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

The Graduates Celebrate First Plasma at MAST-U

The MAST-U team. Source: UKAEA 

 
On 29th October, UKAEA announced that after 7 years of preparations the MAST-U tokamak had achieved its first plasma. MAST-U stands for the Mega-Amp Spherical Tokamak Upgrade. Pretty self-explanatory, right? It’s a tokamak which is spherical and designed to create plasma with currents on the order of megaamperes. From the U for Upgrade, you’d assume it’s just a case of improving current equipment from the original MAST, simple right? Nope, almost 90% of the machine is new! So it’s practically a new machine, with tons more functionality than the original MAST. 

You may have heard of other non-spherical tokamaks like JET, which has set records for fusion power. So you couldn’t be faulted for asking the question ‘why make a tokamak spherical?’ Well, spherical tokamaks come with some major advantages. Due to the smaller ‘hole’ in the centre of the plasma, it’s believed we can get higher power generation at a lower cost compared to normal tokamaks. This is because the magnetic field-generating coils are closer together so the field is naturally stronger and less electricity is required! In addition, the reactors are generally lower in cost as they can be built smaller. 

The history of spherical tokamaks is longer than most at Culham. In 1990 UKAEA built START (the Small Tight Aspect Ratio Tokamak – an informative name) which was the precursor to MAST. MAST was built over the course of 3 years and operated from 1999 until 2013. The tokamak conducted key research into edge-localised mode (ELM) mitigation and the effect of high beta (the ratio of plasma pressure to the pressure from the confining magnetic field) on plasma, and its effect on the rate of fusion. 

 

The MAST-U vessel. Source: UKAEA

In 2007, MAST was operating at its engineering limits and big questions still remained for fusion research. Prime among these was how to handle the exhaust plasma produced in the reaction and how to dissipate that heat. Not to mention the ever-tantalising question of what happens if we put even more energy into it. The proposal was made and it was agreed that MAST would shut down, beginning its journey to upgrade. The challenge: introduce coils capable of higher current (130 kiloamps in the case of the toroidal field coils), raise the operational time to 5 seconds instead of 2, and add an off-axis neutral beam injector to investigate plasma current profile control and mitigation of instabilities. 

Most daunting of all, a novel divertor concept not seen anywhere in the world: the Super-X configuration. An area at the top and bottom of the tokamak capable of accepting plasma exhaust and, through the application of additional magnetic field coils, spread the plasma over a wide area to dissipate heat more effectively. Why have one configuration though when you could have many! By changing the magnetic fields, we can investigate standard (here ‘double null’), Super-X and snowflake divertor modes. These are all candidates for the plasma exhaust solution: how to dissipate heat from the 100–150 million-degree plasma without melting the bottom of the reactor.

 

Magnetic field lines for the potential MAST-U divertor modes. Source: MAST-U Research Plan, November 2019. 

 

The upgrade has experienced its fair share of hiccups during construction and commissioning. We’ll say no more about the source of the mysterious vacuum leak or the elusive earth fault (both now resolved). Big projects are bound to have problems though. Just look at NSTX-U or a football stadium. The difference being MAST-U has ~19km of cabling and fits in a 16m x 19.2m room. 

MAST-U is a testament to the dedication of engineers and scientists to build the bridge fusion desperately needs in its journey toward commercialisation. Seeking to learn more about heat dissipation in the hottest place on earth and whether a change in geometry can potentially shave acres of land off the final power plant size.

 

The MAST-U hall. Source: IOP

Meet the graduates 

The MAST-U graduates Mike, Jimmy, Sarah, Mourmour, and Ellen joined the MAST-U project in 2018 and 2019. Their work has contributed to putting MAST-U over the finish line, with their efforts in commissioning since April vindicated by first plasma this week. Here's how they've been helping. 

 

Ellen Wright, 2nd-year Graduate Electrical/Electronic Engineer: 

I started the graduate scheme last year in the Electrical Engineering department, our team works across many projects at UKAEA but most of my work has been on MAST-U and MAST-UE (MAST-U Enhancement). 

