When I first delved into the fascinating world of plasma physics and plasma chemistry, I was aware, of course, that high-temperature plasmas are essential for controlled nuclear fusion, and low-temperature plasmas of electric discharges are used in fluorescent lights and in microchip fabrication systems. However, I could not have imagined that this field could have an impact on aerospace engineering. Over the last few decades, I witnessed the emergence of plasma-based concepts for enhancement of aircraft and spacecraft performance, and I have actively participated in some of these developments.
Low-Temperature Nonequilibrium Plasmas
Before diving into its applications, let’s briefly touch on what plasma science is. Plasma, often called the fourth state of matter, consists of a soup of ions, electrons, and neutral particles. Found naturally in stars, lightning, and auroras, plasma can also be created and controlled in laboratory and industrial settings. While hot, fully ionized, plasmas are essential to controlled fusion, it is the low-temperature plasma that is relevant to industrial and aerospace applications. Low-temperature plasmas typically exist in electrical discharges that can be created by DC, AC, RF (radio-frequency), or microwave sources, or by complex voltage waveforms such as, for example, repetitive nanosecond-duration high-voltage pulses. Low-temperature plasmas are weakly ionized: the ionization fraction, i.e. the fraction of atoms or molecules that are ionized, typically ranges from about 10-8 to about 10-3. The gas temperature, or, more precisely, the temperature of random translational motion of the atoms and molecules, is typically from room temperature to about 1000-2000 K. (Note that even 1000-2000 K is ‘cold’ compared to that in plasmas used for fusion where the temperature is typically on the order of 100,000,000 K). However, low-temperature weakly ionized plasmas are in a state of extreme thermodynamic nonequilibrium: although the gas is relatively cold, the electron temperature is typically on the order of 1-10 eV (10,000-100,000 K), and the vibrational temperature of the molecules is typically about 2,000-5,000 K. Due to these high electron and vibrational temperatures, energetic excited states of molecules and atoms are efficiently generated, and chemical reactions are stimulated even at room temperature. Such excited states and chemical reactions form the foundation of the plasma use in fluorescent lights and microchip fabrication systems, and can be quite useful in aerospace applications such as engine combustion and flame holding. Other unique properties of plasmas, such as electrical conductivity and responsiveness to electric and magnetic fields, along with the ease with which these properties can be rapidly changed by altering the applied voltage or its waveform, make plasma a versatile medium for solving complex problems in aerospace engineering.
Plasma in Aerodynamic Control
One of the most exciting applications of plasma physics in aerospace is aerodynamic control. Traditional methods of controlling airflow around an aircraft, such as flaps and ailerons, rely on mechanical systems that have slow reaction time and add weight and complexity. Plasma actuators, by contrast, offer a fast-acting and lightweight, with no moving parts, alternative.
There are two principal mechanisms of plasma effects on the flow. The first mechanism relies on plasma-created body forces. For example, in a dielectric barrier discharge (DBD), plasma electrons quickly attach to the dielectric surface, leaving the bulk plasma positively charged. The non-neutral plasma experiences the electric field created by the applied voltage and acquires momentum which, in turn, is transferred to the bulk non-ionized molecules in ion-molecule collisions. This body force mechanism is effective in delaying flow separation and increasing L/D (lift-to-drag) ratio in low-speed flows.
The second mechanism utilizes Joule heating in the plasma. At steady state, the heated regions act as obstacles to the flow, creating virtual shapes that can be tuned and turned on/off. One application of such plasma-generated virtual shapes is the so-called virtual cowl, a plasma-heated region upstream of the cowl at the inlet of a scramjet engine. The virtual cowl can increase the air capture by the scramjet inlet, which is particularly important in off-design flight conditions. Other virtual shape applications include plasma riblets (or plasma-enhanced riblets) capable of reducing turbulent friction drag and plasma virtual roughness elements that would delay the onset of cross-flow instability and thus delay laminar-to-turbulent transition on swept wings.
