Characterization of Magnetic Field in Hypersonic Flow Using Tunable Diode-Laser Absorption Spectroscopy

A nonintrusive optical method for quantitatively measuring externally applied magnetic fields in high-enthalpy flows is developed, and its applicability under hypersonic conditions is demonstrated. Aerodynamic heating during atmospheric entry is reduced in magnetohydrodynamic aerobraking through modification of the shock structure by an external magnetic field. At high magnetic Reynolds numbers, deformation of the magnetic field is predicted, and the associated reduction in thermal protection effectiveness has motivated experimental measurement of magnetic-field distributions in hypersonic flows. An optical diagnostic that combines absorption spectroscopy with the Zeeman effect is applied to hypersonic experiments conducted in the HEK-X expansion tube at the JAXA Kakuda Space Center. The Ar I 794.8 nm transition is selected for its suitability for observing Zeeman splitting, and the argon plasma formed behind a shock wave traveling at approximately 8.9 km/s is measured. The test conditions are arranged so that the magnetic Reynolds number remains below unity, enabling direct comparison with an independently known static magnetic field. Clear absorption spectra and well-resolved Zeeman splitting are obtained at multiple positions, and the plasma path length along the optical axis is estimated to be approximately 24 mm. Path-averaged magnetic-field strengths inferred from the spectra are found to agree with finite-element predictions within a relative error below 3.1 percent. The agreement indicates that the combination of the Zeeman effect and absorption spectroscopy can serve as a reliable nonintrusive diagnostic method for high-enthalpy, short-duration hypersonic flow.