For MAST-U I have been involved in the plasma control system (PCS), I have been documenting the PCS interface, assisting with some new control algorithms and am working on converting a simulation that could be used to test the system without having to connect it to the machine. I have also been lucky to be involved in some cable termination work, a good opportunity to practise practical skills! All of this work has been very enjoyable and rewarding. 

This was an exciting time to join MAST-U in the final stages of commissioning, I am looking forward to seeing operations and everyone's hard work paying off and facilitating important research (and also all the future inevitable problem-solving opportunities!). 

 

Mourmour Man-Friel, 2nd-year Graduate Mechanical Engineer: 

I started the graduate scheme as a graduate mechanical engineer in the MAST-U operations team on September 2019. Before lockdown my role was mainly to support the activities going on in the Load assembly team; during lockdown I’ve been working on MAST-U Enhancement Chilled Coolant System (CCS) for the centre column and assisted in writing a literature review for STEP. 

The Load Assembly team is responsible for MAST-U vessel internal structures, vessel supports & coils as well as some of the supporting systems. My role varies from producing calculations and supporting documents to assembling components and inspecting the vessel for faults. I had the opportunities to do the following: leak testing on the vessel; help in producing a report on condensation levels in the block house (the room MAST-U is contained in); assembling piezo valves and leak testing them. 

I found that opportunities like this really help me to understand concepts. I’m very excited to go back onsite and to see the machine running, as well as carry out work required for MAST-UE CCS project because it is something we’ve been working on during lockdown. 

 

Sarah Parry-Wright, 2nd-year Graduate Electrical Engineer: 

I am Sarah Parry-Wright a Graduate Electrical Engineer working in the Power Supplies Group on MAST-U. I did an MEng in electrical and electronic engineering at Swansea University and started with UKAEA last April. My work here has involved the installation and upgrade of the radial field power supply, which powers the P6 coils controlling the vertical position of the plasma in the machine. I have been heavily involved in the commissioning of most of the coil supplies we have for MAST-U and am currently working in the control room with the operations team working towards getting ready for our 1st plasma. 

My work has been very exciting and fulfilling throughout my time at UKAEA and I have learned so much while thoroughly enjoying working with an amazing team. 

 

Michael Robson (Brother Nature), Graduate Mechanical Engineer: 

As a graduate mechanical engineer (with a background in physics), I’ve been working on getting diagnostics into the blockhouse (our equivalent of JET’s torus hall) and ready for operations. Working with a varied team I’ve been organising not only the mechanical side of bolting a camera to the vessel but also the electrical, networking and data acquisition of the diagnostic as a complete system. Many of the diagnostics I work on are spectrometers that separate light into its constituent wavelengths. This data tells us about what material is in the plasma (like impurities) as well as how much there is and how it’s moving. 

Alongside this work I’ve been involved in a graduate project designing a test cubicle for the electrical safety course and organising a tour of STFC Harwell’s facilities for the graduates. 

The most difficult thing I’ve encountered while helping with commissioning is the awful puns a member of the data acquisition team comes up with. 

Jimmy Measures, Graduate Software Engineer: 

As a graduate software engineer, I’ve worked on many different projects, from designing and creating data analysis tools to work on MASTU’s plasma control systems. The majority of my work has focused around writing and testing code for the Vertical Controller (or Z Controller as its more commonly known), which controls the power supply to one set of coils. This in turn changes the shape of the magnetic field and is used to alter the vertical position of the plasma and elongation. This device will hopefully enable more scientific research into the plasma’s behaviour during fusion. 

I have also worked on a graduate project to create a site-wide digital logbook. The aim is to prevent the loss of notes and make collaborating easier. On the side of working at UKAEA I also created and organised Grad’Lympics (with a lot of help from other graduates). A 5-month long competition of 14 events, from football to a quiz, to determine which out of the Oxford or Abingdon graduates were best. To find out who won please refer to previous blog posts from March 2019.

