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starship-design: Plasma power

Johnny Thunderbird writes:
 > Folks are too worried about making a static sort of fusion reaction,
 > contained in a can so they know just where it's happening. The number one
 > fact about space is that there's plenty of room. That means you can use
 > accelerators to produce fusion, which is easy, rather than try to compress,
 > contain and confine an energetic plasma, which is hard. Using particle
 > accelerators is the only way we know that these reactions happen, and these
 > fusions have been done in detail, decades ago, in the energetic beams of
 > accelerators. Fusion research on Earth is concentrated on making it happen
 > in a can, and that's hard. To use fusion for space drives, we can use the
 > well-known accelerator research, because we don't mind if our entire
 > reaction is going thataway real fast, because that is just what we want to
 > do: our entire purpose is to make it go thataway real fast.
 > In that aspect, all Earthbound fusion power projects are false leads, and
 > trying to pursue the ephemeral deuterium-tritium reaction, in particular, is
 > a big red herring. 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.

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.