Re: Quick question about a STL trip - and another...

On Aug 25, 11:53 pm, "n...@xxxxxxx" <Alien8...@xxxxxxxxx> wrote:
On Aug 25, 7:39 am, IsaacKuo <mech...@xxxxxxxxx> wrote:

On Aug 23, 5:45 pm, "n...@xxxxxxx" <Alien8...@xxxxxxxxx> wrote:

On Aug 23, 8:43 am, IsaacKuo <mech...@xxxxxxxxx> wrote:
The basic acceleration mechanism is simple absorption/scattering.
The X-ray acceleration laser is tuned to the k-edge transition of
Tungsten (this is very short wavelength and allows for an
extremely long ranged laser). About a third of the x-rays get
absorbed by the chip's tungsten while two thirds gets randomly
scattered. The x-rays mostly pass through the carbon parts.
The result of this is acceleration almost perfectly away from the
laser.
Chip velocity WRT laser? Doppler frequency shift puts laser
wavelength where?

The doppler shift is a concern. The X-ray laser can be tuned
to a large degree, but there will plausibly be a need to swap
out wiggler segments a few times.

I haven't been keeping up with wiggler designs. May I assume we're
talking basically a descendant of the Halbach array?

http://en.wikipedia.org/wiki/Image:FEL.png

If so, may I also assume you've replaced the (presumably) permanent
magnets with electromagnets? If so, you can selectively drive them so
as to change the field configuration to chirp (more reasonably rapidly
downstep) the output wavelength to match the apparently (as seen from
the laser) decreasing absorption frequency of the chips.

It's plausibly best to use permanent magnets. Electromagnets
can be a bit more powerful, but they consume power and have
waste heat to worry about. Superconducting magnets can also
be more powerful, but they may be more trouble than it's worth.

The method of swapping them out which I was thinking of was
to physically have extra wiggler sets, and to physically swap them
out.

The acceleration run involved is quite long. The main limiting
factor on acceleration is waste heat radiation. For a micron
thick tungsten sail, the acceleration limit seems to be maybe
2.7m/s/s. At this rate, it takes over one and a half years to
reach half the speed of light. It may take a decade of
acceleration to reach the desired relativistic speed.

So, I'm thinking in terms of swapping out hardware maybe
once every few months. The really expensive stuff is the
power supply and accelerator hardware. In comparison, the
wigglers and the zone plates are not expensive.

Better would be some sort of nonresonant microwave structure the
would do the bunching. ISTR an idea for lining a conventional cavity
with negative-index 'stuff' so that the waves were confined but
without preferred modes. Can't find it right now; has a weird name.

It may be that some sort of microwave or laser beam can
be a better alternative for the wiggler.

The zone plate will also
need to be swapped out a few times. While a simple binary
zone plate wouldn't need to be swapped out, it's only half as
efficient as a sinusoidal zone plate. A sinusoidal zone plate
depends upon partial transparency, so the absorption rate in
the zone plate material is critical. This is similar to the problem
of chromatic distortion in traditional lenses. Using two layers
of different materials can compensate and allow for a wider
sweet spot, but at some point you'll want to swap out for a
different zone plate.

If the plate and its mountings are sufficiently robust you might be
able to get away with a mechanical zoom sort of arrangement, where the
chips are released in bursts and the plate moves to keep the laser's
appropriate wavelength properly focused on the moving target for
maximum acceleration.

Each zone plate will use a modest amount of movement in order
to stay focused on the chip swarm. This movement is also
required in order to compensate for the change in beam
frequency.

So, a burst of chips is released into the beam path ahead of the
zone plate, the laser spits out a chirp of X-rays, and the zone plate
moves forward (what, a small fraction of a meter?) to put the changing
foci on the centroid of the accelerating burst of chips which absorb
the X-rays and go on their merry way; repeat as desired.

Each chip swarm may take years of acceleration. For a short
range journey of less than ten light years, there will be only
one swarm. For the sort of multi-century journey discussed in
this thread, there may be several swarms. However, the mission
profile I suggest is for the chip swarms to be deployed by a
secondary starship.

How large a zone plate were you thinking, and would a somewhat
smoother mechanical resonance-induced motion of the plate be better?

A 3km diameter zone plate can focus a 70keV photon beam
onto a 50m diameter swarm out to 1/3 light year (enough to
boost up to 45%c).

BTW, how were you planning to build your plate? Naively I'd guess
concentric sine-profile thickness rings of extra-pure tungsten
deposited on a carbon (diamond) disk (you have to refine several
metric sh*tloads of both for the chips anyway), which should be fairly
robust.

