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thin-walled cylinder tubes of different sizes made of copper and aluminum under axial compression test were performed in an Instron machine. The diameter to thickness (DIt) and length to diameter (LID) ratios were varied from 18.4 to 50.76 and 1 to 3 for both metals respectively. All these tubes deformed in several modes and the effects of both ratios on this failure mode are appropriately studied. Load-displacement charts as a comparison behavior between copper and aluminum metals are presented. The amount of absorbed energy with maximum and average collapse loads are tabulated to show which of the two metal’s behavior is better in terms of energy absorption principle. Therefore, results presented are of help in understanding the collapse behavior of circular tubes under quasi-static axial loading when used in energy absorption dissipation systems.

In countries where wind energy is less feasible, an energy harvesting device such as highway wind turbines can still be used for applications where less power is needed. This energy can be harveste...

Multiphoton microscopy of cellular autofluorescence and second harmonic generation from collagen facilitates imaging of living cells and tissues without the need for additional fluorescent labels. Here, a compact multiphoton endomicroscope for label‐free in vivo imaging in small animals via side‐viewing needle objectives is presented. Minimal invasive imaging at cellular resolution is performed in colonoscopy of mice without surgical measures and without fluorescent dyes as a contrast agent. The colon mucosa is imaged repeatedly in the same animal in a mouse model of acute intestinal inflammation to study the process of inflammation at the tissue level within a time period of ten days, demonstrating the capabilities of label‐free endomicroscopy for longitudinal studies for the first time.

Physical review / A 99(4), 043423 (2019). doi:10.1103/PhysRevA.99.043423, We apply THz-field streaking to temporally resolve the ultrafast ionization of neutral cluster vs atomic targets exposed to intense ultrashort soft x-ray pulses from a free-electron laser (FEL). In the experiment pristine Xe and mixed Xe/Ar clusters as well as Xe atoms are ionized in the vicinity of the $4d→ɛf$ giant resonance of Xe. We compare the relative streaking delay between the center-of-mass oscillations of electrons that have final kinetic energies in the spectral region of the Xe($4d$) photoline. Our results show that clusters are ionized at the beginning of the 100 fs FEL pulse as supported by calculations of target frustration in the focal volume. We have identified a significantly larger 40 fs relative streaking delay between mixed Xe-Ar clusters vs Xe atoms compared to a 15 fs relative delay found for pristine Xe clusters. This is attributed to the high sensitivity of our spectroscopic measurement to the degree of condensation of the cluster target. Our results show that THz streaking is a powerful technique to temporally study electron emission from extended targets under intense FEL radiation on time scales that are significantly shorter than the FEL pulse duration., Published by APS, Woodbury, NY

Physical review letters 122(7), 073001 (2019). doi:10.1103/PhysRevLett.122.073001, Time delays for atomic photoemission obtained in streaking or reconstruction of attosecond bursts by interference of two-photon transitions experiments originate from a combination of the quantum mechanical Wigner time and the Coulomb-laser coupling. While the former was investigated intensively theoretically as well as experimentally, the latter attracted less interest in experiments and has mostly been subject to calculations. Here, we present a measurement of the Coulomb-laser coupling-induced time shifts in photoionization of neon at 59.4 eV using a terahertz (THz) streaking field (λ=152 μm). Employing a reaction microscope at the THz beamline of the free-electron laser in Hamburg (FLASH), we have measured relative time shifts of up to 70 fs between the emission of 2p photoelectrons (∼38 eV) and low-energetic (, Published by APS, College Park, Md.

Synchrotron radiation news 26(6), 27 - 32 (2013). doi:10.1080/08940886.2013.850384, The free-electron laser (FEL) sources FLASH in Hamburg, LCLS at Stanford, and FERMI in Trieste provide XUV to soft X-ray radiation (FLASH and FERMI) or soft to hard X-ray radiation (LCLS) with unprecedented parameters in terms of ultrashort pulse length, high photon flux, and coherence. These properties make FELs ideal tools for studying ultrafast dynamics in matter on a previously unaccessible level. This paper first reviews results obtained at FEL sources during the last few years in the field of magnetism research. We start with pioneering experiments at FLASH demonstrating the feasibility of magnetic scattering at FELs [1, 2], then present pump–probe scattering experiments [3, 4] as well as the first FEL magnetic imaging experiments [5], and finally discuss a limitation of the scattering methods due to a quenching of the magnetic scattering signal by high-fluence FEL pulses [6]. All of the presented experiments exploit the X-ray magnetic circular dichroism effect [7, 8] to obtain element-specific magnetic scattering contrast, as known from synchrotron experiments [9–12]., Published by Taylor & Francis, Philadelphia, Pa.

