As hypersonic flight rapidly evolves from a futuristic concept to a tangible reality, several physical phenomena that were once considered somewhat exotic become central to the performance and control of hypersonic vehicles. Among these, thermochemical nonequilibrium (TCNE) in shock and boundary layers has emerged as a critical factor that not only shapes the aerothermodynamic environment but also strongly affects plasma-assisted flow control (PAFC) strategies.
Traditionally, flow control in hypersonic regimes relies on geometric shaping or mechanical actuation—methods that are inherently passive or relatively slow. Plasma-based methods, on the other hand, offer fast, active, and potentially non-intrusive control capabilities, enabling manipulation of shock-boundary layer interactions, mitigation of flow separation, reducing peak heat loads, and aerodynamic control (pitch, roll, and yaw). However, to harness plasma actuators effectively, it is imperative to understand the microscopic environment in which they operate—an environment often governed by extreme nonequilibrium.
Nonequilibrium in Shock Layers and Magnetic Heat Shield
In high-speed, high-altitude flows, especially at Mach numbers exceeding 12, the air entering the shock layer at the vehicle’s leading edge does not have enough residence time to equilibrate thermally and chemically. As a result, the temperature characterizing random translational motion of molecules jumps to high values (from 3,000-4,000 K at Mach 12-14 to 10,000-30,000 K for atmospheric reentry), but molecular vibrational, rotational, and electronic modes are lagging behind, reducing the rates of dissociation and ionization that one would expect at the high translational temperatures.
On the other hand, one of the most effective plasma-based control strategies for reentry vehicles is the so-called magnetic heat shield: interaction of the moving ionized (and thus electrically conducting) gas through a static magnetic field at the vehicle nose or leading edge generates electric currents and Ampere forces which increase the shock stand-off distance and reduce the peak heat flux at the surface. This concept could help with aerobraking during reentry and could also extend the lifespan of reusable hypersonic vehicles.
Performance of the magnetic heat shield critically depends on the electrical conductivity of air in the shock layer, which, in turn, is determined by the ionization fraction (i.e. by the percentage of molecules and atoms from which the electrons have been removed) and by the temperature of free electrons. But as we discussed above, these parameters are out of equilibrium in high-altitude reentry shock layers. The sub-equilibrium ionization fraction would reduce the effectiveness of magnetic heat shield. In order to fully understand this, very advanced simulations codes that couple hypersonic gas dynamics with detailed kinetics of processes occurring with molecules, atoms, and electrons must be developed.
Theoretical models for molecular vibrational excitation, dissociation, and chemical exchange reactions exist and have been incorporated into computational fluid dynamics (CFD) codes. However, understanding of the detailed kinetics of ionization processes in high-temperature air is lacking. Apparently, in addition to vibrationally excited molecules, electronically excited nitrogen and oxygen atoms play an important role in ionization, and quantitative information on formation and quenching of the excited atoms is what researchers are working on.
Plasma Discharges in a Nonequilibrium Boundary Layer
Plasma discharges in supersonic and hypersonic boundary layers can enable dynamic control of those boundary layers. Generation of localized plasma regions and application of proper electric and, possibly, magnetic fields could enable dynamic control of boundary layers, especially if the plasma is generated in a repetitively pulsed regime with the pulse repetition frequency matching one of the resonant frequencies of the boundary layer. Potential benefits such as flow separation delay or shifting turbulent pulsation to higher frequencies thus reducing dynamic loads on the structure.
Since the temperature in hypersonic boundary layers, high as it may be (2,000-3,000 K), is still insufficient for strong thermal ionization, the ionization has to be created by running an electric discharge. However, when deployed in a thermochemically nonequilibrium flow, the discharge regime and its fundamental parameters—such as breakdown voltage, reduced electric field (E/N), and electron energy distribution function (EEDF)—are altered.
For instance, consider a scenario in which a nanosecond pulse is applied across electrodes in a boundary layer with high translational temperature (~3,000 K) but low vibrational temperature (~500 K). The electric field required for breakdown is higher than that in equilibrium air at the same pressure, due to the underpopulation of vibrational states, which typically lower the ionization threshold via stepwise excitation. Furthermore, the electron-impact ionization rates and electron-ion recombination rates are modified under such nonequilibrium. Such a regime is quite unusual and interesting for the physics of electric discharges.
Conclusion
While plasma actuators in equilibrium or quasi-equilibrium flows have been well studied, the next frontier lies in understanding and exploiting plasmas and electric discharges in thermochemical nonequilibrium that exists in hypersonic flows. Moreover, deeper understanding of such plasmas and discharges is essential for the development of plasma flow control technologies.