JET DTE2 Results

JET DTE2 Results

An exciting moment for Culham and the global fusion effort! 

 

Pictured: Fernanda Rimini (left), head of plasma operations for JET, at an interview with Sophie Raworth (right) for BBC1's Sunday Morning.

Recently the world’s largest operational tokamak, the Joint European Torus (JET) situated at UKAEA’s Culham site, recently completed the “DTE2 campaign” and announced some exciting results. Since then, these results have been reported by the media – including the BBC, Sky News, CNN, The Times and The Sun – which is no surprise as this is some of the biggest news from JET since breaking the world record back in 1997.

But what on Earth was the DTE2 campaign and what do the results actually mean? Hopefully this blog will give you a general idea of why everyone is very excited down in Oxfordshire!

Let’s start with a little history. JET is a tokamak, which means it is effectively a doughnut of plasma surrounded by magnets. It has been operating since 1983 and is a research tokamak so it gets “rented out” to EUROfusion to do different experiments related to furthering our understanding of fusion energy and how to keep the plasma in the shape we want it. One of these long running experiments was called the “DT campaign” which ran in 1997.

It’s called DT because this is the fuel which it uses; a mix of two hydrogen isotopes, deuterium and tritium. These are like regular hydrogen, but with one and two added neutrons respectively. We use these fuels because when they combine (by fusing the two nuclei), they release a lot of energy which is the whole purpose of our fusion experiments! We don’t use tritium very often because it is currently quite rare and expensive, but future machines will be able to produce their own tritium from a reaction with lithium.

JET’s first experiment with tritium was called the Deuterium-Tritium Experiment 1, or DTE1 for short, and this proved for the first time that this fuel mix would work well, and that the tokamak could hold a plasma for 5 seconds. In JET’s case, 5 seconds is the maximum time because the copper magnets get too hot after this, and need to be switched off to cool down, a problem which newer machines with super-conducting magnets will not have.

Now looking back to the recent results from DTE2, which as the name suggests is a successor to DTE1. Similarly to DTE1, we wanted to show how we could still hold the plasma for the maximum 5 seconds, even with all the internal upgrades to JET since 1997 to make JET effectively a prototype for its successor called ITER. We managed to do this, as well as get almost three times the maximum energy out of the plasma while holding it; an extremely promising sign for the future progress of fusion! 

Video: In-reactor footage of the record-breaking JET pulse. Note that the bulk of the plasma is invisible to the human eye. The pink visible part is the plasma edge, which is cool enough to emit in visible light.

Something that is brought up a lot is the Q factor. This is the fusion energy gain factor which is the ratio of fusion power produced to the power required to keep the plasma in steady state (ratio of energy out to energy in). In this experiment we were not trying to make Earth-shattering improvements to this Q factor; we know that with a larger vessel that we will be able to produce energy, so instead what we were looking for was a good confinement time. This time round we cared far more about keeping these high energy plasmas in the plasma state for as long as possible. This goal becomes clearer when the Triple Product (an important fusion energy concept) is considered. The Triple Product relates the energy we could get out of a tokamak to the density, temperature, and confinement time of the plasma inside. JET is already one of the hottest places in the solar system (when operational) and increasing the density much more is not really possible on this size scale, so increasing the confinement time is the better option.

So, from that perspective, let's have a look at the results. In the DTE2 campaign we managed to sustain a high-power plasma for a full 5 seconds and on top of that, the energy of that plasma pulse was 2.68 times that of those ran in 1997. As a small bonus this also broke the current energy record, so we are also making progress at an impressive rate even if it wasn’t the aim of this experiment.

And the big question, did the DTE2 campaign solve fusion?!?!? Well, no but it was never meant to solve fusion. JET is a research tokamak so was never specifically meant to be used to produce fusion energy for public consumption and it is not equipped to be able to hold onto a plasma long enough to reach breakeven (matching the energy in to the energy output), or actually produce electricity from any energy it did output. We are already confident that a bigger vessel such as ITER will be able to hold the plasma for longer, and now with this data we know that at the start of a plasma pulse we can sit comfortably at a high-energy output scenario.

In conclusion, this data is a big milestone in our quest to realise a safe, sustainable, low-carbon future of energy production; we have shown the world that this “mini-ITER” works, and we are now ready to finalise the full-size version in France. This is not the end of JET as we still have another couple of years of very exciting experiments, but it is extremely rewarding to share this result with the world as the culmination of two decades of progress. So, a big well done is definitely needed for the hard work from everyone involved in the DTE2 campaign!