Scientific reports 7, 40736 (2017). doi:10.1038/srep40736, Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas., Published by Nature Publishing Group, London

The paper is a mini review on the authors own work related to laser induced processes at metal nanoparticle/oxidic support systems. A number of aspects will be addressed ranging from the nano particle size dependence on the efficiency of the photochemical process, the influence of low coordination sites of the nano particles and dynamical processes such as energy transfer between adsorbates, light induced particle changes or spill over processes. The paper reviews nano and femtosecond laser experiments as well as quantum state resolved experiments. Results will be compared to typical findings from simple oxidic supports and metal single crystals. This review may be of interest for future photocatalyst development for applications such as solar fuel synthesis and related problems.

One of the most challenging tasks for extreme ultraviolet, soft and hard x ray free electron laser photon diagnostics is the precise determination of the photon pulse duration, which is typically in the sub 100 fs range. Nine different methods, able to determine such ultrashort photon pulse durations, were compared experimentally at FLASH, the self amplified spontaneous emission free electron laser at DESY in Hamburg, in order to identify advantages and disadvantages of different methods. Radiation pulses at a wavelength of 13.5 and 24.0 nm together with the corresponding electron bunch duration were measured by indirect methods like analyzing spectral correlations, statistical fluctuations, and energy modulations of the electron bunch and also by direct methods like autocorrelation techniques, terahertz streaking, or reflectivity changes of solid state samples. In this paper, we present a comprehensive overview of the various techniques and a comparison of the individual experimental results. The information gained is of utmost importance for the future development of reliable pulse duration monitors indispensable for successful experiments with ultrashort extreme ultraviolet pulses

At the ELBE accelerator a unique super-radiant THz source is currently under developement. It aims at delivering fourier-limited THz pulses with pulse energies of up to 100 microJ at reprates of up to 500 kHz (cw). This corresponds to transient electric elds in the GV/m regime or transient magnetic elds in the few T regime. First results from the commissioning are discussed.

At the Helmholtz-Zentrum Dresden-Rossendorf near Dresden a quasi-cw low-energy electron linear accelerator based on superconducting radiofrequency technology is operated successfully for more than 10 years. The ELBE accelerator is driving several secondary radiation sources including 2 infrared free electron lasers. A new addition will be a THz facility that aims to make use of super-radiant THz radiation. In its final form the THz facility shall consist of one coherent diffraction radiator and one undulator source which provide high-field THz pulses at unprecedented repetition rates. While the medium term goal is to establish a unique user facility for nonlinear THz science, the THz sources are already used as a test facility for novel diagnostic techniques on quasi-cw electron beams. The progress of the developments is reported and an outlook into future challenges and opportunities is given., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Free-electron lasers (FELs), with their high photon flux, short pulse lengths, and coherent radiation, are ideal tools for studying complex materials, simultaneously on sub-picosecond time and on nanometer length scales. In particular, by exploiting the magneto-dichroic transitions at the L or M edges of Fe, Co, Ni or Gd, temporal and spatial studies of magnetic systems can be performed. Infrared-pump–XUV-probe experiments performed at a FEL have revealed that the optically induced demagnetization in magnetic maze-domain structured Co/Pt samples is also accompanied by a spatial response. In this, and other studies using optical (IR) pumping, the magnetization is manipulated by heating of the spin system, i.e. through ultrafast electronic excitations. Using terahertz (THz) radiation for pumping gives a unique possibility to investigate the influence of ultra-short pulses of strong magnetic fields on the magnetization dynamics since the THz photons do not generate direct electronic heating and because the THz phase can couple directly to the magnetic moments. Hence, the use of THz radiation could bring one a step closer to being able to control magnetism on ultrafast time scales.

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br-2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br-2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br-2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.

Within the ELBE upgrade towards a Center for High Power Radiation Sources (HSQ), a mono energetic positron, a liquid lead photo neutron source and two new THz sources have been installed at the superconducting electron linac at ELBE. A variety of established as well as newly developed electron beam diagnostics were installed and tested. In this paper we want to present first results achieved with the currently existing prototype beam arrival time and bunch compression monitors (BAM, BCM) as well as one versatile EOS set-up. Based on these future developements and upgrades are discussed.

