Friday, July 31, 2015

Fusion Power

Could fusion power be the environmentally clean, renewable and endless energy supply we've been dreaming of? After all, our Sun, a natural fusion reactor, warms our entire planet and makes it habitable. Perhaps fusion power could be our ultimate ticket away from polluting, and finite, fossil fuels. Before I go further, I want to clarify that I mean "hot" fusion. Cold fusion, which you likely heard splashed about in the news a few years ago, has been all but debunked in scientific circles and no peer-reviewed papers on it have been published for over a decade. Wikipedia does a good job of telling the cold fusion saga (click the clink above). The science makes no sense. Here we will explore the science behind fusion power and where the technology currently stands. As a warm-up for this article you might want to try Nuclear Weapons: Understanding Nuclear Binding Energy. There I explain how fusion and fission work. Still, I go through the basic concepts here.

The science of the fusion reaction itself is quite well understood but the technology required for harnessing it as a useable energy source on a large commercial scale faces significant challenges. That said, many private companies are currently developing what could be the first large-scale commercial nuclear fusion reactor, and several competing technologies are being tested.

What's Fusion and How Does It Work?

So far, in the Energy Science series, we've explored the energies released by explosive chemical reactions, nuclear fission reactions and nuclear fusion. The electromagnetic force, which is the fundamental force involved in all chemical reactions, including all the combustion reactions used in fossil fuel energy, is 137 times weaker than the strong force. The strong force, aptly named, is the fundamental force responsible for the binding energy between nucleons inside the nucleus of the atom. This nuclear binding energy is released during both fission and fusion reactions. Fission energy has been harnessed for electrical power for decades. Binding energy released by the fission of heavy unstable atoms such as uranium-235, in the form of thermal energy, heats steam, which turns the turbines in a nuclear power plant.

Energy is Released When Atoms Break or Fuse Depending on the Atom

During nuclear fission, large atoms split into smaller atoms, releasing great amounts of energy, which can be transformed into electrical energy. During nuclear fusion, energy is released when atoms combine or fuse into larger atoms. The elements involved in fusion and the energies released, make fusion an attractive alternative to fission energy. If we look at the various elements, we find that small atoms, with small nuclei, release excess binding energy when they fuse together and large atoms with large nuclei release binding energy when they split apart, or undergo fission.

Atomic Nuclei: Two Climbs and a Peak Of Stability

We can see this trend when we look at the two climbs in the binding energy graph below. As large atoms on the right side split, some atomic mass is lost (called mass defect) and released as energy. Examples are induced fission reactions in nuclear reactors and atomic fission bombs as well as the spontaneous decay of radioactive (unstable and large) atoms into smaller stable atoms.
                                               

As small atoms on the left side of the graph fuse together, there is also a mass deficit and energy is released. The steep climb means that lots of energy is potentially available from fusion.

Iron-56 (Fe-56), is at the top of the graph called the peak of stability. It is the most common isotope of iron, possessing the lowest mass per nucleon of all the elements. The nucleons in iron have lower mass because part of their mass comes from their potential energy and they are in the lowest possible potential energy (most stable) state. However, nickel-62, not shown in the graph, has the highest binding energy per nucleon. Its binding energy is just slightly higher than that of iron-56 but iron-56 has a slightly lower mass nucleus because it is more rich in lower mass protons than in higher mass neutrons. It's a common misconception that iron is the most tightly bound nucleus of all and this graph exacerbates that. This small technicality aside, both elemental isotopes are so stable and so tightly bound that neither will fuse or split into smaller atoms, unless an enormous amount of energy is applied to them.

The elements with the most potential for fusion energy are isotopes of the smallest nucleus, hydrogen: hydrogen-1 (sometimes called protium), deuterium (H-2) and tritium (H-3), shown below left. Protons are red dots and neutrons are black dots. The electron is a blue dot.

Notice the sharp upward spike at helium in the graph above. This means that when smaller nuclei fuse into helium they find a potential energy state that is unusually stable. The helium-4 nucleus balanced by two protons and two neutrons, is an unusually stable arrangement and this means that lots of binding energy is released when smaller, less balanced, nuclei fuse into helium.

It might be surprising then to learn that fission actually releases far more energy per reaction (about 200 MeV (million electron volts) per split) than fusion does (between about 18 and 28 MeV per fusion). Deuterium/tritium fusion (shown below right, popping out an excess neutron (n) in the process) releases "just" 17.6 MeV, still an enormous amount of energy.

