The Copernican

We’re one step closer to nuclear fusion but still miles away

The Sun is powered by a massive nuclear fusion reaction that's been churning out energy for almost 5 billion years.  

My favourite source of limitless energy lies in fiction, in Arthur Clarke’s The Songs of Distant Earth to be exact. In the book, Clarke describes a spaceship called ‘Magellan’ powered by zero-point energy, where energy is pulled out of nothing (or out of other dimensions – but since those dimensions are otherwise inaccessible, their existence would mean nothing to our dimension).

Clarke wasn’t entirely wrong – as usual – with his vision: using zero-point energy, or >vacuum energy, is a scientifically viable possibility, albeit not in the way he’d imagine it. It requires tremendous advancements in technology to achieve. Perhaps he knew that, too: the novel is set in the late 40th century, a time by which humankind is likely to have at least fully understood how to produce more energy than is consumed in producing it.

In 2014, the only Earth-bound candidate (apart from >Frank Wilczek’s time crystals) in a position to lay claim to this honour is nuclear fusion. This is a phenomenon already at work in the hearts of stars, but in laboratories on Earth, scientists are still grappling with getting minute details right so they can achieve sustainable nuclear fusion.

Blowing the fuse

On February 12, a team from the $1.2-billion National Ignition Facility (NIF), California, announced that they’d breached the first step: producing a fusion reaction that released more energy than it consumed, over experiments in September and November, 2013. Viewed against a historical backdrop that started in the early 1980s, this is a remarkable achievement. Viewed against a futuristic ‘frontdrop’, it pales in comparison to what should come next.

At NIF, scientists practice one of two known techniques to achieve nuclear fusion, at least if simulations based on theoretical models are to be believed: inertial containment. >The principle is simple. Atoms of hydrogen are heavily compressed inside a very small capsule until they fuse together to form atoms of helium, releasing large amounts of energy. This is how a nuclear fusion reaction is triggered.

But in order to make it practicable, scientists have to make this reaction continue and sustain it. To get there, the reaction has to be controlled in such a way that more atoms of hydrogen and helium use some of the heat produced to compress themselves further, producing another fusion reaction, and so on. Beyond this stumbling block, needless to say, is the panacea to most energy problems conceivable by humankind.

Not exactly the reaction we’re looking for

Before we start speculating, however, it’s important to get some things right about the NIF achievement. For starters, they achieved “fusion fuel gains exceeding unity”, according to their paper. If fusion fuel gain is less than unity, then the amount of fuel produced divided by the amount of fuel consumed is a number less than 1. At unity, the value of the fraction is of course 1. Exceeding unity, therefore, means more fuel was produced than was consumed. The operative clause here is ‘fuel consumed’.

The folks at NIF used strong lasers pulsing for a few nanoseconds to deliver trillions of watts of energy to the contents of the capsule. However, not all the energy is consumed by the atoms but only a fraction. And if they have achieved fusion – which they have – it means the amount of energy produced by fusion was greater than the fraction they consumed, not greater than all the energy they were given. According to their >paper published in Nature, the total energy delivered by the lasers was 1.9 megajoules while the reaction produced about 17 kilojoules.

"Only about 0.5 per cent of the laser energy makes it into the DT fuel. Implosions work as pressure amplifiers trading energy away in exchange for higher central pressures." said Dr. Omar Hurricane, the lead author of the published paper, to The Hindu.

Even so, they were missing something here that’d make the process more efficient. This is where the history comes into play.

To get inertial containment right, scientists have to broadly look out for three things. First, the lasers have to be designed to perfectly deliver specific quantities of energy over carefully described intervals. Second, the lasers produce X-rays inside the capsule that then energise the atoms – the X-rays have to be as symmetrical as possible to act evenly. Third, the contents of the capsule have to be as spherically arranged as possible to minimise instability.

