Number 249, November 21, 1995 by Phillip F. Schewe and Ben Stein
PLASMA INSTABILITIES AT THE CENTERS OF TOKAMAKS have been greatly brought
under control in two separate experiments. Physicists at the TFTR tokamak
at Princeton and at the DIII-D device at General Atomics in San Diego have
shaped the magnetic fields inside their fusion reactors so that the deuterium
plasma confined there assumes a hollow profile. In this "reversed
magnetic shear" mode, the central electron density rose by a factor
of three above previous levels. Controlling and maintaining a high plasma
density and high temperature are important parts of the longterm effort
to develop commercial electricity-generating plants powered by thermonuclear
fusion reactions. According to the TFTR researchers, "This regime
of operation holds promise for significantly improving the tokamak reactor
concept and can lead to a dramatic increase in the performance of present
tokamaks." (F.M. Levinton et al. and E.J. Strait et al., two articles
in the upcoming 11 December 1995 issue of Physical Review Letters; journalists
can obtain copies from AIP Public Information at physnews@aip.org)
THE INFRARED SPACE OBSERVATORY (ISO) , just put into orbit by the European
Space Agency, will extend the work performed a decade ago by NASA's IRAS
satellite, but with greater sensitivity and with a 10-fold improvement
in spatial resolution. ISO will look at infrared light in the 3-200 micron
wavelength range. This corresponds to targets at temperatures between 10
and 1000 K, objects such as cool stars, brown dwarfs, the interstellar
medium, and distant infrared galaxies. ISO's eccentric orbit keeps the
craft out of Earth's radiation belts for 16 hours each day. (New Scientist,
4 November 1995.)
ACOUSTIC TIME-REVERSAL MIRRORS (TRMs) are devices that record a sound
wave emanating from a source and generate a new one that behaves as if
the original were travelling backwards in time. Previously TRMs had been
rigorously tested for sound propagating through fluids such as water or
air. For example, shouting "too" at the device would yield a
reversed acoustic wave (sounding something like "oot") that converges
backward towards the speaker's mouth. The principle has now been shown
to be valid in solids by a team at the University of Paris (Didier Cassereau,
Didier.Cassereau@loa.espci. fr). Demonstrating TRMs in solid objects has
been more difficult because there are two types of sound waves that propagate
through solids: longitudinal and transverse. Therefore, sound produced
in a solid object will result not in a single wavefront but in at least
two that travel at different speeds. The Paris researchers first use the
TRM to send an ultrasonic wave into the solid sample. Then, the echo reflected
back (say, from a defect in the sample) is detected by the TRM, which utilizes
a network of rodlike transducers that both record the incoming echo and
then broadcast a time-reversed version. This signal, in turn, reflects
from the defect, an echo returns to the detector, and so forth, in an iterative
process leading to a clearer location of the defect. TRMs have potential
applications for detecting tiny metallic defects in airplanes and for locating
and destroying kidney stones. For example, shining ultrasound waves through
a patient with a kidney stone produces distinctive echoes from the stone.
The TRM would record the echoes, and then generate a reversed wave, sending
back sound energy that would travel back to the stone. (Paper 1pPA5, Acoustical
Society of America Meeting, St. Louis, Monday, November 27)
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