This really got the programme managers at the Energy Research and Development Administration (ERDA) interested. Here was something that could fill the role of a serious contender to the tokamak. It was little more than a year since the Middle East oil embargo had sent fuel prices through the roof and politicians were searching obsessively for anything that could become an alternative energy source – alternative to importing oil from the Arabian peninsula. Livermore hoped to jump on that gravy train and quickly drew up plans for a large-scale version of 2XIIB, to be called the Mirror Fusion Test Facility or MFTF, and the ERDA agreed to fund its construction.
While 2XIIB was undoubtedly a success, its end-magnets were still leaky and so many had doubts that MFTF would really make the grade as a power-generating reactor. Even using the most optimistic projections, a full-scale MFTF could only achieve a very modest gain – it would produce slightly more power than was used to keep it running. Hence there was considerable pressure on Livermore, during the design of MFTF, to come up with some technique to stop the leaks and improve the gain.
A solution was found in 1976, simultaneously with researchers in the Soviet Union, with a system that came to be known as a tandem mirror. In such a machine, the single magnets at each end are replaced by a pair of magnets separated by a short straight section. The two magnets and the plasma confined between them form a ‘mini-mirror’ system and this proved to be a more effective plug than a single mirror alone. To test the idea, Livermore persuaded ERDA to fund the construction of another experiment, smaller than MFTF, called the Tandem Mirror Experiment. They built TMX and sure enough they showed that the double-magnet end plugs reduced leakage. The only trouble was that now the Livermore researchers had to redesign MFTF. They chose to shorten the existing MFTF and make it into one of the end plugs, so they now needed another whole MFTF as the second end plug and a new central section. The new design, dubbed MFTF-B, was significantly larger and more expensive than the original one but, having got this far, ERDA agreed to press ahead.
While tandem mirrors reduced leakage, there was still room for improvement. In 1980, researchers at Livermore came up with another idea: put a third magnet at each end of the machine and this would produce a double plasma plug at both ends to further block escaping plasma. To test the idea they upgraded the TMX machine with the extra magnets and it did produce a more effective plug. That in turn led to another redesign of MFTF-B to add extra magnets.
By now MFTF-B had turned into a monster of a machine. The plasma tube and all the magnets at each end were enclosed in a stainless steel vacuum vessel that was 10m in diameter and 54m long. You could easily drive a double-decker bus down the middle of it. When it started operating it would need a staff of 150 to tend to it and was expected to consume $1 million of electricity every month. The elaborate end plugs were far from the simple mechanisms that had attracted many people to mirror machines in the first place. Some researchers joked that, like a tree’s rings, you could tell how old a mirror machine was by how many magnets had been added to the ends.
In all it took nine years to build MFTF-B at a cost of $372 million. On 21st February, 1986 staff and guests gathered for the official dedication ceremony. The Secretary of Energy, John Herrington, had travelled over from Washington along with other DoE staff and he commended the Livermore team for their work. But it wasn’t the joyous occasion everyone had been expecting.
The political climate in the mid 1980s was very different from a decade earlier. Ronald Reagan had come into the White House in 1981 and had aggressively cut public spending. The high oil prices and frantic search for alternative energy sources of the 1970s were now just a memory. To the Reagan-era DoE, funding a second type of fusion reactor just to provide competition for tokamaks was an expensive luxury. So the day after congratulating Livermore on its achievement, DoE shut the doors on MFTF-B without ever having turned it on. A few years later it was dismantled for scrap and to this day the scientists, engineers and technicians who spent years working on the machine do not know if it would have worked.
What is the moral of this story? Fusion energy isn’t inevitable. No fusion machine, no matter how much has been spent on it, is safe from the budget axe. The giant machines of today – the newly completed National Ignition Facility and the partially build ITER – could suffer a similar fate to MFTF-B. The search for fusion energy is expensive and it will only continue if politicians and the public want it and need it.
