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Re: starship-design: LINAC efficiency
Having a hungry linac may be fun and exciting, but we will need to figure
out what to feed it. I favor the ram scoop method pioneered by Bussard.
The structural requirement for a current loop meant to serve as a magnet, is
that the current carrying conductor, which in all cases should be operating
in superconducting mode, need have the mechanical rigidity to resist
collapsing into its own magnetic field. Neglecting for the moment any axial
components of force, its radial, inwardly-compressive component could be
compensated by rotation. The superconducting ring defining the magnetism,
can thus be weaker mechanically, than a non-rotating ring which produces a
magnetic field of the same strength. That means it can be less massive, so a
gracile magnet ring, in place of a more robust ring, can be a durable
starship component. This becomes a very desirable feature, when very large
scale constructions are considered.
Very large scale magnets are useful for ram scoops. As we are all quite
aware, none of us want to try designing a starship which carries all its own
reaction mass, fuel and energy from launch point. That's one of the things
we can agree just won't work. We have to pick up some fuel, and/or some
reaction mass, and/or some energy along the way somewhere, somehow. We can
get express shipments from our friends back home for some of these things,
but it seems to me best, not to count on home base assistance for more the
first fraction of the journey. After that, we will need a way to live off
the land, so to speak, even though interstellar space offers really sparse
pickings. Since we know a way to make gracile magnet rings real big, we can
push them along in front of the ship with radiation pressure from lasers.
The interstellar plasma, with a partial pressure I expect to be about one
proton/electron pair per cubic centimeter, will be concentrated by this
magnetism into a brisk refreshing breeze on the ship's bow.
Hopefully neutral atoms will also be ducted in to us by the winds stirred up
by our magnets. This is the time to introduce our our mass spectrometry, by
which we can magnetically deflect the electrons and ions away from the
neutrally charged interstellar gas. This gas we ionize, and merge it with
the ion and electron streams, flowing so swiftly by. Carefully separating
the protons from the ions which astronomers call "metals", we duct the
protons into our linac, to be booted out the business end, at relativistic
velocity, as reaction mass. The remaining ions are mined for lithium, to
feed our fusion reactor. (I mentioned proton-lithium fusion in a post last
week.) Anything heavier, we keep with us in case it might be useful.