Caltech professor Paul Bellan has been studying plasma jets for two decades, revealing unexpected behavior in "cold" plasma. Beran initially proposed a collision avoidance mechanism theory for electron acceleration. He later overturned this theory through simulations and found that some electrons rarely lose energy when passing close to ions, thus continuing to accelerate and produce X-rays. This discovery is of great significance to understanding solar flares and nuclear fusion experiments, challenging traditional plasma theories.
A Caltech plasma jet experiment led by Paul Bellan reveals new electron behavior that could help understand solar flares and nuclear fusion energy.
For about 20 years, Caltech professor of applied physics Paul Bellan and his research team have been creating jets of magnetically accelerated plasma, a conductive gas made of ions and electrons, in a vacuum chamber large enough to hold a person. (Neon lights and lightning are everyday examples of plasma).
In a vacuum chamber, wisps of gas are ionized by voltages of several thousand volts. A current of 100,000 amps then flows through the plasma, creating a strong magnetic field that shapes the plasma into a jet of about 10 miles per second. High-speed recordings show that the jet passes through several different stages within tens of microseconds.
The plasma jet looks like an ever-growing umbrella, Beran said. Once it reaches one to two feet in length, the jet goes through an unstable phase, causing it to transform into a rapidly expanding corkscrew. This rapid expansion triggers another, faster instability that creates ripples. The ripples choke the jet's 100-kilowatt current, much like putting your thumb on a water pipe restricts the flow and creates a pressure gradient that speeds it up. The jet current creates an electric field strong enough to accelerate electrons to high energies.
Surprising discovery of plasma behavior
These high-energy electrons were previously identified by the X-rays they produce in jet experiments, and their appearance was surprising. This is because, according to conventional understanding, the ejected plasma is too cold for electrons to be accelerated to high energies. Note that "cold" is a relative term: While this plasma is about 20,000 Kelvin (35,500 degrees Fahrenheit) -- much hotter than anything humans typically encounter -- it is nowhere near the temperature of the sun's corona, which is more than 1 million Kelvin (1.8 million degrees Fahrenheit).
So the question is 'Why do we see X-rays?'
Cold plasmas are thought to be unable to produce high-energy electrons because they are too "collisive," meaning electrons cannot travel very far before colliding with another particle. It's like a driver trying to race through a highway jam. A driver might slam on the accelerator but only travel a few feet before hitting another vehicle. In a cold plasma, electrons accelerate by just a micron before colliding and slowing down.
The model that Beland's group first tried to explain the phenomenon suggests that a fraction of the electrons manage to avoid collisions with other particles during the first micrometers of their flight. According to this theory, electrons can be accelerated to slightly higher speeds, and once accelerated, they can travel some distance before encountering another particle with which they might collide. Some of these now faster electrons will in turn temporarily avoid collisions, allowing them to reach higher speeds so that they can travel further, creating a positive feedback loop that allows the lucky few electrons to travel further and faster, reaching high speeds and high energies.
While the theory was compelling, Beran said, it was wrong: "People realized that the argument was flawed because electron collisions were not really about collision or non-collision. They were actually deflecting a little bit all the time. So there were no collisions or non-collision of electrons."
New insights from computer simulations
However, high-energy electrons appear in the cold plasma of jet experiments. To find out why, Beran developed a computer code that calculates the behavior of 5,000 electrons and 5,000 ions constantly deflecting each other in an electric field. To figure out how a small number of electrons reach high energies, he tweaked the parameters and observed how the electrons' behavior changed.
When electrons are accelerated in an electric field, they pass near the ions but never actually touch them. Occasionally, an electron zips through an ion, transfers energy to an electron attached to the ion, and slows down, causing the "excited" ion to emit visible light. Since the electrons only occasionally pass this close, they usually just stray slightly away from the ion without exciting it. Most electrons suffer this energy leakage from time to time, meaning they never reach high energies.
As Bellan tweaked his simulations, a number of high-energy electrons emerged that were capable of producing X-rays. He added: "The lucky few electrons never get close enough to the ion to excite it, and they never lose energy. These electrons are continuously accelerated in the electric field and eventually gain enough energy to produce X-rays."
If this behavior is seen in plasma jets at Caltech's lab, it's likely to be seen in solar flares and astrophysical settings as well, Beran said. This could also explain why unexpected high-energy X-rays sometimes appear in fusion energy experiments.
"For a long time, people saw something they thought was useful fusion. It turned out that it was fusion, but it wasn't really useful fusion," he said. "It was a strong transient electric field generated by an instabilities that accelerated some particles to very high energies. That might explain what was going on. It wasn't what people wanted, but it was probably what happened."
A paper describing the work appears in the Oct. 20 issue of the journal Plasma Physics and was presented Nov. 3 at the 65th Annual Meeting of the American Physical Society's Division of Plasma Physics in Denver, Colorado.
Compiled source: ScitechDaily