Deuterium/deuterium fusion releases 23.85 MeV and the fusion of four hydrogen nuclei releases 28.3 MeV energy. In contrast a typical fission event releases about 200 MeV and it doesn't matter much which atom or isotope (more precisely put) is involved. So, why do we even want to pursue fusion energy, when fission energy, with seemingly more bang per buck, is already developed and available?

Why Go Fusion?

1) Fusion delivers a LOT of energy in a very small package.

One part of the answer becomes obvious when we look at the energy outputs instead on a per nucleon basis or a per gram of fuel basis, keeping in mind that much smaller nuclei are fusing compared to the big heavy nuclei which are split. One gram of deuterium, for example, releases an astounding 1012 J (joules) or 275 million kcal (kilocalories) of energy during fusion, whereas one gram of uranium-235 releases 20 million kcal of energy (less than one tenth of the energy per gram) when it undergoes fission.

I should mention that these numbers, however, can be a bit misleading. The fusion reaction takes place in a fluid, a gas-like plasma state, while fission takes place in a solid. Even though the energy output on a per gram basis is much higher, the energy density of the fusion reaction is lower than a fission reaction, so the over ten times greater output is closer to six times greater output in practice.

Consider fossil fuels for comparison. Any typical fossil fuel releases around 10 kcal per gram of fuel. That seems paltry in comparison to nuclear fuels, but it is interesting to keep in mind that compared to man and horse power, the high energy density of fossil fuels such as gasoline, jet fuel and diesel, all used in internal combustion engines, revolutionized the transportation of people and goods over the last 80 or so years and this is perhaps one of the most significant reasons that North America evolved toward the affluent urban/suburban-based lifestyle that most of us currently enjoy. Liquid oil and natural gas reserves are easy and relatively cheap to access and transport, delivering affordable energy to almost everyone, and it has revolutionized how we live. India and China are beginning to enjoy the same benefits (and the drawbacks - think of the smog in Beijing for example!).

2) Fusion does not pollute the environment.

The oil era has come at a cost and it will end in a matter of decades, no matter what we do. The combustion of fossil fuels is releasing unprecedented amounts of carbon dioxide into the atmosphere, causing rapid climate change and acidifying the oceans - effects that according to most climate experts are causing a global extinction event while extra energy in the atmosphere caused by the CO2 greenhouse effect is already causing extreme weather events such as heat waves, drought and flooding. Disruption of agriculture might be our biggest long-term threat to survival. Even if Earth had enough fossil fuel reserves to last us into the next century, we now have fairly solid evidence from multiple sources that our climate, and our biosphere which includes us, cannot handle more fossil fuel-based pollution. Perhaps it's more accurate to say the climate will handle change just fine but we won't be able to adapt to it. Regardless, at current rates of use, most fossil fuels, especially liquid oil, will run out in decades.

3) Fossil fuels now widely used worldwide are going to run out, probably in decades.

It may not seem like it right now with a worldwide oil glut and depressed prices, but oil reserves worldwide are reaching their peak according to some experts, even while new reserves such as fracked oil, natural gas and bitumen become more accessible. Fossil fuel is, after all, a finite resource. All of the fossil fuels we use - oil, natural gas, coal - formed more than 300 million years ago during the carboniferous period. Long before the age of dinosaurs, land was covered in swamps with huge leafy plants and seas were filled with algae, which are tiny one-celled plants. As they died they formed vast thick layers of peat. Over hundreds of millions of years and under intense pressure as it got buried, the peat was squeezed and transformed into hydrocarbons - coal, oil and natural gas. All the Earth's fossil fuel reserves come from this single unique period in Earth's history.

One way or another the fossil fuel age will be brief in terms of human history, and we will be forced to turn to alternative energy sources in order to power our future lifestyles. Meanwhile, many of our lifestyles need to be adjusted toward sustainability but that is food for another day's thought.

