vol 9, issue 6

Fusion energy: promises and problems

16 June 2014
By Rebecca Pool
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NIF’s target chamber

Under inspection: the inside of NIF’s target chamber

NIF’s optics assemblies, mounted on the lower hemisphere of the target chamber

NIF’s optics assemblies contain optics for beam conditioning, colour conversion and colour separation

Prof Steve Cowley

The Culham Centre’s Prof Steve Cowley

Deuterium-tritium fuel pellet

The centimetre-scale hohlraum houses the all-important deuterium-tritium fuel pellet

As physicists edge closer to sustainable fusion, we ask what's next for the industry?

Earlier this year physicists at the US National Ignition Facility achieved a world first for laser-driven fusion by releasing more energy through their nuclear reaction than absorbed by the fuel to start with.

This so-called fuel gain marks a crucial step towards sustainable fusion and has been heralded worldwide as a major breakthrough. But the goal of creating a self-sustaining reaction that produces more energy than it consumes – ignition – is still a long way off.

Ignition has long been considered the holy grail of inertial confinement fusion, that is, fusion driven by lasers. The aim is for a massive laser pulse to confine and compress the fuel – a plasma of deuterium and tritium – fusing the nuclei of these hydrogen isotopes to release far more energy than invested in the fusion fuel in the first place.

The National Ignition Facility (NIF), an inertial confinement fusion facility at Lawrence Livermore National Laboratory in California and home to the world's largest laser system, had been chasing ignition for some five years, without success. Following the facility's completion in 2009, researchers set out to reach this goal within three years. Failure ensued but the US Congress granted more time to explore what was going wrong.

A new fleet of physicists was drafted, a different approach taken, and so fuel gain followed. As physicist and team leader Omar Hurricane says: "There is more work to do and physics problems that need to be addressed... but our team is working to address all the challenges."

Inertial confinement fusion

Put simply, fusion isn't easy. In the case of inertial confinement fusion, the plasma fuel must be confined under pressures of the order of 1011 atm for ignition to take place. Plasma physics theory dictates the researchers will need to boost fusion yields at least 100-fold to reach ignition – their best results still produced less than 1 per cent of the energy of the laser pulse – and the facilities required to achieve this are mindbogglingly complex.

As Hurricane explains, NIF comprises a 192-beam laser system that delivers up to 1.9 MJ of light onto a centimetre-scale target, in this case a gold cylindrical radiation cavity known as a hohlraum, that holds a pellet of fusion fuel.

The laser energy is focused onto the hohlraum which converts this to X-rays, pressurising the outer shell of the capsule. The capsule implodes, pressurising and confining the fuel so the deuterium and tritium fuse, liberating the fusion energy.

To achieve fuel gain, the team altered the timing of NIF's laser pulses. In the past, researchers had applied low-foot laser pulses to the hohlraum, in which the X-ray energy delivered to the fuel pellet remained relatively low before rapidly increasing. This approach led to the plastic shell surrounding the fuel breaking up, mixing with the fuel and limiting output energy.

In a bid to stabilise implosion and prevent pellet-fuel mixing, Hurricane and colleagues adopted a high-foot approach. The lasers are shaped to supply energy early on in compression, allowing heat to be delivered to the fuel before it compresses significantly.

This limits total compression, leading to more stable implosions. As Hurricane says: "Essentially we make the hohlraum hotter earlier on in the implosion, which effectively fluffs up the capsule shell and reducing instabilities."

But this softly, softly approach worries many onlookers. As Dr Mark Hermann from the Pulsed Power Sciences Center, Sandia National Laboratories, wrote in Nature at the time: "How far the high-foot implosions can be pushed remains an open question. The goal of ignition will require a nearly 100-fold increase in fusion yield over these results."

And as Professor Steve Cowley, director of the UK-based Culham Centre for Fusion Energy, says: "With a high-foot set-up you push the [pellet] a little earlier and a little more softly, squeezing it in a softer and less aggressive way. This is good as it doesn't produce as many instabilities, but you'll never get to fusion in this way."

