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In January SSRL hosted the SLAC/DESY "International Workshop on Interactions of Intense Sub-Picosecond X-ray Pulses with Matter". Researchers from five countries and 12 different institutions attended the two-day event organized by SSRL staff members Roman Tatchyn and John Arthur, DESY staff members Gerd Materlik and Jupp Feldhaus, and ESRF staff member Andreas Freund.
The workshop was associated with two ongoing design studies of high energy, linac-driven Free Electron Lasers (FELs) intended for operation at Ångstrom wavelengths. The project under study at SLAC, the Linac Coherent Light Source (LCLS), will utilize a laser-driven rf photocathode gun injector and 1 km of the SLAC linac to drive an ultra-low-emittance 5-15 GeV electron beam through a 100 meter long undulator. The LCLS will produce coherent radiation with a fundamental energy range of ~900-8500 eV. The DESY project (TESLA FEL) has a similar goal, but it is basing its beam acceleration on superconducting linac technology, which should allow it to operate with a higher duty cycle (average output power). Both devices will initially operate as amplifiers in the so-called Self-Amplified Spontaneous Emission (SASE) mode, producing similar 100-300 femtosecond long X-ray pulses featuring full transverse coherence, 10-100 GW of peak power, and unprecedented peak power densities of 1015 to 1016 W/cm2.
Over the period 1992-1996 there have been several well-attended scientific workshops, hosted by both SSRL and DESY, on the potential scientific applications of this new category of radiation. A major concern common to most of the participating scientific communities has been the question of how the extraordinarily high peak power densities of the LCLS and TESLA FEL will affect the various experiments that have been proposed for these facilities. Specific concerns include: 1) damage to beam line optics and instrumentation, 2) damage to samples, 3) non-linear effects, and 4) saturation, and other loading effects on the physical processes being studied. The goal of the January workshop was to systematically address these concerns and to begin to formulate R&D plans and collaborations for addressing and resolving these physical issues.
The workshop program was structured as a sequence of invited tutorial presentations, contributed talks, and discussion sessions. The tutorial lectures reviewed the status of theory, experimentation, and computer simulation of the interactions of ultra-short, ultra-intense radiation pulses with matter. The contributed talks covered a number of topics related to experimental techniques and instrumentation, and the discussion sessions were spent reviewing the presentations and attempting to formulate plans for meaningful theoretical and experimental work.
Several conclusions were reached by the end of the workshop. Most importantly, representatives from LLNL, LANL, and other institutions expressed their intent to seek support for the extension of their existing theories and computer codes into the short-wavelength parameter regime of the LCLS. This would allow detailed calculation of the effects of X-ray FEL radiation on materials. This work is a natural outgrowth of programs at these laboratories based on high-power optical lasers; extension of the work into the X-ray region may uncover new fundamental physics that can be tested experimentally at LCLS, and it will also be useful for predicting the performance of LCLS optical components such as mirrors, crystals, and absorbers. As a result of the workshop, SSRL has entered into cooperative research agreements in this area with LLNL and LANL.
A second conclusion concerns the possibility of using existing sources to approximate the parameters of the LCLS beam for demonstration experiments. The consensus was that insertion devices on existing storage rings probably could not attain the required parameters. However, useful demonstration studies could possibly be performed with a partially completed LCLS using an undulator substantially shorter than the one required for the full-scale LCLS.
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