Re: Non-extinctionary interplanetary warfare

On Aug 13, 7:31 am, IsaacKuo <mech...@xxxxxxxxx> wrote:

I'm not the laser expert, so I have no idea whether such laser
systems could be cheap enough to seriously compete with
locally generated tether power. Basically, the question is
whether it's worth the expense to beam power from, say,
Io orbit to Ganymede orbit? Or does it makes more sense
to use tethers directly for local power at Ganymede? Or
maybe it makes more sense to transfer raw resources into
Io orbit for industrial processing?

It probably depends on your tech assumptions. A 10% efficient solid
state laser, beaming light that can only be captured by germanium
photocells, doesn't sound particularly cheap or useful. Cheap, high
efficiency, phase locked diode lasers tuned to just above the silicon
band edge, collected with cheap, high efficiency* thin film silicon
photocells, is a lot more practical. So, I guess the question is, do
you want beamed power?

Luke

* Silicon has a funny band structure that, at first glance, limits the
efficiency of collection to about 35% - due to momentum**
conservation, you can't absorb photons with less than 3.2 eV of
energy, but the 3.2 eV electrons get thermalized to a non-radiative
part of the band at 1.12 eV, so each electron only gets you 1.12 eV
out. Germanium has the same problem, but with different (smaller)
numbers. If you can't get around this problem, you need to go to a
direct bandgap semiconductor for high efficiency absorption, which
tend to use more exotic elements. It may also be possible to do
clever things with absorption via phonons to get around the "obvious"
limitations of silicon. Finally, there is diamond, which is made with
a common element, should be able to get about 75% efficiency of
turning light energy into electric energy, but has been traditionally
difficult to make.

** More properly wave number, since true momentum is not a good
quantum number in periodic systems like crystals, but the
correspondence is close enough as to make little difference in a
discussion like this one.

.

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