Search Results (10 total)

We report on a resonant magnetic scattering experiment using soft x-ray pulses generated from a free-electron laser (FEL). The free-electron laser was operated at a fundamental wavelength of 7.97 nm and radiation at the fifth harmonic originating from self-amplified stimulated emission at 1.59 nm with an average energy of 4 nJ per pulse was detected. We demonstrate the feasibility of resonant magnetic scattering at FEL sources by using a Co/Pd multilayer as prototype sample that was illuminated with 20-fs-long soft x-ray pulses tuned to the Co L₃ absorption edge at 778.1 eV (1.59 nm).

Coherent diffractive imaging for the reconstruction of a two-dimensional (2D) finite crystal structure with a single pulse train of free-electron laser radiation at 7.97 nm wavelength is demonstrated. This measurement shows an advance on traditional coherent imaging techniques by applying it to a periodic structure. It is also significant that this approach paves the way for the imaging of the class of specimens which readily form 2D, but not three-dimensional crystals. We show that the structure is reconstructed to the detected resolution, given an adequate signal-to-noise ratio.

We present a numerical investigation of the Brownian motion and diffusion of a dumbbell in a two-dimensional periodic potential. Its dynamics is described by a Langevin model including the hydrodynamic interaction. With increasing values of the amplitude of the potential we find along the modulated spatial directions a reduction of the diffusion constant and of the impact of the hydrodynamic interaction. For modulation amplitudes of the potential in the range of the thermal energy the dumbbell diffusion exhibits a pronounced local maximum at a wavelength of about 3/2 of the dumbbell extension. This is especially emphasized for stiff springs connecting the two beads.

The relation between saddle points of the potential of a classical many-particle system and the analyticity properties of its thermodynamic functions is studied. For finite systems, each saddle point is found to cause a nonanalyticity in the Boltzmann entropy, and the functional form of this nonanalytic term is derived. For large systems, the order of the nonanalytic term increases unboundedly, leading to an increasing differentiability of the entropy. Analyzing the contribution of the saddle points to the density of states in the thermodynamic limit, our results provide an explanation of how, and under which circumstances, saddle points of the potential energy landscape may (or may not) be at the origin of a phase transition in the thermodynamic limit. As an application, the puzzling observations by Risau-Gusman et al. [Phys. Rev. Lett. 95, 145702 (2005)] on topological signatures of the spherical model are elucidated.

The VUV and soft x-ray free electron laser FLASH (former VUV-FEL) is a user facility at DESY (Hamburg). In order to optimize the performance of the facility, an accurate characterization of the electron beam properties is essential. The transverse projected emittance, one of the important parameters characterizing the quality of an electron beam, is measured using a four monitor method with optical transition radiation monitors. A normalized rms emittance below 2 mm mrad for a 1 nC beam has been measured. In this paper we describe the experimental setup, data analysis methods, and present experimental results.

The Fermilab photoinjector produces electron bunches of 1–12 nC charge with an energy of 16–18 MeV. Detailed measurements and optimization of the transverse emittance have been carried out for a number of beam line optics conditions, and at a number of beam line locations. The length of the bunches has also been measured, first for an uncompressed beam (as a function of the charge) and then for a compressed beam of 8 nC charge (as a function of the 9-cell cavity phase). These measurements are presented and compared with the simulation codes HOMDYN and ASTRA.

Amorphous hydrogenated silicon suboxides (a-SiO_x:H) deposited by plasma enhanced chemical vapor deposition have a band gap which can be tuned from 1.9 to 3.0 eV by varying the oxygen content [O] from 0 to 50 at. %. n- and p-type doping is realized by adding PH₃ and B₂H₆, respectively, to the source gases SiH₄, H₂, and CO₂. Alloying with increasing amounts of oxygen reduces the average coordination number 〈r〉 from a value close to 4 (a-Si:H) to ≈2.7, which gradually approaches the ideal value of 〈r〉=2.4 for network glasses. This goes along with a softening of the amorphous SiO_x network, i.e., a reduction of the mechanical hardness of the material, which is also predicted by rigidity percolation theory. Also the incorporation of dopant atoms into electrically active, fourfold coordinated sites becomes more unlikely with increasing [O]. As a consequence, n- and p-type doped SiO_x shows increasingly intrinsic character for higher oxygen concentrations. Doping fails for values of 〈r〉<3 and the doping efficiency tends towards zero. Thus, an overall fourfold coordination was found to be a crucial requirement for efficient doping in amorphous semiconductors.

An alternative layout of the TESLA linear collider (Technical Design Report, DESY Report No. 2001-011, 2001), based on weakly coupled multicell superconducting structures (superstructures), significantly reduces the investment cost due to a simplification in the rf system of the main accelerator. In January 1999, preparation of the beam test of the superstructure began in order to prove the feasibility of this layout. Progress in the preparation was reported frequently in Proceedings of TESLA Collaboration Meetings. Last year, two superstructures were installed in the Tesla Test Facility linac at DESY to experimentally verify methods to balance the accelerating gradient in a weakly coupled system, the stability of the energy gain for the entire train of bunches in macropulses, and the damping of higher order modes. We present results of the first cold and beam test of these two Nb prototypes.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30–100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95–105 nm.

We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.