Fig. 1: The experimental apparatus and the axial magnetic field profile.

a Cross section of the central part of the ALPHA-2 experimental apparatus. In yellow and orange are shown the electrodes used to generate the electric fields for the Penning-Malmberg trap. The superconducting magnets that generate the magnetic-minimum trap (octupole and mirror coils) and boost the axial magnetic field to 3 T in the two side regions (left and right solenoids) are shown in blue and pink. The external, 1 T solenoid is not shown. Lasers for Be⁺ Doppler cooling can be injected along an off-axis path at 2.3 degrees through vacuum windows at either end of the apparatus (purple path), or along an on-axis path through a vacuum window onto movable 45-degree mirrors that can be positioned at either end of the central apparatus (cyan path). The orange electrodes in the left and right regions are azimuthally segmented to allow for the rotating wall (RW) technique (see text). The Be⁺ source (not shown) is placed on the same linear translator as the left side mirror²⁰. Microchannel plate/phosphor assemblies used for imaging extracted particles (not shown) are placed on the same translators as the mirrors on both sides. The central portion of the drawing is to scale except for the annihilation detector, whose axial length is marked by purple bars. Position sensitive detectors are used for stabilizing the cooling laser beams. b The corresponding longitudinal magnetic field (on-axis) that is maintained during Be⁺ assisted H̅ accumulation.