Probably something more like printing tungsten powder ink
onto some sort of foil ***. The zone plate doesn't need
to be rigid. Nor does it need to stand up to the same level
of heating as the laser sails, since it has a much larger area.

On the other hand, it's not strictly necessary to keep the laser
tuned to the k-edge transition of tungsten on the laser sails.
The laser sail chips will function as long as the tungsten base
absorbs more of the photons as the carbon semiconductor
electronics.

Yeah, but as you say below you're looking at a 'realistic' final
efficiency in the low single digits; might as well squeeze out what
you can.

There's no loss in efficiency as far as the thrust level is
concerned. In all cases, the amount of thrust is pretty
much equal to the beam power divided by c. Whether
the tungsten or the carbon absorbs a photon doesn't
change the amount of thrust from it. Where it makes a
difference is on board power generation for the chip
electronics and the differential waste heat radiation
thrust for stationkeeping maneuvers. A modest change
in this efficiency isn't really going to change things
significantly.

Chip deceleration rate = ? Alters absorption how?

Chip deceleration rate? What would cause the chips to
decelerate?

I see I left out some text. Presumably the chips will not always be
traveling through perfect vacuum; the local ISM will slow them down a
bit as they plow through it.

The ISM will more or less not slow them down at all. We're
talking about violently relativistic collisions. A stray hydrogen
atom will punch through a chip, probably taking a few atoms
along with it. The lost atoms will be ripped away too quickly
to produce a significant amount of braking thrust.

Instead, the ISM will eat away at the sail like a machinegun
riddling the chip with atom sized bullets. This is why I suggest
deploying the chip swarm from a secondary starship rather
than from the launch system. With a cruise journey of several
centuries, my gut feeling is that the cumulative effects of the
ISM will not be pleasant on tiny little micron thick sail chips.

Now that I think about it that's probably irrelevant. The distance
over which the chips will be absorbing energy from the beam will be
fairly short; farther downrange they'll still be in it but won't be
able to absorb any of it.

I actually think in terms of x-ray laser beams being useful
out to 4.3light years or more. At these ranges, the beam
wouldn't be used for accelerating the chip swarm, but it
would be used to power and guide them for stationkeeping.
Thus, the chip sails can be well lined up for the starship.

How are you producing the X-ray beam? Could an X-ray FEL (no idea
how to build one, sorry) be frequency-agile enough to make the above
irrelevant?

The type of laser I assume is a SASE X-ray FEL which is
focused by a sinusoidal zone plate spaced far away from
the main laser. In principle, this type of laser can be up to
50% efficient because the zone plate absorbs half of the
incoming light while focusing the other half. However, it's
doubtful that the XFEL will be anywhere near 100% efficient.
A more plausible number would be maybe 2% to 10%,
halved to maybe 1% to 5%.

Aaargh. Well, you did say you had a dedicated nuclear reactor for
power.

Actually, the nuclear powered laser I mentioned before is for
a different purpose.

I think nuclear power will be too expensive compared to
solar power to be used for the main chip sail accelerator.
The powerful x-ray laser I'm talking about above would be
solar powered. The initial costs would probably be much
less than nuclear, and the marginal costs would be
negligible. A solar powered laser complex could be used
to launch probe after probe. A nuclear powered laser
complex could be used once and then likely as not would
never be used again due to the cost of refueling it.

The nuclear powered laser I mentioned before would
actually be used only once, and it's used in the middle
of deep interstellar space where solar power is not an
option. This laser would be used to guide and power
small chip impactors. These impactors don't get their
kinetic energy from this laser. Instead, they get their
kinetic energy from being on board the secondary starship.
This laser only needs to power the chip sails enough
for stationkeeping onto a neat even line.

With the multi-century generation ship missions being
discussed in this thread, it makes sense for the chip
swarm to spend most of the journey stored in a
secondary starship. There, they can be protected from
the ISM by the starship's magnetic field and plasma
field.

The initial acceleration of the secondary and primary
starships is another question, of course. Since it takes
years and many light months to accelerate a chip swarm
up to speed, I usually don't recommend a relativistic
kinetic impact powered rocket for the initial acceleration.
However, these starships are going to be cruising for
centuries, so waiting an extra decade for the initial
acceleration run might be worth it.

Normally, I favor either a large ribbon-like laser sail or
relativistic particle beam propulsion. The latter should
be REALLY good; the only problem is that we can currently
only guess at how technically difficult it may be to focus
an atomic beam over such long distances.

Isaac Kuo
.

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