Written by Jimmy Measures and Mike Robson

Spectroscopy – more than meets the eye

by Jamie Zacks

 

There must be hundreds, if not thousands, of diagnostic techniques used by scientists around the world, but one that I quite admire is spectroscopy. 

Now there are some scientists who would automatically assume I’m talking about mass spectroscopy, which is a valuable tool for measuring the mass of particles in a system, so you can figure out which atoms or molecules are present.   I, however, am talking about the splitting of light into its constituent wavelengths.

There are stories you hear of tailors who can accurately guess your size from one brief glance at you, and other tradesmen who take one quick look, and can see the problem.  For me, the beauty of spectroscopy is that is it essentially just taking a look, all be it with some slightly more fine-tuned instrumentation than the human eye.  Spectroscopy is, on the whole, passive (I’ll come to some examples considered non-passive later):  Light is emitted by a system, and we just put a device in there to collect some of it.  It’s what we can do with this information which I think is rather impressive.

Figure 1 - Spectroscopy principles

How do we use Spectroscopy at JET?

On JET we have spectrometers covering a large range of the electromagnetic spectrum, from infra-red (~1000 nm) through to the visible (400-700nm) down to the UV (300nm), VUV (Vacuum Ultra Violet ~10-200nm)), XUV (<10nm), XRay(<1nm).  Due to the high temperatures inside the JET plasma, most of the visible emission comes from the edge region.  As you move further into the centre of the plasma, it gets hotter, and the wavelengths seen are usually much shorter, until there’s very little spectral emission we can detect.  Spectrometers can actually take a number of guises.  Some have a grating and will separate light over a specific wavelength range, others may just be a narrow-band filter (a few nm) and a detector.

The first and most obvious use for spectroscopy is to identify the atoms and molecules emitting the light.  The differing electronic structures of particles emit light with a unique set of wavelengths, acting as a signature.  In tokamaks like JET or MAST, we know that the plasma is mostly made of deuterium which gives a pinkish colour due to the main spectral lines at 656nm (in the red), 486 (blue) and 434 (violet). However there are a number of other elements that can be seen in the plasma, and spectroscopy allows us to look for the weaker sources of light.

Figure 2 - Hydrogen Balmer spectrum

In some JET plasma pulses, it has been possible to see carbon, beryllium, tungsten, copper, iron, helium, oxygen, nitrogen and neon.   A lot of these appeared due to their presence in the vessel components, and are often monitored for each pulse.  If the JET plasma is experiencing problems, one possibility is that impurities are causing it, so being able to identify the impurity through spectroscopy is very useful.  During a recent failure of a vacuum isolation valve between JET and one of the neutral beam injection (NBI) beamlines, impurities were seen when the NBI plasma heating system was firing.  This was quickly identified as being copper thanks to one of the spectrometers, and although it didn’t identify the source completely, it showed operators the level of the problem. When copper is being put into the plasma by NBI, it means something is getting damaged, and this could cause a water leak from the extensive cooling system on JET.

Figure 3 - Spectra for various elements

Is it really so simple?
 

Sometimes the impurities present in the plasma aren’t so easy to identify.  If an element has been seen in the plasma previously and has a simple reason for being there, one or two of the stronger lines it emits can be monitored.  If, however there is an impurity problem but none of the usual suspects is showing high levels, things can get a bit more complicated.  When monitoring just a couple of lines, you don’t need to look at the whole wavelength range captured by the spectrometer and recorded on a CCD (Charge coupled device) camera or PDA (Photo-diode array).  You just have a program to analyse the sections of the data that you know the line falls on.   When you don’t know what the problem is, you have to start analysing the whole spectrum by eye, looking for unusual lines that aren’t accounted for by the normally seen impurities.  If you find a line, there is still a lot to do to identify the source of it.  You need to find which materials emit lines at that particular wavelength, and then look for the other lines they would emit, to confirm the identification.  This can be quite a tedious task when you don’t know what you’re looking for, and all the different elements, with all their different ionisation stages can produce a lot of lines throughout the electromagnetic spectrum.

