Saturday, February 15, 2014

Laser Fusion Breaktrhough at LLNL?

Fusion Breakthrough at LLNL?
Charles Jordan

After hearing a talk by the head of Livermore Edward Moses last year, I retain my long term negative feeling about the National Ignition Facility approach to energy generation through fusion, but am encouraged that the problem is just a very difficult one, rather that an impossible one.  The achievement of scientific break-even is a breakthrough in small letters. Other methods tend to have closer contact between scientific and engineering break-even.  In laser fusion, the difference is huge.   Most methods have been fusing tritium and deuterium into helium and neutrons  for a long time.  

One of the latest approaches was to reduce the time of the implosion.  The wavelength of the high power CO2 lasers at LLNL is 1.06 micrometers.  That gives time for the target to react to the pressure from the light and resist it, boiling off electrons, developing hydrodynamic instabilities, etc. So they tripled the laser frequency (divide the wavelength by 3) to make the impact shorter in time.  Now they have managed to compress the gas inside the micro-balloon of glass containing the deuterium and tritium without breaking it and have been fortunate that the alpha particles which are produced (deuterium + tritium gives alpha + neutron) feed back their kinetic energy into the reaction to a certain extent.  More of that is needed.

Not bad, but the gorilla in the room is this.  The laser system which does this can be fired once or twice a day.  In order to achieve engineering break even, it needs to fire once or twice a second.  And there is no method that I know to cool down the 192 glass-amplified beams in a building bigger that a football field at a rate that keep the temperature constant at a value which doesn't crack the laser glass. Note:  A small defect in the glass or a variation in the intensity across the face of the beam can generate a nonlinear amplification in the glass which will destroy the laser after one shot.

At Ed Moses’ talk, I was waiting to throw as much cold water on his project as I could.  Some years ago, I had been in the control room of the laser in preparation for an experimental test, but I had to step outside just as the shot happened because the real source of revenue and the reason the lab still exists is that the Defense Dept. which wants to maximize yields on hydrogen bombs without dropping one.  This is the only way they can test various ideas with the moratorium on atomic testing.  The National Security guys let me back in as soon as the shot was over.

But in his talk, Mr. Moses surprised me.  He pulled out a model of a laser which can generate 15  kilowatts of energy continuously.  I got to hold it and look at the details (just a model).  It is about 2.5 inches long by 1/2 in square.  My mouth fell open and I asked him some questions about whether this thing actually worked and, though he was positive about it,  he wasn't bullish about something which would change the whole ball game.  You would have to use many of them, but they are small.  The whole apparatus becomes drastically smaller, there is no glass to cool, the micro-balloons can be dropped into the laser focus easily one or two per second.  Of course there is a massive engineering problem of getting the energy of the neutron, which carries away most of the energy of the fusion, transformed into heat and removed from the implosion chamber.  Absorb the neutron's kinetic energy in a molten salt like sodium or potassium and put the liquid out to heat exchangers?  Are there material problems and residual radiation levels due to the large amount of energy generated  in a small space making frequency of repair a major impediment to using this fusion method?
Then I asked another friend of mine at the talk, Richard Muller, a Berkeley physics professor who has been in the news lately testifying in Congress about climate change, whether the laser was a real option and he said the laser had a lot of problems.  He should know since LBL and LLNL physics people get together a lot.
If such a laser were developed, that would be a breakthrough in capital letters.
200 such lasers could provide 3 MJoules per second of input while the shot just reported was 1.8 MJ per 4 hours. 
Breakeven would be the production of 8x10^17 14-MeV neutrons.  That's a gain of 1 which is of no use. Obtaining gain of 10 is non-trivial and requires many more of those tricks I mentioned before, but if it were possible, the glass laser output would average 1.25 kilowatts.  With the continuous lasers ( say two hundred which would be about the same number of beams as the present glass laser), the rate could approach 30 MWatts.  But the machine is smaller and 33 modules together would yield a Gigawatt before converting that heat into electricity, which is what you need for a large power generation plant.  The smaller size of 30 MWatts is about the size of General Electric gas fired turbine generators and could be distributed around with the advantage of low fuel cost.  The complication of radiation, complexity, and the cost of the input electrical energy from the cooling towers are daunting.

Here's hoping, but in my opinion, cold fusion is just as likely to actually bring fusion energy into the picture.

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