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

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.

SNIFfing for Negative Ions

By Ingrid Turner 

CCFE is home to two leading nuclear fusion tokamaks, JET and MAST. In order to achieve hot enough conditions required for fusion energy production and the more interesting experiments, it is vital that we have additional heating systems. One of the most important of these is the neutral beam heating system.

On JET this system has the potential to produce 35MW power in a single pulse. There are 16 positive ion neutral injectors (PINIs) on JET, and 2 on MAST. Each PINI is injected with gas which is then heated by hot filaments (metal wires). A plasma is created by putting a large electric current (arc) through this hot gas. Metal grids which are kept at a high voltage then cause the positive ions in the gas to accelerate away from the plasma. Positive ions cannot be injected straight into the tokamak, due to the large magnetic fields used. These ions are therefore neutralised first (given back an electron by sending them through more gas) and hence we have our neutral beam which is injected into the tokamak.

Unfortunately this process wastes a lot of power because only 25-30% of the ions can be neutralised, and the remaining ones have to be dumped in the form of waste heat. This gets worse for higher voltage beams, therefore it would not be economical to use these injectors on future machines such as DEMO.

Thankfully there is a solution: negative ion neutral injectors. The development of negative ion beams is crucial for fusion energy, as these have a much higher neutralisation efficiency, at around 60%. ITER, the next experimental tokamak which will be even larger than JET, already plans to use these, as it will require much higher power beams of the order 1MV as opposed to the current 125kV on JET. However there are a number of issues with negative ions, for example they have a short lifetime because their electron can be easily removed. There is also a problem with using caesium for making it easier for neutral atoms to pick up an electron, as it is a highly reactive material which is not ideal for maintenance and development of the injectors.

The Small Negative Ion Facility (SNIF) based at CCFE is a project sponsored by CCFE’s Technology Programme, and is used to study and develop negative ion beam production for beam systems on future machines beyond ITER, under work contracts for Eurofusion.  It is different to the PINIs in that it uses a radio frequency (RF) plasma source as opposed to an arc source. This works by using a flat spiral antenna which is used to excite and heat the source gas. SNIF uses high voltage grids much in the same way as they are used on PINIs, with an additional biased plate used to suppress any electrons which may get pulled through from the plasma. The final negative ion beam is then extracted and travels through a vacuum tank where it lands on a copper beam dump (mimicking a future tokamak).

There are several diagnostic systems in place on SNIF which we can use to do experiments and look at the beam power and divergence. There is a Langmuir Probe which is a measuring device inserted into the plasma and measures the plasma density, electron temperature and electric potential in the source, as well as a visible light spectrometer, which is used to measure the intensities of emission spectral lines of the hydrogen in the source. For looking at the beam, there are two cameras mounted on the vacuum tank, and finally we have the beam dump at the end of the tank which has thermocouples (temperature sensors) wired in so that we can measure the temperature rises and check that the beam is hitting the dump in the middle.

Recently I have been involved in analysing the diagnostic data, in particular from the thermocouples and the cameras. The aim of this is to obtain an accurate beam profile and to get a good idea of what the beam current is. Currently the temperature rise model we have does not fit the thermocouple model well, so I am looking into alternative materials for the beam dump. I have also conducted studies to look at how the beam width varies with different parameters, for example RF power and extraction voltage. From this we have been able to obtain the optimum parameters i.e. at which the beam width is narrowest. This has been done both experimentally and also by modelling the system and comparing the results.

In the near future SNIF will be used for many experiments, for example, testing for alternative materials to caesium in injectors, and possibly looking into energy recovery to increase the efficiency of beam systems. There are also many studies still to be conducted for the RF source which I hope to be involved in, including using the Langmuir probe to collect data. We may even try and put a tile in the way of the beam with an infrared camera as an alternative to the copper beam dump. In short, there is a huge potential for development of SNIF, and we are hopeful that ideas and results from this system will be implemented in future beam systems.