Re: Big magnetic fields

"Paul F. Dietz" <dietz@xxxxxxx> wrote in message
news:scOdnfIXIP1rkkzfRVn-jw@xxxxxxxxxx
> Logan Kearsley wrote:
>
> > In that case, while the plasma won't compress anymore on its own,
there's
> > nothing to prevent you from adding more energy to compress it further,
> > possibly by means of a magnetohydrodynamic engine.
>
> The problem here is that you have to do work on the plasma.
> Lots of work. And it still doesn't get you close to high
> density.
>
> And think about what you're proposing: dump lots of energy
> into the plasma, *just so you can radiate it away*. If the

What does that matter, as long as the expenditure is less than the energy
gained via fusion, or antimatter annihilation, or whatever other process one
happens to be using? Anyway, I wouldn't plan to just radiate it away- I
would plan to keep as much of it as possible in the plasma (either by simply
not removing it at all, or by extracting some of the heat with a heat engine
and using that to power a magnetohydrodynamic engine to linearly accelerate
the exhaust), so as to maximize the exhaust velocity.

> vehicle is traveling at relativistic speeds, the energy will
> be far higher than the fusion energy you might get out of
> the plasma. Hell, at moderately relativistic speeds the
> energy is greater than the rest energy of the ions once
> they've been brought to rest and cooled.

Aha! That's I was looking for. So, the physical limit one hits is a point at
which the energy required to compress the plasma is greater than the energy
that can be extracted by fusion. That makes sense.

Now, I just have to wonder if there's a way to engineer around that....

> No, if you want to get rid of the entropy, you need to
> radiate it when the plasma is still cool. This means
> you need to radiate before it has been highly compressed.
> (I don't care what mechanism does the compression; none
> of those mechanisms can avoid the 2nd law.)

Now, I still don't see why one would want to cool it at all, unless the
energy can be added back in again, as one wants the highest exhaust velocity
possible. But assuming that one does need to cool the plasma, and building a
long enough spacecraft is infeasible, how about this:

Rather than feeding directly into the compressor/reactor, the ramscoop feeds
into a set of reverse cyclotrons- the ions and free electrons are put on
circular track in powerful magnetic fields, such that they dump their energy
into the fields and possibly could be used to generate electrical power. The
just-injected particles would be moving along the outer edges of the tracks,
and eventually would be collected at the inner edges once they've lost
enough energy, after which they can be sent on to the compressor/reactor.

-l.
------------------------------------
My inbox is a sacred shrine, none shall enter that are not worthy.


.

Relevant Pages

  • Re: Big magnetic fields
    ... >>>compressing the ISM at all (use extremely low mass grids that induce ... >>>plasma waves in the passing ISM, and add energy electromagnetically, ... > means you're employing dynamic pressure to compress the gas. ...
    (rec.arts.sf.science)
  • Re: Big magnetic fields
    ... > so much kinetic energy, and as you compress that energy turns into heat. ... > When all of it has been turned into heat, the incoming plasma has ...
    (rec.arts.sf.science)
  • Re: Big magnetic fields
    ... nothing to prevent you from adding more energy to compress it further, possibly by means of a magnetohydrodynamic engine. ... *just so you can radiate it away*. ... If the vehicle is traveling at relativistic speeds, the energy will be far higher than the fusion energy you might get out of the plasma. ...
    (rec.arts.sf.science)
  • Self-contained interstellar vehicle
    ... maximum compression of the incoming protons is limited by the kinetic energy ... of the ship, or if we take the ship as the rest frame the kinetic energy of ... the plasma up to fusionable levels. ... antimatter rocket that carries hydrogen fuel as well as antihydrogen fuel) ...
    (rec.arts.sf.science)
  • Lab Experiments Mimic a Stars Energy Bursts (Forwarded)
    ... Lab Experiments Mimic a Star's Energy Bursts ... work may help scientists study topics such as nuclear fusion as a possible ... When the lasers are turned off, the plasma begins to heat up. ... Beryllium ions are frequently used in NIST research on topics such as ...
    (sci.astro)