Twenty-three years later on 31st March, 2009 Livermore held another dedication ceremony, this time for the National Ignition Facility. NIF wasn’t about to be shut down but it was under enormous pressure to perform. NIF’s funders at the National Nuclear Security Administration (NNSA), a part of the Department of Energy, wanted payback for the huge cost of the machine, and wanted it soon. A few years earlier, while NIF was still being constructed, NNSA officials and senior researchers drew up a plan to get to ignition on NIF as quickly as possible, so as to provide a springboard for all the things they planned to do with the facility: weapons research, basic science and, of course, fusion energy.
Called the National Ignition Campaign (NIC), it began in 2006 and included designing the targets for NIF shots and simulating what was likely to happen to those targets. Other labs were also involved in the NIC, including Rochester University’s Laboratory of Laser Energetics and its Omega laser as well as the Z Machine at Sandia National Laboratory which studies inertial confinement fusion with very high current pulses rather than lasers. These facilities were able to try out at lower energy some of the things that would eventually be done at NIF.
By the time of NIF’s inauguration the NIC was three years old and researchers were confident that ignition was within their reach. There was a lot of calibrating and commissioning to do on NIF so it would be well into 2010 before researchers could do shots on targets filled with deuterium-tritium fuel that would be capable of ignition, but the NIF team said they may well reach their goal before the end of that year.
NIF is a truly astounding machine. Its size alone takes your breath away: the building that contains it is the size of a football stadium and ten stories high. Inside is a laser so big and so powerful it would make a James Bond villain weep. Among all the brushed metal, white-painted steel superstructure and tidily bundled cables there is the hum of quiet efficiency. The place has a feeling of a huge power, ready to be unleashed.
The heart of the machine is a small unassuming optical fibre laser that produces an unremarkable infrared beam with an energy measured in billionths of a joule. This beam is split into forty-eight smaller beams and each is passed through a separate preamplifier in the shape of a rod of neodymium-doped glass. Just before the beam pulse arrives, the preamplifiers are pumped full of energy by xenon flashlamps and this energy is then dumped into the beam as it passes through. After four passes through the preamplifiers the energy of the forty-eight beams has been boosted ten billion times to around 6 joules. The beams are then each split into four beamlets – giving a total of 192 – and passed through to NIF’s main amplifiers. These are what takes up most of the space in the facility’s cavernous hall, being made up of 3,072 slabs of neodymium glass (each nearly a metre long and weighing 42 kilograms) pumped by a total of 7,680 flashlamps. After a few passes through the amplifiers the beams, which now have a total energy of 6 megajoules, head towards the switchyard.
The switchyard is a structure of steel beams which supports ducts and turning mirrors to direct the beams all around the spherical target chamber so that they all approach from different directions. The chamber itself is 10m in diameter and made from 10cm-thick aluminium with an extra 30cm jacket of concrete on the outside to absorb neutrons from the fusion reactions. This protective sphere is punctured by dozens of holes: square ones for the laser beams and round ones to act as viewing ports for the numerous diagnostic instruments that will study the fusion reactions. Before the beams enter the chamber they must pass through one final but crucial set of optics. These
are the KDP crystals which convert the infrared light produced by the Nd:glass lasers, with a wavelength of 1,053 nanometres, first to green light (527 nm) and then to ultraviolet light (351 nm) because this shorter wavelength is more efficient at imploding fusion targets.
Finally the beams, which for most of their journey have filled ducts 40cm across, are focused down to a point in the middle of the target chamber where they must pass through a pair of holes in the hohlraum each 3mm across. For all its size and brute force, the laser’s end result must be needle-fine and extremely precise. The hohlraum is held in the dead centre of the target chamber by a 7m-long mechanical arm. This positioner must hold the target absolutely steady in exactly the right spot with an accuracy of less than the thickness of a piece of paper. The arm also contains a cooling system to chill the target down to -255°C so that the deuterium-tritium fuel freezes onto the inside wall of the capsule.
Preparing for a shot: Inside NIF’s reaction chamber showing part of the positioner arm and, at its tip, a hohlraum.
