Number 288, September 26, 1996 by Phillip F. Schewe and Ben Stein
SONOLUMINESCENCE FLASHES CAN HAVE A DIPOLE SHAPE. Sonoluminescence (SL)
is a process in which sound waves strike a tiny gas bubble, trapped in
a liquid, causing the bubble to oscillate and emit picosecond pulses of
light. The mechanism by which the bubble converts and concentrates sound
energy into light energy is largely unknown. An experiment at UCLA by Seth
Putterman and his colleagues shows now that the SL light emission pattern
can have a dipole shape. This implies that the collapse of the bubble is
not spherically symmetric. Furthermore, the dipole pattern can persist
for a period equivalent to 100 bubble cycles (Keith Weninger et al., Physical
Review E, Sept. 1996.) Meanwhile, scientists at MIT and the University
of Marburg (Germany) have put forward a theory which addresses the new
data. Michael Brenner (brenner@math.mit.edu) and his collaborators assert
that the large energy focusing of the SL process can be explained as the
storage of acoustic energy over many oscillations (and not just one bubble
cycle as in the standard shock theory of SL); essentially, the bubble is
a storage tank, patiently soaking up acoustic energy before re-emitting
the energy in the form of sharp light pulses. This model, the MIT researchers
say, accounts for the persistence of the dipole pattern in the UCLA observations,
and points to the possibility that successive light flashes may not be
independent but actually correlated in some way. (Michael Brenner et al.,
Physical Review Letters, 14 October 1996; journalists can obtain the articles
by contacting physnews@aip.org.)
PROBING ARTIFICIAL PLASMAS 6 TIMES DENSER THAN AT THE CENTER OF THE
SUN is the goal of scientists working on the proposed National Ignition
Facility (NIF), a U.S. research center where scientists plan to study nuclear
fusion and other processes involving extremely dense plasmas. Expected
to be funded and completed by the early part of next decade, NIF will use
high-power lasers to achieve, among other things, inertial confinement
fusion, in which a deuterium-tritium (D-T) fuel pellet is compressed to
extremely high densities. In experiments that aim to achieve self- sustaining
fusion reactions, the densities are expected to reach up to 1000 grams
per cubic centimeter (compared to the 150-160 g/cc at the center of the
Sun and the 40-50 g/cc reported for D-T plasmas at existing facilities
such as OMEGA in Rochester and ILE in Japan.) NIF would therefore create
the densest conventional plasmas ever to exist on Earth. To determine what
goes on at the center of such hard-to-penetrate plasmas, researchers at
MIT, Livermore, and University of Rochester have proposed studying "tertiary
protons," which are created at the end of a three-step process starting
with the fusion of deuterium and tritium nuclei. These tertiary protons
would be energetic enough (~30 MeV) to escape the dense plasma and reach
a network of detectors. Energy losses of protons emerging from all different
directions can show whether the fuel pellet compresses uniformly (important
for maximizing the efficiency of fusion reactions) and also yield information
on the density and size of the region in which fusion occurs. In the near
future, the researchers plan to study tertiary protons created in D-T reactions
at OMEGA. (R. D. Petrasso et al., Physical Review Letters, 23 September
1996)
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