Engineering A Star – Nuclear Fusion, Tokamaks and Stellarators

A Stellar Reactor

Greifswald, Northeastern Germany, 2016.  Physicists at the Max Planck Institute have been racing to find a way of producing sustainable, clean energy with a stable nuclear fusion reactor. The challenge? Re-creating the Sun’s powerhouse on a much, much smaller scale.

Hailed as a breakthrough for the Max Planck Institute for Plasma Physics (IPP), the stellarator is a cutting-edge new device designed to mimic the Sun’s interior and release an abundance of nuclear energy, without generating the volumes of toxic waste that accompanies the splitting of atoms in classic nuclear fission.

The Max Planck Institute called it Wendelstein 7-X (W7-X), and as reported back then, it really works.

Nuclear Fusion

During the 1950s, physicists began chasing the dream of nuclear fusion.  But the extreme temperatures involved and the difficulty of controlling plasmas, mean progress has been slow.

Fusion works on the principle that energy can be released by forcing together atomic nuclei, rather than by splitting them as in the case of the nuclear fission reactions that drive our nuclear power stations.

What is Nuclear Fusion?

In a fusion reaction, merging two forms of hydrogen – tritium and deuterium – yields helium plus a neutron and energy. Source: ScienceNews

Existing nuclear energy relies on a process called fission, where a heavy chemical element is split to produce lighter ones.

Nuclear fusion works by combining two light elements to make a heavier one.

Fusion is the process that powers the Sun, along with active “main sequence” stars, and other high magnitude stars.

Nuclear fusion is a reaction in which two or more atomic nuclei of hydrogen are brought in close enough proximity to form one or more different atomic nuclei and subatomic particles (neutrons and/or protons).

The difference in mass between the products and the reactants is manifested by the release of a very large amount of energy.


The mass defect in nuclear fusion is equal to 3.23 x 10^-29 kg.
Equations for the Reaction of Nuclear Fusion  The masses are given in unified atomic mass units (u) – defined as 1/12th of the mass of a carbon C-12 atom.

This mass difference results from the difference in atomic binding energy between the atomic nuclei before and after the reaction.  It is fundamental to the energy release.

Ultimately, this energy could be used to heat water and drive steam turbines to generate electricity.

The effort to achieve nuclear fusion has been likened to building a star on Earth.

Building a Star on Earth

The Superconducting Wendelstein 7-X Stellarator in 2011 (Greifswald, Germany).  As of 2015, the W7-X reactor is the largest stellarator device.  It was designed to achieve enclosure of up to 30 minutes of continuous plasma discharge in 2021, demonstrating continuous operation of a fusion powered reactor: . Source: Max Planck Institute

A chamber was constructed to contain a super-heated plasma – essentially a cloud of loose, electrically charged energetic gas particles.

The plasma was created using a microwave laser, a complex set of magnets and a mere 10 milligrams of helium.

The design of W7-X differed significantly from the tokamak fusion devices used elsewhere.

Schematic diagram of the W7-X Stellarator Source: Max Planck Institute

The German nuclear fusion experiment produced a special super-hot gas which scientists hope will eventually lead to clean, cheap energy.

The plasma cloud lasted 1/10th of a second and its temperature was about 1,000,000°Celsius.


Hotter Than The Sun

The project at Greifswald cost over 1 billion Euros.

Its aim is to heat hydrogen nuclei to about 100 million °C – the necessary conditions for fusion to take place like in the Sun’s interior, thus reproducing the environmental conditions of the Sun’s interior.

Inside a star, gravitational pressure is in equilibrium with heat radiation pressure.  Source: ResearchGate

In the core of the Sun, huge gravitational pressure allows this to happen at temperatures of around 10 million °C.

At the much lower pressures that are achievable on Earth, temperatures to produce fusion need to be much higher – above 100 million Celsius.

No materials exist that can withstand direct contact with such heat.

So, to achieve fusion in a lab, scientists have devised a solution in which a super-heated gas, or plasma, is held inside a doughnut-shaped magnetic field.

To achieve this, they used deuterium – a heavier type, or isotope, of helium.

Stellarators and Tokamaks

Tokamak Field Lines and Currents Schematic Diagrams Source: Stanford University

A tokamak provides the required twist to the magnetic field lines, not by manipulating the field with external currents, but by driving a current through the plasma itself.

The field lines around the plasma current combine with the toroidal field to produce helical field lines, which wrap around the torus in both directions.

Although they also have a toroidal magnetic field topology, stellarators differ from tokamaks in that they are not azimuthally symmetric. Instead, they have a discrete rotational symmetry, often fivefold, like a regular pentagon.

