Physicists capture elusive plasma instability in unprecedented detail

For the first time, scientists have ‘photographed’ a rare plasma instability, where high-energy electron beams form into spaghetti-like filaments.

A new study, published in Physical Review Letters, outlines how a high-intensity infrared laser was used to generate a filamentation instability – a phenomenon that affects applications in plasma-based particle accelerators and fusion energy methods.

Plasma is a super-hot mixture of charged particles, such as ions and electrons, which can conduct electricity and are influenced by magnetic fields. Instabilities in plasmas can occur because the flow of particles in one direction or within a specific region can be different from the rest, causing some particles to group up into thin spaghetti-like filaments.

Known as a ‘Weibel-like current’ instability, these filaments can generate their own magnetic field that further destabilises the rest of the plasma.

“The reason we are particularly interested in instabilities is because they tend to mess up the applications, like injecting energy into plasma to trigger fusion,” said Dr Nicholas Dover, a research fellow at Imperial College London’s Department of Physics and the John Adams Institute for Accelerator Science.

“Normally, we want to avoid instabilities, but to do that we need to understand them in the first place,” he said.

Creating spaghetti-like filaments in plasma

In this experiment, researchers fired a high-intensity laser into an initially stationary plasma to create a high-energy electron beam. The photons in the laser can give an energy boost to the electrons in the plasma, kicking them in the direction of the laser.

If the plasma was perfectly stable and uniform, this electron beam would be able to pass through smoothly, like fast cars weaving between a smooth flow of traffic.

Instead, researchers saw that it disrupted the plasma, triggering small fluctuations that caused some areas to have more or fewer electrons than others. As the electrons clumped together and generated thin filament, which then further destabilised the rest of the plasma.

“The more magnetic fields you generate, the more the instability grows and then the more magnetic field generates,” said Dr Dover, “It’s kind of like a snowball effect.”

Creating the perfect snapshot

Scientists have long inferred this instability from indirect effects, but observing it directly has been a challenge. This study marks the first time it has been captured in a laboratory.

Researchers from Imperial’s John Adams Institute for Accelerator Science collaborated with the Stony Brook University and Brookhaven National Laboratory in New York.

The laboratory utilised two synchronised lasers with different wavelengths: a one-of-a-kind high-intensity, long-wave infrared laser (housed at Brookhaven’s Accelerator Test Facility) and a shorter wavelength optical probe laser.

The first created the electron beam which drove the instability, while the second captured images of it.

Typically, standard lasers struggle to penetrate plasma up to a certain density, making it difficult to observe inside its structure.

However, Brookhaven’s long-wave infrared CO2 laser enabled the researchers to control where energy was deposited in the plasma, allowing the electrons to travel into regions where they could still be observed with a visible laser probe. By synchronising the optical laser, researchers captured detailed images of the instability.

Scientists generated the plasma using gas targets – short bursts of gas released into a vacuum chamber – which allowed them to precisely tune the density of the plasma they created by adjusting the gas pressure in the chamber. By adjusting the density, the researchers could also study how the size of the filaments changed. These fine adjustments resulted in unprecedented close-up images of the instability.

“We were really amazed by how good the photographs were because with optical lasers, it's really hard to take nice photographs of the plasma,” said Dr Dover.

In the future, Brookhaven’s Accelerator Test Facility plans to upgrade the optical laser, allowing the researchers to capture clearer, more precise pictures in shorter time intervals. This will let them observe laser-plasma interactions in real time rather than only analysing the aftermath.

If we can actually crack that, then it can have really big applications, especially in radiotherapy. Professor Zulfikar Najmudin Deputy Director of the John Adams Institute

Professor Zulfikar Najmudin, Deputy Director of the John Adams Institute, highlighted the potential applications of this research: “[Brookhaven] are keen to demonstrate particle beams energetic enough for radiobiology experiments.”

He explains that achieving 10 MeV energy levels in such a small gas target of just a few hundred microns, is virtually unheard of in other interactions: “If we can actually crack that, then it can have really big applications, especially in radiotherapy.”