Clearly these concerns raise doubts over whether the next critical milestone of ignition can ever be achieved at NIF; some suggest the high-foot approach should be ditched and NIF's lasers reconfigured, again. And while Hurricane doesn't disagree, he maintains his team will keep trying.

"The high-foot implosion should be viewed as a strategy that evolves... and the research is to find which tactics work and which don't," he says. "So for now, there are many things that are worth trying before we face a reconfiguration of the lasers, albeit more laser energy would be helpful."

Tipping point

Reconfiguration or not, in the latest raft of NIF experiments the researchers triggered all-important alpha-particle self-heating and evidence for the 'bootstrapping' required to accelerate the deuterium-tritium fusion reactions to 'run away' and ignite. Bootstrapping takes place when the alpha particles, or helium nuclei, produced during deuterium-tritium fusion deposit energy back into the fuel, heating it and exponentially increasing the rate of further fusion reactions.

As Hurricane explains, the key to getting alpha particle heating is for the fuel to have a high areal density, a property physicists refer to as rho R.

"In our implosions, a high rho R was achieved by compression, and to get the best compression, we need our implosions to have a high velocity [hundreds of kilometres a second] and to be as spherical as possible," he says. "We also don't want too much entropy in the DT fuel as this makes the fuel more difficult to compress."

With this in mind, Hurricane and colleagues are now focusing on optimising implosion speed, shape and fuel entropy. Hurricane says modest modifications to the existing laser pulse would lower fuel entropy, while new pellet shell materials, such as beryllium, high-density carbon and boron carbide, could increase implosion speeds.

The team is also investigating alternative hohlraum designs to better control compression shape. Trials on depleted uranium hohlraums, rather than gold structures, are underway while others have suggested trying an egg-shaped hohlraum, or even ditching the cavity completely.

Hurricane believes all options are worth investigating, and highlights how NIF's experimental results now correlate with computer simulations, which will help select the best trials in the future.

But as the physicist says: "We can now rely on these tools to inform us which approaches may be more promising. This throws mud on the assertions that the simulations are missing some sort of mystery physics."

The latest NIF experiments have cleared a hurdle on the road to ignition and encouraged fusion scientists around the world. And while ignition is still a long way off, Hurricane is confident his team will see results, saying: "Achieving ignition at NIF will demonstrate what it takes to make inertial confinement to work."

For NIF, the road to fusion probably stops at ignition. The world's largest system of lasers was only ever designed to be an experimental facility to demonstrate the principle of inertial confinement fusion and not to generate energy for any length of time, let alone operate as a power plant.

As Professor Bob Bingham, chair of the Central Laser Facility at UK-based Strathclyde University says: "The laser architecture is wrong. We need a laser system with high repetition rates, a completely new chamber with lithium blankets to breed tritium, heat exchangers and so on. NIF is not an energy machine."

While the fusion lessons learned from this and a handful of other laser-based fusion facilities around the world will feed into a facility more consistent with fusion power generation, this is still decades away.

"Ignition could be [up to] five years away," says Prof Bingham. "But my generation is never going to benefit from [commercial laser-based] fusion, although the generations to come will."

Fusion alternative

Many alternative approaches to achieving ignition have been conceived. Magnetic confinement, in which a strong magnetic field confines the hot plasma, is a hot favourite with the doughnut-shaped 'tokamak' forming the crux of future magnetic confinement fusion reactors.

Today, the Joint European Torus (JET), located at the Culham Centre for Fusion Energy, UK, is the largest and most powerful tokamak in operation, famed for releasing a hefty 16MW of power from a total input power of 24MW in 1997. The tokamak now serves as a blueprint to build the much more powerful France-based International Thermonuclear Experimental Reactor (ITER), which aims to release 500MW of energy for 500 seconds, by the early 2020s.

As Culham's Prof Cowley explains: "The engineering milestone is when the whole plant produces more energy than it consumes. ITER will be the first to do this."

This is still a long way from commercial fusion. The next step is to design a commercial fusion reactor. Prof Cowley and his team have been talking to a number of UK companies about commercial fusion.

