Fusion Energy

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Abstract

The Sun is a colossal source of energy. In fact, it outputs an estimated 384.6 yottawatts[1] of energy in the form of electromagnetic waves. This power is produced by the process of fusion within the Sun’s core; therein can be found isotopes of hydrogen. Within these isotopes, the nuclei fuse to produce energy as well as other particles.

If we were to recreate and control this simple yet powerful set of reactions, we would have in our hands a way to generate incredible amounts of sustainable and carbon-free energy[2].

What is fusion and how does it occur?

Within a star’s core, there is an abundance of hydrogen isotopes which go through a set of reactions to release the energy. The net reaction takes four protons from the hydrogen nuclei and fuses them into a helium-4 nucleus while also producing two positrons, two neutrinos, two gamma photons as well as a net energy output of 25 Mega electron Volts (MeV).

4 11H ➔ 42He + 2 01e + 2 γ

This makes the Sun a massive source of neutrinos and gamma rays which are part of the energy output; the positrons instantly react with electrons to produce more gamma rays.

The reaction is very simple, however, some difficult obstacles must be overcome before we can start to simulate the Sun’s energy production here on Earth.

Reactions such as fusing two deuterons (hydrogen-2 nuclei) require energy inputs of 3.6 MeV per reaction. This is to reach temperatures of up to 10 000 000 K which is needed to give the nuclei enough energy to react. The need for this intense supply of kinetic energy is to overcome the electrostatic repulsion between two nuclei. Each nucleus has a net positive charge due to its protons, and these like charges repel. However, this can be overcome if the nuclei get close enough for the strong nuclear force to come into play. This force will overcome the repulsion and bind the two nuclei together into the newly formed helium-4 nucleus, releasing energy as it does so.

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Figure 1: Cross-section of the sun

The environment at the core of the Sun provides these huge energy requirements due to its high pressures. The gravitational force of the Sun draws all its mass down towards the centre, crushing the core to a relatively small volume. The core is put under a very large amount of pressure and this allows the hydrogen nuclei to obtain enough kinetic energy to move sufficiently close to each other to give rise to the strong nuclear force. After this takes effect, the nuclei speed towards each other to fuse into a helium nucleus and release energy[3].

Now, the challenge we face is to build a reactor that can give hydrogen nuclei the same amount of energy as the pressure in the core of the Sun. This seems like a mountainous task, but many scientists are working together to overcome it and welcome a new era of sustainable energy production.

The ITER Agreement

Nearly 30 years ago, a group of powerful nations decided to collaborate on a project to develop a system that was capable of simulating the fusion that occurs in the Sun’s core to produce potentially limitless and sustainable energy. The idea of the development of fusion energy for peaceful purposes was first proposed in 1985 at the Geneva Superpower Summit by General Secretary Gorbachev of the former Soviet Union to the US President Reagan.

After a year, the European Union, the Soviet Union, Japan, and the USA agreed to work collaboratively on the project. The development started in 1988 with initial designs. This led to a chapter of more focused engineering design work being conducted until the final design was agreed upon in 2001.

The People’s Republic of China and the Republic of Korea joined the journey in 2003 followed by India in 2005. That was also the same year in which the fusion reactor location was decided: Aix-en-Provence, France.

The very next year, the ITER Agreement was signed in Paris by the representatives of the seven member-countries. The construction of the experiment facility began in the year of 2010. Today, 2000 people are working in conjunction to develop the world’s most advanced magnetic confinement system that can be used for nuclear fusion, and they call it a Tokamak[4].

What is a Tokamak?

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Figure 2: Generator diagram

Inside a power plant, the motion of a spinning turbine is converted into electrical energy using a generator. The turbine is attached to a large magnet which spins as the turbine spins. The magnet is inside a cylindrical space bounded by many coils of conducting wire. The spinning of the magnet causes a constantly changing magnetic field which induces current – the flow of electrons – in the coil of wire.

Figure 3: Tokamak design

A Tokamak is an experimental machine that works in a similar way to a nuclear reactor. It is designed with the function of harnessing the energy produced by fusion to heat a volume of water into steam to turn a turbine[5].

How does a Tokamak work?

Figure 4: Tokamak with plasma

The main component of a tokamak is its doughnut-shaped vacuum. Under the effect of intensely high temperature and pressure, gaseous hydrogen fuels are converted into plasma. Plasma is what provides the environment for light elements – such as hydrogen – to fuse in the core of a star and this is what a tokamak is designed to copy.

The shape and movement of the dangerously hot plasma can be controlled by the massive magnetic coils placed around the vessel. This control means that the plasma can be kept away from the vessel adding a layer of safety to the reactor.