Here in Alberta it is impossible to talk about energy without paying tribute to our burgeoning oilsands industry (where bitumen or heavy crude oil comes from). I certainly respect the big business of oil in our province and fully appreciate how it impacts me economically. Alternative energy is a tough sell in a country that is a net fossil fuel exporter. 10% of our GDP (gross domestic product) comes from those exports. It is also true that a great deal of very costly infrastructure around the world is built to support the internal combustion engine and to switch all of that into electric motor and/or hydrogen fuel cell technology would be extremely challenging. But the facts remain. The fossil fuel age is ending and it seems only pragmatic to start shifting our resources to alternative energy sources now. We could funnel some revenue from fossil fuel exports to develop renewable energy industries in Canada, creating a more diverse future energy plan that would put less pressure on us to develop the energy intensive and highly polluting oilsands.

4) Fusion delivers a constant controllable supply of electricity to the grid

While solar power, wind power, hydrogen fuel cell technology, biofuels and geothermal may play important roles in our future energy budget, nuclear power delivers far more energy than any other source. And, like fission nuclear power, fusion power would deliver a constant supply of electricity to the grid, something that wind and solar power, by their very nature, cannot, as there are cloudy and still days when power production is reduced. The downside, one that seems inevitable as we look to the future, is that all these energy sources ultimately end up as electricity, meaning that all transportation, now largely fueled by fossil fuel, must go electric - unless the hydrogen fuel cell can be also part of the new energy solution. Regardless, our extensive fossil fuel infrastructure has a limited life.

5) Fusion fuel is readily available abundant hydrogen.

It seems at first glance that fusion also has an additional huge plus going for it. The reactants, or fuel, for fusion are as abundant as hydrogen, the most abundant element in the universe. This picture gets a bit more complex when we look at sources of different hydrogen isotopes a little later on. To compare with fission, uranium-235 and, less commonly, plutonium-239, the two reactor fuels, are a finite resource. The Nuclear Energy Agency estimates that there are enough accessible reserves worldwide to run nuclear power plants for 200 more years at the current rate of consumption. However, nuclear power contributes only a fraction of our current energy, about 14% currently consumed worldwide, so those reserves would be depleted far sooner if fission energy became more prevalent.

6) Radioactive products indirectly produced by fusion are short-lived compared to spent nuclear fuel.

Fusion also has a huge plus in that its waste product is non-polluting inert nontoxic helium. In a fission reactor there is always the problem of how to store all the still-radioactive spent nuclear fuel. However, if deuterium-tritium fusion is used, then free and very energetic (about 14 MeV) neutrons are also created. This is the isotope combination currently used in all fusion power research because it is most readily available and because it has the lowest ignition threshold, something we will discuss later on. With this fuel, it is not the neutrons themselves but what they strike that could be a problem. They may strike any number of other elements in the shielding used around a fusion reactor and induce fission in those elements, reactions that would likely produce a variety of dangerously radioactive products in the reactor material - a problem to address when an old reactor is inevitably shut down and dismantled. Because of this process called neutron activation, many difficult to predict emissions such as gamma, alpha and beta radiation as well as various radioactive fission products would be created indirectly during the fusion reaction. Neutron bombardment also leads to embrittlement of any material used to confine the reaction, leading to questions of how to make a vessel durable enough to be commercially viable.

Still, neutron emission is a far safer scenario than all the radioactive products created in a fission reactor. In a fission reactor, the chain reaction can potentially continue in an uncontrolled runaway reaction should the cooling system fail, for example. If pressure builds up from super-heated gases and water during a failure, then a pressure explosion can hurl radioactive products high into the atmosphere creating extensive widespread and long-lasting damage. Even if a fission reactor is completely shut down, the core continues to react and produce significant heat. In a fusion reactor, there is no danger of a runaway reaction because if any containment failure should occur, the very high pressure/temperature environment required to sustain fusion will be instantly lost and the fusion reaction, as well as neutron emission, stops. There is also no radioactive waste to deal with although the reaction core, upon plant shutdown, will be radioactive. In this case, however, the profile of the radioactive products is expected to be different from that of fission. They will be isotopes with shorter half-lives, so that the fusion core will stay radioactive for about 50 years compared to 5000 years in the case of spend fission fuel rods.

What's Next?

All in all, nuclear fusion power, with its readily available and abundant fuel supply, hydrogen, seems to be the hands-down answer to our future global energy needs - if the reaction can be maintained and confined in a safe and economical way. Fusion energy is well worth looking into, for a number of reasons, but along with it's promise come great hurdles to climb as well. I'll look at those in a moment.

