Electron Beams...

Figure 1: Using the Pegasus facility at UCLA, Maxson and colleagues [1] have compressed electron pulses to below 10 fs in duration. In this facility, a relatively long electron pulse (green) produced by an electron source (not shown) enters a specially designed linear accelerator (copper, left), whose electromagnetic fields act to compress the pulse several meters downstream from the accelerator. Here, the pulse can be made so short (below 10 fs) that it would “outrun” all atomic motion in molecules (blue) and materials. In an electron diffraction or microscopy experiment with pulses like this, atoms are effectively frozen in place during an exposure (yellow). [Credit: APS/Alan Stonebraker]

Topics: Electromagnetism, High Energy Physics, Modern Physics, Particle Physics

When you read the term “electron beam,” what comes to mind? If you are a physicist or have a background in physics, it may be the great J. J. Thomson—discoverer of the electron—followed by a vision of an old television or oscilloscope powered by cathode-ray tubes. Very twentieth-century stuff. If this is your view, a new study by Jared Maxson at the Pegasus radiation facility at the University of California, Los Angeles, and colleagues [1] should help make clear how electron beams are one of the primary enablers of twenty-first-century science. Ultrashort electron-beam pulses—less than 10 fs long in this case—are enabling forms of atomic-level dynamic imaging that were previously restricted to the realm of thought experiments [2].

It is difficult to overemphasize the impact that electron beams have had on scientific developments over the last century, or the impact they are expected to have over the next century. While electron beams are currently out of favor in high-energy physics, because of the move from electron-positron colliders to hadron colliders at CERN, Fermilab, and other laboratories, they are central to other areas of science. For example, when we want to perform detailed examinations of the structure of molecules and materials, electron beams are at the forefront. Transmission electron microscopes and scanning electron microscopes are remarkably efficient instruments for generating and measuring an enormous range of signals that reveal the structure of materials. These signals come from both the elastic and inelastic interaction of electron beams with materials, and modern materials science is unthinkable without these instruments. When operated at cryogenic temperatures, these same instruments enable 3D reconstructions of proteins, viruses, organelles, and even whole cells [3]. Complementary work can also be performed using synchrotron light sources or free-electron lasers [4], which produce x-ray and infrared beams that are themselves generated from pulsed relativistic electron beams circulating in storage rings or linear accelerators. The size of the facilities that host these instruments, and the properties and limitations of the instruments as sources of x-ray and infrared radiation, are largely determined by the properties of such electron beams.

APS Physics Viewpoint: Electron Pulses Made Faster Than Atomic Motions
Bradley Siwick, Department of Physics and Department of Chemistry, Center for the Physics of Materials, McGill University, 801 Sherbrooke St. West, Montreal, Quebec, H3A 0B8, Canada