"A design centre could be funded at Culham that brings together the academic, public-sector researchers, like ourselves, with industrial partners to design the first commercial reactor," Prof Cowley says. "We're not that far away from being able to produce electricity and once we've produced this, we've then got to grind away at the engineering until its commercially viable."

Plans for a prototype commercial reactor – DEMO (DEMOnstration Power Plant) – to build a bridge towards the first commercial reactor are underway. While ITER's goal is to produce an eight to nine-minute-long, 500MW burst of power, DEMO intends to produce at least four times that.

Building on the expected success of ITER, Prof Cowley reckons DEMO would take a decade or so to build. "So we're talking the 2040s for the first electricity," he says. "When will it make a big impact on the market? Well that will be the next half of the century."

As always with fusion, the timescales are lengthy, but then the rewards remain unrivalled by any other form of energy generation. And despite his clear allegiance with magnetic confinement, Prof Cowley believes both inertial and magnetic versions are necessary for a future with fusion.

"We have waited 60 years to get close to controlled fusion and we are now close in both magnetic and inertial confinement research. We must keep at it," he says.

Likewise, Prof Bingham agrees both inertial and magnetic confinement fusion are crucial. For one, he envisages magnetic confinement providing base-load generation while inertial systems will be switched on and off according to peaks in energy demand.

Crucially, he says the world needs both to maintain healthy competition. "I honestly think fusion is extremely important for mankind," he says. "Populations are increasing and countries are becoming more affluent. Fusion can provide a reliable and secure energy source that's going to continue for many thousands of years." 

Further information

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The detail behind fusion

Fusion isn't easy. The fusion fuel – hydrogen isotopes of deuterium and tritium – is held in a pellet in a centimetre-scale hohlraum. The pellet is a spherical capsule of plastic, some 2mm in diameter, precisely fabricated and shaped – at a cost of $1m – to optimise performance. Deuterium and tritium are added to the pellet as a plasma gas and cryogenically cooled forming a layer on the inside of the sphere, only 70 microns thick.

At NIF, roughly 50 MJ of electricity pumps the lasers to release 1.9 MJ of laser energy that is then focused through a series of amplifying optics to deliver a carefully controlled laser pulse – lasting less than 2 x 10-8s – to the centimetre-scale hohlraum.

The hohlraum converts the laser power into X-rays, of which, a fraction – around one-tenth – are absorbed by the pellet, generating around 100 Mbar of pressure in the outer shell of the pellet, called the ablator.

According to NIF researcher Omar Hurricane, this ablator pressure is delivered as a series of weak shocks, enough to implode the capsule, which then amplifies pressure from 100 Mbar to many hundreds of Gbar; billions of atmospheres.

Pressurising the fusion fuel in this way creates a high density, high temperature – more than 50 million kelvin – hot-spot some 60 microns in diameter, in which the deuterium and tritium fuse and liberate fusion energy.

Crucially, and as Hurricane highlights: "[In recent NIF experiments], this fusion energy liberated by the fuel exceeded the energy delivered to the fuel by implosion."

Breeding tritium

To sustain a fusion reactor, a continual source of the rare hydrogen isotope, tritium, will be necessary. Professor Steve Cowley from Culham is confident this is not a problem.

"We don't expect to run fusion reactors on the existing tritium as there's only about 25kg in the world," he says. "But what we do expect is to breed our own tritium."

Tritium can be produced within a tokamak when neutrons escaping the plasma interact with lithium contained in the tokamak's blanket. This is the module that covers the instrument's large stainless steel vacuum vessel that provides the vacuum for the fusion reaction.

The concept has never yet been trialled and will take place for the first time on ITER; Cowley is confident it will work. "You may only need 100g of tritium to get a fusion reactor started," he says. "Then it will start breeding its own tritium and will be self-sufficient.

"For a fleet of reactors you've got to make sure you have that first 100kg of tritium, but you can, for example, just over-breed in an existing reactor to breed enough for your next set of reactors," he adds.

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