Defense Intelligence Reference Document MHD Air Breathing Propulsion And Power For Aerospace Applications

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and in devices such as the gliding arc, the plasma evolves from near-equilibrium
to highly nonequilibrium during each of the periodically repeating cycles. In
shock and boundary layers during reentry, the plasma is near thermal
equilibrium while being diffuse. The primary reason for this behavior is that the
ionization in those shock and boundary layers exists without any electric field
and thus is not subject to arcing instabilities.
Plasma Features
What features or properties make weakly ionized plasmas interesting for
propulsion and aerodynamic applications? The most obvious feature is heating-a
consequence of Joule dissipation in an electrically conducting medium placed in
an electric field. As a heating element, plasma has important advantages
compared with conventional heaters. For example, even a surface electric
discharge can effectively heat the gas flow much farther from the wall than a
wall-imbedded conventional heater would. Microwave and laser beams can
create plasmas and heat the gas even far from any surfaces, and the volume and
shape of the heated region can, in principle, be adjusted. Since heated regions
can significantly alter the flow by making the gas flow mostly around them,
plasmas can form switchable, controllable, and tunable virtual bodies or
surfaces. Such virtual surfaces can be deployed on demand for drag reduction,
aerodynamic control (when applied asymmetrically), and optimization of engine
inlet performance, to name a few. It is the localized and transient deployment of
plasma virtual surfaces that results in the most interesting and complex
interactions with gas flows while saving energy compared with large-volume,
steady-state plasma utilization, and thus is especially promising for applications.
Another useful application of plasma heating is ignition. This may seem trivial;
after all, spark plugs in conventional internal combustion engines are well-
developed thermal plasma devices. However, thermal plasma ignition for
scramjet engines is not that simple, since the ignition system would have to
prevent the plasma from being easily blown away by the supersonic flow, and
even if this problem is resolved, if not properly (and quite ingeniously) designed,
the igniter would cause an unacceptably strong perturbation to the flow and loss
of the stagnation pressure and would require extremely high power. As an
example, plasma igniters based on subcritical microwave discharges are quite
sophisticated.
Besides heating, the presence of charged particles is another obvious, and very
important, feature of plasmas. Charged particles can be acted upon by electric
and magnetic fields, and this action can be transferred to the bulk gas by ion-
molecule collisions. Thus, magnetohydrodynamic (MHD) and
electrohydrodynamic (EHD) interactions can be utilized to exert forces and to
decelerate or accelerate the gas in both inviscid core flows and viscous boundary
layers. The magnitude of such interactions depends on the ionization fraction and
the magnetic or electric field strength.
MHD Interactions
The ionization fraction can be quite high in shock and boundary layers at very
high Mach numbers (such as those in reentry flight), or just downstream of
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