We don’t often play with them, but plasmas are a central part of modern life. The plasmas that we can create on Earth, however, are unlike some of the plasmas that are thought to exist in space—the ones in space appear to include antimatter. Those plasmas are hard to observe, and as a result, we don’t think we understand them very well.

That makes a pair of recent publications on the trapping of positrons—the positively charged cousin to the electron—very interesting. While they don't get as far as exploring plasmas with antimatter, they do show that we can trap the antimatter long enough to make some.

Plasma: A fluid that is not a fluid

Before we get to the good stuff, we need to trudge through some well-explored fields. A plasma is basically a gas, but the gas consists of ions—atoms that have had one or more electrons removed—and free electrons.

In most cases, the number of positive charges is balanced by negative charges, so the plasma is neutral. At any given moment, though, small charge imbalances exist between different parts of the plasma. That charge imbalance generates a force, which drives the electrons to move rapidly. This motion overcompensates for the charge imbalance, creating new charge imbalances that keep the plasma in constant motion. Moving charges also generate magnetic fields, which complicate matters even more, since these also modify the motion of charges.

This mostly involves the electrons, since an ion is at least 2,000 times heavier than an electron. While the electrons zip around like kids at a sugar picnic, the ions are a drunk dad that falls asleep by the grill.

The slower motion of the ions destabilizes the whole plasma. If all the electrons leave a region, it takes a while for the ions to start to follow. By the time they’ve started to move, the electrons are back, pulled in by their attraction to the positive charge. Then the electrons are gone again, leaving the bewildered ions still trying to catch up.

A plasma of equals

In the center of the galaxy, the plasmas seem to be somewhat different. Gamma ray observations show that there is a large amount of positron-electron annihilation, indicating that the plasma is made of positive and negative charges of equal mass. We don’t actually know how that sort of plasma behaves, but calculations tell us that it is probably very different from an ion-based plasma.

To create something to study, we need to trap positrons in a magnetic bottle. Electrons can then be added to create an electron-positron plasma. Seems simple, but there's an issue: the rarity of positrons. To solve this problem, a group of scientists in Germany have created a very good positron trap.

The researchers noted that the Stellarator—a fusion device—loses electrons as they drift across the magnetic field (normally electrons travel along magnetic field lines). This drift is driven by electrons colliding with each other. These collisions send the electrons moving perpendicular to both the magnetic field and an electric field that the electrons create.

The researchers arranged a set of electrodes to recreate that electric field. But the electrodes have to provide a very precise profile in relation to the magnetic field lines of the trap. It’s not stated in the article, but I suspect that a lot of time was spent on this problem. Stellarator designers faced a similar problem, so the researchers could probably work with a modified version of the code used to optimize the Stellarator (the research was conducted at the same institution).

The result, though, was spectacular: the researchers were unable to detect any positron loss during the trapping process.

The trap was also pretty awesome. It combines magnetic fields and electric fields to confine the positrons. This allowed the researchers to take advantage of the new electric field technique to controllably load and unload the trap.

The trap holds the positrons for about a second, which is pretty amazing. But the researchers wanted to know why they only got a second. So a second paper was devoted to looking at the positrons once they were inside.

The trapped positrons follow a kind of spiraling orbit around the trap center. As the particles move around, there is a chance of being pinged out of the trap by a collision. For instance, there are always some neutral atoms floating around, and hitting one of them will fling a positron out of the trap. More importantly, any imperfection in the shape of the electric or magnetic field will also lead to particle loss. Loss via that mechanism would indicate a flaw in the trap design.

The researchers were able to examine loss from the trap by modifying the fields produced by the electrodes that guide and help trap the positrons. They were able to determine that most of the positrons are lost by colliding with neutral gas atoms. So the trap is pretty close to as good as it can get.

Next step (I hope): add electrons and learn about positron-electron plasmas and the center of our galaxy.

Physical Review Letters, 2018, DOIs: 10.1103/PhysRevLett.121.235003 and 10.1103/PhysRevLett.121.235005. (About DOIs)