As the aerospace community continues to push the boundaries of atmospheric flight and access to near-space environments, the search for compact, energy-efficient, and fast-response flight control and propulsion enhancement mechanisms has increased interest in an old but often underestimated tool: the microwave discharge plasma. In the unique thermodynamic and chemical environments found in hypersonic flight and near-space regimes, microwave discharges offer a suite of attractive properties—non-intrusive generation, volumetric energy deposition, and resilience to harsh flow conditions. Yet, their operation in these high-enthalpy nonequilibrium flows introduces complexities that are only now beginning to be understood in detail.

Revisiting the Physics of Microwave Discharges

Microwave discharges operate through the interaction of an oscillating electromagnetic field—most commonly in the 2.45 GHz frequency range, but also at lower (~1 GHz) and higher (~10 GHz) frequencies—with free electrons in a gas. Energy is transferred from the field to the electrons, which in turn ionize and excite molecules and atoms via collisions. In contrast to many other types of electric discharges, microwave discharges do not need electrodes and can be generated remotely, in flowing environments, and in geometrically complex domains using properly configured antennas. This makes microwave discharges well-suited for both localized and distributed energy depositions, a requirement for aerodynamic manipulation over large areas such as control surfaces or engine inlets.

However, operating such discharges in rarefied, nonequilibrium conditions—typical of altitudes above 30 km and flow speeds exceeding Mach 8—requires rethinking many of the simplifications inherited from low-temperature plasma physics.

Nonequilibrium Effects in High-Speed Flowfields

At high altitudes and high flight velocities, the ambient air density is low, and the residence time of the fast flow through the plasma region is short. This combination produces a strong thermochemical nonequilibrium: for instance, in hypersonic shock layers, translational temperatures may be elevated (2,000–5,000 K), while vibrational, rotational, and electronic modes lag behind significantly. This affects not only the bulk chemistry but also the microscopic behavior of the plasma discharge.

In particular, the electron energy distribution function (EEDF), a key determinant of ionization and excitation rates, becomes markedly non-Maxwellian. The reduced collision frequency at low pressures enables the electrons to gain energy from the field more efficiently, but also alters the relative importance of inelastic collisions. For example, vibrational excitation—often a precursor to ionization in stepwise schemes—is suppressed in nonequilibrium flows where vibrational levels are sparsely populated. This increases the threshold electric field needed for discharge ignition and alters the breakdown characteristics.

Moreover, the interplay between convective timescales (flow transit) and reactive timescales (chemical or ionization relaxation) means that the plasma often remains in a “frozen” state—chemically non-equilibrated and dynamically evolving.

Once the microwave discharge is ignited, the nonequilibrium is changed: the electron temperature is dramatically increased since the free electrons receive energy from the electromagnetic field, and these hot electrons efficiently excite vibrational and electronic states of molecules.

One interesting phenomenon that occurs at low density is the non-local nature of the plasma discharge. It takes many electron-molecule collisions to get the electron energy to a steady-state, and thus a characteristic length for electron energy relaxation is much longer than the mean free path. Therefore, the electron energy in microwave discharges in rarefied air respond not to the local electric field strength, but to the electric field in the relatively wide vicinity of this point.

Applications: Flow Control and Propulsion

Microwave discharges are emerging as promising tools for two key applications in high-speed flight: flow control and plasma-assisted propulsion.

For flow control, the volumetric and sustained nature of microwave discharges makes them uniquely suited to boundary layer modification. By selectively heating regions near the surface, they can locally alter viscosity, delay flow separation, or even change shock-wave/boundary-layer interaction (SWBLI) characteristics. The challenge lies in achieving sufficient energy deposition with minimal power consumption. One promising approach is to modulate the microwave power, and particularly to use microwave pulses repeated at a certain frequency. In this approach, the low duty cycle reduces the time-average microwave power, and the pulse repetition frequency can be selected so as to be in resonance with certain flow phenomena and thus to cause a strong effect on the flow despite the low average power.

Plasma-heated regions, even when steady, can serve as virtual bodies or surfaces, deflecting the flow. For example, a microwave-heated region generated upstream of the cowl of a scramjet inlet creates a virtual cowl which can increase the air capture in the inlet and thus increase the engine thrust without the need for physical movable parts.

In propulsion, particularly for near-space or high-altitude cruise platforms, microwave-assisted ignition and combustion stabilization have been explored. More recently, microwave discharge propulsion systems—where thrust is generated by direct plasma expansion or by adding momentum to the flow—are under investigation. These systems avoid electrodes and can operate with air-breathing capabilities at altitudes where conventional combustion fails.

In scramjet propulsion, in addition to the problem of long ignition delay time, there is also a problem of slow (compared with the supersonic flow speed) spreading of the flame across the combustor duct. It has been shown that microwave discharges can successfully initiate combustion thus solving the ignition delay problem.

However, it turns out that microwave field that is not strong enough to generate a plasma can substantially increase the flame propagation speed. This non-trivial effect relies upon the fact that the flame itself produces ionization in chemical reactions near the flame front. When the free electrons at the flame front receive energy from the microwave field and transfer this energy to the gas molecules, the local heating at the flame front accelerates the flame propagation. Additionally, the local increase in electron temperature slows down electron-ion recombination and thus increases the ionization fraction, further enhancing the effect of microwave heating on flame propagation.

To conclude, microwave discharge plasmas may not be new, but in the context of modern aerospace challenges, they are more relevant than ever. And as we continue to uncover the intricacies of plasma-gas interaction in rarefied, high-speed environments, they may well become indispensable tools in the next generation of flight.