous line using the experimental CCD $13\mu \mathrm{m}$ resolution, dotted line using the analytical calculation $1\mu \mathrm{m}$ resolution. HWS set with holes of $150\times 150\mu \mathrm{m}$. Spectral selection with a 500 nm thick oxidized aluminum filter.