Written by: Amy Bleasdale, Hermione Salter, Alistair McShee

All About the Active Gas Handling System

Many across UKAEA, not just amongst the graduates, will never get to see inside the building housing the active gas handling system and the labyrinth of equipment that makes JET the only fusion device in the world that can currently run with tritium. The invitation for a visit was therefore eagerly snatched up by graduates wishing to see inside JET’s fuel processing annex. James O’Callaghan and Hannah Todd, 2nd year graduates, have previously given tours to their graduate peers last year and now it was time to induct the new cohort into the mysterious world of tritium. 

The Active Gas Handling System or ‘AGHS’ is currently readying itself for DTE2 (the 2nd deuterium-tritium experiment). This has involved an immense amount of effort from the H3AT department (Hydrogen-3 Advanced Technology) who are upgrading and commissioning systems that have not been operated in full since 1997 when the first DT campaign broke the record for fusion power produced at 16.1 MW. With tritium capability restored, JET is poised to break the record for fusion power in the DTE2 campaign by the end of 2020. 

But first, a quick 1-0-1 on tritium. Tritium is an isotope of hydrogen. In tritium world, ‘normal’ hydrogen (1H) is termed protium and heavy hydrogen (2H) is called deuterium. Super heavy hydrogen — so called because of its extra two neutrons — is tritium. These extra neutrons make tritium nuclei unstable and radioactive. Hydrogen-3 decays to Helium-3 by β- decay — the conversion of a neutron to a proton and emission of an electron and an antineutrino. Hydrogen’s small nuclei can permeate through the atomic lattice of materials. This causes tritium to be retained in pipework and vessels, reducing the efficiency of the process and increasing the tritium inventory in the material. Additionally, decay to 3He can cause structural holes and the emitted electron (Beta particle) can go on to irradiate the rest of the material. Tritium preferentially exchanges with protium in H2O to form HTO. Use of oils and plastics is avoided in AGHS as tritium exchanges with the protium in the hydrocarbon chains. These are a few of the challenges faced by those designing a tritium compatible fuel cycle.

The Active gas Handling system contains subsystems all with different roles to play in the fusion fuel cycle. There are some key systems that must be present. The first system is responsible for taking the gas from the JET tokamak back to AGHS. For this we use a cryogenic pump — much quicker than mechanical pumps at low pressures. The second is impurities processing. Exhaust gas from the JET tokamak must be processed to remove everything that isn’t hydrogen such as water, hydrocarbons, nitrogen, helium and oxygen. This is achieved using a variety of chemical modules that can process and remove a specific type of impurity. To obtain the optimum 50:50 D:T plasma, an isotope separation system is needed. This consists of cryogenic distillation columns and a gas chromatograph. The pure deuterium and tritium can then be stored ready for re-injection into the tokamak.  

Due to its radiological properties the systems must be designed to limit any release of the radioisotope to personnel and the environment. The plant purges system secondary containments, detritiates exhaust gases before release to the environment and monitor the composition of the gas at each stage. This is achieved by our Over Under Pressure Protection System, which purges outer containments, and our Exhuast Detritiation System, which removes tritium by producing tritiated water. This water is processed to recycle the tritium back into the fuel cycle by our new system, the Water Detritiation System. AGHS is continuously staffed during a D-T campaign. Operators in the control room must process the exhaust ready for the next pulse, respond to any alarms and know the protocol for different emergency situations.  

But what is it like to work in such a specialised chemical plant? Tritium engineers are faced with very different problems to the rest of the chemical industry. Oils and plastics are out; 316 stainless steel is in. Parts are bespoke and their tritium capability unknown. The control room is intense and busy. Operators must be aware of everything that is going on to protect people, environment and plant. Tritium scientists are advancing the technology for future fusion machines like ITER, DEMO and STEP. Small scale experiments are performed to prove viable technologies; tritium’s unique properties are not easily modelled. Working with tritium is just as weird, rare and volatile as the isotope itself. 

What did the graduates think of this one-of-a-kind experience? 

A very informative tour. And by far the cleanest chemical plant I've ever been in. 

I really enjoyed this tour — the guides were very knowledgeable and were able to answer all my questions about an area of plant I was very unfamiliar with.  This area of plant has cryogenics, radiation and stainless steel galore — all wondrous to behold. 

The storage solution for tritium was interesting and unexpected. 

I remember being amazed by the amount of Chemical Engineer work that went into a physics experiment. Learning about the Gas Chromatography and the Cryogenic Distillation to recycle unused deuterium and tritium was fascinating.