Quantum beat measurements of weak magnetic fields in plasmas

Accurate measurement of magnetic fields is critical in numerous plasma environments, ranging from astrophysical systems to fusion energy research. While fusion systems may employ techniques such as polarimetry or the motional-Stark effect, the typical approach for measuring magnetic fields in low temperature laboratory plasmas employs perturbative probes (i.e., magnetic sensing coils) that are prone to ambient electromagnetic noise. A purely optical approach of resolving individual Zeeman-split σ-peaks from full velocity distribution function (VDF) measurements using laser spectroscopic techniques, e.g., laser induced fluorescence, exists. However, transient magnetic field effects are difficult to ascertain with this method since the measurement of the full VDF is inefficient and time consuming, especially in pulsed laboratory experiments where each data point used to construct the VDF is composed of several plasma pulse discharges.

In this work, an alternative laser-based technique known as quantum beat spectroscopy is employed to measure weak magnetic field strengths with sub-Gauss resolution with near single shot acquisitions from the Zeeman-split electron energy states of the (²P^o_1/2)4p ²[1/2] state of neutral argon and the 1s3p ¹P^o₁ state of neutral helium. This technique is both spatially and temporally localized, providing sub-millimeter and sub-microsecond resolutions. Unlike LIF, this technique is Doppler-free and does not require sweeping of the laser wavelength, making this approach ideal for transient events such as probing the reconnecting magnetic field strength in laboratory magnetic reconnection experiments. Additionally, this work extends the diagnostic method to obtain the direction of the magnetic field in addition to its magnitude.