Image courtesy of EFDA www.scienceinschool.org - the hydro At these temperatures ing the temperature profile of the fuel, profile ing the temperature to the cooler core its scorching from create can edges, so that researchers for fusion the optimum environment to occur. gen fuel becomes the fourth state of plasma. Measuring the tem- matter, of a plasma that is ten times perature some hotter than the Sun presents challenges – you can’t simply insert a conventional thermometer: it would Things are be melted in microseconds. further complicated because plasma is - Amazingly enough, scientists and ciently than the Sun, even though this ciently than the Sun, even though this in the temperatures creating requires ten heart of the fusion vessel that are of the Sun. times hotter than the core engineers have devised ways of heat- fuels to these tempera- ing hydrogen from them and then preventing tures, them melting the vessel by controlling magnetic fields (as with very strong described in Rüth, 2012). Fundamen- methods tal to these experiments are not only for monitoring the reactions 2012) but also for measur (see Dooley,

Issue 26 : Spring 2013 I he Joint European Torus (JET) Torus he Joint European fusion is the world’s largest experiment, pioneering energy

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I The temperatures in a fusion temperatures The are ten times higher vessel than at the core of the Sun. methods for producing plentiful, clean producing for methods power with the same method used by the Sun: fusing light atoms such as the heavier together to form hydrogen – in the UK atom helium. In fact JET, successor ITER, being and its larger built in the south of France – aims to - effi operate thousands of times more

44 T By Phil Dooley, EFDA-JET By Phil Dooley, in the classroom. easy task. Find out how it’s done – and even simulate it even done – and it’s out how easy task. Find Measuring the temperature inside a fusion reactor is no inside a the temperature Measuring 200 million degrees 200 million A thermometer that goes to that goes A thermometer

Image courtesy of AMIDAPHAT / Flickr Science topics

Figure 1: A stripped-back model of the JET experiment, showing only the temperature- measuring systems. Electron cyclotron emis- sion (pale purple), LIDAR / Thomson scatter- ing (red), charge exchange (bright green), and

X-ray spectroscopy (pale green). Image courtesy of EFDA Image courtesy of EFDA

The path of the LIDAR signal through the plasma: the laser enters horizontally from left. A detector at the top of the vessel picks up light from this laser, which is scattered by electrons. Physics

Physics The four methods involve concepts from several areas Chemistry of physics and chemistry, including optics, electromag- Optics netism, mechanics, energy and atomic structure. In ad- dition, a classroom activity based on the demonstration Electromagnetism of the Doppler effect is presented, which partially ex- Energy plains some of the four methods. Mechanics The Science in School series of fusion articles, of which Atomic structure this is one (see the resources list), has great interdisci- Ages 17+ plinary potential for upper secondary-school students, This article describes four methods that are used by the as the articles could be used to discuss fusion energy: world’s largest fusion energy experiment to deduce the as a source of future sustainable energy, how it works, temperature inside the fusion vessel. Due to the high and its advantages and disadvantages. temperatures involved, any thermometer would melt Mariana Martinho, Portugal

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www.scienceinschool.org Science in School I Issue 26 : Spring 2013 I 45 end upatadifferent temperature! The systems differently toions–andcan electrons canrespond totheheating to create hotionsthatwillfuse,but of theelectrons. Thekeytofusionis positive ionsformedbytheremoval stripped from atoms,andtheheavier particles: electrons thathavebeen made up of two very different charged Image courtesyofMark Tiele Westra 46 I frequency againstintensity. calculated by correlating plasma temperature canbe so thedistributionof not discussedhere,and from othermeasurements strength isalready known signal. The magneticfield plasma, thestronger temperature. The hotterthe are travelling –that is,their speed atwhich theelectrons tion isdeterminedby the The intensityoftheradia- higher frequency emission. tighter spiral andthereforea the helix,which resultsina the doughnut),tighter of thetorus–holein field (i.e.towards thepole The stronger themagnetic quency oftheiroscillation. emitting radiation atthefre- trons follow helicalpaths, in thetorusmakeselec- Figure 2: The magneticfield Hydrogen Science inSchool Neutron Proton Electron Fusion reaction Positron Electron Gammaray Neutrino Positron I Issue26:Spring 2013 Gamma ray Deuterium Neutrino

Image courtesyofEFDA methods increases thereliability of checking theresults ofdifferent ture (figure 1onpage45)–cross- methods fordeducingthetempera- physicists havedevelopedmultiple a fusionexperimentsignificantly. trons andionscanaffect thesuccessof complex interactionsbetweenelec-

