Physics News Update, Number 457

Number 457, November 15, 1999 by Phillip F. Schewe and Ben Stein

LASER LIGHT IN, 50-MEV PROTONS OUT. At next week's meeting of the American Physical Society Division of Plasma Physics in Seattle, three groups will independently announce their ability to generate powerful, intense streams of ions by shining ultrashort laser pulses on tiny spots of solid material. Potentially, this approach offers an alternative to bulky, expensive ion accelerators for producing high-velocity ions useful for cancer therapy and electronics manufacturing. Using a single pulse of light from Livermore's Petawatt laser, the most powerful in the world, researchers at that laboratory (Scott Wilks, 925-422-2974, wilks@icf.llnl.gov) have reported generating 30 trillion protons with energies up to 50 MeV, from a tiny spot approximately 400 microns in size. Using a tabletop terawatt laser one-thousandth the power of the Petawatt, University of Michigan researchers (Donald Umstadter, 734-764-2284, dpu@umich.edu) produce 10 billion protons with about a tenth the energy of those reported at Livermore. In addition, the Michigan team has announced that they can produce a confined beam of ions pointing roughly in the direction of the laser beam. Employing the VULCAN laser at the Rutherford Appleton Laboratory, researchers there (Karl Krushelnick, Imperial College, kmkr@ic.ac.uk, 011-44-594-76-35), generated lead ions with energies up to 420 MeV (and protons up to 17 MeV). The mechanism behind each demonstration is similar. A single laser pulse strikes a thin target, ejecting electrons which form a cloud of negative charge around the back of the target. The cloud pulls positively charged ions from the back of this target and rapidly accelerates the ions to high energies. All of this occurs over a very short distance--almost 1 MeV/micron for protons in the Livermore case, which is orders of magnitude higher than conventional ion accelerators. (Papers FI2.04, O1.11, QO1.12, QO1.13, JP1.74 at meeting; Meeting program at http://www.aps.org/meet/DPP99/baps/; Figures at Physics News Graphics.)

20,000 LEAGUES UNDER THE FERMI SEA Recently Stanford and UC Santa Barbara physicists used two alternating-current voltage sources to skew the quantum states in a tiny semiconducting quantum dot in such a way as to produce (without any net applied bias) a nonzero current through the dot. This was an experimental realization of a "Thouless pump" (named for David Thouless), which pumps electrons much as an Archimedian screw pump lifts water (Switkes et al., Science, 19 March 1999; see also the commentary in the same issue by Altshuler and Glazman). Now, Mathias Wagner (Hitachi Cambridge Laboratory, 011-44-1223-44-2911, wagner@phy.cam.ac.uk) and Fernando Sols (Universidad Autónoma de Madrid) predict that a similar principle will also apply to electrons far beneath the Fermi-sea surface. The Fermi surface or Fermi level represents (in an abstract space in which all electrons are described by their momentum vectors) the highest energy an electron may possess--at zero temperature--in the conduction band of a metal or semiconductor material. Conduction electrons, those that stray from their home atoms, are usually drawn from electrons very near the Fermi surface. Electrons with lesser energies, and occupying rungs further down on an energy-level diagram, are said to reside in the "Fermi sea" and normally do not effectively contribute to the current. Wagner and Sols suggest that with high enough ac power, the resulting pump current might actually consist mostly of electrons from far beneath the Fermi-sea surface. These subsea currents would be largely immune from temperature effects (just as submarines are less vulnerable to surface storms), a very useful property in the electronics world. (Wagner and Sols, Physical Review Letters, 22 November 1999; get text from Select Articles; see figure at Physics News Graphics.)