It is generally argued that the development of stellarators is less advanced than that of tokamaks, although the intrinsic stability they provide has been sufficient for active development of this concept. The three-dimensional nature of the field, plasma and vessel makes it much more difficult to carry out either theoretical or experimental diagnostics with stellarators.

Even harder to design is the divertor – the section of wall that receives the exhaust power from the plasma in a stellarator.

W7-X Plasma Container Chamber Diagram Source: Max Planck Institute (translated to English)

The out-of-plane magnetic coils, commonly found in many modern stellarators, and possibly all future ones, are also much harder to manufacture than the simple, planar magnetic coils which suffice for a tokamak, and the utilization of the magnetic field volume and strength is generally poorer than in tokamaks.

Unlike tokamaks, however, stellarators do not require a toroidal current, so that the expense and complexity of current drive and the loss of availability and periodic stresses of pulsed operation can be avoided.

Additionally, there is no risk of toroidal current disruptions. It might be possible to use these additional degrees of design freedom to optimize a stellarator in ways that are not possible with tokamaks.

Burning Plasma

Lab fusion reactions notoriously consume more energy to initiate than they output.

Once you get a reaction going, you can theoretically get more energy out of fusion than you put in.

There has been a longstanding effort to crack fusion power because it promises an unlimited source of clean energy.

Almost limitless energy.

Fast forward a bit…  Last year, US physicists confirmed that they achieved a stage in nuclear fusion called “burning plasma”.

The World is now one step closer to achieving nuclear fusion.

NIF (National Ignition Facility)

Inside the NIF reactor. Source: LLNL

This stage of nuclear fusion was seen in experiments carried out at the NIF based at the Lawrence Livermore National Laboratory (LLNL) in California, United States.

NIF is the size of a sports stadium.  It operates the World’s largest and highest-energy laser system.  It can generate temperatures in the target of more than 180 million degrees Fahrenheit at pressures of more than 100 billion Earth atmospheres.

In August 2021, NIF reported they had reached about 70% of the way towards plasma ignition.

The scientific achievement was described in two papers published in the academic journal Nature.

Burning plasma occurs when fusion reactions become the dominant heat source in the process, rather than energy introduced from outside.

To this end, NIF used a powerful laser to heat and compress hydrogen.

The 192 laser beams – the highest-energy example in the World – are directed towards a pepper corn-sized capsule containing deuterium and tritium – different forms of the element hydrogen.

The Formula for the Density of Lead (Pb)

This compresses the fuel to 100 times the density of lead (Pb) and heats it to 100 million degrees Celsius – effectively hotter than the centre of the Sun.

Heating the target like this generates the plasma – the high-energy particle “cloud”.

In the plasma, electron particles are stripped from atoms, leaving  only the parts known as atomic nuclei.  These can fuse together, generating energy in the process.

When fusion reactions become the dominant source of heating in the plasma, rather than the laser energy required to start the process, the heat provides the energy for even more fusion.


“In these experiments we achieved, for the first time in any fusion research facility, a burning plasma state where more fusion energy is emitted from the fuel than was required to initiate the fusion reactions, or the amount of work done on the fuel.”

Annie Kritcher, NIF Physicist, LLNL

“Experiments over decades have produced fusion reactions using large amounts of ‘external’ heating to get the plasma hot – now, for the first time, we have a system where the fusion itself is providing most of the heating.”

Alex Zylstra, NIF Physicist, LLNL


It was a key milestone on the way to even higher levels of fusion performance.

Previous attempts to reach this stage were limited by challenges in controlling the plasma shape.

The researchers came up with an improved experimental design involving the use of capsules that can hold more fuel and absorb more energy while containing the plasma.

Even when burning plasma is achieved, energy is still lost from the process.

This is one of the last remaining milestones before NIF’s bigger goal of “ignition” and self-sustaining energy production.

But there is evidence this deficit can be overcome in the future, especially as plasmas are scaled up.

During ignition, the energy released through fusion reactions exceeded that delivered to the fuel by the laser.

JET (Joint European Torus)

The UK-based JET laboratory is the World’s largest and most advanced tokamak.  The site at Culham in Oxfordshire has been pioneering fusion research for nearly 40 years.

Schematic Diagram of JET Energy Storage Flywheel and Photograph.  Source: Semantic Scholar

At Jet, the power draw from the main grid is limited to 575 MW.  Two 500 megawatt flywheel generators are used to run the experiments. 

Each 775-ton wheel can spin up to 225 rpm and store 3.75 GJ, equivalent to the kinetic energy of a 5,000-ton train moving at 140 km per hour (87 mph).