Intensity Despite thesechallenges,plasma Helium-3 Frequency Hydrogen Helium-4 places intheSolarSystem. what goesoninsideoneofthehottest be confidentthattheyare incontrol of your measurements –sothattheycan Speed trap! LIDAR emission Loop-the-loop: electroncyclotron Electron temperature instead ofradiowaves.Light from the except thatituseslaserlight (LIDAR) era tomeasure thespeedofparticles, system similartoapolicespeed cam electron temperature. other systems,givesusaprofile ofthe the magneticfieldstrength, created by Combining thiswithaspatialmapof ture foreachmagneticfieldstrength. frequencyagainst tempera- tellsusthe ralling frequency. A scanofintensity stronger thefield,higherspi varied magneticfieldinthevessel: of theelectron temperature, duetothe waves thattheyemit. are, themore intenseare themicro- therefore faster-moving –theelectrons emission (figure 2).Thehotter–and creates microwaves calledcyclotron along themagneticfieldlines,which charged, electrons are forced tospiral charged particles.Becausetheyare the effect thatmagneticfieldshaveon JET’s second ‘thermometer’ usesa JET’s second‘thermometer’ The microwaves alsoyieldaprofile relies on The first‘thermometer’ www.scienceinschool.org - - Science topics

different angles through the plasma. This is similar to the creation of 2D Energy production in the Sun: two computed tomography images from hydrogen nuclei fuse to form a multiple individual X-rays. deuterium nucleus, a positron and a neutrino. The positron quickly Ion temperature encounters an electron, they anni- hilate each other, and only energy Unfortunately LIDAR is not an remains. The deuterium nucleus effective way of measuring ions. goes on to fuse with another hy- This is because the process of Thom- drogen nucleus to form helium-3. son scattering relies on the induced In the final step, two helium-3 oscillations of a charged particle. The nuclei fuse to form helium-4 and heavier ions oscillate less – they are two hydrogen nuclei. not tossed about by the laser light waves as much as the lighter elec- trons. The greater mass of the ions also means that their cyclotron frequency laser is scattered by the electrons in a moving away. The faster the electron is too low to be useful – the waves are process known as Thomson scattering; moves, the bigger the frequency shift. too long to give a precise measure- if the electrons are moving, then the The cumulative effect of the many ment and they happen to coincide scattered light will be Doppler shifted electrons in the plasma – some mov- with the plasma’s natural absorption (figure 3). We are more familiar with ing towards and others away from the frequency so they do not escape the Doppler shifts of sound: the sound detector – is that the original narrow plasma cleanly.

from passing cars has a slightly higher frequency band of the laser light is In addition, the hydrogen ions in a Physics pitch as they move towards us than broadened (see inset in figure 1). plasma effectively become invisible, when moving away. Similarly, if light The extent of broadening tells us the because all their electrons are stripped is scattered by moving electrons, its speed of the electrons, and hence their off, disabling the common mechanism frequency (colour) will be Doppler temperature. of radiation creation – electrons jump- shifted to higher frequencies for the A two-dimensional profile of the ing between orbits. electrons moving towards the detec- temperature is created by combining However, a route to working out the tor and to lower frequencies for those data from a number of beams fired at temperature has been found via impu-

lxpin / iStockphoto of EFDA of a esy esy ourt urt e c co ag ge Im a Im

Fusion energy research is inspired How do scientists measure the by the Sun, which fuses light atoms such temperature in a fusion vessel? A as hydrogen into heavier atoms, releasing conventional thermometer would energy in the process. melt in microseconds.

www.scienceinschool.org Science in School I Issue 26 : Spring 2013 I 47 Image courtesy of EFDA X-Ray eyes: tungsten impurities A second method for measuring the temperature of the ions – again based on impurities – has recently been commissioned at JET: a new X- ray detector. Tungsten inevitably gets knocked off the new tiles in the wall and contaminates the plasma in small amounts. Unlike the light atoms, hot tungsten typically retains about half of its 74 electrons even in the extreme heat of the core of the plasma, and so does not become invisible – these electrons jump between electron shells and emit X-rays. Doppler broaden- ing of this X-ray spectrum caused by the movement of the ions allows the temperature to be calculated (as in figure 3). The temperature profiles from these four systems are essential for analys- A B C D ing the effectiveness of the heating systems in use at JET – some of which heat electrons, while others work on the ions. The measurements also give Figure 3: The Doppler effect in Thomson scattering. The laser light, entering from vital information about how energy the left, is absorbed and then re-emitted by electrons, and a detector above the in the plasma behaves in different beam picks up the fraction of light emitted upwards. The motion of the electrons relative to both the laser beam and the detector causes the scattered light to be circumstances, including how the Doppler shifted from the original laser frequency either to higher or to lower electrons and ions interact with each frequencies, depending on the direction of motion. other. Using this knowledge, JET’s sci- Electron A is moving towards the laser and therefore absorbs blue-shifted radia- entists and engineers can manipulate tion, while electron D is moving the opposite way, and absorbs red-shifted light, the plasma to maximise the energy which is then scattered at the absorbed wavelength. Electrons B and C are not confinement, thereby creating and moving relative to the laser, so the light they absorb is at the original frequency. maintaining optimal conditions for However, because of their motion towards and away from the detector, respec- tively, the re-emitted light is Doppler shifted to the blue or to the red, as shown. fusion.

