Digest Number 509

--- "victoriatangoman tango_dancer@...
> --- In ssi_list@... Ian Woollard
> > Depends on how it is constructed. If it is kept
> down to 1kw/m^2
> then its
> > no death ray. Provided the lenses are small
> enough, it's
> physically
> > impossible to get the light too concentrated.
> >

Lasers cause eye damage at very low levels, so I doubt
they will ever be approved for beaming towards Earth.
Take a look at the US Code of the Federal Regulations
(I forget the CFR number but I can research it) which
govern how much free laser energy is allowed for
public exposure (administered by the FDA).

It is tiny compared to the allowable microwave levels.

>
> OK, I'm missing something here. If we keep the
> energy density to
> 1,000 watts/m^2 compared to the 1,360 watts/m^2 of
> sunlight,
> comparing efficiencies of 70% and 20%, respectively,
> then the laser
> would be 157% more efficient than solar PV.
>

On thing here makes a difference. The sun angle
across the PV arrays on the lunar surface constantly
changes, and is usually lkess than the 1,360 w/m^2.
To maintian constant max power the PV array must have
expensive and heavy steering equipment.

Whereas a rectenna does not need to be steered, and
always gets maximum power.

> So here's what I don't get: you still need PVs on
> the lunar surface
> to convert the laser light, albeit at a higher
> efficiency but now
> you need a large laser, power source and radiator in
> orbit, or at
> L1. Is this less expensive or complex? I don't know.
>

It is basically the same as a microwave SPS, except
that a laser is used instead of a maser. The size of
the radiaiton depends on the amount of waste heat.
Not sure about that, is a laser as efficient as a
klystron ? The advantage is that the transmitting
"antenna" (a reflector mirror probably) is a lot
smaller.

> Also, a laser that can focus a beam for 30,000 miles
> is considered
> dangerous, isn't it? Also, the laser will be

Yes, getting FDA approval for a big laser for pointing
top Earth will be a major challenge. There are very
strict laws on the books governing exposure of workers
and the public to stray laser light.

Even getting a 1 Watt laser approved involves a lot of
paperwork and testing, especially if it is infra-red
(invisible).

> producing a prodigious
> amount of energy but focusing it across a large area
> to achieve a
> weak energy density, but how difficult would it be
> to change the
> optics and increase the energy density on short
> notice?
>

Making the beam narrow requires increasing the size of
the transmitting reflector. But it would be a net
cost saving, because we want to make the rectenna as
small and lightweight as possible.

We are probably limited by human safety limits.
However, on the Moon (as opposed to Earth) there is no
local population, everybody will be employees of the
development organization, so they can be controlled by
contract to stay away from the rectenna when in use.

There can be security procedures to make sure nobody
goes near the rectenna by accident.

Maybe that will allow us to operate at a higher power
level than we would on Earth.

> If your answer to the above points is that they are
> indeed valid
> concerns about weaponization, then is there really a
> need to keep
> the energy density down to 1,000 m^2? If the
> concerns are satified,
> then why not pump it up to 10-20,000 watts/m^2?
>

Reducing the beam size means increasing the size of
the transmitting antenna / reflector.

This is known as the Raleigh diffraction effect.

But there is a positive ROI, making things on the
Moon small (e.g. rectenna) at the cost if increasing
things at L-1 is a good trade off.