European Commission logo
English English
CORDIS - EU research results

Creating an electron-positron plasma in a laboratory magnetosphere

Periodic Reporting for period 4 - PAIRPLASMA (Creating an electron-positron plasma in a laboratory magnetosphere)

Reporting period: 2022-02-01 to 2023-07-31

The visible Universe is predominantly in the plasma state. On Earth, plasmas are less common, but they find many applications in industry and are also studied with the goal of providing an abundant energy source for mankind through fusion energy. The behavior of plasmas studied thus far, in particular those that are magnetized, is very complex. The complexity manifests itself, first and foremost, as a host of different wave types, many of which are generically unstable and evolve into turbulence or violent instabilities. This complexity and the instability of these waves stems to a large degree from effects that can be traced back to the difference in mass between the positive and negative species, the ions and the electrons. In contrast to conventional ion-electron plasmas, electron-positron (pair) plasmas consist of equal-mass charged particles. This symmetry results in unique behavior of the pair plasmas, a topic that has been intensively studied theoretically and numerically for decades but experimental studies are only just starting.

These studies are not only motivated by a desire to test our theoretical understanding of plasma physics: Strongly magnetized electron-positron plasmas are believed to exist ubiquitously in pulsar magnetospheres and active galaxies in the Universe, and the entire Universe is believed to have been a matter-antimatter symmetric plasma in its earliest epochs after the Big Bang.

The international APEX (A Positron Electron eXperiment) Collaboration aims to create and study of the first long-lived and confined pair plasmas on Earth, by means of combining novel techniques in plasma and beam physics. This project was a core element of the APEX Collaboration, and it majorly advanced progress on the road map toward e+e- pair plasmas. During its course, we designed and constructed a compact levitated dipole trap (a confinement device for charged particles involving a magnetic field similar to a planetary magnetosphere, but without losses along the magnetic field lines). In parallel, we developed techniques to fill this magnetic field geometry with low-energy positrons from the world-leading steady-state positron source (and also with readily available electrons). To accumulate the number of positrons required, we contributed to the development of accumulators for positrons that will deliver large pulses of positrons, to be used not only for the pair plasma experiments associated with this project but for other experiments that use positrons to study solid matter. To support these experimental efforts, we developed and exploited theory and simulation tools and techniques for the interpretation and extension of the experimental results and the prediction and design of future experiments.
The work performed as part of this project advanced several parallel paths.

To enable us to reach the required number of positrons to create a pair plasma, we and collaborators adapted, upgraded, and rebuilt an existing positron trap and accumulator system, plus an extension of this system that will permit accumulation of more than 10x as many positrons in a “multi-cell” trap. This will provide intense, high-quality e+ pulses not only for production of pair plasma, but also for other experimenters who use positrons to study solid matter.

Second, we performed an extensive and highly successful series of experiments in a proto-type dipole trap based on a supported permanent magnet. In this setup, for example, we injected and trapped positrons from the world's leading slow positron beam (the NEutron-induced POsitron source MUniCh, operated at the FRM II neutron source), including with an electron cloud/plasma already in the trap. These experiments validated our strategy for getting charged particles into the trap and also showed that they have good confinement properties once injected.

Third, we designed, built, and nearly finished commissioning the levitated dipole trap, which is based on a circular, superconducting coil that is magnetically levitated in vacuum. This required developing state-of-the-art engineering techniques for cooling the superconducting coil, inducing current in it, and levitating it in a stable manner for as long as an hour or more.

Finally, we developed numerical tools to simulate particles and plasmas in dipole magnetic fields, from charged particles to fluid equilibria.

The results have been (or, in several cases, are in the process of being) published in scientific journals, in addition to being presented at conferences and workshops. They are also the topic of various public outreach efforts, seminars at colleges and universities, etc. Overall, the project succeeded at demonstrating key engineering and physics requirements along the road map to creating and studying these novel matter-antimatter hybrids that represent an exciting frontier of plasma physics.
Our success in injecting ~100% of the positrons from the world-leading NEPOMUC positron beam (operated at the FRM II neutron source) into our prototype dipole trap (based on a supported permanent magnet), plus the subsequent confinement of a population of positrons for more than one second (limited at this point only by current vacuum conditions), represented important early milestones toward the experimental concept. Not only were these results of significant physical interest in and of themselves (and as such, they were published in several articles in Physical Review Letters, as well as being presented at major physics conferences and highlighted in publications for a broader audience), but they also provided necessary experimental validation of design concepts for the levitated dipole trap.

The design, physics validations, and engineering development of the levitated dipole trap were key elements of this project --- from proof of concept tests of the methods for cooling the two high-temperature superconducting (HTS) coils, to the a demonstration levitation feedback system, to the nearly complete commissioning of the trap. The positron accumulation efforts that are necessary for pair plasma creation were also very much advanced by the support of this project. Also, several important numerical/theory/simulation campaigns were also conducted, from advanced single-particle simulations to calculations of local and global thermal equilibrium states of pure e+ or e- plasmas in a dipole geometry (interim states toward e+e- plasma creation).

The FRM II neutron source was unfortunately not available for the second half of the project duration. Combined with the COVID-19 pandemic and resulting supply chain issues, as well as an earlier-than-planned termination of the grant, this meant that e+e- plasmas could unfortunately not be created by the end of the action. However, extensive progress was made toward this goal --- both experimentally and via theory/simulation efforts --- while also discovering new physics and developing new engineering techniques along the way. Many publications about the work funded by this action are in preparation and can be expected to be published in 2023 (i.e. by approximately when the grant would have ended, if not for the earlier termination due to the departure of this project's PI).

We are proud of our achievements and hold out hope of achieving pair plasmas in a levitated dipole trap in the future (contingent on funding availability).