arXiv subjects

A. Card

Publications and source records attributed to A. Card.

3 recordsLinked to original sources

arXiv2025

Design and operation of APEX-LD: a compact levitated dipole for the confinement of electron-positron pair plasmas

The objective of the APEX (A Positron-Electron eXperiment) project is to magnetically confine and study electron--positron pair plasmas. For this purpose, a levitated dipole trap (APEX-LD) has been constructed. The magnetically levitated, compact (7.5-cm radius), closed-loop, high-temperature superconducting (HTS) floating (F-)coil consists exclusively of a No-Insulation (NI) Rare-earth Barium Copper Oxide (ReBCO) winding pack, solder-potted in a gold-plated-copper case. A resealable in-vacuum cryostat facilitates cooling (via helium gas) and inductive charging of the F-coil. The 70-minute preparation cycle reliably generates persistent currents of ~60 kA-turns and an axial magnetic flux density of B_0 ~ 0.5 T. We demonstrate levitation times in excess of three hours with a vertical stability of sigma_z < 20 um. Despite being subjected to routine quenches (and occasional mechanical shocks), the F-coil has proven remarkably robust. We present the results of preliminary experiments with electrons, and outline the next steps for injecting positron bunches into the device.

physics.plasm-ph↗

arXiv2023

Annihilation-Gamma-based Diagnostic Techniques for Magnetically Confined Electron-Positron Pair Plasma

Efforts are underway to magnetically confine electron--positron pair plasmas to study their unique behavior, which is characterized by significant changes in plasma time and length scales, supported waves, and unstable modes. However, use of conventional plasma diagnostics presents challenges with these low-density and annihilating matter-antimatter plasma. To address this problem, we propose to develop techniques based on the distinct emission provided by annihilation. This emission exhibits two spatial correlations: the distance attenuation of isotropic sources and the back-to-back propagation of momentum-preserving 2-$\gamma$ annihilation. We present the results of our analysis of the $\gamma$ emission rate and the spatial profile of the annihilation in a magnetized pair plasma from direct pair collisions, from the formation and decay of positronium, as well as from transport processes. In order to demonstrate the effectiveness of annihilation-based techniques, we tested them on annular $\gamma$ emission profiles produced by a $\beta^+$ radioisotope on a rotating turntable. Direct and positronium-mediated annihilation result in overlapping volumetric $\gamma$ sources, and the 2-$\gamma$ emission from these volumetric sources can be tomographically reconstructed from coincident counts in multiple detectors. Transport processes result in localized annihilation where field lines intersect walls, limiters, or internal magnets. These localized sources can be identified by the fractional $\gamma$ counts on spatially distributed detectors.

physics.plasm-ph↗

arXiv2021

Injection of Positrons into a Dense Electron Cloud in a Magnetic Dipole Trap

The creation of an electron space charge in a dipole magnetic trap and the subsequent injection of positrons has been experimentally demonstrated. Positrons (5eV) were magnetically guided from their source and injected into the trapping field generated by a permanent magnet (0.6T at the poles) using a cross field E $\times$ B drift, requiring tailored electrostatic and magnetic fields. The electron cloud is created by thermionic emission from a tungsten filament. The maximum space charge potential of the electron cloud reaches -42V, which is consistent with an average electron density of ($4 \pm 2$) $\times 10^{12}$ $\text{m}^{-3}$ and a Debye length of ($2 \pm 1$) $\text{cm}$. We demonstrate that the presence of this space potential does not hamper efficient positron injection. Understanding the effects of the negative space charge on the injection and confinement of positrons represents an important intermediate step towards the production of a confined electron-positron pair plasma.

physics.plasm-ph↗