Optics for Fusion

by Institute of Physics: Oxford Centre


Welcome to the crazy world of fusion optics…

About this Event

IOP Oxford Public Lecture:

Optics for Fusion by Dr Alexandru Boboc from the UK Atomic Energy Authority (UKAEA)


Recorded on 22nd July 2020

This event was hosted as a Public Lecture by the Department of Physics at the University of Oxford (Outreach Team).

How hot is hot? Do you think that a drone camera filming 6000°C lava on a volcano is hot or the core of the Sun at 15 million degrees very hot? How about 100 million degrees?

And how about magnetic fields? Any typical optical mount will cope with magnetic fields of a fridge magnet with 0.001 Tesla or at worst with a very expensive neodymium magnet of one Tesla. How it will do with 10 Tesla field then?

And what would the electromagnetic forces do to an opto-mechanical assembly with plasma of one million Ampers at only a meter away?

Plenty of optics are designed for high vacuum environment. But what would the chance of survival be for advanced electronics and controllers in a neutron flux with energies five times higher than the typical fission plant? Then let’s add x-ray and gamma radiation as well!

Put all these conditions together: this is what an optical system for nuclear fusion needs to withstand.

Alexandru Boboc started his research in the early part of his career at University of Padua, where he completed the PhD. He then moved to Ireland at University of Limerick as Marie Curie Fellow where he worked in nanotechnology.

In 2004 he returned to fusion by joining the UK Atomic Energy Authority as the physicist responsible for the JET far infrared interferometer/polarimeter diagnostic, one of the two key diagnostics for this machine to operate.

JET fusion experiment, located at Culham, Oxfordshire, is currently the only fusion facility able to work with the fusion fuel of the future power plant: deuterium and tritium (D-T).

Alexandru substantially enhanced the diagnostic capability both in terms of performance and reliability over the years, his current focus being on diagnostic preparation for JET D-T campaign, a first one since 1991! Some of his research interests are in laser-based diagnostics design, implementation and operation in harsh nuclear environments, and real-time systems used for safety systems in a nuclear fusion plant.

He is now recognised as an international expert in the field with selection as member of various executive panels and expert groups on diagnostics development including ITER and JT60-SA. Alexandru is a Chartered Engineer and Chartered Physicist with Institute of Optics as well as member of Optical and ISAT groups within IOP.

The event was organised by the Institute of Physics Oxford Centre and Oxford University in conjunction with the Institute of Physics Optical and ISAT groups.

Images copyright UKAEA.

ITER – “the way” in Latin, a worldwide collaboration to build the world’s largest tokamak, a magnetic fusion device that has been designed to prove the feasibility of fusion as a large-scale and carbon-free source of energy based on the same principle that powers our Sun and stars)

JT60-SA – Japan Torus 60 Super Advanced. A fusion experiment designed to support the operation of ITER and to investigate how best to optimise the operation of fusion power plants


The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope Dr Baboc and my readers will forgive any mistakes and let me know what I got wrong.

Everybody needs energy

There are more people living today than have ever been


The graph above shows the world growth through history

Green energy is very desirable now because we need to reduce our carbon emissions and because we are running out of non-renewable energy sources.


But even in a forward-thinking country like Germany, after 20 years of “green” energy incentives, nearly 60% of energy requirements are produced from non-renewable energy sources (40% fossil fuels and 20% nuclear) in 2019.

It is a similar picture in the UK

The problem with nuclear energy isn’t just because of the waste products but because about half of the nuclear power stations are old and need to be retired and there is nothing to replace them.

The problem with “Green” energy


The Sun doesn’t always shine, the wind doesn’t always blow and energy storage is a problem

Tesla installed the largest battery into the electricity grid in Australia in 2017.

129 MWh of energy at a cost of $ 50 million, equivalent to $ 388 per available kWh.

The cost per 1 kWh electricity is generally ~ 5 cents. If the battery lasts up to 1000 charge / discharge cycles, add 38.8 cents per kWh.


129 MWh is only enough to power half of Oxford

How much waste is generated by?

