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  (Source: Fusenet)
The output power has been bumped 10x, gains come thanks to improved alpha particle, neutron yields

It will likely be decades before mankind taps into the sun's power source, using fusion to provide cheap, clean power for mankind's ever evolving ambitious.  While some physicists remain convinced that mankind may not ever be able to harvest reliable power from direct nuclear fusion reactions, U.S. national lab researchers took a step down that path, publish a paper in Nature which detailed "fuel gains" -- small bursts in which power produced eclipsed the power consumed to trigger and drive the reaction.
 
I. Now We're Cooking
 
The research was conducted at Lawrence Livermore National Laboratory's (LLNL) National Ignition Facility (NIF) -- a site that politicians were complaining just two years ago was the "mother of all boondoggles".  But LLNL has since then silenced most of these detractors by doing what all researchers aim to do -- produce results.
 
Late last year it announced that its laser-confined fusion chamber had achieved "self-sustaining burning" for a brief period.  During the test the reactor was roughly energy neutral at a pellet level, albeit at a massive energy deficit when all the support equipment (lasers, etc.) were considered.

NIF Hohlraum
Fusion occurs inside a tiny chamber heated by X-Rays. [Image Source: LLNL]

Refining its technique from a "porcupine" or "low-foot" style laser pulsing technique to a so-called "high-foot" implosion, researchers were able to achieve greater stability over the last several months.

Last year's shots pumped 1.7 MJ (1.8e6 J) of laser energy into the hohlraum -- German for "hollow room" -- a small metal vessel that absorbs and transfers heat to the fusion fuel pellet.

NIF Pellet
A cryogenically chilled hydrogen DT isotope fuel pellet [Image Source: LLNL]


In the latest tests, with the new implosion method researchers were able to pump up the energy to 1.9 MJ, of which about half actually heats the fuel, roasting the 2 mm pellet of deuterium and tritium (fusible hydrogen isotopes, respectively) with an insanely hot X-ray bath.  The amount of energy the tiny pellet endures is comparable to the amount of energy of a one-ton car travelling at 100 miles-per-hour (160 km/h).

NIF pellet
A view shows how the lasers heat the outer gold vessel, frying the pellet towards fusion. [Image Source: LLNL/Nature]

That laser power compresses the hot spot of the pellet under 126-152 Gigabar (trillion bar).  The entire construct is heated to 13-15 MeV (million electron-volts) -- around 3.3 million Kelvin.

That fire is preceded by ice.  The tiny pea-sized pellet was chilled for the "shots" down to 18.3 K (-254.8 ºC).

NIF fuel pellet
The fuel pellet is coated in icy isotopes. [Image Source: APS]

The pulse takes about 20 ns, during which roughly 450 Terawatts of power is transferred to the pellet, or roughly 450 times the power transferred during a lightning strike.  The result is a hot spot, followed by an implosion followed, by the generation of fast alpha particles and neutrons.
 
II. Might as Well be Walking on the Sun
 
In test shots using the new high-foot method, researchers were able to achieve a roughly 1.2-1.4 times gain in the energy produced by the fusion event over the energy that applied to the pellet.  A second shot, at slightly higher power saw 1.8-2.0 times gain.
 
That's 10 times better than the best results to date for deuterium and tritium.


NIF yield
The NIF has been steadily bumping its energy gains. [Image Source: Nature/LLNL]

But don’t get too excited; the experiment did not achieve the ultimate goal of fusion research -- ignition (not that it was supposed to).  Ignition is the point at which a reaction produces more power than it consumes over its lifecycle.  It takes a lot of power to get there. 
 
The capacitors that pump the lasers draw a whopping 422 Megajoules of energy while active.  Less than one percent -- roughly 3 Megajoules -- reaches the infrared stage that directly drives the lasers.  And then roughly another third is lost in transit.

NIF laser positioning
Modern lasers are as much as 15 times more efficient than the capacitor pumped ones used by the NIF. [Image Source: LLNL]


At best the researchers observed around 3.8 MJ -- recovering just under a hundredth of the energy they put into the device, not counting measuring equipment and other indirect power expenses.
 
III. Fast Helium Nuclei Excite, Hint at Future Ignition
 
But as the old saying goes, you have to spend money to make money.
 
While researchers expected the inertial confinement fusion (ICF) to be imperfect, they struggled for much of the last several years with why they were getting results inferior to what their simulations had predicted.  The answer seemed to lie in a flawed firing technique, which gave rise to so-called hydrodynamic instabilities during the implosion, preventing the pellet from reaching its maximum compression.
 
Using the new high-footed firing method researchers appear to have mostly overcome that roadblock.  Their neutron and fast alpha particle radiation has risen substantially, at last approaching the results predicted by theory.  For the first time these particles had sufficient energy to see so-called "bootstrap" effects, enhancing the energy output of the reaction.
Fusion reaction
Bootstrapping is critical to achieving "runaway" long-burn fusion and ignition.

Researchers consider that a crucial step on the road to fusion.  In the researchers journal article abstract they stated optimistically:

We also see a significant contribution to the yield from α-particle self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite.

The results are forcing physicists to take ICF -- long regarded as the inferior counterpart of magnetic confinement fusion (MCF) -- seriously.  Even as work continues on the world's biggest MCF fusion project to date, International Thermonuclear Experimental Reactor (ITER), the NIF is showing it has what it takes to steal the spotlight.

Solar storm
Fusion indirectly provides most of the Earth's chemical stored energy, via the Sun.
[Image Source: NASA]

That's in many regards a vindication of LLNL researcher John Nuckolls, who was first allowed to publicly reveal the compression and heating effects in 1972.  Mr. Nuckolls' theories had been in the crosshairs following earlier tests' inability to produce significant amounts of high-energy atomic particles.  Now some skeptics are becoming more receptive to the idea of ICF as a potential alternative to complement the MCF path.
 
Researchers expect to next cross another critical milestone by pushing the apparatus a bit harder.  Currently the bootstrap heating is narrowly eclipsed by the heating energy from the shot.  If it can surpass that total, the NIF will take another bold step towards making the dream of nuclear fusion power a reality.

Sources: Nature, LLNL, Physics Journal





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