Text 861, 72 rader
Skriven 2006-09-13 17:52:48 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 792
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 792   September 13, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu
        
PARTICLE ACCELERATION BY STIMULATED EMISSION OF RADIATION (PASER for
short), a sort of particle analog of the laser process, has been
demonstrated, for the first time, by a team of physicists from the
Technion-Israel Institute of Technology using the accelerator
facilities at the Brookhaven National Lab. In a regular laser,
photons traveling through an active medium (a body of excited atoms)
will stimulate the atoms, through collisions, to surrender their
energy in the form of additional emitted photons; this coherent
process builds on itself until a large pulse of intense light exits
the cavity in which the amplification takes place.  In the new
proof-of-principle PASER experiment, the active medium consists of a
CO2 vapor, and instead of surrendering their energy in the form of
stimulated photons, the atoms transfer their energy to a beam of
electrons. The electrons stimulate the atoms into giving up their
surplus energy through collisions.  The electrons' energy is
amplified in a coherent way; that is, the electrons are directly
accelerated by a direct and coordinated quantum transfer of energy.
Although millions of collisions are involved for each electron, no
heat is generated.  The transferred energy goes into an enhanced
electron motion.  One could say that here was a laser which produced
no laser light, only a laser-like transfer of energy resulting in
electron acceleration.
It should be said that the electrons began with an energy of 45
million electron volts (MeV) and absorbed only a modest energy of
about 200 thousand electron volts (keV).  The electrons, first
accelerated in a conventional accelerator, were also exposed to a
CO2 laser and also sent through a "wiggler" array of magnets; these
actions served to carve a larger bunch of electrons into separate
micro-bunches, which are timed and modulated in energy in order to
more readily partake of the resonant PASER process in the CO2-filled
resonant cavity a little farther along (see figures at
http://www.aip.org/png/2006/268.htm). Being able to accelerate
electrons with energy stored in individual atoms/molecules, a
concept now demonstrated with the PASER, provides new  opportunities
since the accelerated electrons may prove to be significantly
"cooler" (they are more collimated in velocity) than in some other
prospective acceleration schemes, enabling in turn the secondary
generation of high-quality x rays, which are an essential tool in
nano-science. (Banna, Berezovsky, Schachter, Physical Review
Letters, upcoming article.)

EINSTEIN'S LITTLE MACHINE.  Albert Einstein was the ultimate
theorist, having spun out mathematical explanations of space and
time, gravity, atoms, and quantum phenomena. And yet Einstein also
had his experimentalist side too.  He grew up in a household where
gadgets were all around (his father owned an electrical instrument
factory), and he worked in a patent office where a parade of
detailed engineering drawings came past his view every day.  In fact
he built several practical devices and took out numerous patents of
his own.  One of Einstein's creations, which he called his
"Maschinchen," or little machine, sought to measure voltages at the
level of 0.0005 volts.  This sort of precision is easy to achieve
nowadays but was not possible in 1907, when Einstein developed a
contraption which took charge induced on a metal plate by a weak
nearby potential and then stored it in a special accumulator; the
effect of the small voltage signal could then be multiplied.  Three
known versions of this machine are known to exist, at least one of
which was used in an experiment conducted by Walter Gerlach (who
later worked on the Stern-Gerlach discovery of electron spin).  Now,
two scientists at the University of Ghent in Belgium have performed
computer simulations to show in detail how the Maschinchen worked.
Danny Segers (danny.segers@ugent.be) says that he and Jos Uyttenhove
are building a replica to better explore Einstein's handiwork.
(American Journal of Physics, August 2006)

---
 * Origin: Big Bang (1:106/2000.7)