The Joint European Torus (JET) at Culham has been Europe’s flagship tokamak for nearly four decades. It holds several world records, such as fusion power produced (16 MW, achieved in 1997 [1]) and largest ratio of fusion power out to heating power in (Q=0.67 [1]). It is one of only two fusion machines to ever run experiments with a deuterium-tritium plasma. UKAEA operates JET for the EUROfusion consortium, which runs a research programme involving hundreds of scientists all over Europe.





The latest JET campaign, C38, is a precursor to next year’s DTE2: the first deuterium-tritium (D-T) experiment since 1997. DTE2 will act as a ‘dress rehearsal’ for ITER, allowing new insight into the behaviour of deuterium-tritium plasmas and testing components for ITER in more extreme conditions. With the C38 campaign resuming this month after a 3-month postponement due to COVID-19, we decided to have a look at some of the progress JET has made in recent years, and talk to some of UKAEA’s graduates who are helping to run the world’s largest and most powerful tokamak.





Making progress

This is the 38th experimental campaign on JET, and the total number of experimental pulses is now over 97000. While JET holds many records, most of these were set in 1997, so it seems fair to ask: what has been the point of all these JET pulses since?





The JET control room in 1997 during the first full D-T campaign (source: CCFE), and now during preparations for the second (source: EUROfusion).



The first thing to say is that fusion, and plasma physics in general, is really hard. This isn’t just the ego-stroking sentiment of a physicist but an unfortunate reality. However, this is not to say we have made no progress. A tired stereotype of fusion is that it hasn’t really gone anywhere since the 1980s which, like most stereotypes, has little grounding in reality.



Up until the turn of the century, the record for the triple-product (a measure of fusion confinement) increased at a rate even faster [2] than the famous Moore’s Law, where computer chip processing power doubles every 18 months. The achievements on JET played a key role in this. Since the last D-T campaign, work on JET has focused on testing ITER components and making important advances in plasma control [3].



What the stereotype really hits at is that we haven’t built anything closer to a real fusion power plant than JET in the last 30 years. This is being rectified by the construction of ITER, the next-generation European tokamak scheduled to begin D-T operation in the mid-2030s. ITER should surpass breakeven (more fusion power out than heating power in) and show the viability of other power plant technology, like making tritium using breeder blankets and holding plasmas stable for up to an hour. This would then bridge the gap to EU-DEMO, the hypothetical demonstration fusion power plant marked for construction sometime near the middle of this century, and other energy-producing devices.





Looking to ITER

In recent years, much of UKAEA’s research has gone towards the design of ITER. It is this role which makes JET so crucial, as it can test aspects of the ITER design in a real tokamak. In the past it was imagined we could easily just scale up small-scale fusion machines to create power plants, which has proven to be unrealistic.



The complex behaviour of fusion plasmas means that JET is one of the few places in the world which can generate data for and physically test new technologies for future large tokamaks. Good examples of this are the ‘ITER-like wall’ of beryllium-tungsten tiles installed in 2011 [4] and the Shattered Pellet Injector (SPI) installed in 2019.









The ITER-like walls in JET, coloured by material. Source: https://www.iter.org.





The SPI generates frozen pellets of deuterium and neon gas which are shattered and propelled into the plasma to suppress instabilities (bouts of turbulence which affect the plasma’s performance). These gas pellets rapidly decrease the plasma temperature, dissipating energy and reducing potential damage to the tokamak. This technology will be crucial for ITER, ensuring the much larger and more energetic plasma will not damage the machine during disruptions – sudden terminations of the plasma caused by instabilities.



Getting JET ready for D-T has required monumental effort from many people across site to upgrade and re-commission systems which have been unused for over 20 years. There have also been significant upgrades to the machine since 1997. The neutral beams recently set a new record heating power of 32 MW [5] and record neutron rates have also been broken. This all puts us in a good place to achieve excellent performance which will be invaluable for ITER’s preparations.



It has been 37 years since JET generated its first plasma but there has arguably never been a more exciting time to be involved. Here are the thoughts of some of the UKAEA graduates about what it’s like to work on JET.





- by Tom Wilson, 1st-year graduate, NBI Shift Leader





The graduates at JET



Matthew Magness, 1st-year Graduate, NBI Operator:

I joined the graduate scheme in September 2019 as a mechanical engineer, with my primary work being on STEP – UKAEA’s fusion power plant design project. In November, I began training as an operator for the neutral beam injection (NBI) heating system on JET. This system provides upwards of 30MW of heating power to JET during pulses so is a really critical system to allow JET to achieve its goals during campaigns.

