The main rationale for new X-ray sources is that since their discovery in 1895, X-rays have been the single most powerful technique for determining the structure of all forms of condensed matter. Through increasingly powerful imaging, diffraction, and spectroscopic techniques, physicists, chemists, biologists, and medical doctors, as well as quality-control inspectors, airline passenger screeners, and forensic scientists have resolved the structural detail and elemental constituency on length scales from inter-atomic spacing to the size of the human body. Every day, that knowledge underpins our modern technologies, our health, and our safety. Quite remarkably, for the first 70 of these 119 years, X-ray sources changed little from the original Roentgen tube. Even today, the X-ray technology used in universities, industrial labs and hospitals derives from this primitive electron tube with incremental improvements.
Today, however, the benchmark for X-ray performance is set by large accelerator-based synchrotron radiation facilities, such as the Advanced Photon Source, of which more than 60 exist worldwide. Interestingly, about three-quarters of Nobel Prizes awarded for X-ray-based discoveries used small conventional or rotating-anode-based X-ray sources. Notwithstanding that the most recent 5 prizes have been awarded for work based on large facilities, the wide availability of the small sources, their ease of use in hospital or industrial settings, and their capability to test new ideas without the barriers of schedule, travel, and expense of the major facilities are features which will remain very attractive. As our work over the last decade has shown*, it is now possible to exploit recent advances in accelerator and laser technologies that offer the potential for vastly more powerful compact X-ray light sources (CXLS) at a cost that most modern laboratories and hospitals could afford. X-ray beam performance could eventually approach the performance of the large facilities in general, and in critical parameters such as pulse length and source size the CXLS would surpass them.
The technical approach involves inverse Compton scattering (ICS), the up-conversion of a low-energy laser photon to a high energy X-ray by scattering from a relativistic electron produced in a linear accelerator. The geometry of the interaction is a near head-on collision between the laser and electron beams. The scattered X-rays emerge in the same direction as the electrons. The physics of ICS is nearly identical to spontaneous synchrotron emission in a static magnetic undulator as used at the large synchrotron facilities, but because the wavelength of the laser is much shorter than a static undulator period, the energy required of the electrons to make hard X-rays is orders of magnitude lower, thus reducing the size and cost of the proposed light source by orders of magnitude.
Although the baseline performance will not match the standard of large facilities, these compact light sources are expected to provide X-ray parameters similar to that of bending magnet synchrotron beam lines for a small fraction of the cost, far beyond existing lab source performance. Fluxes are calculated to be 2 x 1010 photons/second in a 0.1% bandwidth, about a factor of 10 higher than the most powerful conventional sources. Larger bandwidths enable even higher photon fluxes for methods which can utilize a broader spectrum of radiation. But more importantly beam brilliances would be a factor of 1000 higher than the best conventional sources. Furthermore, in some parameters such as pulse length of a pico-second or less, and source size of a few microns, this CXLS technology will significantly exceed the present standard at the large synchrotron facilities. Polarization control, and approximate axial symmetry of the source are additional advantages. Unlike large facilities, compact sources generally would have only a single instrument per linear accelerator allowing great freedom to optimize the source for the particular application.
* W.S Graves, et al., Compact X-ray source based on burst mode inverse Compton scattering at 100kHz, Physical Review Special Topics-Accelerators and Beams 17, 120721, (2014).
Layout of the components for the compact X-ray source showing the Yb lasers including the one that produces electrons via photoemission and the cryo Yb:YAG amplifier used for ICS. Accelerator components shown include the RF gun, short linac and transport magnets. The ancillary equipment in the background show all the racks needed to house the RF transmitter and power supplies for magnets, vacuum equipment, and lasers.