Caltech Professor Paul Bellan’s two-decade research on plasma jets reveals unexpected behaviors in ‘cold’ plasmas. Initially theorizing a collision-avoidance mechanism for electron acceleration, Bellan later disproved this through simulations, discovering that some electrons, by rarely losing energy in near-ion passes, continuously accelerate and produce X-rays. This finding, significant for understanding solar flares and fusion experiments, challenges conventional plasma theories. Credit: SciTechDaily.com
“The ripples choke the jet’s 100-kiloamp electric current, much like putting your thumb over a water hose restricts the flow and creates a pressure gradient that accelerates water,” Bellan says. “Choking the jet current creates an electric field strong enough to accelerate electrons to high energy.”
Surprising Discoveries in Plasma Behavior
Those high-energy electrons were previously identified in the jet experiment by the X-rays they generate, and Bellan says their presence was a surprise. That’s because conventional understanding says the jet plasma was too cold for electrons to be accelerated to high energy. Note that “cold” is a relative term: Although this plasma had a temperature of about 20,000 Kelvin (35,500 degrees
The Bellan group’s first attempt at explaining this phenomenon was a model suggesting that some fraction of the electrons manages to avoid colliding with other particles during the first micron of travel. According to the theory, that allowed the electrons to accelerate to slightly higher velocity, and once going faster, they could travel just a little bit farther before encountering another particle with which they might collide. Some fraction of those now-faster electrons would again avoid a collision for a time, allowing them to attain an even higher speed, which would allow them to travel even farther, creating a positive feedback loop that would allow a few lucky electrons to go farther and faster, attaining high speeds and high energies.
But while compelling, the theory was wrong, Bellan says.
“It was realized that this argument has a flaw,” he says, “because electrons don’t really collide in the sense of hitting something or not hitting something. They are all actually deflecting a little bit all the time. So, there’s no such thing as an electron that’s colliding or not colliding.”
New Insights From Computer Simulations
Yet, high-energy electrons do appear in the cold plasma of the jet experiment. To find out why, Bellan developed a computer code that calculated the actions of 5,000 electrons and 5,000 ions continuously deflecting off each other in an electric field. To suss out how a few electrons were managing to reach high energies, he tweaked the parameters and watched how the electrons’ behavior changed.
As electrons accelerate in the electric field, they pass near ions but never actually touch them. Occasionally, an electron whizzes so closely past an ion that it transfers energy to an electron attached to the ion and slows down, with the now “excited” ion radiating visible light. Because electrons only occasionally pass so closely, they usually just deflect slightly from the ion without exciting it. This occasional energy leakage occurs in most electrons, which means they never attain high energies.
When Bellan tweaked his simulation, a few high-energy electrons capable of creating X-rays appeared. “The lucky few that never come close enough to an ion to excite it never lose energy,” he adds. “These electrons are continuously accelerated in the electric field and ultimately attain sufficient energy to produce the X-rays.”
Bellan says that if this behavior occurs in the plasma jet in his Caltech lab, it probably happens in solar flares and astrophysical situations as well. This may also explain why unexpectedly high-energy X-rays are sometimes seen during fusion-energy experiments.
“There’s a long history of people seeing things that they thought were useful fusion,” he says. “It turns out it was fusion, but it wasn’t really useful. It was intense transient electric fields produced by instabilities accelerating a few particles to extremely high energy. This might be explaining what was going on. That’s not what people want, but it is probably what happens.”
The paper describing the work appeared in the October 20 issue of Physics of Plasmas and was presented on November 3 at the 65th Annual Meeting of the American Physical Society Division of Plasma Physics in Denver, Colorado.
Reference: “Energetic electron tail production from binary encounters of discrete electrons and ions in a sub-Dreicer electric field” by Paul M. Bellan, 20 October 2023, Physics of Plasmas.
DOI: 10.1063/5.0167004
Funding for the research was provided by the National Science Foundation and the Air Force Office of Scientific Research.