The first step is to remove any impurities from the vacuum as you only want to fuse the hydrogen fuel. The magnet system is then powered up. The hydrogen fuel is then released into the chamber and a powerful electrical current is run through.

This causes the gas to become ionised as all the nuclei are stripped of their electrons, this mixture then forms a plasma which is confined in a doughnut shape by the magnet system.

The plasma particles gradually increase in temperature as the particles in the chaotic mixture collide with each other and gain more kinetic energy.

The plasma is then heated to a point where the nuclei are at a sufficiently high energy level to overcome their electrostatic repulsion and fuse[6].

How is the Plasma Heated?

The environment inside the Tokamak chamber must reach a temperature of 150 000 000 °C. To put this into perspective, this is ten times the temperature at the centre of the blazing Sun. The plasma must be constantly kept at this temperature to provide the activation energy for fusion.

Three external heating systems that will operate in conjunction to heat the chamber environment to the required level.

Researchers hope to produce a ‘burning plasma’; they aim to produce a plasma in which the helium nuclei – produced by fusion – can provide enough energy to maintain the temperature of the chamber. If this were to happen, then the heating systems could be significantly reduced or possibly switched off entirely which would save incredible amounts of power and money[7].

Neutral Beam Injection

Figure 5: Neutral beam injection system

One of the external heating systems is a neutral beam injection system. This machine was engineered to accelerate a neutral particle to an incredibly high speed to be fired into the plasma and pass on its kinetic energy.

Physicists know that a charged particle experiences a force (and is therefore accelerated) when in a magnetic field. This is one way of accelerating the neutral particle but it requires the particle to have a non-neutral charge. This is why the particle starts off being ionised (by having one of its electrons removed) into a positive ion. The ion will then be passed through a magnetic field which will accelerate it to a high speed. It will then regain its electron by speeding through a cloud of gas before entering the reactor chamber so that it becomes neutral again. The particle must be neutral, otherwise, it will be deflected by the strong magnetic system around the chamber (controlling the plasma) because of it interacts with the magnetic field (similar to how it was accelerated).

However, there is a problem with this method. The particle must be travelling at incredibly high speeds – three or four times that of previous systems – to penetrate deep enough into the plasma for the initialisation of the fusion reactions; positive ions become very difficult to neutralise (before entry into the chamber) at these high speeds.

Therefore, for the very first time, ITER will be using a negatively charged ion to reach high speeds. This ion has its extra electron attached very loosely to the nucleus, so it can be removed with little difficulty. However, this poses a second problem as negative ions are much more challenging to produce and control[8].

Ion Cyclotron Heating

Figure 6: Ion cyclotron resonance heater

The second method of heating is to simply use electromagnetic waves at very high frequencies to transfer heat to the plasma-like how a microwave oven transfers heat to the food inside.

Energy is transferred to the ions in the chamber by a high-intensity beam of electromagnetic waves at frequencies of 40 – 55 MHz[9].

Electron Cyclotron Heating

This works very similarly to the ion cyclotron heating system. This uses electromagnetic waves to transfer energy specifically to the electrons in the plasma. The electrons then transfer this newly gained energy to the ions via electron-ion collisions[10].

The Magnets

https://www.iter.org/doc/www/content/com/Lists/Machine/Attachments/27/magnet_overview_plasma.jpg

Figure 7: Magnet system used in the Tokamak

The magnet system being used in the ITER fusion reactor will be the most enormous and integrated superconducting magnet system to be ever built.

The whole system will contain 10 000 tonnes of magnets; the total stored magnetic energy will be approximately 51 GJ. They will be constructed from alloys of niobium-tin or niobium-titanium. The magnetic system becomes superconducting (allowing electrons to flow with absolutely no resistance) and functional when it is cooled with supercritical helium at a temperature of 4 Kelvin (-296 °C).

The advantage of using superconducting magnets is that they can carry high current and produce tremendously powerful magnetic systems while consuming less power and have a lower cost. This made them an obvious choice for the ITER fusion reactor which already has a very high energy consumption and financial investment.

ITER will be using high performance, internally-cooled superconductors. These are grouped strands of superconducting materials mixed with copper which are cabled together and fitted into a structural steel jacket[11].

Figure 8: Superconducting cable

Other Ideas

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Figure 9: Other designs

There are many other designs for fusion reactors currently being tested. An example is the laser design in which a small pellet of fuel is confined and heated with a laser until it reaches a hot and dense enough state to kickstart fusion.

The hope is that – with many innovative designs in development – our problem-solving skills and ingenuity as engineers and scientists can help us find the best solution to this newly found problem.

Why isn’t this a reality?

