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Laboratory Plasmas

In terrestrial laboratories, experimentalists call on a similar effect
to accelerate bursts of electrons to high energies. The idea is to
excite large-amplitude Langmuir wave by firing brief, super-intense
laser pulses into a plasma. the laser pulses have a pressure that
shoves plasma electrons out of the way. This lets a pulse act like a
sort of speed boat, and in its wake are huge waves whose electric
fields can be used to accelerate electrons to high energies. These
fields are so strong that they would tear apart material structures;
so it is not surprising that particle accelerators based on this
effect could one day be more powerful and much smaller than those that
used in conventional technologies which require such
structures-coppers cavities and the like. Though they have obtained a
number of promising results, plasma researchers are still at the early
stages of harnessing this intriguing effect.

Up to this point, we havenÕt thrown magnetic fields in the cosmic brew
that defines plasma physics. But it is only when magnetic fields are
considered that plasmaÕs astonishing complexity and variety become
fully apparent, and you can understand the intellectual fascination,
the utopian dreams and Kafkaesque nightmare that the substance has
planted in the minds of many a researcher.

A magnetic fieldÕs profound influence on plasma properties owes to its
effect on the orbits of the individual charged particles. Like rings
sliding along wires, electrons and ions for the most part move easily
along magnetic field lines but spiral tightly around them. The
spiraling effect greatly inhibits the ability of charge particles to
travel across field lines.

Perhaps the simplest of all Òplasma confinementÓ devices is a bundle
of straight field lines (produced by external, current-carrying coils)
capped at two endpoints by plates that are electrically biased. Such a
device confines a cloud of just one plasma species-positively charge
protons, say, if the plates are biased positively to repel these
particles as they try to escape along field lines. These simple and
elegant devices will provide researchers with some of their best
opportunities to do basic studies of waves in turbulence in plasmas in
the coming years. Because the confined particles are completely
isolated from contact with the outside world, these devices, unlike
almost any other, can also serve as a ÒbottleÓ for containing
antimatter-such as electrons antimatter counterparts, called
positrons-which would be annihilated in any encounter with ordinary
matter. Plasma researchers have already confined more than 100 million
positrons for up to an hour in such a trap in order to study how they
might behave in the exotic corners of the universe where such clouds
are thought to form naturally.

Externally imposed fields greatly expand a plasmaÕs repertoire of
behaviors, leading to waves with different phase velocities along and
across the magnetic field lines, for example, and introducing new
types of collisionless damping. The truly sublime complexity of the
science doesnÕt become apparent, however, until you realize that
magnetic fields in plasma donÕt generally remain fixed, but squirm
about in turbulent plasma, an equal partner with the fluctuating
electric fields and the particles in determining the overall dynamics.
Magnetic fields, in fact, seem to be the universeÕs ultimate weed,
sprouting spontaneously in the sun and stars, in planetary cores and
in interstellar, interplanetary and intergalactic space. The study of
how electrical currents in the plasma both generate these fields and
are affect by them is one of the most interesting topics in plasma
science today.