Re: Laser-Wakefield Acceleration for Space Propulsion?

[sanman]

I was reading up some more about the laser wakefield acceleration, and
it seems it can be used for ion acceleration, and not just electron
acceleration.

http://www.bnl.gov/atf/Meetings/AAC04/abstracthigh.htm

So by firing some intense brief pulse laser through a dense gas, the
laser locally ionizes the gas molcules and an intense
electric/magnetic wakefield is created which squirts the ions through
it, subjecting them to a very high acceleration gradient along a short
distance.

You guys have mentioned that laser-heating of propellant can produce
very high specific impulses (2000secs, etc), but how do these compare
to what the laser-wakefield effect would do?

Based on the high acceleration gradient compared to electric/magnetic
particle accelerators, would the specific impulse be in the tens of
thousands? Does anyone have any rough numbers?

You've already pointed out that electric thrusters, even with their
high specific impulse, still only produce miniscule amounts of overall
thrust. The best electric thruster only does several newtons, I think.
This is insufficient for earth-to-orbit, especially if you account for
the high weight of the electric thruster itself. Shouldn't people then
use a more accurate figure for specific impulse based on a combination
of engine mass and propellant mass? (the "powertrain", so to speak).
Or better yet, why not a mass-to-thrust ratio, where the mass would be
that powertrain mass.

It seems like the job of a rocket engine is to provide the most DeltaV
per unit of engine+fuel mass, preferably at lowest cost.

Thermally-created DeltaV (eg. combustion, laser-heating, arcjet) seems
to be less efficient, since the hot molecules can be chaotically
shooting in any old direction, hitting and heating the engine walls,
etc.

Electric propulsion seems to be more efficient, since electric and
magnetic fields are specifically accelerating the propellant molecules
in the desired direction of thrust. But the plasma propellant far less
dense than regular combustion exhaust gas, and the mass flow is lower,
so the overall thrust is too low for earth-to-orbit acceleration.

Laser-wakefield acceleration should then be even more fuel-efficient
than electric thrust, relying more on high exhaust velocity than
propellant mass.
But would the much higher exhaust velocity adequately compensate for
the low plasma density to result in adequately high mass flow and
thrust for earth-to-orbit?

The laser pulses used for laser-wakefield acceleration are intense but
brief and also narrow. This means that the volume of plasma being
accelerated is also commensurately small, as compared to the far
larger broader field of a regular electric thruster.

And yet the follow-on effects from each brief laserpulse are supposed
be longer-lived. That's probably because it takes time for a magnetic
field to collapse and also for plasma to recombine. So you are
benefitting from the lagtime, because during that post-pulse lagtime
your laser is off, but your plasma hasn't caught up yet so it's still
feeling it.

The laser wakefield is compared to a ship passing through water,
creating a wake which others can surf on. But what if your ocean was
made of something more viscous than water -- you could create more
wake, or shear. Likewise, perhaps you'd want to select a gas that
would create a larger wake for each pulse to move as many molecules
along as possible (ie. more shear).

Then the next thing you need is to have lots of laser pulses per unit
time and per unit cross-sectional area. So your laser-wakefield
thruster would be an array of laser emitters, with a distribution
density such that all the plasma wakes they produce would barely
overlap, resulting in an efficient arrangement of accelerative wakes
to optimize your mass flow rate(aka "thrust").

If a CCD array can be made sufficiently dense to take in light at high
resolution, then why can a laser emission array be made to emit
coherent light at an appropriate resolution? Can't LEDs be used for
picosecond or even femtosecond lasers?

Comments?

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