There are still many challenges that must be overcome in terms of the capabilities of today’s machines. Current experiments with fusion reactors require much more energy to start the reactions than the energy produced by them. This is giving scientists data to analyse and improve their designs, but it is also a huge loss in terms of the money required to power the reactors themselves.

There is also the issue of money. Countries are investing heavily in this 10 billion dollar project when they could be placing much safer investments into already proven clean energy solutions such as hydroelectric power, wind, and solar. Are they willing to bet their time, money and resources on a solution that could end up failing?[12]

However, the possibility of potentially generating virtually limitless supplies of energy for everyone has kept the leading nations focused on building this mini Sun on Earth.

Why do we need fusion?

As the human population is predicted to reach 10 billion by 2050, there will be a significant increase in the energy demanded (two or three times the current energy demands).

These demands will be difficult to reach with current energy production methods. This is especially true with fossil fuels which are becoming increasingly dangerous to use, yet they still make up the largest sector of energy production.

The development of fusion energy would lead to a potentially unlimited supply of energy; the deuterium required for fusion is abundant in seawater and tritium can be made from fusing lithium. Fusion energy doesn’t produce greenhouse gases or long-term radioactive waste[13] which makes it a clear winner in strategies to meet energy demands.

In times where we are using more and more energy at the cost of the environment, it is clear that fusion energy is the way forward.

Footnotes

  1. UTIA. The Sun’s Energy. UTIA Institute of Agriculture. https://ag.tennessee.edu/solar/Pages/What%20Is%20Solar%20Energy/Sun’s%20Energy.aspx (accessed 03 Sep. 2018)
  2. CCFE. Introduction to fusion. http://www.ccfe.ac.uk/introduction.aspx (accessed 03 Sep. 2018)
  3. Mike O’Neill. OCR A level Physics A 2. (London, Pearson Education Limited, 2nd edition), pages 223-224
  4. ITER. THE ITER STORY. https://www.iter.org/proj/iterhistory (accessed 04 Sep. 2018)
  5. ITER. WHAT IS A TOKAMAK?. https://www.iter.org/mach/Tokamak (accessed 04 Sep. 2018)
  6. ITER. WHAT IS A TOKAMAK?
  7. Kurzgesagt – In a Nutshell. Fusion Powers Explained – Future or Failure. (YouTube, 2016).
  8. (accessed 04 Sep. 2018)
  9. ITER. EXTERNAL HEATING SYSTEMS. https://www.iter.org/mach/Heating (accessed 04 Sep. 2018)
  10. ITER. EXTERNAL HEATING SYSTEMS
  11. ITER. EXTERNAL HEATING SYSTEMS
  12. ITER. MAGNETS. https://www.iter.org/mach/magnets (accessed 04 Sep. 2018)
  13. Kurzgesagt – In a Nutshell. Fusion Powers Explained – Future or Failure.
  14. CCFE. WHY FUSION IS NEEDED. http://www.ccfe.ac.uk/Why_fusion.aspx (accessed 03 Mar. 2019)

Pictures

Title image of a fusion reactor. Source URL: https://icdn2.digitaltrends.com/image/iter-fusion-reactor.jpg [Accessed 03 Sep. 2018].

A cross-section of the Sun. Source URL: https://i.ytimg.com/vi/W1ZQ4JBv3-Y/maxresdefault.jpg [Accessed 04 Sep. 2018].

Generator diagram. Source URL: http://4.bp.blogspot.com/-bp7cWiFL5R0/TvQPCuYuGkI/AAAAAAAAAV4/ag28eDL8ifA/s1600/plant_generator.gif [Accessed 04 Sep. 2018].

Tokamak design. Source URL: https://www.iter.org/mach/Tokamak [Accessed 04 Sep. 2018].

Tokamak with plasma. Source URL: https://www.iter.org/mach/Tokamak [Accessed 04 Sep. 2018].

Neutral beam injection system. Source URL: https://www.iter.org/mach/Heating [Accessed 04 Sep. 2018].

Ion cyclotron resonance heater. Source URL: https://www.iter.org/mach/Heating [Accessed 04 Sep. 2018].

Magnet system used in the Tokamak. Source URL: https://www.iter.org/mach/magnets [Accessed 04 Sep. 2018].

Superconducting cable. Source URL: https://www.iter.org/mach/magnets [Accessed 04 Sep. 2018].

Other designs. Source URL: http://i.gzn.jp/img/2016/11/11/fusion-energy-explained/cap00050.png [Accessed 04 Sep. 2018].

About the author

Tawsif is an A-Level student pursuing a career in aerospace engineering. He has a keen fascination with physics and how it can be employed to engineer machines that solve real-world problems. He has previously conducted projects with Airbus and BRE to gain industry experience; he continues to research into physics and engineering to advance his learning and feed his curiosity.