Re: Orion Drive space battle


Logan Kearsley wrote:

I don't know if anybody has surpassed it yet, but laser diodes have been
constructed with efficiencies of up to 71%. They have high divergence,
though. Possibly that could be fixed with a phased array and/or suitable
application of lenses.

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.

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 eficiencies are, if I remember correcly, 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.

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.

The ever popular neodymnium lasers (such as the Nd:YAG workhorse of
moany modern machining lasers) have a quantum efficiency of on the
order of 90%. This means it is theoretically possible to get a
neodymnium laser to operate at close to 90% efficiency. Other effects
have limited the efficiency of real neodymnium lasers to closer to 10%
to 30% (if you want continuous beams, pulsed beam efficiencies are much
lower).

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).

.

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