|
Number 473, March 3, 2000 by Phillip F. Schewe and Ben Stein
ULTRAVIOLET LASER AT DESY. A free electron laser (FEL) built at the DESY lab in Hamburg by the international TESLA collaboration has achieved a beam of radiation with a wavelength of only 93 nm. FELs normally operate in the following way: a beam of energetic electrons passes through a series of S curves (an undulator) where they are made to radiate light which is stored inside a mirrored cavity. The photons, reflecting back and forth in the cavity, help to stimulate the electrons to radiate even more, thus amplifying the higher-energy light beam. The resultant light is tunable and coherent. At wavelengths below about 150 nm, however, mirrors are not effective and light accumulation cannot occur. Scientists of the TESLA collaboration have now succeeded at DESY in carrying out a scheme suggested 20 years ago: give up the accumulation of light in an optical cavity and let the radiation amplify itself in a single pass as the electrons travel through a very long undulator section, thereby increasingly interacting with the radiation. The product is essentially coherent synchrotron radiation. The TESLA collaboration consists of 38 institutes from 9 countries. Major hardware contributions came from DESY, Italy, France and the USA (US institutes: ANL, Cornell, Fermilab, UCLA). The work with the UV laser is part of an effort to produce an x-ray laser with 6-nm light (by the year 2003). And beam-optics lessons learned might in turn contribute to a more ambitious plan to develop a next-generation linear 500-GeV electron linear collider with integrated x ray lasers called TESLA. (Joerg Rossbach, rossbach@desy.de; www.desy.de/pr-info/News; figure at Physics News Graphics)
SNOWBALLS SURVIVE IN HELLISH CONDITIONS. Many of the unique and unusual properties of liquid water at ambient conditions are due to the ability of water molecules to form hydrogen bonds, which in turn causes the oxygen atoms to be arranged in a three dimensional diamond-like network. However, under extreme pressures the properties of water can change drastically. For example, although water ice normally melts at 0 C at ambient conditions, at a pressure of 10 Giga-pascals (10,000 atm) water remains "frozen" up to 320 C! New computer simulations carried out at the Lawrence Livermore National Laboratory (Eric Schwegler, 925-424-3098, schwegler@llnl.gov) have explored what happens to the microscopic structure of the compressed liquid, in a region of the phase diagram where experimentally determined structural data do not exist. These simulations indicate that when the liquid is squeezed up to a pressure of 10 GPa, the hydrogen bonds and oxygen network are substantially altered. At this high pressure, each water molecule is close packed and surrounded by 12.9 molecules, as opposed to 4.5 neighbors for ambient conditions. (E.Schwegler, G.Galli, F.Gygi, Phys. Rev. Lett., 13 March 2000; figure at Physics News Graphics, also see Select Article.)
MAXIMALLY RANDOM JAMMING. Packing particles into a container has been important since antiquity, when basketfuls of grain were traded or collected as taxation. Packing applies not just to grains of wheat of course, but also to ball bearings, living cells, a variety of granular media, and the placement of atoms and molecules in solids and liquids. Hence packing has become a science, and the maximum fraction of space that can be filled with spheres is a conjectured 74%. This is for an ordered "face-centered cubic" array that looks like a stack of cannonballs or oranges. (Kepler came very close to the 74% figure four centuries ago.) The mathematics for estimating the maximum filling fraction for an array of disordered, or randomly packed, balls is much more slippery. Salvatore Torquato and his colleagues at Princeton consider that the whole problem of random close packing (RCP) is ill posed and have proposed in its place a new concept which they call maximally random jamming, a precisely defined condition in which spheres are deployed in the most disordered way. Computer simulations show that the packing fraction for the maximally jammed state is about 64%. Torquato (torquato@matter.princeton.edu, 609-258-3341) believes that the new model will help to study randomness in many-body systems in general. (Torquato, Truskett, Debenedetti, Physical Review Letters, 6 March; see figure at Physics News Graphics, also see Select Article.)
DARK MATTER UPDATE. At the dark matter detection meeting in Marina del Rey, California last week (Update 473) a group form Gran Sasso, Italy reported detecting evidence for dark matter particles. The Cryogenic Dark Matter Search collaboration (10 US institutions), using a different detection scheme, reported finding no evidence for such particles, and asserted that their results were incompatible with the Gran Sasso finding. (Stanford press release, 2/24. see preprint at http://arXiv.org/abs/astro-ph/?0002471.)
|
