The PAX/APEX experiment for production of a pair plasma
In this activity, technologies are being developed for the creation of a confined pair plasma, composed of an equal amount of positrons and electrons.
Recent years have seen a huge interest in laboratory experiments on electron-positron plasmas. Contrary to intuition, under good vacuum conditions the lifetime of such a plasma is not primarily limited by annihilation of particles with their anti-particles. It could live for minutes or even hours.
Interest in experiments on such plasmas comes both from basic plasma science and from astrophysics. In basic plasma science, fundamentally new results on wave and turbulence dynamics are expected due to mass symmetry between positive and negative charge carriers. In astrophysics, positron-electron plasmas are believed to exist e.g. in the vincinity of quasars and galactic acretion disks.
The apparatus for creating and confining such a plasma is currently under construction in Garching and Greifswald. The actual experiment will be conducted at one the most intense sources for cold positrons, the NEPOMUC beamline at the FRM II research reactor in Garching.
In 2013, prototype studies of A Positron-Electron EXperiment (APEX) for the confinement of electron-positron plasmas were started. At its heart will be a superconducting, levitated current loop producing a dipole magnetic field.
For positrons, available beam currents are by far weaker than those of electrons. Effective transport of charged particles across magnetic surfaces is therefore essential. We have carried out a numerical analysis of an injection scheme into the dipole magnetic field overlaid with an external electric field (Fig. 1). More than 80% of the injected positrons can take long orbit lengths under optimized conditions.
In 2015, the injection of positrons from NEPOMUC into the dipole field of the prototype permanent magnet was demonstrated. After transiting to closed magnetic field lines by the so-called ExB drift, they made at least a half turn around the magnet before annihilating on a target probe.
In the Positron Accumulation eXperiment (PAX), the aim is to slow down, store and accumulate positrons, in order to increase the pair plasma density in the APEX experiment. These techniques are currently being developed in our Greifswald laboratory, in a collaboration with EMAU Greifswald (L. Schweikhard, G. Marx) and UC San Diego (C. Surko, J. Danielson). They will be mated with the experiment at NEPOMUC as soon as they are fully operational. In 2015, the storage of 3*108 electrons in a high magnetic field for over an hour was achieved.