Nuclear energy, whether it is fission or fusion, releases energy on a scale of over a million times that of fossil fuels. Of the two, fission and fusion, fusion wins in terms of output/gram by a factor of more than six times. This means that fusion is the most powerful source of energy available to us. Could we find a way to mimic the Sun's intense energy output by harnessing the fusion reaction for our own use?

Plants Might Have it Right

When we think about mimicking the Sun's fusion energy there is a point not to be missed. We can also harness the Sun's fusion energy as the light and heat of solar energy and use that as part of our energy source. There is potentially far more energy available than we would ever need. Humans consume about 539 EJ (exajoules; one EJ is equivalent to 278 TWh) of energy per year (as of 2010). In the same year, Earth's atmosphere, oceans and landmass absorb almost 4 million EJ of solar energy. That's about 8000 times more energy than we use. Solar energy is currently harnessed using technologies such as photovoltaic systems (solar panels), solar water heating and concentrated solar power.

Plants do a great job of harnessing solar energy, locking sunlight into the chemical bonds of energy-dense sugar and carbohydrate molecules, while releasing no polluting gases into the environment, all done through the process of photosynthesis. I wrote an article on photosynthesis two years ago, and although artificial leaf technology currently seems to be little more than a niche area of research, future breakthroughs might make this technology viable in the future, a topic I think will make an interesting future article.

I think it is interesting to keep in mind that the fossil fuels we rely on now are simply carbohydrates and sugars made by plants transformed into longer organic molecular chains under heat and intense pressure. To release the energy in those chemical bonds, we must combust the hydrocarbon molecules, a process much less elegant and far dirtier than the chemical pathways that the original plants utilized.

Despite It's Promise, Fusion Is Not An Easy Technology

Fission power and fusion power share a technological challenge: how to safely control and harness an enormously energetic reaction.

A fission reactor must maintain a complex cascade of nuclear fission chain reactions at just criticality. A computer system does this by constantly monitoring the reaction rate and lowering or raising the control rods in the reactor. The rods absorb excess free neutrons in the system and slow down the reaction rate. All fission reactions are chain reactions that increase exponentially, so the reactor must always adjust to keep a very delicate balance between prompt-critical and sub-critical reaction states. This is a complex technological undertaking but the fact that the fission reactions can take place under everyday pressures (the highest pressures to deal with are high-pressure steam) and temperatures makes the technology feasible. You just need to bombard an already unstable large nucleus with a free neutron to start off the fission chain reaction and then keep it going at a fixed rate until the nuclear fuel is spent. Meanwhile you capture the heat energy it gives off.

Capturing the energy from a fusion reaction is a very different challenge. I think it might be useful to understand fusion as a kind of phase shift, like ice melting into water and vice versa. Fusion is a process that occurs spontaneously to matter in an extremely high-energy environment, one that we would never encounter in nature here on Earth. You have to reach deep inside a star to find such a process. Simply put, you must apply enough energy to atoms that they get energetic enough to start fusing together into bigger atoms and then you must capture the energy that is released. When you do this to small atoms, a tremendous amount of energy is released. Protostars with sufficient mass spontaneously ignite into an ongoing fusion reaction that is maintained in equilibrium for up to billions of years by the opposing forces of gravity (directed inward, increases pressure so speeds up fusion) and thermal pressure (directed outward, decreases pressure so slows down fusion). These forces constantly balance the interior pressure of the star, which is continuously heating atoms in a plasma state to a fuseable state. It's a wonderfully elegant system that is very challenging to recreate.

With fusion we must supply a great deal of energy to heat atomic nuclei in order to get a payout of far greater energy from the reaction. In other words, the reaction has a built-in energy barrier, called the Coulomb barrier, which must be overcome in order for it to take place. Under everyday conditions, all atoms repel each other because the electrons orbiting the nuclei, having like charges, repel each other. This repulsive force is called the electrostatic force and it pushes atoms apart. Likewise, positively charged nuclei also repel each other through the same force, which is part of the fundamental electromagnetic force. Another force that is hundreds of times stronger than the electrostatic force acts as an attractive force inside each nucleus. This force binds protons and neutrons together very tightly and it is (usually) more than strong enough to overcome the repulsion that protons experience. Without it, atomic nuclei would never have formed in the universe.