For producing 1000MW one year we need …


Coal and oil generate a great deal of waste, CO2, SO2 and NO2

Fission is very efficient but produces a great deal of radioactive waste

Solar is very economic when up and running but it is expensive to set up and a lot of space is required

Fusion is one billion times more efficient than coal and helium is the only waste product (however it needs more electricity to work than is produced at the moment)

Nuclear fusion

The engine of the stars and the energy of the future

Sun burns 600millions tons of fuel every second

Nuclear fusion on Earth

Up to 80% of reaction energy can be converted in electricity


Advantages of Fusion …

Less radio-toxic Waste than Fission (Structural material becomes radioactive in fusion)

No CO2, the only waste product is a small amount of helium

Abundant fuels – Deuterium can be extracted from water and tritium will be produced inside the reactor

Safe – there is no risk of a runaway reaction with fusion

Continuous supply – Fusion will provide reliable power and costs are predicted to be similar to other energy sources

There cannot be economical wars over fuel reserves in case of fusion as no none has the reserves

Fission fuel – how to get it on Earth


Deuterium (or hydrogen-2, symbol 2H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus. Deuterium has a natural abundance in Earth’s oceans of about one atom in 6420 of hydrogen. Thus, deuterium accounts for approximately 0.02% (0.03% by mass) of all the naturally occurring hydrogen in the oceans, while protium accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another

Extracting deuterium from seawater is a simple and well proven industrial process. “Heavy water”, or D2O (water in which deuterium substitutes for hydrogen), is first separated from regular water by chemical exchange processes, and is then submitted to electrolysis in order to obtain deuterium gas.



Tritium (radioactive with a half-life of about 13 years)

Obtained inside a reactor via “breeding” from lithium


Why do we need optical systems on a fusion machine?

JET uses lasers, ultrasound and magnetic confinement


Inside a fusion reactor there are only few grams of fuel at a time



The plasma reaches temperatures of 10-100 million degrees C (much higher than our Sun)

No probes can be inserted inside the plasma

Only laser and microwaves can penetrate the plasma safely without destroying it

Plasma is very difficult to control



Types of optical systems used on fusion

(The list is not comprehensive)


Working conditions for an optical system in a fusion reactor


The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of current fission power reactors.

Existing optical diagnostics

JET Vacuum Ultraviolet Spectrometer

Spectrometers are a very good indicators of reactor wall damage. They show up things that shouldn’t be there and show if everything is working fine


JET Far Infrared Interferometer/Polarimeter Diagnostic

Far Infrared radiation (THz) is the only one for which plasma is a transparent medium like glass


Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm (corresponding to a range of about 20 THz to 300 GHz), which places far infrared radiation within the CIE IR-B and IR-C bands. The long-wave side of the FIR spectrum overlaps with so named terahertz radiation. Different sources use different boundaries for the far infrared; for example, astronomers sometimes define far infrared as wavelengths between 25 μm and 350 μm.


Deuterated cyanide (DCN) FIR laser discharge

JET High Resolution Thomson Scattering Diagnostic

The laser beam is so powerful that it could blind someone on International Space Station




Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism.

JET Imaging Cameras

On JET an advanced 3D mapping with AI detects hot spots on the machine wall in real-time


JET Bolometer


Where is Fusion NOW

Target of JET D-T campaign is full plasma control for 5 seconds at 30MW heating power

• March 2021 – JET Deuterium- Tritium campaign using real reactor fuel mix

• 2025 – First plasma in a fusion reactor called ITER

• 2035 Deuterium-Tritium campaign operating in reactor mode (Q = Pout/Pin >> 1)

• Many countries work on DEMO commercial reactor already

–> UKAEA works on Spherical Tokamak for Energy Production (STEP)


New optical diagnostics are being developed for fusion

ITER – First ever fusion machine to run in reactor mode *



ITER (originally the International Thermonuclear Experimental Reactor) is an international nuclear fusion research and engineering megaproject, which will be the world’s largest magnetic confinement plasma physics experiment. It is an experimental tokamak nuclear fusion reactor that is being built next to the Cadarache facility in Saint-Paul-lès-Durance, in Provence, southern France.

First machine to run in reactor condition with NET plasma energy gain 50MW => 500MW

All optics inside designed to be shielded for radiation, thermal and humans. Nothing that could melt goes into the shielding



Credit www.iter.org/doc/all/content/com/gallery/media/7%20-%20technical/tkmandplant_2016_72dpi.jpg

The pink = the plasma



Next generation of optical diagnostics* ITER Bolometers


A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance.


Real-time tomographic reconstruction with 500 lines!


JET has “only” 48

source https://www.iter.org/newsline/-/3415

Next generation of optical diagnostics*

ITER Dispersion Interferometer

Preliminary design agreed July 2020 to be ready before 2025!


Instead of conclusions



JET is to the first phone as ITER is to the first smartphone

Optics plays a crucial role in providing measurements essential for operating a fusion plant

We need to develop new optics for fusion and here is where new generation of physicists and engineers will play a crucial role

Electricity from Nuclear Fusion is a question of when, not if!