JET smashed its own record for the amount of energy it can extract by squeezing together two forms of hydrogen.

The experiments produced 59 megajoules of energy over five seconds (11 megawatts of power).  Seemingly, not a long time.  But a very long time on a nuclear timescale.

Although it was not a massive energy output – only enough to boil about 60 kettles of water, it was more than double what was achieved in similar tests back in 1997.

The significance of this result is that it validates the design choices that have been made for an even bigger fusion reactor undergoing construction in France.

“The JET experiments put us a step closer to fusion power.  We’ve demonstrated that we can create a mini star inside of our machine and hold it there for five seconds and get high performance, which really takes us into a new realm.”

Dr Joe Milnes, JET Head of Operations

“These experiments we’ve just completed had to work.  This was high stakes and the fact that we achieved what we did was down to the brilliance of people and their trust in the scientific endeavour.  If they hadn’t then we’d have real concerns about whether ITER could meet its goals.”

Prof Ian Chapman, JET CEO

The JET science team had to tune their plasma to work effectively in this new environment.

Joint European Torus (JET) Reactor Schematic Diagram Source: Science Direct

The reactor was designed to demonstrate a lining for the 80-cubic-metre toroidal vessel enclosing the magnetic field that would work efficiently with these isotopes.

For its record-breaking experiments in 1997, the walls of the JET reactor had used carbon, but carbon absorbs tritium, which is radioactive.

For the latest tests, new walls for the vessel were constructed out of beryllium and tungsten, which are 10 times less absorbent.

And for the past 10 years, JET has been configured to replicate the much anticipated ITER set-up.

ITER – The Way Forward

The EU’s main nuclear fusion project is called ITER (International Thermonuclear Experimental Reactor, iter also meaning “the way” or “the path” in Latin).

ITER’s stated mission is to demonstrate the feasibility of fusion power as a large-scale carbon-free source of energy.

The ITER facility is supported by a consortium of world governments, including from EU member states, the US, China and Russia. It is expected to be the last step in proving nuclear fusion can become a reliable energy provider in the second half of this century.

Located at Cadarache, in southern France, it is highly controversial for having already cost [Check!!] more than €10bn, the project will not be fired up until the mid-2020s.

ITER is a tokamak device – a sort of ring-shaped magnetic chamber. Until now, tokamaks were the only way to contain the highly energetic plasma involved in nuclear fusion.

ITER’s toroidal vessel volume will be 10 times that of JET.

The image shows four nuclear fusion reaction equations.
When deuterium and tritium fuse, the two nuclei come together to form a helium nucleus (an alpha particle), and a high-energy neutron releasing 17.59 MeV.  Image: Hyperphysics

The French lab’s preferred “fuel” to create the plasma will be a mix of two isotopes of hydrogen called deuterium and tritium.

It is hoped the French lab will get to break even.

The commercial power plants of the future should then show a net gain that could be fed into electricity grids.

Powering the Planet

If the process of nuclear fusion can be successfully replicated on Earth, it holds the potential of creating a plentiful supply of low-carbon, low-radiation energy.

However, turning fusion into a commercially viable energy source has so far proven elusive. 

First, it must generate safely and sustainably more energy than the amount being put in.  As yet, no one has reached this point, despite an 80-year effort to “build a star on Earth”.

And the process will need to be scaled up, which will mean a delay of perhaps another few decades.

Many technical challenges remain.


Herein lies the problem.

The need for carbon-free energy is urgent – and the government has pledged that all electricity in the UK must be zero emission by 2035.

It means using nuclear, renewables and energy storage.

TAE Technologies Team Photo. Many fusion physicists are only just at the start of their science careers. Image: Klimatupplysningen

With nuclear fusion, the world is in for a very long game.

Among the 300 scientists working as JET, a quarter of them are in the early part of their careers.  They will carry the baton of future research.

“I want to deliver fusion first, but anyone who does it is a hero,” Dr Michl Binderbauer, TAE Technologies CEO


In Europe, these challenges are being worked on by the Eurofusion consortium, which comprises some 5,000 science and engineering experts from across the EU, Switzerland and Ukraine.

The United Kingdom is a participant too.  But its full involvement in ITER will require first for post-Brexit Britain to “associate” with certain EU science programmes.

Currently, JET cannot actually run any longer because its copper electromagnets get too hot.  JET is likely to be decommissioned after 2023.

For ITER, internally cooled superconducting magnets will be used.  ITER will begin plasma experiments in 2025.

From safety regulations to the complexity of geopolitics, the challenges are immense.  Some in the fusion community hope that new thinking and disruptive technologies could help shatter this paradigm.