>Date: Fri, 3 Mar 2000 15:59:55 -0500 (EST) >From: AIP listserver <[EMAIL PROTECTED]> >To: [EMAIL PROTECTED] >Subject: update.473 > > >PHYSICS NEWS UPDATE >The American Institute of Physics Bulletin of Physics News >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, >[EMAIL PROTECTED]; www.desy.de/pr-info/News; figure at >www.aip.org/physnews/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, >[EMAIL PROTECTED]) 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 >www.aip.org/physnews/graphics. 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 ([EMAIL PROTECTED], 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 www.aip.org/physnews/graphics. 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.) >
