Changing Physics: Plasma Jets With Unexpected X-Ray Emissions

Caltech Professor Paul Bellan’s two decades of research into plasma jets has revealed unexpected behavior in ‘cold’ plasmas. Having first theorized a collision-avoidance mechanism for electron acceleration, Bellan later disproved it through simulations, discovering that some electrons, by rarely losing energy in nearby ion channels, continue to accelerate and produce X-rays. This finding, important for understanding solar flares and fusion experiments, challenges conventional plasma theories. Source:

at Caltech plasma The jet experiments led by Paul Bellan revealed new electron behaviors, contributing to the understanding of solar flares and fusion energy.

For about 20 years, Caltech Professor of Applied Physics Paul Bellan and his team have been creating magnetically accelerated jets of plasma, an electrically conducting gas composed of ions and electrons, in a vacuum chamber large enough to can hold a person. (Neon signs and lightning are everyday examples of plasma).

In that vacuum chamber, tiny gases are ionized at a few thousand volts. One hundred thousand amps will flow through the plasma, producing a strong magnetic field that will shape the plasma into a jet traveling at about 10 miles per second. High-speed recordings show that the jet transitions through several distinct stages in a few tens of microseconds.

Bellan says that the plasma jet looks like an umbrella that has grown in length. Once the height reaches one or two feet, the jet goes through an instability that causes it to turn into a rapidly expanding corkscrew. This rapid expansion causes a different, faster instability that creates ripples.

“The ripples choke the jet’s 100-kiloamp electricity, like sticking your thumb in a water hose that stops the flow and creates a pressure gradient that speeds up the water,” Bellan said. “The throttling of 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 produced, and Bellan said their presence was a surprise. That’s because conventional wisdom says that jet plasma is too cold for electrons to be accelerated to high energies. Note that “cold” is a relative term: Although this plasma has a temperature of about 20,000 Kelvin (35,500 degrees. Fahrenheit)—much hotter than anything humans normally encounter—is nowhere near the temperature of the Sun’s corona, which is more than a million Kelvin (1.8 million degrees F.)

“So, the question is, ‘Why do we see X-rays?'” he said.

Cold plasmas are thought to be incapable of generating high-energy electrons because they are “collisional,” meaning that an electron cannot travel very far before colliding with another particle. It’s like a driver trying to drag race in freeway gridlock. The driver may hit the accelerator but only travel a few feet before hitting another car. In the case of a cold plasma, an electron accelerates only about a micron before colliding and slowing down.

The first attempt by the Bellan group to explain this phenomenon was a model that suggested that some fraction of electrons managed to avoid collisions with other particles during the first micron of travel. According to the theory, that allows the electrons to accelerate to a slightly higher speed, and once faster, they can travel a little further before encountering another particle that they can collide with. Some fraction of the now faster electrons will again avoid the collision for a while, allowing them to reach a higher speed, allowing them to travel even further, creating in a positive feedback loop that allows a few lucky electrons to go forward. and faster, achieves high speed and high power.

But while pressing, the theory is wrong, Bellan said.

“It turns out that this argument has a flaw,” he said, “because electrons don’t really collide in the sense of hitting something or not hitting something. They all actually deviate a little bit all the time. Therefore, there is no such thing as an electron colliding or not colliding.”

New Insights From Computer Simulation

However, high-energy electrons ACT seen in cold plasma in the jet experiment. To find out why, Bellan created a computer code that calculated the actions of 5,000 electrons and 5,000 ions moving steadily away from each other in an electric field. To find out how to manipulate certain electrons to achieve high energy, he changes the parameters and observes how the behavior of the electrons changes.

As the electrons accelerate in the electric field, they pass near the ions but never touch them. Sometimes, an electron gets so close to an ion that it transfers energy to an electron attached to the ion and slows down, with the now “excited” ion emitting visible light. Because the electrons rarely travel too far, they usually deflect slightly away from the ion without exciting it. This occasional leakage of energy occurs in most electrons, which means they cannot gain high energy.

When Bellan tweaked his simulation, some high-energy electrons appeared capable of producing X-rays. “The lucky few who don’t get close enough to an ion to excite it don’t lose energy,” he added. “These electrons are continuously accelerated by the electric field and eventually reach enough energy to produce X-rays.”

Bellan says that if this behavior occurs in plasma jets in his lab at Caltech, it probably also occurs in solar flares and astrophysical situations. This may also explain why unexpected high-energy X-rays are sometimes seen during fusion-energy experiments.

“There’s a long history of people seeing things that they think are useful integrations,” he said. “It turns out to be fusion, but it’s not useful. It’s a very intense transient electric field created by instabilities that accelerates some particles to a much higher energy. Maybe this explains if what’s going on. That’s not what people want, but maybe it’s going to happen.”

A paper describing the work appeared in the October 20 issue of the Physics of Plasmas and 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.

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