An international team of scientists has photographed plasma instability for the first time, revealing striking new images of electron beams forming spaghetti-like filaments.
A research team from Imperial College London‘s John Adams Institute for Accelerator Science and the Brookhaven National Laboratory used a high-intensity laser to produce and capture the filamentation instability. Their findings have potential applications that include plasma-based particle accelerators, fusion energy, and even healthcare.
The Fourth State of Matter
Plasma is one of the four fundamental states of matter, along with solid, liquid, and gas. It consists of a combination of charged ions and electrons that can conduct electricity and are influenced by magnetic fields.
When the flow of particles in a plasma region differs from the rest or moves in a single direction, instability occurs. This can lead to the formation of long, thin filaments, known as a “Weibel-like current” instability. These filaments can trigger a runaway effect, generating their own magnetic fields and further destabilizing 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 a Plasma Filament
The team targeted a stationary plasma with a high-intensity laser for their experiment, generating a concentrated, high-energy electron beam. When the laser’s photons interacted with the plasma’s electrons, they boosted the electrons’ energy and pushed them toward the laser.
The electron beam would pass through smoothly in a perfectly stable, uniform plasma. However, due to a lack of uniformity, disruptions occurred. The beam created small fluctuations in areas with varying electron densities, leading to the formation of filaments. As electrons began clumping together, they increased the destabilization.
“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.”
Scientists have long speculated about instability of this type, but only indirect effects have ever been observed. This is the first time researchers have captured the effect in a laboratory.
Capturing Instability in Imagery
To photograph the instability, the team synchronized two lasers operating at different wavelengths. Brookhaven developed a unique long-wave infrared laser with an unprecedented intensity to create the electron beam responsible for generating instability. A more conventional short-wavelength optical probe laser was then used to capture images of the event.
One major challenge in studying plasma instabilities is that plasma densities are typically too high for standard lasers to penetrate. To overcome this, the researchers employed a shadowgraph technique to visualize density variations and measure the filaments. The Brookhaven laser allowed the team to control where the electrons traveled within the plasma by directing energy deposits. This capability enabled them to position the electrons in a region where the optical probe laser could observe them.
To set up their experiment, the scientists first injected short bursts of gas into a vacuum chamber to create gas targets. This process allowed for precise adjustments to plasma density, which the researchers fine-tuned to assess its impact on filament formation. By carefully controlling these conditions, they were able to capture extreme close-up images of the instabilities.
“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.
Mastering Instability
The next step for researchers at the Brookhaven Accelerator Test Facility is to improve their imaging capabilities. They plan to upgrade the optical laser to capture clearer images and shorter time intervals, allowing for real-time observations rather than relying solely on post-experiment data analysis.
“If we can actually crack that, then it can have really big applications, especially in radiotherapy,” Said Professor Zulfikar Najmudin, Deputy Director of the John Adams Institute. “[Brookhaven] are keen to demonstrate particle beams energetic enough for radiobiology experiments.”
The paper “Optical Imaging of Laser-Driven Fast Electron Weibel-like Filamentation in Overcritical Density Plasma” appeared on January 17, 2025, in Physical Review Letters.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.