Date: Thu, 24 Jul 1997 08:10:26 +0000 (GMT)
From: aufsj@IMAP2.ASU.EDU
Subject: ??? Plasma mirror propulsion...
Organization: Arizona State University
Newsgroups: sci.physics,sci.physics.plasma


I have posted previously on possible electronic warfare
applications for low temp plasma mirrors. It seems, however, that there
may be other more peaceful uses for them, possibly including use as a
means of propulsion for spacecraft.
I don't think this design can be engineered at this time, but I
am mainly wondering if there are any "theoretical" objections or if
perhaps it is contrary to the laws of physics. It is a bit sci-fi, but
sounds like a damn elegant design (if it would work--post opinions).

A low temp plasma is formed by running an electrical current
through a gas. This plasma will have a "critical frequency", energy
below this frequency is reflected while energy above this frequency
passes through. Could this be exploited as a means of propulsion?
Suppose that there is RF energy being radiated from two
direction, one behind and one in front of the mirror (a simple case). If
one were to accelerate the mirror towards one of the sources then it
seems that more and more of the energy from "behind" would be doppler
downshifted (i.e. "bounce" off the mirror) while the energy from in front
would be more and more doppler upshifted (and thus pass through the
mirror with minimal interaction). If the energy 'bouncing off' from
behind imparted energy on the plasma mirror system this would tend to
accelerate it further, leading (perhaps?) to a situation where as the
mirror accelerates more and more energy 'behind' it is shifted below the
critical frequency and thus there is more energy being imparted on the
mirror from this direction --- even while energy coming from 'in front'
is being upshifted and thus passes through the mirror.
In other words, a "plasma sail". (is this an old idea I just
haven't heard about?).

If one could accelerate the sail to relativistic speeds, would it
be possible to reach a speed whereby light from 'behind' is shifted down
into the RF energy range, ie from terahertz to about 10 gigaghertz --
even while light from ahead is upshifted incredibly above the critical
frequency and passes right through? What speeds would be necessary to
shift visible light from behind down to 10 gigahertz or so? Could the
sail, if given only a slight impulse, accelerate to reach these speeds?

The energy 'imparted' by bouncing off the sail would not be great
(especially at lower speeds) but would it not eventually accelerate the
sail as long as the plasma was in some way linked to the physical
structure enclosing it (I'm thinking along the lines of electromagnetic
coils placed in spacecraft).

Basically, this idea sounds very elegant. All you need is a low
temp plasma and some structure that contains it and any other payload.
Even in an environment in which energy is coming in from all directions
(say, background energy from the 'big bang') one could pick a direction
and sail, the "discontinuity" caused by the sail passing energy in one
direction only, creates an unbalanced force, and Newtons rules would seem
to apply :-).


Is there some big fundamental flaw I'm missing? Has this idea
been fleshed out before?

regards,

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