From: aufsj@IMAP2.ASU.EDU
Subject: #1. Possible EW apps for Plasma Mirrors ???
Organization: Arizona State University
Newsgroups: sci.physics.electromag,sci.physics.plasma


I was doing some back of the envelope stuff the other day and
came up with some ideas that seem, at my first (and out of my specialty!)
glance to be possibly promising. I'm not a physicist, or even an
electrical engineer, having actually entered the subject via the back
door of computer science. So if I don't get the nobel (or a big grant)
perhaps y'all will at least get a good laugh. :-)

It seems to me that plasma mirrors of the rather unexciting kind
(i.e. we're not talkin' fusion here) might have some very valuable
applications with regards to contemporary electronic warfare. I haven't
been able to find much evidence of anyone even looking at the subject
very hard, so I am wondering what I'm missing--as I admitted earlier this
is somewhat new ground for me.
A plasma mirror reflects electromagnetic radiation whose
frequency is less than the "plasma frequency". Generally, a plasma mirror
can be as effective as a metal mirror in reflecting microwaves. Compared
to a lot of things we manufacture today, a plasma mirror "system" would
seem to be fairly simple. But why go to that trouble, if it just works as
well as a metal plate? That is the whole point....
I've pulled out the NRL Review of 1993 for some representative
data ( God bless the Naval Research Laboratory, and say hello to the
guys at code 9100 for me ). "A Plasma Mirror for Microwaves" describes
the construction of a very basic plasma mirror and a few baseline tests.
The main thrust behind this was the thought that perhaps such a mirror
could be used to provide radars with an agile scan mechanism. What I
propose are a couple of leaps beyond that application, to some EW uses.
Such a mirror (produced by passing a current through low pressure
gas with some bells and whistles attached) could be accomplished by
"energizing a selected linear array of emitting points in a two
dimensional matrix cathode, and/or by changing the direction of the
magnetic field". Also "different shapes cold be formed by shaping the
cathode and the magnetic field".
The key here is that the mirror can be created rapidly and ended
rapidly, and then reformed.....ad nauseum. In addition, there is the
possiblity that shaped mirrors could be formed. In addition, as to
practicality, I am predicting based on the parameters for this test (a
few amps, 150 mTorr, 150 gauss) that making plasma mirrors--or more
properly the tubes/cells that they exist in---is not going to be too
difficult. Once again, I'm not talking about the exotic fusion type
plasmas here.
The mirror discussed in this experiment was not very large,
approx 15 cm square. However, it seems that considerably larger ones
could be constructed ( I'll use an arbitrary 1 square meter ), or perhaps a
number of smaller mirrors could be joined together in a 'beehive' manner
to present a fairly unified "front" for the reflection.


Doppler Nullification:

The plasma layer that acts as the "mirror" exists in the space
between between a linear cathode on the 'bottom' of the cell and the top
of the cell serves as the anode. A current is passed through the cell
such that the desired "plasma frequency" is reached, and now the plasma
will reflect RF energy below that frequency. This much seems straightforward.
However, what if you "swept" the linear cathode from the front of
the cell to the back of the cell ( using, say, a linear array )? If the
plasma field 'follows' the sweep of the cathode (even with a delay, only
the rate is important) then the effect is to reflect the RF energy *but*
with a doppler shift that is a function of the sweep rate of the cathode
as well as motion of the cell itself. In other words, depending on what
sweep rates one could attain, one could return an arbitrary doppler
shift. For a radar generally in front of the cell, sweeping towards the
back would tend to cancel detection of forward motion, while sweeping
forwards would tend to overstate the actual speed of the cell, and
leaving the field stationary within the cell makes the return the same as
cell speed.
For example, a cruise missile heading towards a ship might have a
"cell" in its nose (say 1 meter square). As it gets close to the ship it
activates the plasma mirror. The missile is moving towards the ship at
880 feet per second (I seem to recall that's just below Mach 1) but it is
"sweeping" the cathode from the front to the back at the same 880 feet
per second rate. The mirror is formed, it is 'dragged' back as it follows
the cathode, and when it reaches the back a new mirror (i.e. charged
area) is created at the front of the cell and then it is dragged back and
the process repeats. The numbers would be a factor of many things, I won't
go into the math of various scenarios, but the numbers don't seem (to me)
to be unrealistic at all.
The result? Imagine the missile heading towards a ship with a
scan mode Phalanx system. In the best case, since a large portion of the
missiles radar reflective area is 'covered' by the zero-doppler
deception, the system gets a "stationary" (or no detectable) return and
does not fire on the missile [well, in the simplest case. actual Phalanx
parameters are a bit more complicated and confidential]. In a second
case, the opponent radar might detect 'non-protected' portions of the
missile like the tail and wings (and body if not directly aligned) as
well as the doppler spoof return. In this case, he may have no idea which
is the correct value, screwing up all sorts of calculations. You could
dither the "spoofed" portion of the return (that provided by your doppler
mirror) in order to throw off your opponent, especially with a priori
knowledge of his systems. He detects a target, but is it coming at him
at 500 mph, or going away at 300 mph, or changing speed by 100 mph every
time his radar sweeps across it? An ability, even if marginal, to throw
off the doppler calculations of hostile radars would seem to be a
capability in great demand.
The plasma mirror would appear to have some great virtues in this
regard. Unlike various "active" and "repeater" and "mimicry" systems
that try and fool doppler, the mirror is *physically* altering the
doppler return of *whatever* signal the opponent throws at it (assuming
you have the plasma frequency set high enough). Unpredictable repetition
intervals, frequency agility, weird intrapulse modulations----all would
be moot because your mirror is just like a dumb steel plate, albeit one
that can be made to appear to be moving at any velocity at any given time.
Presumably, one wouldn't want to bother making great big
cells/tubes that could hold big plasma mirrors. One possible answer, as I
stated earlier, is to join them in a beehive fashion. But even if that
isn't practical, there are some reasons it seems that even the ability to
cover relatively small areas with such "plasma mirror" might be useful:
A) relatively small things, like a head on view of a missile (the
one you hope an enemy is going to have). As a bonus, the rapid formation
times for a plasma mirror would allow you to put a screen between the
missile radar antenna (a big return item) and the opponent. You could
drop the plasma freq momentarily and the missile radar could scan
"through" the cell, then raise the freq and do the doppler spoof and an
opponent gets fouled up returns. Smallish UAVs might be able to be
screened. Re-entry vehicles? Spacecraft? Imagine denying an opponent
doppler on those toys....
B) aircraft. It would be kind of hard to shield a 747 perhaps.
But the key factor would seem to be the proportion the radar energy
returned by the plasma mirror as compared to the rest of the aircraft. In
other words, extremely low RCS (buzzword: Stealth) aircraft would seem to
be well served by such technology. When you are talking about an
aircraft with a probably -60dbsm value the return from a 1 meter square
mirror (which can be pointed for optimum return) is going to loom pretty
large. This would be *particularly* true if the mirror were shielding
some highly reflective portion of the aircraft. Ideally, you would
"overwhelm" the valid return with your spoof return (by forcing an
extremely high dynamic range) or at least, as described earlier, dither
it to confuse him and throw him off.

Well, what part of the big picture am I missing? It seems that
at least some doppler spoofing should be attainable, yet I haven't heard
of it being done. I'm certain I'll hear why soon..... :-)

regards,

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Steven J Forsberg at aufsj@imap2.asu.edu Wizard 87-01