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Plasma experiments bring astrophysics down to Earth

Plasma experiments bring astrophysics down to Earth

New laboratory technique allows researchers to replicate on a tiny scale the swirling clouds of ionized gases that power the sun.
December 1, 2015

Intense heat, like that found in the sun, can strip gas atoms of their electrons, creating a swirling mass of positively and negatively charged ions known as a plasma.

For several decades, laboratory researchers sought to replicate plasma conditions similar to those found in the sun in order to help them understand the basic physics of ionized matter and, ultimately, harness and control fusion energy on Earth or use it as a means of space propulsion.

Now Stanford engineers have created a tool that enables researchers to make detailed studies of certain types of plasmas in a laboratory. Their technique allows them to study astrophysical jets—very powerful streams of focused plasma energy.

Writing in Physical Review Letters, mechanical engineering graduate students Keith Loebner and Tom Underwood, together with Professor Mark Cappelli, describe how they built a device that creates tiny plasma jets and enabled them to make detailed measurements of these ionized clouds.

The researchers also proved that plasmas exhibit some of the same behavior as the gas clouds created by, say, firing a rocket engine or burning fuel inside an internal combustion engine.

Their instrument, coupled with this new understanding of the fire-like behavior of plasmas, creates a down-to-earth way to explore the physics of solar flares, fusion energy and other astrophysical events.

“The understanding of astrophysical phenomena has always been hindered by the inability to generate scaled conditions in the laboratory and measure the results in great detail,” Cappelli said.

Plasmas 101

Charged particles generate electric currents and magnetic fields as they move. This means that engineers can apply electromagnetic forces to interact with and influence plasmas — something they cannot do with ordinary or neutral gases.

“This is why plasmas are both interesting and complicated,” Loebner said.

Some simple technologies rely on plasmas. For instance the light in a neon sign is produced by a plasma effect. When the electrons in the plasma flip between moving freely and being bound to an atom, they release the photons that we see as different colors of light.

But the Stanford researchers wanted to study more energetic and highly ionized plasmas of the sort that might be found in a solar flare.

Toward this end they constructed a device the size of a mini-fridge. They used it to apply an electric field to a small amount of gas and to accelerate the gas’s electrons through a rifle-like tube. These electrons crashed into neutral atoms, knocking still more electrons loose as the particles continue to gain energy, like a rolling snowball starting an avalanche. This process ionized the gas and produced a plasma.

A probe within the tube provided detailed observations to prove that the plasma exhibited two behaviors that scientists have long observed in combusting gases. These two behaviors are deflagration and detonation. A deflagration is gas that expands away from the burning site, such as thrust exiting a rocket engine. A detonation is a gas burning under pressure that creates a shock wave.

In their laboratory experiment, the Stanford researchers observed that plasmas exhibited both detonation and deflagration behaviors.

“An experimental tool is only as valuable as your understanding of the behavior of the system,” Loebner added. “Knowing that plasmas behave like something we already know, burning gases, will help researchers conceive new experiments about a whole range of interesting plasma phenomena.”

Next Steps

By successfully describing plasma behaviors in the lab, Cappelli and his team hope to provide important contributions to many areas of experimental research.

For example, their experiments resemble conditions near the walls of the international fusion reactor project known as ITER, an enormous planned facility in southern France. Fusion, whether in a reactor or in the center of the Sun, requires sufficiently heated and confined plasma such that colliding nuclei will fuse together, releasing energy.

Aerospace engineers working on plasma-based propulsion systems for rockets and shuttles would be interested in their results as well. These spacecraft generate thrust by using electromagnetic fields to accelerate and direct the plasma, ejecting it from the engine. NASA’s Dawn probe used a similar ion propulsion engine to reach the solar system’s asteroid belt earlier this year.

This research also has implications for solar storms, which can eject giant clouds of magnetized plasma. If a solar eruption were to reach the Earth, it could disrupt satellites and communication systems, potentially creating havoc. Considering the possibility of such a situation, government and commercial groups naturally have an interest in understanding how the plasma would behave.