Imperial College researchers simulate accretion disks around black holes in the lab

Imperial College researchers have created a spinning disk of plasma in the lab, simulating the disks around black holes and star formation.

The experiment is modeling more precisely what happens in these plasma disks, which could help researchers discover how black holes grow and how collapsing matter forms stars.

As matter approaches black holes, it heats up to become plasma – a fourth state of matter composed of charged ions and free electrons. It also begins to rotate, in a structure called an accretion disk. The spin causes the centrifugal force to push the plasma outward, which is counterbalanced by the black hole’s gravity pulling it in.

These glowing rings of orbiting plasma pose a problem – how does a black hole grow if matter is stuck in orbit rather than falling into the hole? The leading theory is that instability in the magnetic fields in the plasma causes friction, which results in a loss of energy and a fall into the black hole.

The primary way to test this was to use liquid metals that could spin, and see what happens when magnetic fields are applied. However, because the minerals must be contained within the tubes, they are not a true representation of free-flowing plasma.

Now, researchers at Imperial have used the Massive Ampere Generator for the Machine for Plasma Immersion Experiments (MAGPIE) to spin the plasma into a more accurate representation of the accretion disks. Details of the trial were published May 12 in the journal Physical review letters.

Plasma acceleration

First author Dr Vicente Valenzuela-Villaseca completed the study during his Ph.D. in the Department of Physics at Imperial. He said, “Understanding how accretion disks work will not only help us reveal how black holes grow, but also how gas clouds collapse to form stars, and even how we can better create our own stars by understanding the stability of plasma in fusion experiments.”

The team used the MAGPIE machine to accelerate and collide eight plasma jets, forming a spinning column. They discovered that the closer we got inside the rotating ring the faster it moved, which is an important property of accretion disks in the universe.

MAGPIE produces short pulses of plasma, which means that only one rotation of the disk was possible. However, this proof-of-concept experiment shows how the number of cycles can be increased with longer pulses, allowing for a better characterization of the disc properties. The longer experiment runtime will also allow magnetic fields to be applied, to test their effect on system friction.

Dr Valenzuela Villasica said, “We are just the beginning of being able to look at these accretion disks in completely new ways, which includes our experiments and our snapshots of black holes using the Event Horizon Telescope. These experiments will allow us to test the theories and see if they match astronomical observations.”