Speaker
Description
Background
Magnetohydrodynamic (MHD) aerobraking is a promising active thermal protection method for atmospheric re-entry. The Lorentz force generated by the interaction between the ionized shock layer and an applied magnetic field increases the shock standoff distance. This increase reduces the wall heat flux and augments drag through the reaction force [1]. Previous demonstration tests were conducted under low magnetic Reynolds number conditions (Rem < 1), in which the magnetic field was expected to undergo negligible deformation. In realistic flight scenarios, a high magnetic Reynolds number (Rem ≫ 1) is expected for vehicles with a large characteristic length and during high-velocity entry where there is a high degree of ionization. Under high-Rem conditions, the magnetic field can no longer be assumed to be rigidly fixed, but instead is expected to advect significantly with the plasma flow, such that its deformation must be taken into account [2]. Consequently, the Lorentz force may no longer act as intended, and the thermal protection effect and drag augmentation may be degraded. Correctly designing and evaluating MHD aerobraking for large flight vehicles under realistic flight conditions therefore requires capturing this field deformation. Corresponding experiments are required to better understand this phenomenon and to benchmark the sophisticated numerical simulation tools required to simulate this effect. This study describes a technique to measure this deformation experimentally through a non-contact, fast-response magnetic field measurement.
The authors have previously measured the magnetic field around a spherical model in the HEK-X expansion tube at the JAXA Kakuda Space Center by combining tunable diode laser absorption spectroscopy (TDLAS) with the Zeeman effect to exploit the resulting spectral splitting [3]. The split components were clearly observed at the stagnation point at the front of the model, where the flow comes to rest. At observation positions radially away from the stagnation point, the splitting profile became complex because the relative angle between the polarization plane and the magnetic field vector varies. The complex profile indicates that the split components can be enhanced through appropriate polarization control. In this study, the complex spectral profiles arising at locations such as the model shoulder are simplified through polarization control to accommodate a variety of measurement positions, and experiments aimed at detecting magnetic field deformation under high magnetic Reynolds number conditions are conducted in the X2 expansion tube at the University of Queensland.
Methods
Existing HEK-X data are used as the reference condition (Rem < 1), and tests under conditions expected to yield a high magnetic Reynolds number are conducted in the X2 expansion tube (University of Queensland). Argon is used as the test gas, and the Ar I 794.8 nm transition is measured with a DFB laser. The magnetic field is estimated from the splitting width of the absorption spectrum. Utilizing the fact that the splitting profile is determined by the relative angle between the polarization and the magnetic field, a half-wave plate is used to rotate the polarization and optimize the relative angle at each observation position, enhancing the spectral splitting. The derived magnetic field is evaluated by comparison with the static field.
Results and Conclusion
The effectiveness of polarization control was confirmed in a preliminary test using a DC discharge tube that generates a steady-state argon plasma. Varying the relative angle of the polarization with the half-wave plate systematically changed the intensity ratio between the split and unsplit components, demonstrating that the splitting profile can be controlled through polarization control. The results show that the splitting can be clearly resolved even at observation positions away from the stagnation point. Building on these results, tests under high magnetic Reynolds number conditions are conducted in the X2 expansion tube to detect magnetic field deformation as a deviation from the static field distribution. The proposed method provides a foundation for measuring magnetic field deformation under Rem ≫ 1 conditions using absorption spectroscopy with the Zeeman effect.
References
[1] Lefevre, A., Gildfind, D. E., Gollan, R. J., Jacobs, P. A., and James, C. M., "Magnetohydrodynamic Experiments of Total Heat Flux Mitigation for Superorbital Earth Reentry," AIAA Journal, Vol. 60, No. 9, 2022, pp. 5046-5059.
[2] Van Oeveren, S. B., Gildfind, D. E., Wheatley, V., and Gollan, R. J., "Numerical Study of Resistive Magnetohydrodynamics for a Superorbital Entry," AIAA SciTech Forum, AIAA SCITECH 2025 Forum, Orlando, FL, 6-10 January 2025.
[3] Muramatsu, T., Shimamura, K., Kakami, A., Katsurayama, H., Gildfind, D., Yatsuyanagi, S., and Tanno, H., "Characterization of Magnetic Field in Hypersonic Flow Using Tunable Diode-Laser Absorption Spectroscopy," AIAA SCITECH 2026 Forum, Orlando, FL, 12-16 January 2026, AIAA 2026-0676.c1.
Summary
In MHD aerobraking, a high magnetic Reynolds number is expected under realistic flight conditions, particularly for vehicles with a large characteristic length and during high-velocity entry where the degree of ionization is high. Under such conditions, the magnetic field may advect and deform with the plasma flow, and this deformation must be taken into account in system design and evaluation.
This study estimates the magnetic field from the splitting width of the absorption spectrum using TDLAS and the Zeeman effect, while polarization control is used to clarify the spectral splitting at different observation positions.
The proposed method enables stable magnetic field measurements away from the stagnation point and supports experimental detection of magnetic field deformation under high-Rem conditions, contributing to the validation of numerical simulation models.