Dr Boboc would like to thank his UKAEA colleagues for all the information provided for this talk and in particular to Oliver Moore and James Davies that with IOP Oxford and Physics Department of Oxford University made this Webinar possible.

How to find out more…

On the web:

www.ccfe.ukaea.uk www.euro-fusion.org

By email: Communications@ukaea.uk


Questions and answers

1) why does JET need to operate at 5 to 10 times hotter than the sun when the sun achieves fusion at its lower temperature?

The sun burns at lower temperature because are incredibly dense and enormous gravity forces (confinement). The nuclei are already squished close together to the energy needed to fuse is relatively low, therefore the temperature required is not that high than in conventional Earth experiments.

In a tokamak, the plasma is much thinner, and the force of the magnets is not enough to fuse the atoms, therefore much higher temperatures are required.

2) What would plasma look like if you were able to see it safely?

Plasma is transparent in visible radiation, in infrared and x-ray looks different. There are plenty of pictures of the plasma at www.euro-fusion.org

3) How will the fuel be introduced into the torus and how are any waste products removed when the reactor is running continuously?

The Deuterium is introduced either in form of gas or frozen pellets with a pellet gun. Tritium is generated as a co-reaction product inside breeding tiles that have a high content of Lithium

4) How can the reactor withstand such heat? Is there a cooling system and how does it work? And what would an accident/meltdown of a fusion reactor look like, what would be the consequences?

No material can sustain 1 millon oC, not to mention 100. The plasma is kept in suspension by the magnetic fields in the middle of the reactor. Active cooling systems are envisaged around all the machine, the latest technology being a liquid metal coolant technique.

5) Do you believe that fusion will be our final source of energy, or will we need to develop new methods far in the future?

Fusion is the ultimate source of energy; however, I believe that by the end of the century it could contribute towards 30% of electricity production.

6) How would you collect the energy from fusion reactions?

The neutron generated from the reaction will be stopped by the coolant systems on the machines and these will transfer the heat to standard steam electrical generators as in the case of nuclear fission plants.

7) Will the UK still have a notable contribution to ITER after Brexit? It’s associated with the EU isn’t it?

There are plans to associate in ITER as an Euroatom associated member. Euroatom membership is not only for EU states.

8) Fission created a large amount of political backlash due to concerns of safety, will Fusion put these people at ease?

Fusion was never publicised properly until recently and is hard to clean the stigma linked with the term “nuclear”. It is important to educate people more of the advantages of fusion and much, much, lower risk than fission.

9) What considerations have been taken for constructing an alignment preserving optical system? some of the mirror holders for example would be susceptible to misalignment between power up and continuous operation.

Most of ITER optical systems are envisaged to be provided with automatic mechanism composing tilting mechanisms combined with quadrant detectors inside a PID controller with real-time control.

10) I find it difficult to believe that you stated 80% of the energy is converted to electricity. If you are talking about a machine like ITER which is effectively a large kettle and the laws of thermodynamics suggest more like 30% Julian Marks

I said up to 80%, of course is ideal at this point, how much will be actually converted in electricity due to losses is impossible to predict at this moment as we never had an experiment with net gain power (reactor mode).

11: what chemical do you use for cooling?

Liquid nitrogen, liquid helium, water, special coolant and also metal coolant envisaged at the moment etc

12) How does laser confinement fusion work, and how do the diagnostics differ?

There are plenty of videos and presentation of this, the main principle is that many powerful lasers (google NIFS) focused so much that the light will cause a shockwave to a pellet made of D-T that will fuse this. However, it is extremely inefficient method at the moment.

13) How are the extreme temperatures in the ITER reactor achieved?

By ohmic heating (magnets), radio frequency, neutral heating systems (the main one)

14) how is the suspension of the plasma realised?

By means of magnetic fields. Another example are the bullet (Maglev) trains that are suspended in air by strong magnetic fields.

15) Strictly no coatings were mentioned for optics due to temperatures around 400 oC.

All optics in the vicinity of 25 m of machine will have no coatings and most of the mirrors are metallic. The component in diagnostics areas would have coatings as normal as this will be accessible to humans on daily basis.

16) How could the use of optical fibres help?

Using specific imaging techniques with the appropriate lenses, curved mirrors, the cameras will be able to see the area of interests from far distance via fibres from area where there the radiation levels and neutrons do not pose a risk to the equipment (ex. pixel burning)

For anyone who missed the talk, or just thought it was so good they want to see it again, here is a link where you can do just that:



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