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Outer Space in a Bottle
On the ground floor of the Rehabilitation Center on Veteran Avenue in
Westwood, amid the treatment and recovery rooms, sits a small piece of
outer space, glowing like the aurora borealis. The hospital is the unlikely
location of the Large Plasma Device (LAPD), a tube some 10 meters long
in which the plasmas that permeate the universe can be re-created for use
in experiments.
The LAPD, so named to discourage interlopers, is the brainchild of Walter
Gekelman, a UCLA professor of physics and astronomy. Gekelman has long
been interested in the properties of astrophysical plasma, a form of matter
that fills the apparently empty space between the stars. Astrophysical
plasmas are created when gases become ionized -- their atoms stripped free
of their electrons. The free, negatively charged electrons and positively
charged nuclei of astrophysical plasma, while only sparsely distributed
throughout outer space, have significant effects on phenomena such as the
propagation and generation of radio waves through space. Electromagnetic
waves ripple and writhe in astrophysical plasmas, creating what Gekelman
calls a “strange electromagnetic weather.”
The plasma phenomena most visible from Earth are auroras, the shimmering
curtains of light seen in the northern sky created when the solar wind
interacts with the plasma that surrounds the Earth. Astrophysical plasmas
are of interest not only to astronomers and physicists trying to better
understand the structure of the universe, but also to engineers who want
to improve satellite communications. During the Gulf War, for example,
disturbances in the plasma surrounding the Earth, caused by a solar storm,
cut off communication between Washington and Kuwait.
Most of our knowledge of astrophysical plasma phenomena comes from satellite
observations. The problem with these measurements, Gekelman explains, is
that a satellite “is like a cork in the ocean, taking its measurements
from a single point in motion.” To make matters worse, the plasma phenomenon
itself may also be in motion. This limited perspective puts scientists
in the place of the proverbial blind men trying to describe an elephant
-- forced to infer the big picture from isolated details. Frustrated by
this limitation, Gekelman decided the best way to observe astrophysical
plasmas in detail was to re-create them in the laboratory.
 Simulating phenomena that normally take place over hundreds of kilometers
involves some sleight of hand, even in a large laboratory. The 10 meter
tube in Gekelman’s laboratory -- one meter in the LAPD may represent a
kilometer of outer space -- contains a plasma that is 10 million to 100
million times denser than those found in space. Phenomena in a denser plasma
exactly mimic over relatively small laboratory distances the phenomena
that take place over the much larger reaches of space.
The magnetic fields that shape astrophysical plasma phenomena are re-created
by a series of massive ring shaped magnets, 68 in all, spaced six inches
apart along the length of the tube. Each magnet contains 60 meters of copper
and weighs in excess of half a ton. To save money, Gekelman and his students
built the magnets themselves.
The LAPD, which held its first plasma in 1989, has been used to understand
many phenomena first glimpsed by satellite experiments. For example, rockets
sent to investigate the north polar region’s aurora borealis observed long,
thin tubes punched through the plasma and aligned with Earth’s magnetic
field. When these holes were struck by whistler waves (magnetic ripples
in the plasma that often accompany lightning and that sound to radio operators
like whistling), they mysteriously shot out high energy particles and emitted
another kind of plasma radio wave. Gekelman’s group was able to re-create
this strange phenomenon in the LAPD and perform the kinds of detailed measurements
that would be impossible with rocket observations.
The LAPD gives scientists an unprecedented, detailed, three dimensional
picture of plasma phenomena. It has also been used to investigate issues
in plasma physics that are of interest to researchers trying to harness
fusion energy. In a fusion reactor, a plasma must be heated to hundreds
of millions of degrees Celsius. One way of doing this might be to fire
radio waves into a plasma. This can be achieved on a small scale in the
LAPD, pointing up the flexibility of Gekelman’s creation, a little flask
of outer space with lots of down to earth uses.
Power Plays...
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