Measuring Magnetic Fields and Patch Potentials in a Penning-Malmberg Trap with an Electron Plasma Reservoir

Prof. Fajans and his graduate students investigated and implemented a sequence of novel ideas that have significantly changed how the ALPHA collaboration traps antihydrogen and controls systematic errors. Dr. Celeste Torkzaban discovered the SDREVC technique, where the total charge and rotation frequency of a nonneutral plasma are precisely controlled by simultaneously applying rotating wall compression and evaporatively cooling the plasma. Then Dr. Eric Hunter realized these reproducible plasmas could be used as "reservoirs," where a sequence of smaller "electron clouds" could be extracted from a larger electron plasma. Today, magnetometry at ALPHA is done by exciting a cyclotron resonance in these electron clouds. Additionally, the magnetron motion of these clouds was used to measure the electric field produced by patch potentials. This measurement resolved a long-standing question about why lasers degrade the plasma trap. During the ALPHA-g measurement of the effect of gravity on antihydrogen, the magnetron frequency of electron clouds was used to measure the rapidly changing magnetic field used to "drop" antihydrogen. In a future planned upgrade to the ALPHA-g measurement, a segmented electrode will be used to move electron clouds off-axis to obtain a more complete 3D map of the magnetic field.