Ultrastable superconducting magnet system for a penning trap mass spectrometer

A custom-designed magnet/cryostat system is described which has demonstrated remarkably improved field stability over previous designs. To shield from external magnetic noise, a custom-fabricated flux-gate device remotely senses the changes in magnetic field and cancels them out at the site of the magnet/cryostat via a 1.7-m-diam Helmholtz coil. To provide further shielding, the basic superconducting solenoid includes a passive flux-stabilizing coil. To stabilize internal field shifts, the temperature of the materials in the immediate vicinity of the solenoid (which have a temperature-dependent susceptibility) is stabilized via the new cryostat geometry and by controlling the pressure of the evaporating liquid helium to a few parts per million. As a result, the total system now has a composite shielding factor of approximately ${10}^4$ and an overall temporal stability on the order of 17(2) parts in ${10}^{12}$ per hour. This instrument, the heart of our new Penning trap mass spectrometer, has recently been used to determine a preliminary value of the proton’s atomic mass to an accuracy of 1.4 parts in ${10}^{10}.$ With the new magnet/cryostat system, this spectrometer now has a potential resolution which exceeds 2 parts in ${10}^{11}$ with 100 hours of data using a single carbon 4+ ion.