The contents being made to implode are a mixture of >deuterium (D) and >tritium (T) – both isotopes of hydrogen. They are coated as a fine patina on the insides of the capsule, which is made of gold. When laser pulses strike the gold, it emits X-rays that then driven the isotopes inward at such energy and speed (almost 1.12 million km/hr) that they are forced to fuse. Because the flux of X-rays can’t be possibly perfectly controlled, the focus was mostly on getting…

The perfect shot into the perfect capsule

Earlier, scientists shot laser pulses at the capsule in two stages, >totalling four shocks. The first stage, called the “foot”, was used to generate a lower X-ray intensity for a prolonged period of time before rapidly ramping up to a higher energy. As announced in another paper on February 5, Dr. Hye-Sook Park and her colleagues at the Lawrence Livermore National Laboratory, where the NIF is housed, changed this.

Instead of a low-foot profile, this team switched to a >three-shock high-foot one: the initial X-ray burst was set to a higher intensity.

This step was motivated by a discrepancy in previous experiments. The scientists’ theoretical calculations and computer simulations showed that the low-foot profile should have initiated nuclear fusion… but experiments disagreed. In search of an explanation, they suspected that the contents of the capsule were being blown apart by the slow X-ray delivery before they could get to the compression stage. The simplest way to forestall this premature detonation was to pack in more heat with the fuel before proceeding to compression – ergo, the high-foot profile.

Dr. Hurricane added that "the instabilities of the imploding capsule have been greatly reduced with the high-foot technique", but they hadn't been fully eliminated either.

On the downside, because a lot of the energy would be delivered first-up, the total possible compression is reduced. On the upside, Dr. Park and her colleagues reported that the number of >alpha particles released in the first fusion reaction was higher than ever before. These particulate clumps are responsible for initiating a chain reaction, and getting more of them initially means the likelihood of sustainable fusion increases.

As Dr. Hurricane told The Hindu, "We have progress towards ignition as indicated by getting a significant contribution to the yield (a near doubling) coming from 'self-heating' where the reaction starts to heat itself, further accelerating the fusion reactions. Without this process, ignition would not occur. Still more work is needed to get closer to an igniting state."

NIF now knows the high-foot profile is the way to go because, with it, theory and experiment agree. And on this path, what the team will need to get right is removing all the tiniest sources of turbulence as much as they can. This was the subject of another paper, >published last week in Physical Review Letters, which reported a 50 per cent increase in yield with an enhanced capsule design against previous designs.

The other fusion projects

As the NIF team moves on, there are other nuclear fusion experiments afoot. The biggest operational one with more funding and international participation – called the >International Thermonuclear Experimental Reactor, is based out of Cadarache, France. Because of its prodigious scale and relatively lesser research cohesion among participants, its reactor is expected to be built only by 2019. Once operational, its magnetic containment ( >tokamak) reactor will make it the largest experimental fusion facility of its kind in the world. The European Union is forking out 45 per cent of its $21.87-billion cost while six other countries, including India and the USA, are footing 9 per cent each.

All things considered, the interest in nuclear fusion is alive and kicking despite progress being made at a necessarily tedious pace and at the cost of billions of dollars. After the laser facility at NIF came online in 2009, it set for itself a deadline of September 2012 by which to achieve the ignition of a fusion reaction – and missed, prompting politicians to deprioritise the project and chop funding by $60 million. In the same year, on the other hand, >Russia and >China announced plans for two ‘superlaser’ facilities to replicate inertial fusion.

Once any of them achieves a sustainable fusion reaction, countries will quickly start designing power plants. Already, engineers at the Naval Research Laboratory, USA, are drawing up plans >for a Fusion Test Facility that will let them experiment with nuclear fusion with an aim to generate electric power. Let’s then give ourselves till the end of this century, eh, to earn the license to dream of Clarke’s ingenuity as the stuff of reality?

( Special thanks to my friend Sreeyuth Lal)

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Printable version | Nov 21, 2020 2:21:37 PM |

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