Measuring Magnetic Fields with a Penning Trap


The Penning trap is widely used for high-resolution mass spectrometry. When combined with cyclotron resonance and fast-Fourier-transform (FFT) techniques, the Penning trap mass has proven to be the most precise and accurate method for determining atomic masses. Physicists used it to accurately measure the mass of rare and unstable isotopes [1]; and chemists have broadly adopted it as one of the most powerful analytical techniques [2].

Now a group at the National Superconducting Cyclotron Laboratory facility at located at Michigan State University has shown that the same device can be used to precisely measure strong magnetic fields.

Magnetic field measurement and mass measurement used the same Penning trap principleMagnetic field measurement and mass measurement used the same Penning trap principleThe idea behind the high-precision magnetometer is to monitor the cyclotron frequency of a reference ion (or group of ions) in a miniature Penning trap and, from its motion, to deduct the magnetic field strength. This operating principle is closely related to that of a typical Penning trap mass spectrometer, except that in this later case the mass of the trapped ion is extracted by monitoring its motion in a precisely known magnetic field.

The Michigan State University group used their Penning trap magnetometer (MiniTrap) to continuously monitor magnetic field strength of a 9.4 T  superconducting magnet to the very high relative precision of 1 part in 108 [3].

The Bollen group is now pursuing the development of an even smaller magnetic field probe, also based on a Penning trap. This “microtrap” is being designed to operate in environments where ionizing radiation levels are expected to be high. The microtrap will be initially deployed on the Facility for Rare Isotope Beams (FRIB) beamline. This new scientific user facility for nuclear science, funded by the Department of Energy Office of Science (DOE-SC), Michigan State University (MSU), and the State of Michigan is under construction on campus and operated by MSU. FRIB will provide intense beams of rare isotopes that will be used to study basic science, as well as medical and industrial applications.


  1. Georg Bollen, R. B. Moore, G. Savard and H. Stolzenberg, “The accuracy of heavy-ion mass measurements using time of flight-ion cyclotron resonance in a Penning trap”, J. Appl. Phys. 68, 4355 (1990); doi: 10.1063/1.346185
  2. Raymond E. March, “An Introduction to Quadrupole Ion Trap Mass Spectrometry”,J. Mass Spectr., 32, 351-369, (1997)
  3. David L. Lincoln, “First direct Q-value measurement of the neutrinoless double-decay candidate 82Se and development of a high-precision magnetometer”, PhD thesis Michigan State University, Physics (2013)


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