Not quite 10 years ago on JET, there was an impurity problem, again when NBI was running.  This time, however, the copper levels remained unchanged, leaving a number of people scratching their heads.  Eventually, using spectroscopy, the impurity was identified as titanium, which has no business being inside JET!  It was actually found to be from titanium oxide, which is used as a whitening agent in masking tape, a small piece of which had been left on a beamline component, and was slowly being eroded by the neutral beam, sending particles into the JET plasma.

Sometimes trace amounts of impurities are injected into JET on purpose, using either a gas puff, or laser ablation for metallic elements.  Using spectrometers to monitor the locations of these elements helps our understanding of how materials move about in the plasma.

Now, I’ve gone on about the matter of identification of elements a bit longer than I intended, but just wanted to highlight that it’s not easy if you don’t know exactly what you’re looking for.  Now I’m going to talk about the really cool (I only say that slightly ironically) stuff we can get from spectroscopy, in particular on tokamaks.

What else can spectroscopy tell us about plasmas?

The Doppler effect is a pretty well-known effect that if something emitting light is moving fast enough, the wavelength seen will shift red if it’s moving away, blue if it’s moving closer.  This is a pretty key idea in astrophysics and cosmology.  In plasma physics, we have a hot gas emitting light.  This gas will be moving in all directions, so what you see is a broadening of the spectral line, with the width of the line corresponding to the energy and temperature of the emitting material in the plasma.  If your resolution is good enough, and the plasma hot enough, this width can easily be measured, and voila, you have some useful plasma parameters.

Each element has numerous spectral lines associated with it, and the relative intensities of these lines can tell us quite a lot.  If, with computer power, we can accurately model the mechanisms that excite the electrons in an emitting atom, we can often calculate plasma electron temperatures and densities, ionisation and recombination rates, and densities of other species that interact with the emitting species.  The types of mechanism considered will be thermal excitation, collisions with plasma electrons, excitation by light, or collisions with other elements.  Some parameters such as the electron temperature and density can be measured by alternate methods, but most of these will be invasive to some degree, which means we may affect the behaviour of whatever we are measuring.  As I said earlier, the beauty of spectroscopy is that it can be passive. 



Figure 4: Spectrometer on the Small Negative Ion Facility (SNIF) at CCFE. Looks at molecular and atomic hydrogen spectrum.

If we look at the spectrum emitted by a molecule, there are a lot of small lines due to the vibration and rotation of the molecule, and again, line ratios can help find the energy in these.
 

Figure 5: Example of hydrogen molecular spectrum on SNIF

Similar techniques, along with some very impressive telescopic instrumentation, which just require us to look with our eyes, have given us so much information about what is taking place out in the solar system and the universe beyond, as well as inside a tokamak, such as understanding the power produced by the sun – vital if we want to recreate it!

Figure 6: Example of how Thompson Scattering spectra are used

 

Not so passive spectroscopy…

Just a quick note on some of the non-passive techniques that use spectroscopy:  In tokamaks these can be grouped into Thompson Scattering, and Charge Exchange Spectroscopy.  The former fires a laser through the plasma, and as the light is scattered by the plasma electrons, it is Doppler shifted.  Analysis of the shifts was one of the first techniques used on a tokamak to obtain the electron temperature.  The intensity of light scattered can also give the density.  Charge Exchange spectroscopy, and some derivatives such as Motional Stark Effect spectroscopy, make use of the interaction of the neutral beam heating particles with the plasma particles.  The light emitted can be analysed to give velocity and rotation information for the plasma.

So, in conclusion, I just want to say that it’s impressive that we can learn so much by essentially just looking. 

Thanks for reading.

