How can Weakly-Ionized Plasmas Extend Aircraft Capabilities?  
Weakly-ionized plasmas are commonly used in manufacturing processes (surface treatment), in the lighting industry (neons), or in consumer electronics (plasma tvs). In the last decade, weakly-ionized plasmas have also been considered as a means to enhance the capabilities of aircraft either through plasma aerodynamics or through MHD power generation.
What is a weakly-ionized plasma?
A weakly-ionized plasma (a.k.a. weakly-ionized gas) can be defined as a plasma in which the ionization fraction is rather low, in the range 10$^{-8}$ to 10$^{-4}$. Weakly-ionized gases do not necessarily exhibit a high temperature: the gas temperature ranges from 4,000 to 10,000 K in high current arc discharges for welding, but can also be as low as 200-300 K in low current glow discharges for light bulbs [1]. While the temperature of the gas remains relatively low, the temperature of the electrons typically reaches very high values in the range 10,000-40,000 K, resulting in significant thermal non-equilibrium. Weakly-ionized plasmas can be created within the air flowing around an aircraft through a multitude of techniques such as potassium seeding, electron beams [2], electric fields [3, 4], microwaves [5], or laser beams.
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How can weakly-ionized plasmas be useful in enhancing the capabilities of aircraft?
One important physical effect associated with weakly-ionized gases is heating. Significant heating occurs within a weakly-ionized plasma due to the latter having a low ionization fraction and hence a low electrical conductivity. Due to the low conductivity, the plasma opposes significant resistance to the flow of current, and this results in large amounts of heat deposited to the gas. Compared to conventional heaters, a plasma has some advantages. For instance, microwaves, electron beams, and surface electric discharges can all heat the airflow at a much higher distance from the wall than conventional heaters would. Due to their lower density, the heated regions effectively become virtual bodies by deviating the flow around them. Another advantage of plasmas is to permit the rapid change of the size and shape of these virtual bodies which can be deployed on demand for performance enhancement of the aircraft such as aerodynamic drag reduction or inlet efficiency optimization (see for instance Refs. [6, 7, 8]).
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Other than heating, are there other features of weakly-ionized gases that can be useful?
In addition to heating, the presence of charged particules (electrons and ions) is another important characteristic of plasmas. Forces can be exerted on charged particules by electric and magnetic fields and these forces can be transferred to the bulk of the air by ion-neutral and electron-neutral collisions. Thus, magnetohydrodynamic (MHD) and electrohydrodynamic (EHD) interactions can be used to exert forces and to accelerate and or decelerate a weakly-ionized plasma.
What is MHD (magnetohydrodynamics)?
The MHD interaction consists of either adding or extracting energy to a flow through the Lorentz force. The Lorentz force is proportional to the product between the current flowing in the gas and the magnetic field applied on the flow.
How can MHD be used aboard an aircraft?
The MHD interaction can be particularly useful in generating electrical power aboard an aircraft through a so-called MHD generator. While the MHD generator has been known for many years to be a viable power generation device (indeed, it has been used successfully as a low-weight and high power generator by extracting kinetic energy from rocket exhausts [9, 10, 11]), it is only recently that it has been envisaged as a means to provide power aboard flight vehicles [12, 13]. The MHD generator is expected to be especially efficient for aircraft flying at hypersonic speeds: due to the power generated being proportional to the product of the force exerted on the flow and its velocity, MHD can provide a low-weight and high-output power generator while inducing little additional drag if used on a flow travelling at hypervelocities. Preliminary estimates show that a power of tens or even hundreds of megawatt per cubic meter could be produced in scramjet or ramjet-powered aircraft [14]. The energy extracted could then be used to power on-board flight navigation equipment or a megawatt-class energy weapon. Alternately, the power generated could be deposited as heat in strategic locations of the flowfield to improve the aerodynamic performance of the flight vehicle.
Another possible application of the MHD interaction is the bypass of the flow kinetic energy from the inlet to the nozzle such as in project AJAX [15, 16] or as in other derived concepts [17, 18].
Other possible applications of the MHD interaction process include the observed drag reduction over blunt bodies when an external magnetic field is applied to a conducting incoming flow (see Ref. [20] for instance), the control of the shock positioning in a scramjet inlet to achieve shock-on-lip condition over a relatively wide Mach number range [21] while keeping the geometry fixed, or the suppression of boundary layer separation in the inlet of supersonic aircraft. Indeed, preliminary analytical studies shed hope that the separation of the boundary layer by shockwaves can be largely suppressed by applying a Lorentz force on the flow in the direction opposite to the adverse pressure gradient created by the shockwave.
What is EHD (electrohydrodynamics)?
The EHD phenomenon consists of an electric field acting on a non-neutral plasma. Because the plasma is not neutral, the electric field induces a force on the charged particules. The force is then transferred to the bulk of the gas through collisions between the charged particules and the neutral particules.
What are some aerospace applications of EHD?
While the EHD (or ion wind) phenomenon has been known for several decades, it has not been applied to aircraft until recently: It is only in the last decade that the dielectric barrier discharge (DBD) — a device making use of the EHD phenomenon — has been deployed on aircraft (see for instance Refs. [22, 23, 24, 25]). The DBD plasma actuators have been shown to be particularly useful in preventing or delaying boundary layer separation [26], in enhancing jet mixing [27], in keeping the flow attached on turbine blades [28], and in triggering laminar to turbulent transition. For instance, by using a series of DBD devices positioned at the leading edge of a delta wing, experimental studies have shown that it is possible to delay separation and therefore to increase the critical angle of attack of the airfoil [29]. Due to its low cost, low weight, and its effectiveness at controlling the airflow on wings or in inlets, the DBD plasma actuator is considered one of the most promising technologies in plasma aerodynamics that have a possible commercial application in the near future.
References
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