The catch here is that the strong force has a very short range of influence - about the diameter of a medium size nucleus. This is why very large atoms are unstable. The repulsive electrostatic force gets weaker over a much longer distance than the strong force does, so proton-proton repulsion becomes significantly more disruptive and destabilizing as you go up in proton number and nucleus size. This is why I put the word 'usually' in brackets above. Only in an extremely high-energy environment will atoms have enough kinetic energy to get close enough to overcome this Coulomb barrier. The atoms essentially bang into each other with enough energy to fuse. We bang atoms together inside an atom smasher. Or, many nuclei in a plasma state are simply forced very close together and fuse under tremendous pressure and this is what happens inside a star.

When two nuclei get close enough to experience the attraction of the strong force they spontaneously fuse together into a larger nucleus. In theory any nuclei will fuse if given enough energy. Even iron and larger nuclei will fuse but the fusion reaction of large nuclei will absorb energy (it is endothermic), while the fusion of small nuclei releases energy (it is exothermic). A strongly exothermic reaction is the one we want. Hydrogen nuclei are the easiest nuclei to fuse (they fuse at the lowest temperature) because they are small and there are less repulsive electrostatic charges to overcome.

The two main challenges in making a fusion reactor are 1) getting atoms energetic or hot enough and 2) confining the reaction (you must constantly keep the nuclei squeezed close enough together to maintain the temperature and the fusion reaction). Stars do it naturally because of their enormous size and the physics of being in the vacuum of space, but in order to make a fusion reactor ourselves we must imput a great deal of energy to heat and confine the plasma before we can output energy.

Making a "Bottle" To Hold Fusion

There are two general methods scientists are looking at in order to make a commercial fusion reactor. They are in effect based on two different kinds of "bottles" that are capable of containing the most energetic reaction in the universe. Neither bottle can be made of any physical material as it would never withstand fusion conditions.

And this, not surprisingly, is where things get tricky. In a fusion reactor we are basically creating the Sun in miniature. No physical material can withstand the temperatures involved in fusion. If a physical container of any sort did come into contact with the reaction, the reaction would almost instantly lose thermal energy to the material and fusion would come to a dead stop (and as you might be guessing this is also a built-in safety feature). Fortunately there are at least two general ways we can still make a very effective container. A mass of fusing atoms has special physical properties that we can exploit. We can suspend the reaction in a vacuum and hold it there by applying specific forces to the plasma. In space, gravity does the job but here on Earth we can use the force of electromagnetism. A promising method is called magnetic confinement. Or, we can let the physics of inertia make our bottle for us by directing all the energy of the reaction into the dead centre of the vacuum, a method called inertial confinement. We will explore both methods but first a brief description of the plasma fuel itself.

The atoms we are interested in are VERY hot, over 100 million°C. They are so hot they are no longer intact atoms but plasma. When atoms are heated they gain energy. The electrons in each atom begin to absorb energy and move to higher and higher excited states. Eventually the electrons, one by one, become so energetic that they fly away from the nucleus all together, leaving the nucleus entirely stripped of electrons, while the electrons, now free, fly around at great speed. This is the (hot) plasma state. As the plasma is heated further the electrons fly around even faster and the nuclei themselves gain more kinetic energy. If you heat atoms without confining them, they will bang into each other once and then just fly off at great speed thanks to their kinetic energy, just as particles in a collider do when they collide. Without confinement you will never be able to continue to heat the plasma to ignition temperature (we will explore this term in a moment but for now think of it as the point when they will fuse). You must both energize the plasma AND confine it at the same time. The plasma being tested in all experimental fusion devices is deuterium/tritium plasma. This combination yields less fusion energy than hydrogen-1 fusion for example, but the reason for choosing deuterium/tritium is two-fold. First, it is easy to obtain; it's abundant in seawater. Tritium is present naturally only in trace amounts, so it has to be bred in a fission reactor OR, and this is the goal of a commercial fusion reactor, it can be created within the reactor itself. Neutrons escaping the plasma will interact with lithium contained in what are called blanket walls (neutron absorber walls) of the reactor, and this reaction will continuously create new tritium that can be collected. Second and probably most important, deuterium-tritium has a lower ignition temperature then either deuterium/deuterium or hydrogen-1 so it is easier to obtain fusion conditions.

