We describe the design concepts for a potential future source of femtosecond x-ray pulses based on synchrotron radiation production in a recirculating electron linac. Using harmonic cascade free-electron lasers (FEL's) and spontaneous emission in short-period, narrow-gap insertion devices, a broad range of photon energies are available with tunability from EUV to hard x-ray regimes. Photon pulse durations are controllable and range from 10 fs to 200 fs, with fluxes 107-1012 photons per pulse. Full spatial and temporal coherence is obtained for EUV and soft X-rays. A fiber laser master oscillator and stabilized timing distribution scheme are proposed to synchronize accelerator rf systems and multiple lasers throughout the facility, allowing timing synchronization between sample excitation and X-ray probe of approximately 20-50 fs.
For the determination of absolute photon fluxes from high-intense, pulsed VUV and soft X-ray sources like free-electron lasers, a gas-monitor detector system based on the photoionization of rare gases was developed. A prototype system was successfully used for the characterization of VUV free-electron laser radiation at the TESLA test facility (phase 1) in Hamburg. Pulse-resolved measurements at peak powers of more than 100 MW at a wavelength of 87 nm were demonstrated. In order to provide a photon-beam diagnostic of VUV-FEL radiation during phase 2 of the TTF project, a set of four new detectors has been constructed, based on the prototype. The new detector system can be used not only for intensity measurement and monitoring, but also for measuring the beam position. The detector set was calibrated in the Radiometry Laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II. The calibration was performed using spectrally-dispersed synchrotron radiation at low intensities and a semiconductor photodiode as a transfer standard.
Most proposed linac-based light sources, such as single-pass free-electron lasers and energy-recovery-linacs, require very high-brightness electron beams in order to achieve their design performance. These beam requirements must be achieved not on an occasional basis, but rather must be met by every bunch produced by the source over extended periods of time. It is widely assumed that the beam source will be a photocathode electron gun; the selection of accelerator technique (e.g., dc or rf) for the gun is more dependent on the application.
The current state of the art of electron beam production is adequate but not ideal for the first generation of linac-based light sources, such as the Linac Coherent Light Source [ ] (LCLS) x-ray free-electron laser (X-FEL). For the next generation of linac-based light sources, an order of magnitude reduction in the transverse electron beam emittance is required to significantly reduce the cost of the facility. This is beyond the present state of the art, given the other beam properties that must be maintained. The requirements for current and future linac-based light source beam sources are presented here, along with a review of the present state of the art. A discussion of potential paths towards meeting future needs is presented at the conclusion.
A stigmatic spectrometer for the 2.5-40 nm EUV region has been realized. The design consists of a grazing-incidence spherical variable-line-spaced grating with flat-field properties and of a spherical mirror mounted in the Kirkpatrick-Baez configuration that compensates for the astigmatism. The spectrum is acquired on a fluorescent screen and intensified CCD detector, that can be moved along the spectral focal curve to select the spectral region to be acquired. The spectral and spatial resolution of the system have been characterized by using the emission from an hollow-cathode lamp and a laser-produced plasma. At present, the instrument is installed at the VUV-FEL at DESY for the spectral monitoring of the FEL beam in the 20-45 nm region.
We present a simple setup for obtaining high resolution, sub-micron images using high harmonic generation (HHG) in a hollow-core waveguide as a light source. We demonstrate imaging with illumination at a wavelength of 30 nm using an all-reflective, double-multilayer mirror setup and a CCD camera as a recording device. For the magnifications of up to 50x used here, the all-reflective setup has advantages over zone plate microscopes because of the much larger working distances that allow for imaging of plasmas. This setup has also a throughput that is higher by at least a factor of three compared to zone-plate microscopes, and presents the additional advantage of preserving the temporal pulse width of the harmonics because diffractive optics are not used. This work demonstrates the feasibility of high-spatial-resolution, time-resolved, EUV imaging of plasmas and other objects using a tabletop compact light source.
We demonstrate the use of a tabletop capillary-discharge Ne-like Ar laser emitting nanosecond duration pulses at a wavelength of 46.9 nm for investigation of radiation damage mechanism and damage threshold in Sc/Si extreme ultraviolet multilayer mirrors. To vary the emission load at the surface of the mirror under test the intense 0.13 mJ laser pulses were focused using a spherical Sc/Si multilayer mirror to obtain fluences ranging from ~ 0.01 to >10 J/cm2. Single spots and large area patterns (2x2 mm2) were irradiated depending on the type of surface analysis technique employed. Damage threshold fluences of ~ 0.08 J/cm2 were measured for Sc/Si coatings deposited on both borosilicate glass and Si substrates, compared to the 0.7 J/cm2 found necessary to damage a bare Si substrate. The use of scanning and transmission electron microscopy, and small-angle X-ray diffraction techniques revealed the thermal nature of the damage mechanism. These results are relevant to the use of the Sc/Si mirrors in combination with newly developed high power EUV sources, and provide a benchmark for their further improvement.
The planned Linac Coherent Light Source (LCLS) will operate at a rate of 120 x-ray pulses per second. Therefore, when considering effects leading to the damage to its optics, one has to be concerned not only with a possible damage within one pulse, but also with effects accumulating during many pulses. Two of such effects: a thermal fatigue, and the intensity-dependent radiation damage, have been identified and analyzed in this paper. Constraints on the admissible fluence per pulse have been derived and ways for decreasing the impact of the multi-pulse effects have been suggested.