References Dooley P (2012) Seeing the light: rities in the plasma; while undesirable in a process called charge exchange. monitoring fusion experiments. in large quantities, traces of impurities This involves shooting a beam of neu- Science in School 24: 12-16. www. can be useful for this purpose. tral hydrogen atoms into the plasma scienceinschool.org/2012/issue24/ at high speed. When these atoms fusion Partner swapping: charge encounter a carbon ion, sometimes an Rüth C (2012) Harnessing the ­exchange electron jumps from a hydrogen atom power of the Sun: fusion reactors. 22 One of the most common contami- across to the carbon, which then emits Science in School : 42-48. www. nants in plasma is carbon, which was a nice sharp spectrum that is easy to scienceinschool.org/2012/issue22/ JET’s wall lining until 2010. Although measure. Because of the high tempera- fusion the carbon tiles have been replaced ture, the carbon atoms are moving Web references with beryllium and tungsten, there very fast in all directions, so the sharp are still traces of carbon in the plasma. frequencies of the spectrum are then w1 – Learn more about EFDA-JET. Usually the carbon is invisible, like the spread out in the same way as the See: www.efda.org/jet hydrogen, but it can be made visible scattered LIDAR signal is (figure 3).

48 I Science in School I Issue 26 : Spring 2013 www.scienceinschool.org Science topics

w2 – EIROforum is a collaboration between eight of Europe’s largest inter-governmental scientific re- search organisations, which com- Demonstrating the bine their resources, facilities and Doppler effect expertise to support European sci- ence in reaching its full potential. As To create your own Doppler shift, you will need a small battery-­ part of its education and outreach powered sound source that can emit a long tone or set of tones of a activities, EIROforum publishes constant pitch. A small alarm clock or a mobile phone with a single- Science in School. To learn more, see: tone ring or alert will do nicely. Then you need a long sock or stocking www.eiroforum.org – the longer the better – and lastly, plenty of space! Resources Standing in the middle of your space, start the alarm clock ringing, then drop it into the stocking and swing it around your head as fast Warrick C (2006) Fusion – ace in the as possible. Other people will hear the pitch varying as the sound energy pack? Science in School 1: 52- source moves towards and then away from them. If you are doing the 55. www.scienceinschool.org/2006/ swinging, you will notice no difference, as the sound source is moving issue1/fusion. neither towards nor away from you, but at right angles to you. y classroom activit This article is part of a series of articles about fusion published in Science in School. www.scienceinschool.org/ fusion For an introduction to the electromag- ______netic spectrum – and how it is used If you enjoyed this article, why not in astronomy – see: browse the other cutting-edge Physics Dr Phil Dooley is the news and educa- Mignone C, Barnes R (2011) More ­science articles in Science in School? tion officer at EFDA-JET. He was than meets the eye: the electromag- See: www.scienceinschool.org/­ born in Canberra, Australia, and netic spectrum. Science in School cuttingedge completed a PhD in laser physics at 20: 51-59. www.scienceinschool. the Australian National University. org/2011/issue20/em To escape academia he took a job in IT in Rarotonga, Cook Islands, for 18 months, before returning to More about EFDA-JET Australia and working in software training. His love of science drew him back to physics, this time as a communicator, running the school The Joint European Torus outreach programme at the Uni- w1 (JET) investigates the potential of versity of Sydney. In October 2011 ­fusion as a safe, clean and virtually limitless Phil joined the EFDA-JET team in energy source for future generations. It can create the conditions Oxfordshire, UK. (100-200 million °C) in the plasma sufficient for fusion of deuterium and tritium nuclei to occur, and it has achieved a maximum fusion power out- put of 16 MW. As a joint venture, JET is collectively used by more than 40

European fusion laboratories. The European Fusion Development Agreement To learn how to (EFDA) provides the platform to exploit JET, with more than 350 scientists use this code, see and engineers from all over Europe currently contributing to the JET pro- page 57. gramme. EFDA-JET is a member of EIROforumw2, the publisher of Science in School. To see all EFDA-JET-related articles in Science in School, see: www.scienceinschool.org/efdajet

www.scienceinschool.org Science in School I Issue 26 : Spring 2013 I 49