Plasma, this "soup" of naked nuclei and free electrons, can be manipulated. Magnetic confinement is the more developed of the two approaches, and it can be used to hold, control and even heat the plasma. Intact atoms have no net charge and do not display their inner magnetic properties except under certain circumstances where the atoms are ordered in a material in a certain way. However, now that the atomic charges are separated, the plasma "soup" responds very well to both electric and magnetic fields. In fact, the plasma itself creates magnetic and electric fields and, as a fluid, it conducts both external electricity and magnetic fields. The motion of this fluid, which can be described or modelled using a combination of fluid dynamics and Maxwell's equations, is actually self-organizing. The motion creates fields, which in turn control the movement of the plasma.

Magnetic Confinement: The Tokamak System

The Tokamak Fusion Test Reactor (TFTR) at Princeton University, in operation between 1982 and 1997, took advantage of the torus-shaped geometry used by the tokamak developed in the USSR in the 1950's. In 1993, the test reactor produced an output of 5.6 million watts of power in a controlled fusion reaction. However, more energy than that had to be put into the device. Still, progress has been made between that and the Joint European Torus (JET) in England, which reached an output of 1.7 million watts in 1991 (still requiring more input than output gained). Under development right now, using the information gained by these projects as well as numerous other projects around the world, is an international effort that is equipped to deal with the cost and the complexity of fusion reactor research. The International Thermonuclear Experimental Reactor (ITER) in southern France hopes to demonstrate feasible commercially viable fusion power by building further on the tokamak technology to achieve an output of 500 MW for every 50 MW input.

For both magnetic confinement and inertial confinement reactors, achieving good outputs of energy is a significant technical challenge. How do you capture even a fraction of the thermal energy of the reaction without physically interfering with it and slowing it down? Energy in the form of heat emitted by the fusion reaction must be captured indirectly and efficiently in a usable form, and it appears that neutrons are the key. In magnetic confinement setups, fast moving neutrons escape the magnetic confinement because they are electrically neutral and are immune to magnetic and electric fields. Their kinetic energies are absorbed by a meter-thick neutron-absorbing lithium blanket that surrounds the chamber but does not touch it. The lithium blanket does three jobs. It prevents harmful neutron radiation from escaping. It absorbs the kinetic energy of the neutrons produced by the fusion, and it breeds new tritium that can be used as fusion fuel. The blanket heats up and the heat is transferred to a coolant liquid flowing through it. The hot coolant can be used to heat water into steam which, like a fission power plant, can turn a turbine and create electricity. One challenge is to keep the hot plasma itself from contacting the blanket walls because that would dissipate the heat and slow down the particles so that the fusion reaction could not be maintained.

In magnetic confinement, both electric and magnetic fields are used to heat and squeeze hydrogen plasma. This does two jobs for the price of one but it presents a challenge. The field lines of an external magnetic field will put a Lorentz force on the plasma that is perpendicular to its field lines and this allows the plasma to leak out the ends of the field lines and strike the blanket wall. A torus-shaped magnetic field tackles this inevitable problem. It is a doughnut shape (see below) that forces the field to curve around to form a closed loop. Then a perpendicular magnetic field is superimposed on it (from the inner field coils shown below). It keeps plasma contained and it seems to be the best configuration for magnetic confinement. This tokamak device is shown below.
Abteilung Offentlichkeitsarbeit - Max-Planck Institut für Plasmaphysik
The two fields create magnetic field lines that follow spiral paths (shown as thin yellow lines) around the torus that are very effective in reigning in the plasma particles.

Although there are a number of toroidal confinement systems under investigation, the tokamak device seems most promising. A strong electric current is induced in the plasma using a central solenoid (which also contributes to the perpendicular magnetic field). The current heats the plasma to about 10 million°C. A separate heating device shooting intense beams of neutral atoms into the plasma is used to heat it further, up to 100 million°C, or fusion temperature. You can keep the plasma in a fusion state indefinitely as long as you keep injecting new fuel into the system.

Inertial Confinement

Magnetic confinement seeks to keep nuclei confined close together and very hot for an extended period of time, while inertial confinement operates under a different premise. With this technology, nuclei, usually in the form of a tiny deuterium/tritium pellet like the one shown below left, are blasted very fast and very hard, giving them no time to move away from each other. To blast the nuclei most test reactors aim powerful lasers right at the fuel or right around the fuel in order to cause an implosion that forces the nuclei to fuse. Typically you then need a steady stream of these pellets delivered to the target area, several per second for example, so that a steady output of heat and neutron radiation would result. There are other methods of inertial confinement being tested as well, such as the pinch method, where a strong current is sent through the plasma to generate a magnetic field so intense that it squeezes the plasma to fusion. And later in this article we look at small fusor devices that use a strong voltage drop to slam nuclei together in the centre.

