Re: starship-design: Plasma power

["Johnny Thunderbird" <jthunderbird@nternet.com>]

----- Original Message -----
From: "Steve VanDevender" <stevev@efn.org>
To: <starship-design@lists.uoregon.edu>
Sent: Wednesday, June 21, 2000 6:20 PM
Subject: starship-design: Plasma power


> Johnny Thunderbird writes:
>  > Folks are too worried about making a static sort of fusion reaction,
To make a fusion space drive, all we need is the mature
>  > and well-studied technology of fusion induced by beams from
accelerators.
>  >
>  > How long can a linac (linear accelerator) be in space? There's plenty
of
>  > room, right?
>
> I find that a deeply strange perspective, given that the idea of "fusion
> in a can", as you call it, is to produce the temperatures and pressures
> needed to make fusion efficient, or even possible.  The amount of fusion
> you can get in an environment where you can't even keep the atoms close
> together is going to be rather small.  Physically I don't see any way
> for you to reliably get a high proportion of atoms fused out in some
> diffuse tail of gas that you're bombarding with other atoms.
>

In the context, which is of an MPD arcjet with boron enrichment from the
anode which strikes the arc, we are not speaking strictly of a "diffuse tail
of gas" in the jet, for the purpose of the final or "nozzle" magnetic coil
is to collimate the exhaust ions, restricting their flow to a relatively
narrow column in the wake. The fraction of the exhaust plasma which is
neutral gas, will of course tend to escape this organized column, in diffuse
fashion by thermal motion. But thermal motion is very, very slow as seen by
a beam of relativistic protons, so even the hot neutral gases won't get far
from the central jet target zone without getting bashed. Many of them,
anyway. Our plasma is cold, it is positively frozen, when the beam from the
accelerator comes along. We need to take account of the scaling factors in
speed, contrasting the thermal motions we normally think of as pretty
speedy, with particles accelerated to within a gnat's ass of C.  Convert
from keV to Kelvins, that's your comparison.

In the case of the ions, they are constrained to spiral tightly around the
magnetic field lines, at their cyclotron frequency, and they won't be doing
any wandering. They will stay where they're stuck magnetically, while they
are bombarded by this beam of  _relativistic_  protons. The fast things here
are the fusion products, and the impingent beam. The slow things are the
neutral exhaust gases and the ions pinned on their helical tracks, which
might as well be encased in a block of ice by comparison. Temperature and
pressure are not prime requisites for the fusion of nuclei bombarded by a
particle beam; in fact, temperature is completely irrelevant, as we have
seen, and pressure is derived from temperature, so all we really need talk
about is density. Specifically, the fusion will be self-sustaining at a high
enough density of the target particles; but what need have we for a chain
reaction? We're building neither a bomb nor a power plant, but an ephemeral
structure of high energy in deep space. So most of our protons missed,
should we care? We have plenty more protons to toss. The fact that we have
projected them out into cold space, at the highest possible velocity, means
we have derived the maximum thrust obtainable from their mass. Going for
fusion as well, we are reaching for a bonus.

> Also, if your idea is to essentially throw some hydrogen out the back
> slowly and then throw more out the back very, very quickly in the hopes
> it will fuse with the hydrogen you dumped earlier, then the problem is
> that the fusion doesn't happen in a place where you can get thrust from
> it.  If you hold a stick of dynamite against the back of your car and
> detonate it, it will push the car forward (how to avoid blowing the back
> of your car off is left as an exercise for the reader :-).  If you drop
> a stick of dynamite on the road behind you and set it off after it's
> fallen away, it won't push on your car much at all.  Your concept is
> much like the latter car-and-dynamite situation.

The imagery evokes the old NASA fission-bomb propulsion proposal, a dreadful
pulsating drive. Your point is actually that the ship, in front, subtends a
solid arc which rapidly decreases as the energetic reaction gets farther
behind into the jet, which is of course quite valid. The efficiency of the
thrust produced on the ship is vastly decreased for an explosive reaction
far behind the ship. Yet there are cases which decrease the weight of this
objection. Starship designs which have been lately in my consideration, all
have in common a magnetic presence which is effectively many times the size
of the solid portion of the ship, and would effectively provide a much
greater cross-section to intercept the forces produced by trailing energetic
reactions. The particular implementation we consider here, using an MPD
arcjet, has such a built-in magnetic field. It's easier to believe that a
fusion reaction behind the ship would tend to nudge it forward, than to
believe that it wouldn't. Again, this fusion is sought as a bonus, for we
have already made optimum use of two of the most efficient propulsion
techniques we can conceive, the MPD arcjet and the pure thrust of the
relativistic particle beam. Why haggle over the efficiency of the auxiliary?

Serenely,
Johnny Thunderbird