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.
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 experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 10 7 in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise.
Sputter-deposited amorphous carbon coatings of high optical quality and very high radiation stability will be used as reflecting optical elements for free electron laser (FEL) applications in the vacuum ultraviolet (VUV) and soft x-ray spectral regions. Their reflectivity at grazing angles of 2° is typically 94–96% for photon energies between 50 and 250 eV. Using the focused radiation (λ=98 nm) of the VUV FEL at Deutsches Elektronen-Synchrotron DESY the damage threshold of such coatings was determined to 0.07 J/cm2. Ripples with a spatial period of 76 nm were found on the surfaces damaged by the FEL radiation.
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