| |
The laser was invented 50 years ago. About ten years later John Madey of Stanford University published a seminal paper entitled "Stimulated Emisssion of Bremsstrahlung in a periodic Magnetic Field" describing the induced coherent radiation from the motion of a relativistic electron through a magnetic field. This was the birth of the free electron laser, FEL. In 1976 Madey and co-workers demonstrated that the FEL concept worked. The gain was obtained by using the reflection of mirrors in an optical cavity, an approach that for about 25 years limited the available spectral region to the infrared, visible and near VUV spectral region. With the development of the so-called SASE (Self Amplified Spontaneous Emission) concept this limitation was eliminated. The FLASH facility at DESY, Hamburg, is based on SASE and has been in operation for 5 years for users, reaching ever shorter wavelengths and presently operating down to 4.5 nm.
In this talk I will describe recent developments that culminated with the successful first lasing in the spectral region 0.15-1.5 nm at the Linac Coherent Light Source (LCLS) at Stanford in April 2009. The first hard x-ray FEL became a reality. LCLS is based on the last 1000 meters of the 3 km long linear accelerator at SLAC National Accelerator Laboratory (with electron beam energies 4-14 GeV), a 130 meter long undulator to produce the coherent x-rays, and two experimental halls separated by 200 meters. The FEL radiation has unprecedented properties in terms of brilliance, with 1-500 femtosecond pulse widths, 0-4 mJ pulse energies (up to 10^12 photons per pulse),and tunability. LCLS will open up novel research opportunities in a number of fields: high-field and femtosecond physics of atoms and molecules, imaging of nanostructures and biomolecules (including non-periodic structures), diffraction studies of stimulated dynamics (e.g. phase transitions, charge transfer reactions) with pump-probe techniques, coherent scattering of nanoscale fluctuations, and high-energy density science (warm dense matter and plasma physics). LCLS started operation for users in October 2009 and has now three experimental stations in use covering the spectral region 480-9000 eV. The experimental program is still in its infancy but I will illustrate the revolutionary new science with a few examples. In particular I will focus on nanocrystallography where the structure and morphology of 10-500 nm single nanocrystals have been determined in single-shot experiments. I will also describe the plans for the implementation of beamlines for coherent x-ray imaging and x-ray photon fluctuation spectroscopy that will be available in 2011. Finally the plans for upgrading LCLS to include more convenient photon energy tenability, polarization control, and seeding to produce transform limited x-ray pulses.
|