(Courtesy of Lawrence Livermore National Laboratory)
An NIF shot goes like this: the original fibre laser creates a short laser pulse, around 20 billionths of a second long, which then travels through the preamplifiers, amplifiers and final optics before it enters the target chamber as 192 beams of ultraviolet light with a total energy of 1.8 megajoules. This is roughly equivalent to the kinetic energy of a 2-tonne truck travelling at 160 kilometres/hour (100 miles/hour) but because the pulse is only a few billionths of a second long its power is huge, roughly 500 trillion watts which is 1,000 times the power consumption of the entire United States at any particular moment. With that sort of power converging on the hohlraum, things start happening very fast. The 192 beams are directed into the hohlraum through holes in the top and bottom and onto the inside walls. The walls are instantly heated to such a high temperature that they emit a pulse of x-rays. The hohlraum’s interior suddenly becomes a superhot oven with a temperature of, say, 4 million °C and x-rays flying all over the place. The plastic wall of the capsule starts to vaporise and flies off at high speed. This ejection of material acts like a rocket, driving the rest of the plastic and the fusion fuel inwards towards the centre of the capsule.
If the NIF scientists have got everything right, then this inward drive will be completely symmetrical and the deuterium-tritium fuel will be crushed into a tiny blob around 30 thousandths of a millimetre across and with a density 100 times that of lead. The blob’s core temperature will be more than 100 million °C but even this isn’t quite enough to start fusion. The laser pulse has a final trick up its sleeve to provide the spark. If the timing is right then a converging spherical shockwave from the original laser pulse should arrive at the blob’s central hot spot just as it reaches maximum compression. This shock gives the hot spot a final kick to start nuclei fusing. Once the reactions start, the high-energy alpha-particles produced by each fusion heat up the slightly cooler fuel around the hot spot. That leads to more fusions, more alpha particles, the reaction gains its own momentum and –BOOM! – fusion history is made. A faultless shot might produce 18 megajoules of energy, ten times that of the incoming laser beams.
That sequence of events was the goal when Livermore researchers began their NIC experiments in 2010. The first unknown was whether the laser was up to the job. Critics of NIF had warned that laser technology wasn’t ready for such a high-energy machine. They foretold that the amount of power moving through the optics would cause them to overheat and crack; that specks of dust on glass surfaces would heat up and damage them; and that flashlamps would continually blow out and need to be replaced. During NIF’s construction there were problems with exploding capacitor banks and flashlamps, and the whole system for preparing and handling the optical glass had to be redesigned to keep it as clean as a semiconductor production plant. NIF’s designers did their work well. When they finally turned it on and ramped up the power over the first couple of years, the laser didn’t tear itself to pieces. The occasional lamp did blow and some optical surfaces got damaged, but NIF staff had worked out ways to either repair surfaces or to block out a damaged section so that the laser could keep running.
The Livermore researchers knew from earlier machines that getting the laser to work was just the start: numerous hurdles still lay ahead. The first of these was the chaotic environment inside the hohlraum once the laser pulse starts. While the high-energy beams are heating the inside walls they kick up lots of gold atoms that form a plasma inside the hohlraum. If it’s not carefully controlled this plasma can cause havoc, sapping the energy of the incoming beams, diverting them from their desired paths and even reflecting some of the beam back out of the hole in the hohlraum. Such interactions had limited the achievements of earlier laser fusion machines and the NIF team studied the problem carefully and carried out extensive simulations. In the early experiments of the NIC the team mostly managed to keep these plasma interactions under control, largely by avoiding situations that were known to aggravate them.
Another potentially difficult area was the implosion of the capsule. The implosion is an inherently unstable situation because it involves a dense material – the plastic shell – pushing on a less dense one – the fusion fuel, and Rayleigh-Taylor instabilities can lead to fuel trying to burst out of its confinement. Researchers’ first weapon against this is symmetry, hence the careful placement of beams around the hohlraum interior to ensure that the x-rays heat the capsule evenly. Their second weapon is speed: if they can make the implosion sufficiently fast, the plastic and fuel won’t have time to bulge out of shape.
