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NIF achieves milestone: closer to alpha burning but far from net energy gain

Update per 17 Feb, 2014: Nature has released the final, accompanying publication from NIF along the lines we projected in this article in Oct 2013.

BBC reports milestone of NIF; others follow

The National Ignition Facilty (NIF) at Lawrence Livermore National Laboratory in California is a large, inertial confinement device that uses the world's highest energy laser system to compress small capsules of hydrogen isotope fuel to extremely high temperature and pressure. Like ITER, Wendelstein 7-X and other experimental devices, NIF has the aim to achieve fusion ignition with high energy gain.

On October 7, 2013, BBC news was first to release a news article that suggested a major breakthrough towards this goal has been achieved ("Nuclear Fusion Milestone passed"). Other media rushed to pick up the headlines and report this breakthrough in fusion; and in numerous cases, they misinterpreted the actual milestone as having produced net energy.

The tiny D-T capsule inside the target chamber of NIF  [Source: NIF website]

The actual milestone: not one, but two!

The BBC story reported "to understand that during an experiment in late September 2013, the amount of energy released through the fusion reaction exceeded the amount of energy being absorbed by the fuel - the first time this had been achieved". The key here is in the words "absorbed by the fuel", as only a small fraction of the total energy input of NIF is eventually absorbed by the plasma. According to an official press release by NIF, the experiment "yield was significantly greater than the energy deposited in the hot spot by the implosion".

Then it seems the BBC made a mistake here in saying this was the first time it had been done, as this record ratio has in fact already been achieved at an experiment on August 13 (see the date of the above linked press release), where the UV-energy absorbed by the fuel was ~5 kJ and the released fusion energy ~8 kJ. This is one milestone that has been achieved.

The second milestone in fact concerns a record neutron production, attributed in a NIF memo (shot was dated Sept. 28 and the memo was dated Sept. 29, 2013) to be made during an experiment in late September. From Sciencemag we learned that this 28 September shot actually produced 5x1015 neutrons, 75% more than any previous shot. In an email from NIF we learned that this shot absorbed 12 kilojoules of energy and released 14 kilojoules of energy. NIF reports the success was mainly achieved by altering the shape of the laser pulse to make it deliver more power at the beginning.

Now this is an important milestone, and gets us a step closer to reaching a burning plasma (that is self-heated by alpha particles). To reach that goal, NIF reports that the fusion yield needs to get a factor 4-5 higher still. But the overall energy balance of NIF is still low (see table below) and it can be debated how far it is from reaching ignition.

The definition of ignition

Ignition of NIF could be considered as the ratio of fusion energy released to laser energy brought into in the hohlraum (a cylinder that contains the capsule). The next question that arises then is what exactly is the laser energy brought into in the hohlraum: is it the laser input, is it the amount of laser energy absorbed by the hohlraum (which is less than 85% of the total laser output)?

The absorbed heat of the laser then causes the hohlraum to re-emit the energy as intense X-rays (with an efficiency around 15% or less), which on its run are absorbed by the solid deuterium-tritium target. So in a another definition of ignition, the amount of energy of the X-rays absorbed to implode the capsule is compared to the released fusion energy. We believe that this is the measurement that was used for the experiment no Sept 29: the energy yield was more than the X-ray energy absorbed to implode the capsule'.

Approximate energy efficiency of different process steps of NIF:

Input energy of the laser
(xenon lamps are powered by a capacitor bank)

422 MJ

Laser Infrared output
(amplified IR light of the laser)

3.6 MJ
Laser UV output
(about 50% is left after conversion to UV)

1.8 MJ

Laser energy absorbed by the hohlraum
(theoretical prediction: about 85% is left after the X-ray conversion in the hohlraum)
<1.5 MJ
Laser energy absorbed by the outer layers of the DT target pellet
(theoretical prediction: about 15% of the X-rays are absorbed by the outer layers of the target)
<220 kJ

Actual energy absorbed by the DT target pellet
(based on report that more energy for this shot was released than UV-energy that is absorbed in the DT-target).

<14 kJ

Energy out:

Energy released by fusion reactions
(fraction 3.3x10-5 of input energy of the laser)

~14 kJ

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