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A new super bright type of X-ray source is being developed by scientists at the US Department of Energy's Argonne National Laboratory in Illinois. The proposed free electron laser oscillator (X-FELO) can produce X-rays that are one hundred million times brighter than any currently operating laboratory X-ray source. Argonne distinguished fellow Kwang-Je Kim explains how present technology uses undulators to create bright X-ray beams through spontaneous emission at ANL's Advanced Photon Source (APS) and other facilities around the world. Efforts to increase the brightness of such sources has focused on X-ray free electron lasers. One approach exploits self-amplified spontaneous emission (SASE), which could amplify the spontaneous emission of X-rays by a factor of a million or more in a single pass. Someone using SASE will see X-ray brightness whch is ten billion times brighter than a spontaneous emission source during one hundred femtoseconds of pulse duration. On the average, however, the brightness of SASE is higher by about ten thousand times than the brightest APS source currently being delivered. In contrast, in an X-FELO device, first proposed by R. Colella and A. Luccio at a Brookhaven National Laboratory workshop in 1984 uses an optical cavity consisting of two or more Bragg reflectors and carries a pulse of electrons into an undulator as normal, but the X-ray pulses generated are then reflected back into the undulator entrance by Bragg reflectors--a pure single crystal of carbon (in the form of diamond) or α-aluminium oxide (sapphire). Whichever material is used it must have a total loss per pass of less than 20 percent and the reflectivity of each Bragg mirror must be well over 90%. The X-ray pulse then connects with the next electron bunch and travels back along the undulator. Kim explains that this process repeats indefinitely in spiralling feedback loop with the X-ray intensity growing each time until equilibrium is reached. Although the basic concept of X-FELO was introduced more than twenty years ago, the high-quality, low intensity electron beams required for the device under development only emerged within the last few years. For example, the electron beams from Cornell's proposed energy recovery linac will be suitable for an X-FELO. The brightness, or more precisely the spectral brightness, of the emerging X-ray beam is proportional to the intensity of coherent photons per unit spectral bandwidth. While the intensity of an individual X-ray pulse from an X-FELO is lower by about three orders of magnitude than an SASE pulse it is at an extremely narrow bandwidth, three to four orders of magnitude finer than those produced by SASE. Moreover, the pulses come with a repetition rates higher by two to four orders of magnitudes higher than in SASE. An X-FELO user will therefore be able to use an X-ray brightness that is higher on average by about six to eight orders of magnitude brighter than the APS and other synchrotron radiation sources and three to four orders of magnitude brighter than the proposed SASE technology. Kim and colleagues anticipate that such characteristics will make X-FELO very useful in several fields of research. For example, the inelastic X-ray scattering and nuclear resonant scattering experiments currently undertaken on beamlines at the APS are severely limited by the small X-ray flux in the millielectronvolts (meV) bandwidth. An X-FELO will enhance the flux by six to eight orders of magnitude, which will have the effect of reducing the data collection times by the same factor, meaning experiments will be 1 million to 100 million times faster, in other words. Such improvements in the X-ray characteristics also mean that X-FELO is ideally suited to bulk-sensitive, hard X-ray photo-emission spectroscopy, such as that used in time-resolved measurements of the Fermi surface of complex materials including high-temperature superconductors. "Several groups from around the world are working to develop the high-quality electron beam necessary for the oscillator," Kim adds. Kim and Argonne senior scientist Yuri Shvyd'ko published details of their work with physicist Sven Reiche of the University of California Los Angeles in the 20th June issue of Physical Review Letters. There, they report details of a free-electron laser oscillator generating X-rays with wavelengths of about 1 Å, which they explain is feasible using ultralow emittance electron beams of a multi-gigaelectronvolt energy-recovery LINAC, combined with a low-loss crystal cavity. The device, they add, will produce x-ray pulses with 109 photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with an rms bandwidth of about 2 meV, and rms pulse length of about 1 picosecond. Related links:
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