Method based on atomic photoionization for spot-size measurement on focused soft x-ray free-electron laser beams

Abstract

A method has been developed and applied to measure the beam waist and spot size of a focused soft x-ray beam at the free-electron laser FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. The method is based on a saturation effect upon atomic photoionization and represents an indestructible tool for the characterization of powerful beams of ionizing electromagnetic radiation. At the microfocus beamline BL2 at FLASH, a full width at half maximum focus diameter of ͑15± 2͒ m was determined. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2397561͔ Recent progress in developing pulsed high-power vacuum ultraviolet ͑VUV͒ and soft x-ray uv ͑XUV͒ sources, such as higher-harmonic generation ͑HHG͒ sources 1 and free-electron lasers ͑FELs͒, 2 has opened the door to extended research on nonlinear interaction of electromagnetic radiation with matter from the optical region to shorter wavelengths. When focused into a spot of a few micrometers in diameter, the radiation can reach peak irradiance levels of more than 10 13 W cm −2 where nonlinear effects such as atomic multiphoton ionization occur. 3-6 A key point for the understanding and theoretical description of nonlinear processes is, in general, their dependence on irradiance. Therefore, among other quantities such as pulse energy and duration, spot-size determination of focused high-intensity VUV and XUV radiation is mandatory.Recently, two conventional methods have been applied to measure the spot size of focused HHG beams, namely, the knife-edge 7 and the fluorescence screen technique. 1,[8][9][10] Both methods provide information on two-dimensional photon intensity distribution with a spatial resolution of 1 to 2 m. The possibility of applying these techniques to FELs is, however, limited. The FEL pulse energy levels of VUV and XUV radiation may be some orders of magnitude higher than those of HHG sources and can cause radiation damage on fluorescence screens and, in general, on any solid irradiated surface. Moreover, due to its statistical nature, a beam of FEL radiation based on self-amplified spontaneous emission 2 may strongly fluctuate from shot to shot, perpendicular to the propagation axis, and requires spot-size measurements which do not depend on the beam position. In this context, we describe a method which has been used to determine the beam waist and spot size of a focused beam at the XUV-FEL facility FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. 11 It is based on a saturation effect upon photoionization of a rare gas and manifests itself by a sublinear increase in the ion yield as a function of the photon number per pulse. It is due to a considerable reduction in the number of target atoms within the interaction zone by ionization with a single photon pulse and becomes stronger with decreasing beam cross section. The method is indestructible and not affected by fluctuations of the beam position. Moreover, it can easily be realized in any ionization chamber by introducing a ͑rare͒ gas and detecting the photoionization signal as a fun...

The impact of pulse duration on multiphoton ionization in the soft X-ray regime

“…For the measurements at 42.8 eV photon energy, the focus of the beamline BL2 at FLASH was used ( figure 1A) with a FWHM diameter of 15 μm [39]. The FEL beam was introduced into a homogeneous gas target at pressures from 10 − 4 to 10 − 3 Pa.…”

Nonlinear photoionization in the soft X-ray regime

“…3 shows an example of such saturation curve we have obtained on neon atoms at the photon energy of 38.0 eV. The effect was used as a new method to derive the photon beam cross section A and, by that, focus size and beam waist at BL2 [9]. For this purpose, the absolute photon number per pulse was measured with the aid of our calibrated GMD and the measured saturation curves were fitted by Eq.…”

“…Validity of this wave picture of light depends on the ponderomotive energy U p which is the quiver energy transferred to a free electron by the oscillating field. U p increases linearly with the irradiance E, but decreases with the square of the photon energy hω according to: (9) and defines the so-called Keldysh parameter [36]:…”

“…While for the first photons within a pulse 100% of the targets are available to be photoionized, the target number may already be strongly reduced for the last photons of a pulse which explains this type of non-linearity. In the simplified case of a rectangular beam profile, the number N ion of ions generated per pulse is described by an exponential saturation function of the photon number per pulse N ph [9]:…”

“…It concerns, in particular, the photoelectric effect by highly intense X-rays. Therefore, we have studied at the new Free-electron LASer in Hamburg FLASH [2,3] atomic and molecular photoionization from a fundamental point of view [9][10][11][12]. In continuation of former work in the vacuum ultraviolet [13][14][15][16][17][18], we have quantitatively investigated space-charge effects, saturation due target depletion, sequential and direct multi-photon ionization, and strong-field phenomena in the soft X-ray regime.…”