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From the early days of free-electron lasers (FELs), the idea of lasing at wavelengths where gain media had not previously existed was one of the most attractive aspects of these devices. The spectral region in the soft X-ray was both inviting, due to the exciting physics one could do with such a source, and challenging, due to the need for a very bright electron beam (high charge density), very high gain to overcome cavity losses, and the cost of providing all this at very high electron beam energies with high-duty-cycle electron beams. Electron beam brightness is defined as the ratio of the peak current to the product of the horizontal and vertical normalized emittances. The normalized electron beam emittance is the product of the rms transverse size and momentum spread. High gain is usually obtained using a very long undulator, which requires a very small energy spread in addition to the small transverse emittance. Electron beams in the 1970s generally provided either low peak current or large energy spread and very low charge density. Even the VUV spectral range was considered too difficult for the accelerators and optics available then. Early FELs therefore operated in the infrared range. FEL oscillators were eventually pushed into the visible and ultraviolet but could not push into the VUV due to either low gain or poor mirror reflectivity. [ABSTRACT FROM AUTHOR]

We have previously reported on the analytical calculations of mirror distortion in a high-power FEL with a near-concentric cavity. This analysis allowed us to estimate the power level at which the FEL interaction would be affected, though no exact theory of FEL power vs. distortion exists at this point. Recently we have directly measured the mode size and beam quality as a function of power using a resonator with a center wavelength of 5 μm. The resonator mirrors were calcium fluoride. This particular material exhibits a large amount of distortion for a given power but, due to the negative slope of refractive index vs. temperature, adds almost no optical phase distortion on the laser output. The mode in the cavity can thus be directly calculated from the measurements at the resonator output. The presence of angular jitter produced raw measurements inconsistent with cold cavity expectations. Removing the effects of the angular jitter, we derive results in agreement with cold cavity measurements. The resulting indicates that the FEL power is limited for the IR Demo FEL when the Rayleigh range grows by a factor of 4.3. This corresponds to spherical aberration of 0.15λ per mirror. [Copyright &y& Elsevier]

Abstract: Titanium was laser nitrided by means of free-electron laser irradiation in pure nitrogen atmosphere. The variation of pulse frequency and macropulse duration of the free electron laser resulted in δ-TiN x coatings with different thickness and different micro- and macroscopic morphologies. The coatings revealed, characteristic values for hardness, roughness and crystallographic texture, which originate from the growth mechanism, the solid–liquid interface energy and the strain. Further investigations showed that the dendritic growth is beginning at the surface and that the alignment of dendrites is normal to the surface. A correlation of the texture with the time structure of the laser pulses was found. Numerical simulations were performed and compared with the experimental results. The simulations can explain the experimental results. [Copyright &y& Elsevier]