PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 632 April 9, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

FIRST FUSION AT THE Z MACHINE was announced this week at the April meeting
of the American Physical Society in Philadelphia.  For the first time,
Sandia National Laboratories' Z facility in New Mexico has created a hot
dense plasma that produces neutrons associated with nuclear fusion.
According to Sandia's Ray Leeper (rjleepe{at}sandia.gov), the neutrons emanate
from fusion reactions within a BB-sized deuterium capsule placed within the
central target in the Z facility, itself about a third of a football field
in diameter. While tokamaks cause fusion reactions to occur by confining
plasmas in large magnetic fields, and laser facilities focus intense beams
on or around a target, Z applies a huge pulse of electricity (about 12
million joules) with very sophisticated timing. The pulse creates an intense
magnetic field which crushes an array of 360 tungsten wires into an
ultra-light foam cylinder to produce x rays. Striking the surface of the
fuel capsule embedded in the cylinder, the x-ray energy produces a shock
wave that compresses deuterium gas within the capsule, fusing enough
deuterium to produce neutrons.  Sandia researchers measured a yield of
approximately 10 billion neutrons, around the expected energy of 2.45 MeV,
corresponding to a very modest level of nuclear fusion (about 4 millijoules
of energy).  The deuterium capsule reached a temperature of about 11.6
million Kelvin and was compressed from a diameter of 2 mm to 160 microns.
The whole compression took about 7 nanoseconds.  Providing outside
commentary, Cornell University's David Hammer (hammer{at}ece.cornell.edu) said
the Sandia group performed pretty much a full set of tests to verify that
they had achieved nuclear fusion.  The ZR (Z-Refurbished) facility, an
upgrade scheduled to go online in 2006, is slated to attempt scaled-up
fusion experiments.  While the Z approach to fusion is a promising,
straightforward, and potentially robust method, researchers caution that
they are at the start of a very long road in investigating its feasibility
as a fusion power source.

FIRST LIGO SCIENTIFIC RESULTS.  With two controlling partners, MIT and
Caltech, and two branch offices (two completely independent detectors)
located in Washington State and Louisiana, the Laser Interferometer
Gravitational-Wave Observatory (LIGO) is essentially a giant strain gauge.
In the LIGO setup laser light reflects repeatedly in each of two
perpendicularly oriented 4-km-long pipes.  A passing gravity wave will
distort the local spacetime, stretching very slightly one of the paths while
shrinking the other, causing the interference pattern of the two merging
laser light beams to shift in a characteristic way.  LIGO does not measure
static gravitational fields, such as those from the sun or the Earth itself.
 Rather it strives to see ripples in spacetime radiated by such events as
the inspiral of two neutron stars toward each other, a phenomenon which
would typically produce a strain in the LIGO apparatus as large as one part
in 10^20.   That is, a passing gravity wave is expected to change the
distance between mirrors some 4 km apart by about 10^-18 meters, a
displacement 1000 times smaller than a proton.  Such a measurement
represents a physics and engineering feat of great delicacy.  But at long
last the LIGO team has prepared its instrument and at this week's APS
meeting, reported its first official results from the initial "science" run
conducted over 17 days in September 2002.
In this first run no gravitational wave events were observed, but palpable
knowledge was gained as to what the sky should look like when viewed in the
form of gravity waves.  So great is LIGO's sensitivity that it has been able
to set the best upper limit on the output of gravitational waves from three
of the four prime source categories.  These four expected waveforms are as
follows: bursts from sources such as supernovas or gamma bursters; chirps
from inspiraling objects such as coalescing binary stars; periodic signals,
perhaps from sources like spherically asymmetric pulsars; and a stochastic
background source arising from gravity waves originating from the big bang
itself. LIGO deputy director Gary Sanders (Caltech,
sanders_g{at}ligo.caltech.edu) said that in three of these four categories,
had set new upper limits on the rate at which gravitational waves were being
produced.  In the coalescing binary category, for instance, LIGO has
established an upper limit of 164 per year from the Milky Way, a factor of
26 better than the previous limit.  Erik Katsavounidis (MIT,
kats{at}ligo.mit.edu) said that LIGO could establish an upper limit on
periodic signals from bright pulsars with a sensitivity of about 10^-22.
Sheila Rowan (Stanford Univ and Univ Glasgow) spoke of future operations at
LIGO.  First of all, the second scientific run currently underway will be
some ten times more sensitive than the first run, the one being reported at
the meeting.  If in the first science run LIGO was essentially sensitive to
gravity waves from the whole of the Milky Way, then in the second science
run (conducted Feb-Apr 2003), featuring a ten-times improvement in
sensitivity, the region of space patrolled would effectively reach out to
about 15 million light years, a realm that includes the nearby Andromeda
galaxy.  (For more information about LIGO and a complete collaboration list,
see www.ligo.caltech.edu ) In its search for gravity waves, LIGO (which with
about 440 scientists is as big as the large particle physics experiments
underway at accelerators) is also collaborating with other interferometer
devices such as GEO (in Germany, www.geo600.uni-hannover.de ) and TAMA
(Japan).

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PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources.  It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
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