In order to measure the photon flux of highly intense and extremely pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) radiation in absolute terms, we have developed a gas-monitor detector which is based on the atomic photoionization of a rare gas at low particle density. The device is indestructible and almost transparent. By first pulse-resolved measurements of VUV free-electron laser radiation at the TESLA test facility in Hamburg, a peak power of more than 100 MW was detected. Moreover, the extended dynamic range of the detector allowed its accurate calibration using spectrally dispersed synchrotron radiation at much lower photon intensities.

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular current short wavelength FELs can uniquely drive nonlinear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only can FELs be characterised and benchmarked but also the natural departure point in the hunt for nonlinear behaviour of atomistic systems bathed in laser fields of ultrahigh photon energy. In this topical review we illustrate with specific examples the gamut of apposite experiments in atomic, molecular physics currently underway at the SCSS test accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford).

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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...

Abstract. At the free-electron laser FLASH, multiple ionization of neon atoms was quantitatively investigated at 93.0 eV and 90.5 eV photon energy. For ion charge states up to 6+, we compare the respective absolute photoionization yields with results from a minimal model and an elaborate description. Both approaches are based on rate equations and take into acccout a Gaussian spatial intensity distribution of the laser beam. From the comparison we conclude, that photoionization up to a charge of 5+ can be described by the minimal model. For higher charges, the experimental ionization yields systematically exceed the elaborate rate based prediction.PACS numbers: 42.50 Hz, 32.80.Rm, 32.80.Fb, 41.60.Cr With the construction of X-ray lasers during the last years applying the free-electron laser (FEL) concept of self-amplified spontaneous emission (SASE) [1,2,3,4], a number of experiments have become feasible which elucidate the principles of photon-matter interaction in a new parameter regime ([5, 6] and refs. therein). The combination of high frequency and high intensity allowed in particular new insight into the photoelectric effect and the photoionization process [7,8,9,10,11,12,13,14,15]. Even simple photoionization involves in a strict sense multi-electron dynamics as soon as the photoelectron is not ejected dominantly from the highest occupied orbital rendering a comprehensive theoretical description difficult. For more complex atomic targets such as xenon, multiphoton absorption in the energy range of the giant resonance poses in addition the question, which role collective electron excitations play in the high frequency -high intensity domain [10,16]. Due to these difficulties, the role of electron ‡

The results of a study of ultra-rapid (flash) sintering of oxide ceramic materials under microwave heating with high absorbed power per unit volume of material (10–500 W/cm3) are presented. Ceramic samples of various compositions—Al2O3; Y2O3; MgAl2O4; and Yb(LaO)2O3—were sintered using a 24 GHz gyrotron system to a density above 0.98–0.99 of the theoretical value in 0.5–5 min without isothermal hold. An analysis of the experimental data (microwave power; heating and cooling rates) along with microstructure characterization provided an insight into the mechanism of flash sintering. Flash sintering occurs when the processing conditions—including the temperature of the sample; the properties of thermal insulation; and the intensity of microwave radiation—facilitate the development of thermal runaway due to an Arrhenius-type dependency of the material’s effective conductivity on temperature. The proper control over the thermal runaway effect is provided by fast regulation of the microwave power. The elevated concentration of defects and impurities in the boundary regions of the grains leads to localized preferential absorption of microwave radiation and results in grain boundary softening/pre-melting. The rapid densification of the granular medium with a reduced viscosity of the grain boundary phase occurs via rotation and sliding of the grains which accommodate their shape due to fast diffusion mass transport through the (quasi-)liquid phase. The same mechanism based on a thermal runaway under volumetric heating can be relevant for the effect of flash sintering of various oxide ceramics under a dc/ac voltage applied to the sample.

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Yb:(LaY)2O3 ceramic samples have been sintered to almost full density in a fast microwave heating process with zero hold time. Rapid densification is observed at heating rates ranging from 50°C to 2400°C/min. The estimated value of the power absorbed in the materials per unit volume is from 10 to 400 W/cm3, which is similar to the processes of flash sintering under an applied dc/ac voltage. The microstructure of the sintered samples exhibits traces of a liquid‐like intergranular phase. The observed flash microwave sintering effect is associated with both preferential microwave absorption and enhanced mass transport within the softened grain boundaries arising due to an elevated concentration of defects and impurities therein. The volumetric nature of microwave heating gives rise to thermal stresses which can act as an additional driving force for sintering. The advantage of the microwave flash sintering process is that no electrodes are needed to supply the power to the articles undergoing sintering.