The largest most energetic device built to date, with the most powerful lasers in the world, is the National Ignition Facility in the United States, shown below. This is the very futuristic device you may have seen depicted in Star Trek Into Darkness standing in as the Starship Enterprise's warp core. It fills up a big room.


192 laser beams are aimed at a single tiny spherical capsule (like the photo above left). Coated on the inside of the capsule is a microns-thick layer of frozen deuterium/tritium. When the lasers fire, they compress the capsule by about 35 times, driving its contents to densities more than high enough to fuse. The continuing problem with this technology is that all the energy required to set up the system and run the incredibly intense lasers is more than the system delivers. Also, the setup has not been able to achieve ignition. This is a state in which the heating process delivers not just enough energy to initiate fusion but enough energy to sustain fusion by starting a fusion chain reaction. That is the ultimate goal for any fusion reactor. In a working chain reaction scenario, the fuel is so rapidly compressed that it emits a significant number of very energetic alpha particles (helium nuclei). In adjacent fuel layers, these alpha particles release their energy as heat. If they release enough heat, the adjacent layers will also begin to fuse, emitting more alpha particles and so on, eventually burning up all of the fusion fuel and releasing an enormous amount of heat energy. Achieving this kind of efficiency in practise is very challenging.

Many additional technical questions remain about the system. For starters, at least with a laser system, the lasers must be precisely aimed to heat the sphere evenly and the sphere must be perfectly spherical (again in the effort to create efficient ignition). Otherwise the compression of the fuel will be uneven and it will not all be burned. To make the design even more challenging, how lasers interact with plasma is not completely understood (but on the upside there may be secrets to increasing laser efficiency locked in there too, as lasers are energy intensive devices by nature).

Still, in 2013, a capsule at this facility gave off more energy than the laser energy applied to it, and that is an encouraging breakthrough. Fast ignition, a new development, might help achieve ignition as well as significantly reduce the amount of input laser energy required. In this case, a perfectly spherical blast is not the goal. Instead, the laser system brings all the fuel to maximum compression like before but then a second ultra-high power pulse delivers a single pulse just to one side of the pellet. This second energy pulse heats one side of the pellet past fusion temperature and starts a chain reaction that travels through the pellet. A number of projects to test this approach are in development.

Fun With Fusors: A Table-top Device to Impress Your Friends and Scare Your Neighbours

Ironically, while a large number of scientists have been trying to develop a commercially viable fusion reactor, perhaps just as many amateur fusion enthusiasts have been successfully building their own table-top fusion devices, and some of these small designs, often called Farnsworth-Hirsch fusion reactors or fusors for short, can even be useful sources of free neutrons. These projects give us the impression that full-scale nuclear fusion energy is just around the corner but unfortunately, as we've seen, big hurtles face us such as achieving ignition, maintaining a cascade reaction, and efficiently capturing the potentially enormous amount of heat energy released.

A tabletop fusor is based on an inertial electrostatic confinement scheme that uses kinetic energy to cause fusion. A strong electric field heats atoms to fusion temperature. The fusion products themselves (especially the fast free neutrons that are emitted) are not commercial contenders because (1) relatively few are emitted and (2) there is no viable way to use their kinetic energy to generate power because in this setup the energy is quickly lost through radiation and conduction. Still, projects like these (you can find several how-to's online and this link is just one of them), though potentially dangerous and a huge draw on your power bill, will introduce you in a hands-on way to vacuum systems, plasma physics, radiation safety and high-voltage devices - I'd love to build one of these myself!

A General Schematic

This is how a fusor works: The setup is basically a cathode inside an anode inside a vacuum. In (1) below, the fusor contains two concentric cages - the cathode (blue circle) is inside the anode (red circle). This creates a strong electric field so the charges in the deuterium gas that is injected into this vacuum tube separate into plasma (a cold plasma at this stage). Positive ions (protons and a few other nuclei as it's not a perfect system) in the plasma are attracted to the inner anode. They fall down a large voltage drop that is created by the strong electric field of the cathode/anode (it's like a battery except in a battery, negatively charged electrons move in the opposite direction) (2). The electric field accelerates the ions and heats them because the kinetic energy of particles is basically heat.

