AAWSAP DIRD, MHD Air Breathing Propulsion and Power for Aerospace Applications, November 2010

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The average energy cost, W;, of ionization varies greatly depending on the
ionization method. For example, in conventional glow-like discharges of large
volume at moderate or high pressure, the ionization cost is ~10,000 eV (i.e.,
three orders of magnitude higher than the minimum ionization energy [10-15
eV]). This is due to the low average electron energy ( ~ 1 eV) and to the
dominant losses of electron energy in inelastic collisions with air molecules. This
is why a highly efficient ionization technique must be used in order to give cold­
air MHD devices a chance to be viable. High-energy electron beams represent
such a technique. Generated in vacuum electron guns and injected into air
through either thin foil or a differentially pumped window, energetic (>1-50 keV)
electrons produce many more low-energy plasma electrons, so that the average
ionization cost is only W; =34 eV. This ionization efficiency is theoretically the
best.
Of course, electron beam systems are quite difficult to work with due to fragile
foils or massive differential pumping facilities; X-ray generation is also not
helpful for flight applications. But even putting these important practical
problems aside, and even with the lowest possible cost per electron, the
requirement that a cold-air nonequilibrium MHD device uses significantly less
power for ionization than it extracts from (in the generator case) or adds to (in
the accelerator case) the flow imposes a severe constraint on the maximum level
of ionization and conductivity. Calculations show that the maximum ionization
fraction is on the order of 10-6 and the maximum conductivity is on the order of
1 mho/m. With this low conductivity, substantial (5~0.1 or higher) MHD
interaction parameters can only be reached with magnetic fields higher than
several Tesla (i.e., 10-20 Tesla). The weight and volume of a magnet then
makes such flight devices quite impractical, unless a breakthrough in magnet
and materials technologies occurs resulting in ultralightweight magnets with
B~l0 Tesla.
As an example of potential use of nonequilibrium cold-air MHD devices with
ionization by e-beams, we note the studies of MHD scramjet inlet control
performed by one of the authors of this survey and his Princeton University
colleagues. These theoretical/computational studies showed that indeed, with
proper optimization, MHD interaction at the compression ramp upstream of the
scramjet inlet can restore the shock-on-lip (SOL) condition at Mach numbers
higher than the design Mach number for a given fixed-geometry inlet (Figure 6).
During the MHD operation, the generated electrical power would be enough for
ionizing e-beams, with a significant percentage of the power left to be stored
onboard and used for other purposes. The advantage of MHD inlet control is that
it eliminates the need for a variable-geometry (movable) cowl that would be
associated with a large weight and complexity; the disadvantage is that the
weight and complexity associated with magnets and e-beam systems may
negate the advantages. Systems studies are needed to fully assess the
practicality of this MHD inlet control, and results of such studies would strongly
depend on the state-of-the-art and future advances in lightweight magnet and e­
beam technology. 1s, 19, 20, 21, 22
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