Perhaps the best way to use plasma heating in aerodynamics is to time-modulate the plasma by applying a proper voltage waveform. For example, driving a DBD plasma actuator with repetitive nanosecond-duration pulses creates flow disturbances that enhance the mixing of boundary layers, thus energizing the near-surface flow region and delaying flow separation. This mechanism turns out to be considerably more effective than the body-force mechanism: it can be effective at much higher speeds, including transonic and maybe even supersonic flows.
In general, time-modulation of the plasma can be done with any frequency, which opens a way of tapping into natural resonant frequencies of the flow so that even weak plasma-created disturbances would have a strong effect on the flow.
Revolutionizing Propulsion Systems
Plasma propulsion systems, particularly Hall effects thrusters and ion thrusters, have been developed and deployed decades ago. These systems use plasma to generate thrust with high efficiency and high specific impulse, making them ideal for satellite maneuvering and long-duration space missions. Ion thrusters have already been used in several space missions, including NASA’s Deep Space 1 and the Dawn spacecraft.
While the plasma space propulsion systems operate with strongly ionized (i.e. with ionization fraction >10%) plasmas, the use of weakly ionized plasmas in airbreathing propulsion for aircraft and missiles has been a subject of research in the last few decades. One particularly important potential application is that of scramjet (supersonic combustion ramjet) engines in airbreathing hypersonic vehicles. The flow in scramjet combustors is very fast, a few thousand meters per second, whereas the fuel-air mixing, ignition, and flame spreading are relatively slow. In addition, a pilot flame can be easily blown off by the strong “wind”. These are tremendous challenges in scramjet development. Plasma-assisted combustion studies have demonstrated that plasmas, especially those driven by high-voltage nanosecond pulses, can dramatically speed up both mixing and ignition as well as strengthen the flame holding. Plasma-assisted combustion has also been proven to be effective in lean mixtures. Moreover, microwaves have been shown to significantly accelerate the flame propagation in both laminar and turbulent flames. These highly encouraging results were supplemented by deep insight into the microscopic mechanisms of plasma-assisted combustion, thus creating a firm foundation for practical applications, particularly to hypersonic vehicles.
Enhancing Space Exploration
When a spacecraft enters a planetary atmosphere, the temperature in the shock and boundary layers is so high that the gas is ionized and plasma is formed. If a magnet is placed in the spacecraft, plasma motion in the magnetic field will generate electric currents, generating both heating and body forces. The bow shock stand-off distance will be increased, and the heat flux to the surface at the nose will be reduced. In the late 1950s, continuing into the 1960s, this idea was proposed for spacecraft aerobraking and heat load mitigation in reentry; however, the concept was never practically implemented: a physical heat shield was deployed instead of the “magnetic heat shield” on practical reentry vehicles.
In recent research studies and patents, a new version of the concept of using plasmas and magnetic fields in planetary entry of spacecraft has been explored. In the new version of the concept, the body forces would be asymmetric with respect to the spacecraft axis, thus increasing the L/D ratio (to enable aerodynamic maneuvering) and enabling pitch, yaw, and roll control. The asymmetric character of the body forces can be ensured by either placing a number of individually controlled pairs of electrodes around the circumference or by tilting the magnetic field with respect to the axis.Computational modeling showed that strong effects can indeed be ensured with a modest magnetic field strength for at least some planetary exploration scenarios, including, for example, Neptune. Smaller, but still quite meaningful, effects could be achieved for entry into the Martian atmosphere.
Overcoming Challenges
While the potential of plasma physics in aerospace engineering is immense, the path to widespread adoption is not without hurdles. One of the biggest challenges is the integration of plasma technologies into existing or previously designed systems. Aircraft and spacecraft are built with stringent safety and performance requirements, and introducing new plasma-based components often requires redesigning these systems from the ground up.
Developing these technologies at a commercially viable scale is an ongoing effort that demands interdisciplinary collaboration and significant investment.