Physicists capture the elusive plasma instability in unprecedented detail

For the first time, scientists have “photographed” the rare plasma instability in which high-energy electron beams form in spaghetti-like filaments.

A new study published in the Physical Review Letter outlines how high-intensity infrared lasers were used to generate filamentation instability. This is a phenomenon that affects the applications of plasma-based particle accelerators and fusion energy methods.

Plasma is a very hot mixture of charged particles such as ions and electrons that introduce electricity and can be affected by magnetic fields. Plasma instability can occur because the flow of particles in one direction or in a particular region may differ from the other regions, and some particles may be grouped into thin spaghetti-like filaments.

Known as current instability, like Weibel, these filaments can generate their own magnetic fields that further destabilize the rest of the plasma.

“The reason we are particularly interested in instability is because they tend to ruin applications, such as injecting energy into the plasma to cause fusion,” said Dr. Nicholas Dover, a researcher at the Faculty of Physics at Imperial College London and the John Adams Adams Institute.

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

Create spaghetti-like filaments in the plasma

In this experiment, the researchers first fired a high-intensity laser into the stationary plasma to create a high-energy electron beam. The photons in the laser increase energy into the electrons in the plasma and kick them in the direction of the laser.

If the plasma is perfectly stable and uniform, this electron beam can pass smoothly, like a high-speed vehicle weaving in between smooth traffic flows.

Instead, the researchers saw that it destroys plasma and causes small variations that cause some regions to have more or fewer electrons than others. When the electrons coagulated together to produce thin filaments, this further destabilized the rest of the plasma.

“The more magnetic fields you generate, the more instability it becomes, the more magnetic fields you create,” Dr. Dover said.

Create the perfect snapshot

Scientists have long inferred this instability from indirect effects, but observing it directly has been a challenge. This research was first photographed in a lab.

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

In the laboratory, two synchronized lasers of different wavelengths were used: a unique high-intensity long-wave infrared laser (accommodated at Brookhaven’s accelerator testing facility) and a short-wavelength optical probe laser.

The first created an electron beam that promoted instability, and the second image captured that image.

Typically, standard lasers struggle to penetrate plasma to a specific density, making them difficult to observe within their structure.

However, Brookhaven’s long-wave infrared CO2 laser allowed researchers to control where plasma energy was deposited, allowing electrons to move to areas where visible laser probes can still be observed. By synchronizing the optical laser, researchers captured detailed images of instability.

Scientists used gas targets to generate plasmas – the gas target (explosion of gas released into the vacuum chamber) allowed them to accurately adjust the density of the plasma produced by adjusting the gas pressure within the chamber. By adjusting the density, researchers can also study how the size of the filament has changed. These fine adjustments resulted in an unprecedented close-up image of instability.

“I was really surprised at how good the photos were because it’s really difficult to take a nice photo of the plasma with an optical laser,” Dr. Dover said.

In the future, Brookhaven’s accelerator testing facility plans to upgrade its optical lasers, allowing researchers to capture clearer and more accurate photos at shorter time intervals. This allows you to observe the interaction of lasers and plasmas in real time, rather than analyzing the aftermath.

Professor Zulfikar Najmuddin, assistant director of the John Adams Institute, highlighted the potential applications of the study. “Brookhaven) is keen to demonstrate energetic particle beams sufficient for radiation biology experiments.”

He explains that achieving 10 MEV energy levels with such a small gas target of just a few hundred microns is virtually unprecedented in other interactions. “If you can actually break it, it can have a really big application, especially in radiation therapy.”