A tunable x-ray source hundreds of times smaller than a conventional synchrotron has been successfully demonstrated by an international team of researchers at Max Planck Institute of Quantum Optics. The feat marks an important step toward building x-ray sources that are more widely available to hospitals for advanced medical diagnostics and therapy. The source is not only small in size but also affordable. Its designers hope that their novel approach to creating coherent, ultrafast, pulsed x-rays will open up the tool to fundamental research including drug discovery, materials science, biology and nanotechnology. “Since current synchrotron sources are big in size [typically hundreds of meters in diameter] and very costly [up to $1 billion], only a few of these facilities exist worldwide, and therefore limit the number of users that can benefit from them,” researcher Matthias Fuchs explained. “Our experiment paves the way for a new generation of brilliant, compact x-ray sources with the potential for widespread application in university-scale laboratories.” What is more, the source delivers photon pulses with an intrinsically ultrashort duration, estimated to be only a few femtoseconds. The duration of comparable synchrotron facilities is typically more than three orders of magnitude longer. In this experiment, a laser pulse (red) is focused into a gas cell in which plasma waves accelerate electrons (yellow) to energies of several hundred megaelectronvolts. The electron beam is collimated by a pair of quadrupole lenses. Plasma radiation and the laser beam are blocked by a 15-μm aluminum foil. The electrons propagate through an undulator and emit soft x-ray radiation into a narrow cone along the forward direction (blue). The radiation is collected by a spherical gold mirror and characterized by a transmission grating in combination with an x-ray CCD camera. Stray light is blocked by a slit in front of the grating. The pointing, divergence and spectrum of the electron beam are diagnosed by phosphor screens. Courtesy of Matthias Fuchs, Max Planck Institute of Quantum Optics. In the experiment, which was described in a Nature Physics paper on Sept. 27, 2009, a high-intensity laser is focused into a gas target, where it separates electrons from their atom core to produce a plasma. As the laser propagates through the plasma, it pushes electrons away just like a snowplow and, in combination with the electric fields of the plasma, generates a so-called “plasma wave,” or wave of electrons. The wave trails the laser pulse at nearly the speed of light, exactly the way a water wave trails a boat. The electrons then are focused into a narrow beam by magnetic lenses and fed into an undulator, a periodic magnetic structure that forces electrons to transversely oscillate. This quivering motion causes them to emit an intense burst of radiation in the soft x-ray range. Ultrarelativistic electrons (yellow) are forced on a sinusoidal trajectory by the periodic magnetic fields of an undulator, emitting short-wavelength radiation (red). Driving these synchrotron sources with laser-accelerated electron beams holds promise to reduce their dimensions from kilometers to a university-laboratory size. Courtesy of Thorsten Naeser. Artwork: Christian Hackenberger. By increasing the energy of the electrons, Fuchs and colleagues can decrease the wavelength of the x-rays into the hard x-ray range. Although the experiment is currently at the proof-of-concept stage, the team is hopeful that its x-ray source eventually will become a more accessible alternative to current synchrotron facilities. “This experiment is also a milestone on the path toward tabletop, free-electron lasers, which are based on a similar principle,” project leader Florian Grüner said. “In the short term, with minor improvements to our setup, we expect to have a source that is capable of performing state-of-the art experiments.”