The experimenters use a measure called the experimental ignition threshold factor (ITFX) to chart their progress. The ITFX is defined so that an ignited plasma has an ITFX of 1. For the first year of the NIC, the value of ITFX demonstrated the advances they made. When ignition experiments started the shots achieved an ITFX value of 0.001. A year later it had reached 0.1 – a hundred-fold increase – but there it stalled. The second year of the NIC was plagued by phenomena that the NIF team was unable to explain. Although the target chamber was bristling with diagnostic instruments – nearly sixty in total – to probe what was going on inside, measuring x-rays, neutrons, and even taking time-lapse movies of the implosions, the researchers could not figure out why the capsules were not behaving as the computer simulations said they would. For reasons unknown, a significant portion of the laser beam’s energy was getting diverted from its intended purpose of driving the implosion of the capsule. The capsule shell was also being preheated before the implosion started – perhaps by the stray laser energy – which made it less dense and less efficient at compressing the fuel. The implosion velocity was also too slow. At a fusion conference in September 2011, the DoE’s Under Secretary for Science, Steven Koonin, who oversaw NIF, said that ‘ignition is proving more elusive than hoped.’ He also said that ‘some science discovery may be required,’ which is a polite way of saying ‘we don’t know what’s going on.’
Koonin set up a panel of independent fusion experts to give him regular reports on NIF’s progress. The panel was critical of the schedule-driven approach of the NIC, which specified what shots had to be carried out and when and, if something unexpected arose, didn’t allow any time to explore what was going wrong. In a report from mid 2012 the panel pointed out that Livermore’s simulations of NIF predicted that the shots they were then carrying out should be achieving ignition, but the measured ITFX values showed they were still a long way off. What sort of a guide to progress were the simulations if their predictions were so wide of the mark? As before in laser fusion, it was simulations that led researchers to have inflated expectations.
It had been stipulated when the NIC was devised that if ignition is not achieve by the end of two years of experiments at NIF then the NNSA had sixty days to report to Congress on why it had failed, what could be done to salvage the situation, and what impact this will have on stockpile stewardship. That deadline passed on 30t
h September, 2012 and on 7th December the NNSA submitted its report to Congress. The report admitted that Livermore researchers did not know why the implosions were not behaving as predicted and even conceded that it was too early to say whether or not ignition could ever be achieved with NIF. The NNSA asked for NIF’s funding – running at roughly $450 million per year – to be continued for a further three years so that researchers could investigate why there was a divergence between simulations and measured performance. Significantly, the report also called for parallel research to be carried out on other approaches to ignition as a backup in case NIF failed. These alternatives included pulsed-power fusion at Sandia’s Z Machine, direct-drive laser fusion using Rochester’s Omega and even direct drive on NIF.
At the time of writing, it was not known how Congress would react to this proposal although President Barack Obama’s proposed budget for 2014 suggests cutting NIF funding by 20%. Also, some members of Congress have campaigned for years for NIF’s closure and this admission of weakness could only help their cause. Whatever happens, progress towards ignition looks set to slow because, while the NIC used around 80% of the shots on NIF, from the beginning of 2013 weapons scientists would be getting a bigger share, more than 50%. Many laser fusion experts still believed that NIF can get to ignition, but the question is: when?
Meanwhile, in France, the ITER project was just starting to get moving. Following the ceremony in Paris to sign the international agreement in November 2006 it took almost a year for each partner to ratify the treaty and only then could they officially create the ITER Organisation. But that didn’t hold up excavation of the site. Machinery began clearing trees from land near Saint Paul lez Durance in January 2007. Some rare plants and animals were moved elsewhere; remains of an eighteenth-century glass factory and some fifth-century tombs were preserved. Heavy earth-moving machines arrived in March 2008 and set about carving away at the side of a hill to lower the ground level then shifting the earth downhill to build it up into a level surface. Altogether the diggers shifted 2.5 million cubic metres of material to create a platform of 42 hectares, the area of sixty football pitches. And then everything came to a halt as the new team in the temporary office buildings nearby struggled to get to grips with the machine they had to build.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 26