A Tale of Two Tokamaks - Post the First

By James Edwards

See Post the Second here.

It was the best of times, it was the worst of times. It was 1973. Design work officially began on the Joint European Torus (JET). Several of the pioneers of the first fusion devices had already been thinking about how to make much larger tokamaks than any that had existed before, and now they’d been given the permission to go ahead from the Council of the European Community.
 

The JET design team in 1977.

Design work for JET, headed by Paul-Henri Rebut, started off well and progress was quick, but there came to be disputes over where to build the machine. The two contenders to host the international experiment were Germany, which would host it at Garching, and the UK, which would host it at Culham. Neither country was willing to concede, so over the years following 1973, the design team became dissatisfied with the project’s political procrastination.

However, in October of 1977, a Lufthansa aeroplane had been hijacked by members of the Baader-Meinhof gang and Palestine Liberation Organisation. The GSG 9 (part of Germany’s police force) used stun grenades provided by the UK’s SAS to successfully rescue the passengers while the plane was at Mogadishu, in Somalia. After this event, it’s often said that West Germany and the UK came to an unofficial agreement that JET would be built at Culham. About a week after the incident, construction on machine components started.

The JET Torus Hall being built in 1980.

Final design work progressed well with the renewed vigour of the team, and on the 18^th of May 1979, work started on constructing the JET buildings and infrastructure at Culham. As some of the machining for components had been started earlier than this, it meant that the initial functional tokamak was completed by 1983 on time and on budget and by 25^th of June of that year, the first ever JET plasma popped into being. Allegedly, many bottles of champagne were quaffed in celebration.

First plasma in the (very) bare-bones control room.

The first few years involved experimenting with the plasma and how to operate such a large fusion device (and of course, this still goes on), but since then, there have been several other notable events at JET…

In 1991 on the 9^th of November, JET was the first tokamak to use a deuterium and tritium fuel mixture (also known simply as DT), releasing 1.7MW of power for a short time. The deuterium and tritium fusion reactions occasionally used in JET and planned for more extensive use in ITER are the best balance between power output and ease of producing the right conditions in a tokamak for sustainable fusion.

1993 saw an almost entirely new machine setup, with the introduction of the divertor – a system initially thought to be useful to remove impurities and waste helium, but which also turned out to be extremely helpful in making more efficient fusion plasmas by having an effect on plasma density. JET wasn’t originally designed with a divertor, so the whole system had to be constructed inside the vessel, as it would have taken too long to take the tokamak apart to install it. Luckily, there was enough mechanical engineering expertise (and space!) to add this brand new system without dismantling the whole tokamak.

 

The JET Mark 1 divertor is the flat ring at the bottom of the vessel.

During September in 1997, JET achieved a record amount of fusion power using DT fuel: a peak output of 16.1MW, and a record JET still holds. This is enough to power just over 7400 UK homes*.

More recently, the 4000 or so carbon fibre composite wall tiles inside the vacuum vessel were replaced between 2009 and 2011 with a remote handling system (an industrial-strength version of the RIFT project). The new tiles, acting as a trial run for the planned wall type in ITER, are mainly beryllium and tungsten and have been very successful so far.

 

The ITER-Like Wall in JET, made of beryllium and tungsten.

What about now? Well, we’re preparing for the next set of DT experiments, planned for 2017 – twenty years after the record breakers of 1997. These DT “campaigns”, as they are known, will be gathering data for physicists and engineers to allow them to plan the DT operations for ITER, providing us working on JET with an exciting and ambitious set of goals ranging from materials analysis to plasma control.

And what about the other tokamak I implied in this post’s title? That would be the Mega-Ampère Spherical Tokamak (MAST), and it’ll be featured in a later post!

* Based on average UK household usage of 19MWh per year in 2013. Source: Department of Energy & Climate Change, 2014. Energy Consumption in the UK (2014). Link 

All images courtesy of the European Fusion Development Agreement.