As an operator, my primary role is to monitor and condition the positive ion neutral injectors (PINIs), which provide the heating power, in the lead up to and during pulses, as well as to liaise with the NBI shift leader to ensure we operate at the heating power and duration required. I completed my training for this role in February in preparation for the C38C campaign, however this was delayed until July due to COVID-19, and I am awaiting my first shift of this campaign. For this campaign, I’m hoping we can continue to see reliable high heating power from the NBI system in the lead up to DTE2 in the near future.

Peter Cooper, 2nd-year Graduate, Viewing System Operator:

I’m coming to the end of my second year on the graduate scheme, working as an engineering analyst in the Office of the Chief Engineer. I trained as a viewing system operator (VSO) for JET about a year ago and since then I do shifts about once a week.

The VSO’s role in the control room is essentially to be the eyes for the session leader (SL) and engineer in charge (EIC). The VSO has access to both visible and infrared cameras views from inside the vessel. Some of these infrared views are also used in the vessel thermal map (VTM) which is part of the protection system and can cause an alarm, stopping the pulse if components get too hot. If this happens, it is the VSO’s job to determine if it was a false alarm and inform the SL and EIC what caused the alarm so that they can adjust the next pulse or take the necessary action.

As VSO, I review both visible and protection cameras after each pulse, looking for abnormalities. These could be a variety of things including new, localised, high temperature areas, debris flying around the machine (UFOs) or an unusual pulse termination. I log these abnormalities and if necessary, inform the SL and EIC. Depending on the experiment being performed and equipment being tested, I may be asked to monitor for specific phenomenon such as pellet injection or arcing on the ICRH antennas.

Restart pulses, like the ones going on now to prepare for the upcoming campaign, can be tedious for the VSO, but I’m looking forward to the high-power pulses to come, especially DT!

Edward Litherland-Smith, 2nd-year graduate, Charge-Exchange Spectroscopist:

After graduating with a degree in Physics and Technology of Nuclear Reactors from the University of Birmingham, I started as a Graduate Charge Exchange Spectroscopist; In this role I work on the KS5 core Charge Exchange Recombination Spectroscopy (CXRS) system, as well as maintaining and updating the analysis software CHEAP. This involves assisting in operation of the diagnostic and analysis of the data produced with the rest of the spectroscopy team. During the restart period before we've been working on commissioning the diagnostic ready for the campaign, involving various calibration pulses to produce useful measurements for the coming experiments. Going to higher power regimes in the new campaign will allow new and interesting physics to be explored, some of which will be for the first time!

Hannah Todd, 2nd-year graduate, AGHS Control Room Duty Officer:



I am a chemistry graduate working shifts within the Active Gas Handling system. The Active Gas Handling system is another vital part of JET operations. It controls the collection, processing and redistribution of the hydrogen fuel sources for JET. The system is responsible for safely processing the radioactive hydrogen isotope tritium, other hydrogen isotopes and various exhaust impurity gases. Without the Active Gas Handling System, JET could not operate experiments using the currently most thermodynamically viable fusion reaction of deuterium and tritium.

The demands of a control room duty officer vary day to day. This week I have been involved in liquid helium system commissioning, exhaust gas discharges, deuterium fills of the JET gas introduction modules, making safe gas mixtures to avoid explosive atmospheres, safety alarm testing and a multitude of other jobs. This interaction mainly involves liaising with the operators on plant while we work the remote operating system. The shift team are also responsible for the safety of everyone within the building. We are trained in responses to certain alarms and emergency evacuations/procedures. It can be quite daunting at first but quickly you become familiar with all the sub-systems, standards ops, daily checks and operation of the control system. Being in the control room is the best place to get practical experience and gain knowledge on all the requirements of such a specialised chemical plant.









References and further reading

[1] The Scientific Success of JET,

M. Keilhacker et al., Nuclear Fusion 41, 1925 (2001)

[2] ITER on the road to fusion energy,

K. Ikeda, Nuclear Fusion 50, 014002 (2010)

[3] Advances in understanding and utilising ELM control in JET,

I. T. Chapman et al., Plasma Phys. Control. Fusion 58, 014017 (2016)

[4] JET ITER-like wall – overview and experimental programme,

G. F. Matthews et al., Physica Scripta T145, 014011 (2011)

[5] Record heating power achieved on JET (CCFE, 14/11/2019),

https://ccfe.ukaea.uk/record-heating-power-achieved-on-jet/