We present results of single-shot resonant magnetic scattering experiments of $\mathrm{Co}/\mathrm{Pt}$ multilayer systems using 100 fs long ultraintense pulses from an extreme ultraviolet (XUV) free-electron laser. An x-ray-induced breakdown of the resonant magnetic scattering channel during the pulse duration is observed at fluences of $5\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$. Simultaneously, the speckle contrast of the high-fluence scattering pattern is significantly reduced. We performed simulations of the nonequilibrium evolution of the $\mathrm{Co}/\mathrm{Pt}$ multilayer system during the XUV pulse duration. We find that the electronic state of the sample is strongly perturbed during the first few femtoseconds of exposure leading to an ultrafast quenching of the resonant magnetic scattering mechanism.

Nature Communications 4, 1731 (2013). doi:10.1038/ncomms2754, The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations., Published by Nature Publishing Group, London

The radiation produced by a Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) leaves a specific energy pattern over the length of the radiating electrons. This pattern, the SASE Fingerprint, leads to a spikey energy structure modulation on the order of the coherence length on the electron bunches. Using a dispersion section and an additional radiator this modulation can be used to recreate the SASE pulse envelope in a different wavelength regime (optical afterburner radiation) [1]. Using this unique feature, at the Free Electron Laser in Hamburg (FLASH) we demonstrated that the pulse duration of an XUV FEL can be obtained using standard laser diagnostics (i.e. FROG). Cross correlation with a multitude of different methods show best agreement down to several femtoseconds. [2] Future development reaches from online pulse duration measurement to amplification and jitter-free pump probe experiments. References [1] E.L. Saldin, E.A. Schneidmiller and M.V. Yurkov, Phys. Rev. ST Accel. Beams 13, 030701 (2010) [2] T. Golz, Thesis: Pulse duration measurements at 4th generation X-ray light sources – Frequency Resolved Optical Gating at FLASH (2012)

At the SRF based prototype cw accelerator ELBE a new electron beamline, providing for femtosecond electron bunches with nC bunch charges and repetition rates in the 1-200 KHz regime and with pC bunch charge and repetition rates of 13 MHz, is currently being constructed. The 40 MeV electrons will be used in photon-electron interaction experiments with TW and PW class lasers and the generation of broad and narrow bandwidth coherent THz pulses. Discussed here are ideas for novel online diagnostics of the electron bunch properties (e.g. arrival time and bunch form) based on the time and frequency domain analysis of the emitted coherent THz radiation, but also based on direct measurements by e.g. electro-optic sampling. The suitability of ELBE as a testbed for diagnostic of future cw X-ray photon sources (e.g. energy recovery linacs) will be discussed.

We present a time-resolved infrared (IR) pump and extreme-ultraviolet (XUV) probe diffraction experiment to investigate ultrafast structural dynamics in colloidal crystals with picosecond resolution. The experiment was performed at the FLASH facility at DESY with a fundamental wavelength of 8 nm. In our experiment, the temporal changes of Bragg peaks were analyzed and their frequency components were calculated using Fourier analysis. Periodic modulations in the colloidal crystal were localized at a frequency of about 4-5 GHz. Based on the Lamb theory, theoretical calculations of vibrations of the isotropic elastic polystyrene spheres of 400 nm in size reveal a 5.07 GHz eigenfrequency of the ground (breathing) mode., 12 pages, 3 figures

Superconducting radiofrequency (SRF) accelerator technology allows to accelerate electron bunches not only in considerably longer bunch trains than is possible by normal conducting accelerator technology, but also in principle enables a mode of operation that can circumvent any macropulse scheme and provides relativistic electron bunches with real repetition rates scaling from the kHz to GHz regime [1, 2, 3, 4]. In this proceedings we discuss different approaches foreseen for online electron bunch diagnostic at the new femtosecond electron beamline of the superconducting cw RF accelerator ELBE [11]. In medium term we believe the concepts tested at the femtosecond beamline at ELBE are off great importance for various energy recovvery linac accelerators (ERL) coming online in the next few years [2, 4] as well as facilities like the European X-FEL [5] and FLASH [6] when operated in long bunchtrain mode of operation.

At the srf based prototype cw accelerator ELBE a new electron beamline, providing for femtosecond electron bunches with nC bunch charges and repetition rates in the 1 – 200 KHz regime and with pC bunch charge and repetition rates of 13 MHz is currently constructed. The 40 MeV electrons will be used in photon-electron interaction experiments with TW and PW class laser and the generation of broad band and narrow bandwidth coherent THz pulses. In this paper we outline ideas for novel online diagnostics of the electron bunch properties (e.g. arrival time and bunch form) based on the time and frequency domain analysis of the emitted coherent THz radiation but also based on direct measurements by e.g. electro-optic sampling. The suitability of ELBE as a testbed for diagnostic of future cw X-ray photon sources (e.g. energy recovery linacs) will be discussed.

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\ensuremath{\Delta}E/E={10}^{\ensuremath{-}4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.