Progress of the APEX experiment for creation of an electron-positron pair plasma

Abstract

Electron-positron pair plasmas are an unexplored state of matter predicted to have properties intriguing for plasma physics as well as astrophysics. Here we described recent progress in the APEX collaboration dedicated to the production of a cold, confined electron-positron plasma in the laboratory. We focus on methods to inject positrons produced externally into a magnetic dipole trap, and to manipulate the ensuing trapped positron cloud. These experiments are carried out at the NEPOMUC positron beamline of the FRM II research reactor. Recent progress in producing more intense positron beams is briefly discussed.

Lossless Positron Injection into a Magnetic Dipole Trap

“…5a). Simulations of our earlier experiments [23,33] revealed the two primary loss channels that limited injection in that configuration to ∼ 40 %. First, there was a large loss cone for particles that did not have sufficient perpendicular velocity to magnetically mirror before hitting the negatively biased magnet case (∼ 40 %).…”

“…Biasing top2 (instead of grounding it) also reduces the tolerances in parameter space, though this is a comparatively mild effect, and lossless injection can still be achieved. So as to enable analyses of loss channels and their elimination, as well as separate examination of the injection and toroidal drift processes, simulations of the positron trajectories have been done both with the SIMION software package [32,33] and with AlGeoJ, a home-grown particle mover that implements a modified leapfrog algorithm in Java [34]. Initial conditions can be generated as random draws from energy and spatial distributions matching those measured for the positron beam [30].…”

“…Synthetic diagnostics of the simulations produce both current and count "data" that can then be compared to experimental measurements, with the initial position of the beam center as an adjustable parameter. Further details can be found in the Supplemental Material [31], as well as in other publications [33,34]. Injection efficiencies are reproduced in simulations using voltage and current settings identical to those used in the experiments.…”

Upgrade of the NEPOMUC Remoderator

“…The reactor-based positron source NEPOMUC provides an intense, mono-energetic positron beam for various experiments: The Coincidence Doppler-Broadening Spectrometer (CDBS) [1], the Pulsed Low-Energy Positron System (PLEPS) [2], the Scanning Positron Microscope (SPM) [3], the Surface Spectrometer (SusSpect) [4], and an open beam port for temporary devices, which is presently used for an electronpositron pair plasma experiment [5]. The NEPOMUC primary beam intensity reaches a yield of more than 1.1 × 10 9 e + s −1 [6]. It is transported in a magnetic field of about 4 mT [7] and owns a mean diameter of about 10 mm FWHM.…”

Manipulation of positron orbits in a dipole magnetic field with fluctuating electric fields

“…Efficient injection was realized by adjusting the E × B plate voltages V E×B , bias voltages of the magnet and outer electrodes, as well as steering coil currents. The status of the injection and trapping experiments at the prototype dipole field trap is presented in [23]. After injection, positrons make a toroidal precession motion in the dipole field due to the grad-B and curvature drifts, as well as the gyro and bounce motions along magnetic field lines.…”