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Creating an electron-positron plasma in a laboratory magnetosphere

Periodic Reporting for period 2 - PAIRPLASMA (Creating an electron-positron plasma in a laboratory magnetosphere)

Reporting period: 2019-02-01 to 2020-07-31

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.
This project aims to create and study the first long-lived and confined pair plasmas on Earth. This is now possible by combining novel techniques in plasma and beam physics. We are designing and building a levitated dipole confinement device and will fill it with readily available electrons and low-energy positrons from the world-leading steady-state positron source. To accumulate the number of positrons required, we are developing a positron trap and accumulator that will deliver large pulses of positrons that can be used not only for the pair plasma experiments associated with this project but for other experiments that use positrons to study solid matter.
The work performed to date has focused on three parallel paths. First, we have been adapting an existing positron trap and accumulator system to be installed on the reactor-based positron beamline at the research reactor in Munich. This system will provide intense high quality positron pulses not only for production of pair plasma which is the focus of this project, but also for other experimenters who use positrons to study solid matter. In addition, an extension of this system is under development that will permit accumulation of more than 10x as many positrons in a so-called “multi-cell” trap. Second, we have been performing an extensive series of experiments in which we inject and trap positrons from the reactor-based beam into a proto-type trap that consists of a supported permanent magnet. These experiments have validated our strategy for getting charged particles into the trap and also show that they have good confinement properties once injected. Finally, we have made significant progress in designing the final magnetic trap which will consist of a superconducting coil that is magnetically levitated to produce a magnetic field similar to a planetary magnetosphere. This requires developing a strategy for cooling the superconducting coil, inducing current in it, and levitating it in a stable manner for as long as an hour or more.
Our success in injecting nearly 100% of the positrons from the reactor-based beam (at the research reactor in Munich) into a proto-type trap using a supported permanent magnet, and the subsequent confinement of a population of positrons for more than one second (limited at this point only by current vacuum conditions), represents significant progress toward our final goals. These results are novel and have been published in several articles in the pre-eminent physics journal, Physical Review Letters, as well as being presented at major physics conferences and highlighted in publications for a broader audience.

We anticipate successful completion of the construction and initial operation of a levitated dipole trap by the end of the project. This trap will be fueled by intense positron pulses from a positron trap and accumulator system that will be installed for general use on the positron beam at the research reactor in Munich.
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