PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 641 June 12, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

THE TWISTED ORIGIN OF SPHEROMAKS.  Researchers at the California
Institute of Technology have made important progress in solving a
long-standing mystery concerning the formation of spheromaks,
self-organizing toroidal plasma configurations that are
superficially reminiscent of smoke rings. It is well known that
current-carrying plasmas embedded in an initial seed magnetic field
can form spheromaks.  The formation process is believed to involve
some kind of dynamo process, whereby the internal  magnetic fields
become re-arranged or even amplified so as to achieve a stable
minimum energy state for the internal magnetic forces. (Similar
minimal-energy state arguments help explain why soap bubbles, for
example, tend to be spheres rather than cubes or other shapes.) But
until now, no one has definitively demonstrated just how a plasma
transforms from an unstable, high internal energy configuration into
a spheromak. The new experiment sheds light on the phenomenon by
capturing images of plasmas as spheromaks form. The images show that
plasma currents initially flow in straight lines along a confining
magnetic field. Owing to an effect known as the kink instability,
the plasma currents develop bends that twist into a helix (see image
at www.aip.org/mgr/png ). The helix acts like a coiled current
element, or solenoid, which amplifies the original, straight
magnetic field. Above a certain threshold in the initial magnetic
field, detached plasma spheromaks are formed. The researchers
(contact: Paul Bellan, pbellan{at}its.caltech.edu, 626-395-4827)
confirmed the theory behind the effect by measuring the rapid
amplification of the magnetic field inside developing plasma
solenoids. Spheromaks are potentially promising routes to
plasma-based nuclear fusion, and insight into their formation will
help in the design of future experiments-and possibly even a clean,
safe energy source. In addition, spheromak formation is important
for explaining the behavior of plasma in the solar corona, as well
as understanding the physics of jets that sprout from black holes,
galactic nuclei, and other astrophysical objects. (S. C. Hsu and P.
M. Bellan, Physical Review Letters, 30 May 2003)

A CARBON NANOTUBE COMPOSITE FIBER, made by injecting single-walled
nanotubes into a pipe filled with polyvinyl alcohol to form a gel,
can be spun out into100-meter strands.  According to the scientists
at the University of Texas at Dallas who created the spinning
process, the resulting fibers are "tougher than any natural or
synthetic organic fibre described so far," with a tensile strength
of 1.8 gigapascals.  The 50-micron-diameter fibers are 60% nanotube
by weight and has been woven into a fabric.  In textile form, the
researchers suggest, their composite material could be used for
making distributed sensors, antennas, capacitors, and even
batteries.  (Dalton et al., Nature, 12 June 2003.)

"COLOR FILTERING" AT THE ATOMIC LEVEL.  One of the most astounding
inventions of the late 20th century, the scanning tunneling
microscope, or STM, yields atomic-scale landscapes of electrically
conducting surfaces such as metals. Now, researchers at the Colorado
School of  Mines (Peter Sutter, psutter{at}mines.edu) have demonstrated
a new technique, called "energy-filtered STM," which is analogous to
putting a color filter on an ordinary microscope.  Just as color
filters make it easier to discern desired features in a photograph,
color-filtered STM makes it easier to distinguish between chemically
similar atoms, something that's usually very difficult to do.  It
can even identify specific chemical bonds on a surface. Conventional
STMs employ a metal tip, which, as it turns out, is generally most
sensitive to the highest-energy electrons on the surface.  These
electrons jump or "tunnel" to the tip, giving scientists data to
reconstruct an image of the surface.  This preference for the
highest-energy electrons can be a problem, because it can obscure
the signal from lower-energy electrons, which may be associated with
different atoms or different kinds of chemical bonds.  To address
this issue, the new technique employs an indium arsenide (InAs) tip.
InAs is a semiconductor, and all semiconductors have a "fundamental
bandgap," a range of energies that no electrons can possess because
of the 3D atomic structure of the material.  In the case of a
semiconductor tip very close to a conducting surface, what's more
important is something called a "projected gap," a range of
forbidden energies that appears when the 3D electronic structure is
seen along the tip axis.  So because of the projected gap, electrons
in a certain energy range cannot tunnel to the tip.  Adjusting the
voltage between the tip and sample can shift this projected gap so
that it blocks off the high-energy electrons, making the tip more
sensitive to electrons in lower-energy bonds at the sample surface
(see images at http://www.aip.org/mgr/png ). Researchers can shift
this range of forbidden electron energies repeatedly, to build up,
for example, maps of specific chemical bonds on a surface, and to
analyze how abundant one type of chemical bond is compared to
others.  This technique is now being explored for 'atom-by-atom'
mapping of the composition of alloys of chemically similar elements,
which is important for certain technologies such as thin-film
growth, which often involve nanometer scale variations in the
composition of alloys (Sutter et al., Physical Review Letters, 25
April 2003)

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