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  (Source: 20th Century Fox)
The energy released by the pellet of fuel exceeded the absorbed energy, but was less than the total energy used

The pellet imploded, producing more energy than it absorbed.  As the reaction died down, the physicists sat back to assess their work.  In layman's terms, the Lawrence Livermore National Laboratory (LLNL) researchers' September fusion effort could be summed up with a simple phrase -- close, but no cigar. But celebratory smoke or not, the latest test at the National Ignition Facility (NIF) is a promising sign that mankind is growing closer to harnessing the power source of countless solar systems across the universe -- fusion.

I. Cheaper Than ITER, but Mired in Controversy, NIF Soldiers On

Located in sunny central California on the grounds of the University of California, Berkley (UC Berkley) -- the LLNL's NIF seemed an ideal place for fusion experimentation to occur.  UC Berkley faculty, alumni, and researchers have won 72 Nobel Prizes (including 28 alumni Nobel laureates).  Five of these Nobel Prizes were in physics and belong to deceased faculty.  But UC Berkley also boast three active Noble Prize winners.

But for all the brainpower it's been a rocky road for the NIF.

Construction began in 1997 on the fusion apparatus, which uses the indirect drive methodology, in which the fusion fuel is heated not directly (as in magnetically contained plasma fusion devices), but indirectly via a secondary material.

NIF Laser pre-amps
Pre-amplifiers are pictured pumping up the power to the LLNL's record-setting laser.  The laser's power system is only 1 percent efficient from plug-to-beam. [Image Source: LLNL]

The NIF uses laser-confinement and is competing with the International Thermonuclear Experimental Reactor (ITER) to become the first energy positive fusion device.  ITER is a $17.5B USD international research reactor being constructed in Cadarache, France by an international team from the United States, China, the European Union, India, Japan, the Republic of Korea and the Russian Federation.  It uses a more traditional magnetic confinement scheme -- but it is also substantially more costly than the NIF.

When compared to ITER, the NIF's energy source and confinement methodology is far more exotic, employing 192 parallel laser beams from flashlamp-pumped, neodymium-doped phosphate glass lasers.  The device would focus these beams within its hohlraum -- German for "hollow room" -- a small metal vessel that absorbs and transfers heat to the fusion fuel pellet.  Inside the vessel is a small, spherical fusion pellet about 2 mm in diameter.  The pellet is coated in deuterium and tritium (so-called DT fuel), fusible hydrogen isotopes.  A small amount of DT gas is also inside the pellet's interior.  The pellet is first chilled to ~18 K (-255 ºC).

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

When the lasers hit the vessel, the vessel absorbs them and transmits X-Rays to the pellet, which in turn heats up becoming an explosive plasma.  Pressures of over 100 billion times Earth's atmosphere (~100,000 Megabar) and 3.3 million Kelvin -- roughly nearly 300,000 times the mean surface temperature of the Earth -- are reach as the fuel pellet implodes, creating a shockwave that further compresses the DT fuel, triggering fusion, a process in which alpha particles (2 proton + 2 neutrons with no electrons) are violent created, ejecting neutrons, which trigger the fusion of more hydrogen nuclei into alpha particles.

NIF Hohlraum
The hohlraum, in finished form [Image Source: LLNL]
 
The chamber is designed to measure and contain up to 45 MegaJoules (MJ) worth of energy produced by the ensuing alpha-particle drive fusion chain reaction -- or roughly the energy released by 11 kilograms of TNT exploding.  If the design proved successful, it could be expanded to employ more efficient direct drive reactions and higher energy limits; plus the harvested heat energy from the vessel could be used to produce electricity.

But construction of the facility saw a number of setbacks, taking until 2009 to complete.  At that point the facility was five years behind schedule and the budget had soared from $1B USD to $4.4B USD.

NIF lasers
The NIF was finished 5 years late and way over budget. [Image Source: LLNL]
LLNL researchers promised big results -- saying they should be able to achieve ignition by the end of September 2012, with the help of a 500-terawatt (TW) laser pulse.

But September came and went with no fusion, leading members of Congress to call on the NIF to be shut down.  IEEE Spectrum editor Bill Sweet, a veteran of India's nuclear power development project, blasted the effort arguing that most physicists view laser-contained (aka. "inertial confinement") fusion ignition as a pipe dream.  He argues that most agree that magnetic confinement fusion is far more likely to be realized, though still a difficult problem.  He titled a recent piece on the NIF "The Mother of All Boondoggles?" and in it, he infers that it is.
 

II. Energy Neutral -- at a Pellet Level, at Least

Now LLNL has issued a release saying it has worked out some of the early hiccups in the implosion process and achieved a major milestone.

Earlier fuel implosions were patterned "like a porcupine", according to researchers, so they tweaked the shape of the ultraviolet lasers and cut the durations of their pulse from 15 nanoseconds to 10 nanoseconds.  By doing this the team managed to greatly improve the energy output of the fusion event.
 
NIF building
The NIF is housed in a building the size of 3 football fields. [Image Source: LLNL]

A late August implosion produced a neutron yield of nearly 3e15, or approximately 8,000 joules of neutron energy.  NIF Associate Director Ed Moses comments, "The yield was significantly greater than the energy deposited in the hot spot by the implosion.  This represents an important advance in establishing a self-sustaining burning target, the next critical step on the path to fusion ignition on NIF."

Of course 8 kJ pales in comparison to the 1.7 MJ (1.8e6 J) of laser power used to trigger the fusion.  Even worse, that shot requires up to 3.0+ MJ at the infrared level which drives the UV lasers, and up to 422 MJ at the capacitor level, which pumps the lasers.  From the laser output, only roughly 1 percent of the energy from the capacity pumping stage is used.

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]

Future commercial fusion setups could use newer diode pumped lasers to achieve efficiencies as high as 16-18 percent (using the current state of the art components).  That could cut the required output of the fusion reaction by more than an order of magnitude.

Second as little as 15 percent of the energy is absorbed by the hohlraum, and as little as 15 percent is retransmitted as X-rays to the target pellet.  By switching to a so-called polar direct-drive (PDD) scheme (where the lasers directly sweep over the target pellet), as much as 1/(.15^2) = 44+ times theoretical gain could be observed.

NIF Hohlraum NIF pellet
One potential optimization is to ditch the hohlraum (left) and directly drive the fusion even by targeting the pellet (right) with laser sweeps. [Image Source: LLNL]
 

Cumatively these improvements have the potential to cut the total energy used to around 1 MJ from plug to pellet.  That still would require a dramatic improvement in the reaction performance -- more than two orders of magnitude -- to break even on a plant basis.  But despite its shortcomings and setbacks, the NIF continues to defy doubters and shave off these orders of magnitude.

Papers on the new work have been published in the AIP peer-reviewed journal Physics of Plasmas and the Journal of Plasma Physics and Controlled Fusion.

Sources: Physics of Plasmas Journal, Journal of Plasma Physics and Controlled Fusion, LLNL, BBC News





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