Understanding Plasmas

WHISTLER WAVES are one of the many kinds of waves that exist in plasmas. A whistler wave is shown propagating through density striations in a laboratory plasma. Panel (a) shows the experimental apparatus. Panel (b) shows the plasma discharge, and panel (c) shows the nonlinear wave pattern. This experiment allows controlled observation of nonlinear whistler-wave activity, which is an important process in the Earth's upper atmosphere. [Courtesy, Walter Gekelman, University of California, Los Angeles]

Collective effects become still more interesting when the subject is waves in plasma. In this respect, a neutral gas is a bit of a one-trick pony, supporting only sound waves, which arise out of a seesaw competition between the neutral atoms' pressure and their inertia when the gas is jostled at the right frequency. Such a sound wave eventually dies away as collisions between atoms turn its coherent motions into random, thermal energy in the gas.

EDGE PLASMA of a fusion experiment in the D-IIID tokamak controls impurities and deposits heat on selected areas of the vessel's wall. [Courtesy of General Atomics]

By contrast, a plasma supports a multitude of wave motions, the two simplest being so-called Langmuir waves and ion-sound waves. In Langmuir waves, the ions are nearly stationary and the electrons rapidly oscillate around them like pendulums. The force that swings the electrons back and forth isn't gravity, however, but the strong electric field that temporarily builds up when the negative charge in the local cloud, or "fluid," of electrons doesn't balance the positive charge in the ions. Hence the oscillations, as the disturbed electrons rush back to short out the field, overshoot, and rush back again. The lower-frequency ion-sound waves behave something like the usual waves in neutral gas, with one crucial twist: The waves' speed is now determined by the ion's inertia and the combined pressure of both species. In this wave, the ions and electrons swing in synchrony, tethered together by the electric field that comes about when one or the other species lags slightly behind in the thermomechanical dance.

Most remarkable of all is the ability of a plasma to damp away its waves when collisions between particles are negligibly rare. This collisionless damping occurs when some of the plasma particles are moving with close to the same velocity as the wave phases themselves, so that the particles ride the disturbance like a surfer on a water wave. The particles gain energy while the wave loses energy and damps away. In this way, a wave may be said to "heat" the surrounding plasma. Some theories invoke this mechanism to help explain the mysteriously high temperature of the sun's corona. Certainly one of the most interesting and engaging plasmas in the universe, this prominent solar feature-seen as the bright, wispy halo during an eclipse-has a measured temperature of over a million degrees centigrade, while its only apparent source of heat, the solar surface below, sits at just 6,000 degrees. Perhaps, some theorists propose, the sloshing, turbulent motions at the sun's surface generate waves that propagate upward and dump their energy into the corona.

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