Magnetometry advancements for antihydrogen precision measurements in ALPHA

Experiments on trapped antihydrogen atoms are often conducted in superimposed Penning-Malmberg (PM) and Ioffe-Pritchard (IP) traps. The PM trap manipulates the charged particles necessary for anti-atom synthesis, while the IP trap confines the neutral antihydrogen produced. Tests of charge-parity-time symmetry through spectroscopy and the weak equivalence principle via controlled releases of anti-atoms in Earth’s gravitational field are performed within the trapping magnetic field. Detailed knowledge of the B-field trapping potential is crucial for understanding and enhancing the precision of these measurements.

Currently, magnetometry techniques available to ALPHA experiments without invasive hardware within traps are limited to on-axis locations. Techniques include Electron Cyclotron Resonance (ECR) using microwave pulses to illuminate quickly prepared low-density electron plasmas carefully positioned along the PM trap axis [1, 2], or extrapolations of the magnetron frequency of these plasmas [3]. First, this study presents an increased characterisation on the preparation of the target plasmas to avoid charge extraction instabilities from a reservoir of electrons. Then, we present the results on radial far off-axis control of these plasmas under sectored cylindrical PM electrodes, extending over 80% of the trap radius. Far off-axis ECR is performed on these displaced plasmas within the superimposed traps. The plasmas can be reproducibly restored on-axis within a maximum uncertainty of ~400 um, maintaining acceptable plasma characteristics, and diagnosed.

This technique allows for probing the magnetic field and tuning the 3D trapping potential, thereby addressing uncertainties in experimental data and models, ultimately improving the precision of antihydrogen measurements. Beyond this primary application, the off-axis displacement technique has also been utilised in ALPHA for: fast and precise trap misalignment measurement via magnetron motion centre determination, suppression of spatial magnetron motion amplitudes, and radial off-axis antiproton extraction to minimise transfer losses.

[1] ED Hunter et al. Physics of Plasmas, 27(3), 2020.

[2] ED Hunter et al. Review of Scientific Instruments, 91(10), 2020.

[3] ALPHA. Nature, 621(7980):716–722, 2023.