UC Berkeley

A sequence of electron clouds is extracted from an electron plasma reservoir. These clouds are highly reproducible and their E x B drift motion is nearly identical to that of a single particle, making them useful for measurements of electric and magnetic fields. First, by weakening the trapping potential confining the clouds we observe that they move off-axis, and we use this to measure the electric field due to patch potentials. Next, we measure the total charge of these clouds using small shifts in their magnetron frequencies. The misalignment between the trap electrodes and the external magnet is measured by imaging the clouds from different axial locations in the trap. By combining electron cyclotron resonance with the patch potential measurement procedure, we can measure the magnetic field strength up to a millimeter away from the trap axis. Finally, a new magnetometry technique called electron magnetron phase imaging (EMPI) is used to measure the rapidly changing magnetic field involved in observing the effect of gravity on antihydrogen. In EMPI, the magnetron frequency is measured precisely, and then we observe small changes to the magnetron frequency as the magnetic field decreases. In the process of analyzing the experimental data from each of these measurements, subtleties in the motion of electron clouds are revealed. Some of these measurement techniques help us to understand systematic errors in the ALPHA collaboration's test of the weak equivalence principle. Other techniques are used to inform experimental procedures and help explain the behavior of ALPHA's Penning-Malmberg traps. Most of these ideas could be applied to many Penning-Malmberg traps, provided that they have the ability to image charged particles. Unknown magnetic fields, patch potentials, and misalignment pose difficulties for many experiments, so implementing these cloud-based measurements could benefit other research groups.