An international co-operative project to develop an advanced X-ray source on the basis of Compton back-scattering is in progress. The goal is the re-configuration of the Kharkov Institute of Physics and Technology (KIPT) N-100M storage ring to support the efficient interaction of its electron beam with a high power pulsed-laser cavity. At equilibrium both the electron and X-ray beams' phase space characteristics are determined by a balance of stochastic photon cooling and emission. In this paper the operating parameters and fundamental spectral properties of the novel source are described.
The temporal structure and phase evolutions of a high-gain, self-amplified free-electron laser are measured, including single-shot analysis and statistics over an ensemble of many shots. Excellent agreement with the theory of free-electron laser (FEL) and photon statistics is found. This is an important step towards understanding and controlling the radiation in such FEL pulses.
We summarize the technology status of undulators suitable for Vacuum Ultraviolet (VUV) sources. Planar, biharmonic, multi-harmonic and elliptically polarized undulator designs and performance will be reviewed. The present state of the art in virtual prototyping, manufacturing and tuning will also be discussed.
Laser-based plasma spectroscopic techniques have been used with great success to determine the line shapes of atomic transitions in plasmas, study the population kinetics of atomic systems embedded in plasmas, and look at the redistribution of radiation. However, the possibilities for optical lasers end for plasmas with ne > 1022 cm-3 as light propagation is severely altered by the plasma. The construction of the Tesla Test Facility (TTF) at DESY (Deutsche Elektronen-Synchrotron), a short pulse tunable free electron laser in the vacuum-ultraviolet and soft X-ray regime (VUV FEL), based on the SASE (self amplified spontaneous emission) process, will provide a major advance in the capability for dense plasma-related research. This source will provide 1013 photons in a 200 fs duration pulse that is tunable from ~6 nm to 100 nm. Since an VUV FEL will not have the limitation associated with optical lasers the entire field of high density plasmas kinetics in laser produced plasma will then be available to study with the tunable source. Thus, one will be able to use this and other FEL x-ray sources to pump individual transitions creating enhanced population in the excited states that can be easily monitored. We show two case studies illuminating different aspects of plasma spectroscopy.
For conventional wavelength (UV-Vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (l < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes it possible to investigate how the ablation characteristics depend on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser (Colorado State University, Fort Collins), focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with those measured for ultrashort pulses, we can study the influence of pulse duration on XUV ablation efficiency. Ablation efficiencies measured with short pulses at various wavelengths (i.e. 86/46.9/21.2 nm from the above-mentioned lasers and ~ 1 nm from the double stream gas-puff Xe plasma source driven by PALS) show that the wavelength influences the etch rate mainly through the different attenuation lengths.
We propose the use of a ultra-relativistic electron beam interacting with a few-cycle, intense laser pulse and an intense pulse of the coherent x-rays to produce a multi-MW intensity, x-ray pulses
approximately 100 attoseconds in duration. Due to a naturally-occurring frequency chirp, these pulses can be further temporally compressed.
Parabolic refractive x-ray lenses are high quality imaging optics for hard x-rays. They are well suited for full field and scanning microscopy with hard x-rays. They are robust and can withstand the heat load of the white beam of an ESRF undulator source. The microbeam properties expected for focusing the XFEL beam with refractive lenses are estimated. The stability of the optic in the X-FEL beam is considered. Beryllium parabolic refractive lenses have been installed at the SPPS at SLAC. First results of the commissioning of these optics are reported.
We demonstrated a significant improvement in the resolution of the x-ray streak camera by reducing the electron beam size in the deflection plates. This was accomplished by adding a slit in front of the focusing lens and the deflection plates. The temporal resolution reached 280 fs when the slit width was 5 mm. The camera was operated in an accumulative mode and tested by using a 25 fs laser with 2 kHz repetition rate and 1-2% RMS pulse energy stability. We conclude that deflection aberrations, which limit the resolution of the camera, can be appreciably reduced by eliminating the wide-angle electrons.
Next generation x-ray sources require very high-brightness electron beams that are typically at or beyond the present state-of-the-art, and thus place stringent and demanding requirements upon the electron injector parameters. No one electron source concept is suitable for all the diverse applications envisaged, which have operating characteristics ranging from high-average-current, quasi-CW, to high-peak-current, single-pulse electron beams. Advanced Energy Systems, in collaboration with various partners, is developing several electron injector concepts for these x-ray source applications. The performance and design characteristics of five specific RF injectors, spanning "L" to "X"-band, normal-conducting to superconducting, and low repetition rate to CW, which are presently in various stages of design, construction or testing, is described. We also discuss the status and schedule of each with respect to testing.
Recent measurements of the absolute diffraction efficiency of plane gratings in the conical diffraction mounting (in which the light approaches the grating in the plane parallel to the direction of the grooves) are presented. Three gratings have been tested at the beamline BEAR (Elettra Synchrotron, Trieste) in the 10-130 nm region, showing a peak efficiency as high as 70%. The aim of these measurements is the use of two gratings in the conical diffraction mounting for the realization of a high-throughput time-compensated monochromator for the spectral selection of high-order harmonic radiation produced by the interaction between an ultrashort laser pulse (less than 100 fs) and a gas jet. The monochromatic and ultrashort pulse at the monochromator exit can be used for the injection of a Free Electron Laser. The theory of the time-compensation with gratings will be briefly resumed, the design of the monochromator will be presented, and the results of the measurements at BEAR will be discussed.