starship-design: Fast Ignition

[wharton@physics.ucla.edu (Ken Wharton)]

Hello all...

Well, while the final-boundary condition idea did get tossed around more 
than I expected, I'll end it with the observation that even if those 
sort of objects DID exist, we wouldn't be able to interact with them 
(expect gravitationally and perhaps on a quantum level) without causing 
all sorts of impossible paradoxes.  They would contain information to 
the future of the universe, and if we were to get that information, we 
could then use it to make a paradox by forcing that information to be 
incorrect.  So, based on this fundamental problem, I'd say that, while 
an interesting concept, it's probably not going to get us anywhere.

Just for completeness, I did a little research and found out that people 
HAVE tossed around the idea of inverse-antimatter galaxies de-evolving 
at the end of the universe.  All speculation, though, seems to assume 
that the antimatter is no longer around, and has converted into matter.  
The way this would (supposedly) happen would be though baryon decay.  
According to all grand unified theories, protons (and neutrons) will 
eventually decay, leaving the universe with no stable matter except 
electrons and positrons.  This is symmetric, so the recollapsing phase 
of the universe would then see electrons un-decaying into anti-protons 
and anti-neutrons.  This would relate the proton decay time to the age 
of the universe.  Also--and more interestingly--it might mean that all 
current experiments to detect proton decay may be off-base; they're 
looking for evidence of positrons that the protons decay into, and as I 
mentioned earlier, cosmological antimatter couldn't annihilate with 
matter because it's constrained to exist at the end of the universe.  So 
looking for antimatter annihilations in giant underground detectors may 
not be the way to measure proton decay after all...

Not to drop one subject without bringing up a new one, though, I just 
got back from the first Fast Ignitor Laser Fusion Workshop.  The Fast 
Ignitor is a laser fusion concept that was declassified about three 
years ago, and is a plausible method of igniting a fusion reaction 
without needing Mega Joules of laser energy.  (On the Mega-Joule scale, 
though, the DOE just authorized construction of the 192-beam National 
Ignition Facility.  Cost: 1.2 billion.)  The traditional scheme requires 
that a fusion-fuel pellet (DT) be uniformly illuminated with laser beams 
(or, indirectly, with laser-produced x-rays) until it compresses to the 
densities where fusion reactions will happen on a fast enough time 
scale.  This time scale is simply the time that the fuel spends at these 
high densities due to its own inertia, so this type of fusion is known 
as ICF; Inertial Confinement Fusion.  (As distinct from MCF, 
magnetically confined fusion, i.e. tokamaks)

The way Fast Ignition would work is this: you would compress the fuel 
with the regular lasers to a much lower density.  You need less energy 
in the main laser beams to do this, and there are many laser facilities 
in the world that are already capable of achieving the required 
densities.  Then you'd bring in a separate short-pulse laser; the 
ignitor beam.  This beam would be focused on the edge of the (semi-
compressed) fusion pellet, and would be much more intense than the 
heater beams.  This does not mean more energy, however; intensity is 
simply energy per time, and the ignitor beam would be a very short 
pulse.  Heater beams need to be on the order of 10 nanoseconds long; the 
fast ignitor will be three orders of magnitude shorter; 10 picoseconds.  
Also, the heater beams are not at best focus; the ignitor beam needs to 
be as small a focus as possible.  Focused down, the ignitor beam would 
need to have an intensity of at least 10e21 Watts per square centimeter.  
At these intensities, the ignitor beam will (hopefully) drill its way 
into the plasma of the fuel pellet, dump its energy into a beam of 
electrons, which would then propagate into the super-dense fuel region 
and ignite a fusion reaction in a small part of the pellet.  The fusion 
burn would then spread to the rest of the pellet, and voila:  fusion 
energy.

So how realistic is this scenario?  The ignitor beam is the hardest 
part.  In a few weeks I'll be working on the first-ever experiments at 
10e21 W/cm^2 intensities, using the Petawatt laser here at Livermore.  
However, it's only going to be 500 Joules in 0.5 picoseconds; the Fast 
Ignitor scenario will need the same intensity for at least twenty times 
longer; 10 kilo Joules in 10 picoseconds.  And 50 kiloJoules would be a 
lot nicer.  Scaling up a laser that's already pushing about five 
different limits isn't going to be easy.  There are a lot of other 
problems as well, most of the big ones revolving around the electron 
beam; you'd need Giga Amps of high-density current to ignite the pellet.  
No one's even sure if the required electron beam parameters are 
theoretically possible, let alone how to make such a beam with a laser.

But Fast Ignition would definitely be worth it. It would relax the 
energy requirements for fusion, and also enhance the fusion gain from 
10-30x incident energy in regular ICF to over 1000x with a Fast Ignitor.  
Reducing laser energy also has the benefit that the lasers can fire more 
often; the National Ignition Facility may achieve ignition (by 2005), 
but it will only be able to ignite 2-3 pellets a day!  Not exactly 
useful for a spaceship engine.  

The other good news is that lots of money is being poured into short-
pulse laser experiments as a result of the fast ignitor concept.  Lots 
of universities have short-pulse lasers, and can now directly contribute 
to fusion research without coming to the big Government laser 
facilities.  So there will be a lot more people working on laser fusion 
for the next decade, and who knows, maybe something will actually come 
of it.

At the very far stretch of the imagination, research into ultra-high 
intensity lasers may discover some completely unexpected physics.  The 
electric field of a focused laser goes as the intensity times the 
wavelength squared, and every time a new record is set (as will happen 
next month on the Petawatt) there's always a remote chance for something 
new.  Some of you have mentioned the possibility of extracting energy 
from vacuum and other optimistic ideas; if such a thing is possible 
maybe it will be huge laser fields that do it.  I'll keep everyone 
posted...

Ken