Division of Plasma Physics

Laboratory Plasmas

figure 10

PARTICLE-WAVE SURFING and mixing occur when charged particles interact with waves. A plasma can often be viewed as a layered fluid with the layers having different fluid densities. Waves in the plasma cause these layers to first "surf" and then mix with particles in other layers. The start of the mixing is illustrated below, where different layers are color-coded to visualize the evolution of mixing that often leads to chaos. This physical picture is applicable to many different plasma processes occurring in the laboratory, in space, and in commercial applications. [Courtesy, Michael Mauel, Columbia University]

figure 12

SPIRALING CHARGED PARTICLES move in paths that nearly follow the magnetic field lines. In an important application of magnetic fusion confinement, the field lines thread the inside of a donut shaped volume, never hitting the walls of the confining device. Consequently, the plasma particles spiral about these field lines, and hot plasma at temperatures in excess of 100,000,000 degrees centigrade (the temperature of the sun) can be confined inside a metal chamber whose surface is nearer room temperature.

figure 13

TURBULENT PLASMA STRUCTURES align along magnetic fields and cause spiraling charged particles to wiggle and squirm across the field lines. This fine-scale chaotic motion causes heat to be transported from within the hot plasma core to the much cooler edge. The control of this energy transport mechanism often determines how efficiently fusion (the energy production mechanism that heats stars) can be used for power production on Earth. [Courtesy, W. Lee and S. Parker, Princeton Plasma Physics Laboratory]

In terrestrial laboratories, experimentalist call on a similar effect to accelerate bursts of electrons to high energies. The idea is to excite large-amplitude Langmuir waves 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 both more powerful and tens or hundreds of times smaller than those that use conventional technologies, which require such structures - copper 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 haven't thrown magnetic fields into the cosmic brew that defines plasma physics. But it is only when magnetic fields are considered that plasma's astonishing complexity and variety become fully apparent, and you can understand the intellectual fascination, the utopian dreams and the Kafkaesque nightmares that the substance has planted in the minds of many a researcher.

A magnetic field's 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 charged particles to travel across field lines.

Perhaps the simplest of all "plasma confinement" 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 charged 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 and 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 "bottle" 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 plasma's 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 doesn't become apparent, however, until you realize that magnetic fields in plasmas don't generally remain fixed, but squirm about in turbulent plasmas, an equal partner with the fluctuating electric fields and the particles in determining the overall dynamics. Magnetic fields, in fact, seem to be the universe's 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 affected by them goes under the name of dynamo theory, and it counts as one of the more vigorously developing areas in plasma science today.

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