Re: Orion Drive space battle


Logan Kearsley wrote:
"Luke Campbell" <lwcamp@xxxxxxxxx> wrote in message
news:1153532634.369786.42470@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Laser diodes have been made with efficiencies of 90%+. As you mention,
their beams are crap. No, lenses can't fix it, there's not enough beam
coherence there to work with.

What about phased arrays? AFAIK, the high divergence is a result of their
small size, but a phased array ought to be able to simulate a large
aperture, right?

They would if it was the small aperture that was causing problems.
Instead, diode lasers just have terrible beam quality. They lase on
too many modes, with really poor beam coherance. Maybe someday we'll
be able to make a better diode laser. For today's tech, the main
applications for diodes as laser weapons are to pump solid state
neodymnium lasers.

I wonder what the limits are to FEL efficiency. The basic idea doesn't
necessarily have to use electrons- you might build a Free Proton Laser,
or a
Free Alpha-Particle laser, so perhaps a "focus fusion" reactor could be
coupled directly to a wiggler to generate laser light instead of
electricity.

Single pass FEL efficiencies are, if I remember correctly, something on
the order of 30%. This is not really relevant, though, since all
energy not extracted as coherent photons remains in the electron beam,
and all this electron kinetic energy can be easily recovered as
electrical energy using the same machinery that accelerated the beam in
the first place. For example, if you are using an electrostatic
accelerator, running the electron beam backwards through the
accelerator turns the electron beam energy into usable electricity
(that can be used to accelerate another electron beam, for example).
This has been demonstrated. A lot of people are even more excited by
using linacs to pump FELs, and then running the electron pulses through
the linac out of phase with the accelerating fields to recover the
energy. In this sense you get a near 100% "wallplug" efficiency at
turning electricity into laser beam energy.

Oo, excellent. And FELs are tunable, too. So, the net efficiency, and how
much waste heat you have to dump, will depend mainly on the accelerator
efficiency, and the wiggler electromagnet efficiency. You say below particle
beams are near 100% efficient, so that sounds pretty good.

Yeah, like I said, you get better than 99% wallplug efficiency.

I'm having a bit of trouble imagining how to set up the geometry for
recapturing the electron beam and recovering its energy. With electrostatic
accelerators, I imagine you could just have the whole thing in a straight
line, with one accelerator at each end and a hole in the 'nozzle' end for
the laser beam to emerge from, but that seems less practical for a linac.

Here's a link for a typical energy recovery linac geometry.
http://erl.chess.cornell.edu/
These things are being built now (and some have already been built) to
get x-ray free electron lasers for materials research. Note that if
you put the electron bunch back into the linac in phase with the
accelerating RF fields, you can re-accelerate it rather than decelerate
in to recover its energy. The reason this avenue is not pursued in
modern research is that after many passes, the electron bunches tend to
spread out. You end up with nanosecond long x-ray pulses rather than
fempto- or attosecond long x-ray pulses, meaning less intensity and
lower time resolution (nanoseconds does not let you resolve atomic
motion).

Other charged particles are not very practical for free charged
particle lasers. Radiation occurs when a charged particle is
accelerated. Electrons have a very low mass compared to any other
charged particle, so for a given force, they are accelerated more, and
thus radiate more. In addition, because of their low mass, they go
faster (or, in the relativistic limit, have larger relativistic
effects), which is good for getting them to produce coherent radiation
beams.

So, using other particles requires stronger wiggler magnets. Obviously,
electrons are ideal, but I just wonder if there are cases (such as when you
can get a 'free' alpha particle beam out of a p-B11 fusion plasma) where it
might be useful to accommodate other particles.

For a particle of velocity v, charge e and relativistic factor gamma,
the radiation from acceleration perpendicular to the direction of
motion is proportional to e^2 gamma^4 v^2. For a given energy E and
mass m, gamma=E/(m c^2) where c is the speed of light in vacuum.
Because of the mass factor, you can already see that proton radiation
will be decreased by a factor of 16E+12. Fusion applications tend to
use particles in the keV range, which is not only low energy but well
into the non-relativistic limit for alpha particles (so v << c). These
factors tend to pretty much kill the raidation from anything other than
electrons.

I wonder about particle beam efficiencies, too. Might it be more
efficient,
in terms of energy delivered to the enemy compared to energy lost as
heat,
to use particle beam weapons?

Particle beams are nearly 100% efficient at turning electrical energy
into beam energy. They have other problems, though (such as a
difficulty keeping them tightly focused).

Excellent. Using two accelerators side-by-side to produce a neutral particle
beam would help limit the divergence, but the average particle beam would
still probably have significantly higher divergence than the average laser.

The main limiting factor is that when the heavy charged particle
recombines with an electron, the electrostatic attraction of the
electron gives the particle a "kick" during the recombination process.
This adds random motion, i.e., heat, to the beam. For a proton beam,
conservation of energy indicates that you get a heat of 13.6 eV (the
binding energy of hydrogen) per particle. Since it is late, I'll let
someone else work through the math on the beam divergence this gives,
but I recall it is pretty significant.

On the other hand, particle beams can be much more penetrating, and produce
secondary radiation.

A free electron laser pumped by a linac has no problem generating
x-rays. If you can get good hard x-ray optics, you can get a beam of
highly penetrating radiation. Admittedly, they will not be as
penetrating as a beam of 50 GeV hydrogen atoms, but with diffraction
limited optics you could be killing other spacecraft across the solar
system. The big problem I see is getting diffraction limited optics.

'Twould be interesting to work out under what circumstances each type of
weapon would be better. Lasers probably win for long range no matter what.

They have in all the scenarios I've ever worked, or seen worked, unless
you severely limit laser technology (to, say, mid IR chemical lasers or
some such).

.

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