A note on "temperature" here as I've been talking about heating plasma as well as mentioning hot and cold plasmas: We can say the electrons are "hot" because they have lots of kinetic energy. Temperature is after all just the average kinetic energy of a system of particles. It is more accurate (and simpler for particle physicists) to describe the energy of any particle in electron volts (eV). What you are trying to do here is more akin to making a particle accelerator than to making a hot dense plasma like the one inside the Sun. A hot plasma is a plasma under pressure (inside the Sun) and a cold plasma is not. Intense pressure increases the temperature of atoms so they lose electrons. An intense electric field strips off an atom's electrons. Both end states are plasma.

The electrons miss the inner cage (3) and collide in the center (4) where they hopefully have enough energy to fuse.

Wikihelper2134;wikipedia
The hottest plasma stays safely away from the outermost walls of the chamber so the glass tube stays intact, a good thing as you are dealing with energies roughly equivalent to about 50 million°C at the centre and the whole thing is under pressure (even though the voltage is very low - about 4kV, which is what makes this device workable at home). What you get, especially at lower voltages, is a totally awesome purple blue glowing star-in-a-jar for your living room, shown below. Many of the deuterium atoms are not fully ionized but instead in an excited state where electrons are constantly moving up energy levels and down energy levels (and this is when they emit a blue or red photon).

William Jack;Wikipedia
This seems innocuous enough but the excited deuterium ions in the plasma radiate not only purple-blue visible light but also dangerous ultraviolet and sometimes even X-rays (this is what, in addition to heat, all the energy of fusion transforms into and it's called bremsstrahlung radiation). Harmful radiation is more likely when you use a device that's operating at fairly high voltage. Then, you are more likely to experience hard (very energetic and very dangerous) X-ray radiation as well as fast neutron radiation (which will penetrate deep into your tissues causing damage) and possibly even deadly gamma rays. You need radiation protection. If you make a lower voltage apparatus, you can generally get away with UV eye protection and you will still achieve the cool blue glow and possibly some fusion. However, in all cases you are dealing with deadly voltages, potential vacuum explosions and explosive hydrogen gas. A good safety protocol and good working knowledge of these systems (or a good advisor) is absolutely essential before any construction considerations. You'll likely have to buy a pressurized canister of deuterium gas from a university, but I did find an online source here. It's not dangerous so it's not a restricted product. You will also need some vacuum equipment (likely the most expensive part) and some other components, many of which could be scrounged up from old junk. You can build a low-voltage demonstration-type fusor but it seems to me that you are then building a glorified neon-type light fixture. If you really want to build a bonified kick-ass fusor you will need some way to detect fusion and there are a number of ways to do this. Most devices detect moderated (slowed) or thermal neutrons. You can use a boron-10 lined proportional tube and read it using a Ludlum Model-3 for example, and here I refer you to a nice setup someone has posted on his blog, showing a photo of the reactor and data as proof of fusion. I think the author (Will) might be a high school student but I'm not sure as he posts no information about himself. Impressive anyway!

Conclusion

Unfortunately you cannot scale up a fusor to make it commercially viable. Even if you could, the inner electrode would almost instantly be destroyed in the fusion reaction. Fusion power technology is far more challenging than fission power, more challenging than I think most people realize. It might not come as a surprise then to hear many people bemoan how long it's taking to get a commercially viable fusion power plant up and running. In general, the fusion concept is straight forward enough. Getting a few atoms to fuse is the do-it-yourself project called the fusor. However, a continuously running large-scale fusion plant that is efficient enough to deliver economical energy challenges the best physicists and engineers we have.

The challenges of maintaining very precisely controlled temperatures, pressures and magnetic field parameters in the case of magnetic confinement systems or laser focusing in the case of (most) inertial confinement systems make fusion energy anything but an easy slam-dunk. However, once the technology is perfected, and I am personally confident it will be, fusion reactors will be by far the most powerful and sustainable energy source humanity has ever experienced. Where fossil fuel has been an approximately 80-year answer to our energy needs, nuclear fusion can easily be the energy source we will rely on for thousands of years and more to come.

3 comments:

  1. Hey there! Nice stuff, please keep us posted when you post again something like that!

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