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

TL;DR

APEX-LD addresses magnetically confining low-temperature electron-positron pair plasmas in a compact levitated-dipole geometry. It employs a fully soldered NI ReBCO HTS F-coil inside a resealable cryostat, inductively charged by a C-coil and stabilized by an external L-coil with an FPGA-based feedback controller. The device achieved multi-hour levitation with micrometer-scale vertical stability and demonstrated electron confinement and field-line visualization, plus initial observations of diocotron-type plasma modes in pure-electron plasmas. These results establish a robust, repeatable platform for studying magnetized pair plasmas and outline concrete paths toward injecting positrons and scaling to higher-field, more symmetric configurations.

Abstract

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.

Figures (12)

• Figure 1: Schematic of APEX-LD during the levitation phase of operation. The upper levitation region hosts the F-coil (which produces the purple confinement region), L-coil, laser displacement sensors, thermal shield (green), and particle injection electrodes (pink). The lower preparation region hosts the cryostat (open, lid retracted), C-coil, primary cryocooler, horizontal and vertical translators (arrows indicate motion), and helium injection tubing. The lifter/catcher platform transports the F-coil between the two regions.
• Figure 2: (top) Exploded schematic of the F-coil. The NI ReBCO HTS winding pack is a single-pancake coil of $150$ turns closed by twelve radial bands connected in parallel. The fully soldered winding pack is soldered into a gold-plated copper case. (bottom) A photograph of the manufactured F-coil.
• Figure 3: Cross section of the cryostat CAD model, presented in the closed (sealed) configuration for F-coil preparation. Note the coplanar, concentric alignment of the F-coil relative to the C-coil for good inductive coupling.
• Figure 4: The feedback-stabilized levitation system. The larger diameter L-coil, located above, naturally stabilizes the slide and tilt motions of the F-coil. The feedback loop stabilizes the vertical motion ($\hat{z}$). An electron emitter can be inserted onto outboard field lines to inject electrons into the confinement volume.
• Figure 5: Temperature of the cryostat and HTS coils during F-coil preparation. The F-coil is heated by the lifter above $T_c$ and then rapidly re-cooled using helium gas. The C-coil is kept below $45$ K so it can safely energized during the charging procedure.