Unlocking Secrets of Plasma
Inside Walter Gekelman’s warehouse-sized laboratory in Westwood Village, enough electricity to power a thousand homes pours into a row of 68 magnet rings, each one weighing half a ton.
A steady pulse of brilliant red light flashes from inside the cylindrical machine--as tall as a bus and twice as long. With each pulse, a thimble’s worth of neon gas seeps into the near-vacuum inside the machine and directly into the path of a 500,000-watt electron beam. As temperatures shoot to a quarter-million degrees, a curious form of matter known as a plasma is created. It lasts only a few thousandths of a second. Any longer and the 80-ton Large Plasma Device would melt.
Plasma makes up well more than 99% of the “stuff” of the observable universe. Thin wisps of plasma occupy the vast near-vacuum of interstellar space. Plasma is the main ingredient of stars--dense as iron and 5 million degrees at the sun’s core. Lightning, licks of fire and fluorescent light bulbs are earthly examples of tiny amounts of plasma. Yet this universally common form of matter is a rarity on our relatively cool, dense planet--perhaps why the study of plasma is one of the youngest fields of physics.
Atoms as we normally know them each consist of a central nucleus orbited by charged electrons. This relationship is faithfully maintained even as materials shift from solid to liquid to gas. A plasma, by contrast, is ionized--its atoms “stripped” of electrons.
“I always ask my students what happens if we keep making steam hotter and hotter?” said Gekelman, a UCLA professor. As a material is heated, its atoms vibrate more and more intensely. At extreme temperatures, orbiting electrons are literally shaken loose from their nuclei. “It’s at this point that you have a plasma,” he said.
First described by 19th century physicist Sir William Crookes, plasma wasn’t named until American chemist Irving Langmuir coined the term in 1929. During the 1950s, as physicists worked on testing hydrogen bombs, they realized that the extremely hot plasmas formed by the nuclear explosions could be used to generate energy. Born classified, research on nuclear fusion and plasmas remained secret until 1958.
Today, researchers like Gekelman study the fundamental properties of how waves of energy flow across and through plasma. Among the behaviors the Large Plasma Device is designed to study is the ability of a plasma to, in effect, store a memory of energetic disturbances that pass through it.
Ordinarily, if a material is disturbed repeatedly in exactly the same manner, it will react the same way each time. Drop three identical pebbles into a bucket of water, for example, and the ripples will be the same each time. Not so with a plasma.
Under certain circumstances, disturbances to the plasma change it. The plasma reacts “in such a way that a second, identical pulse may cause a completely different reaction,” Gekelman said.
Built by Gekelman and his colleagues, the Large Plasma Device allows researchers to create and contain plasmas for detailed study of those mysterious phenomena. Making plasma in the lab requires million-degree heat and a vacuum environment. The device uses vast amounts of electricity to ionize gas and generate a magnetic field that keeps the hot plasma from the inner walls. Computer-controlled probes reach into the sealed innards, monitoring the burst of plasma that hangs around for mere thousandths of a second. The machine then takes about a second to cool down enough to generate another plasma burst. The trick is to ensure that each plasma is made exactly the same every time--every second, 24 hours a day for months.
“There is no better and more accessible facility for understanding the underpinnings of plasma science,” said Ron McKnight of the Department of Energy’s Office of Fusion Energy Sciences. The department and the National Science Foundation recently awarded $4.8 million to support the UCLA machine. Next fiscal year, the department will spend more than $9 million on basic plasma research.
Among the topics for which researchers are using the machine is the study of a large and ubiquitous energy-carrying ripple called an Alfven wave. Named after the late Swedish physicist and Nobel laureate Hannes Alfven, it is one type of disturbance to which plasmas react.
Common in outer space, Alfven waves carry solar wind--also a variety of plasma--to Earth from the sun. The waves also are involved with solar flare eruptions that explode with the power of a million 100-megaton nuclear bombs. These solar hiccups can cause a coronal mass ejection in which a section of the sun’s outer plasma, weighing about 10 billion tons, tears off and hurtles into space. Occasionally, such eruptions speed toward Earth on the legs of Alfven waves at speeds in excess of 2 million mph.
When coronal mass ejections collide with Earth’s protective magnetic field, they create the electrical and magnetic discharges known as aurora borealis, or northern lights, and their south pole counterpart, aurora australis. They also can down orbiting satellites and overload ground-based power grids.
A daylong interruption of cell phones, credit card purchases and 40 million pagers in 1998 was blamed on a solar event that disabled a communications satellite. A similar disturbance in 1989 left 6 million people without power for nine hours in Quebec and affected some Northeastern U.S. states.
“Coronal mass ejections are very important in terms of the magnetic storms that may occur at Earth,” said Joe Kunches, chief of space weather operations at the Space Environment Center in Boulder, Colo. “If we could model how Alfven waves travel and interact with our magnetic field, our ability to predict Earth disturbances would be better.”
The sheer size of the Large Plasma Device makes it “the only machine in the world” that enables researchers to make detailed studies of Alfven waves, Gekelman’s main research interest.
He has company. The University of Iowa’s Craig Kletzig, who studies the generation of northern lights, is scheduled to visit the device in April.
But plasma scientists aren’t the only ones flocking to the Large Plasma Device. Gekelman routinely spends Saturday afternoons with high school students experimenting on a smaller plasma machine that he and the students built from spare parts especially for his classroom. “Some come away with little more than an inkling of what’s going on, and some are so smart I figure they should just skip the rest of high school and come right here,” he said.
