The aim of fusion research is the achievement of the conditions required for plasma to start producing energy from the fusion of hydrogen atomic nuclei. It is an extremely challenging task, as plasma heated to hundreds of millions of degrees needs to be confined well enough at sufficient density. The simultaneous task of diagnosing such a plasma, of measuring its characteristics, is not straightforward either! Consider how you would measure, for example, inner plasma temperature. One cannot simply put a sensitive element inside the hot plasma - not only would it sublimate, but more importantly, the experiment would be lost as the plasma would cool down and become impure.
What can be done then? Firstly, one can simply try and observe the plasma from the outside, applying as many different methods as possible and exploiting a great variety of physical phenomena, ranging from atomic effects and nuclear reactions to radiation propagation and electromagnetism. Quite a few tricky computing methods (including tomography, better known in its medical applications) provide information about plasma's internal properties purely from external measurements. Secondly, one can send a tiny harmless probe into the plasma, like a beam of atoms, laser light or a microwave frequency, and observe its behaviour in the hot plasma. In both cases, a good understanding of the physics underlying the measurements is essential to get sensible results.
JET has the most complete set of diagnostics for reactor grade plasmas in the world (Fig. 1), with unique capabilities in measuring the thermonuclear fusion products, ie the fast neutrons, gamma rays and alpha particles (both confined and lost). As it is the only tokamak facility that can use all hydrogen isotopes, absolutely unique diagnostics are also required to measure the plasma isotopic composition. Other major goals of the JET diagnostics are common to big fusion experiments: to determine plasma temperature and density, to measure plasma particle and radiation losses, to find out the magnetic topology and to observe plasma flows and fluctuations. The specificity of JET, in this case, consists of providing conditions for these measurements that are closest to a reactor environment. Below are a few important examples of the diagnostics methods applied at JET.
We human beings have lots of experience in observing visible light - that is, the electromagnetic radiation emitted by atoms. But no light comes from the hot core of tokamak plasmas as proper atoms are extremely rare there - at these temperatures almost all of them will have decomposed into nuclei and free electrons (atoms become fully ionised). One of our essential diagnostic tools is charge exchange spectroscopy (Fig. 2 & 3) which relies on importing atoms into the hottest plasma regions. At JET, a neutral beam heating system launches billions of billions of neutral atoms into the plasma at extremely high velocities. In collisions with the hot plasma they rapidly loose their electrons, quite often by passing them to plasma nuclei (hydrogen ions) or to heavier nuclei ('impurity' ions). Although the ions will soon loose the electrons in subsequent collisions, they can shine light in the meantime! By observing the characteristics of this very distinct light from impurity ions we can tell (thanks to the Doppler effect) what the temperature of the plasma ions is and what direction of flow the plasma has. Even more importantly, these data can be resolved to a precision of one centimeter as the light originates only in regions very close to the neutral beam.
The neutral beam atoms themselves emit light that can be measured separately, as the Doppler effect at the velocity of beam atoms causes a distinct shift in the wavelength (that is in colour) of their light. Moreover this radiated light has specific features due to the fact that the beam atoms cross very rapidly through a strong magnetic field - its characteristic spectral lines are split and polarised. By measuring these features we can determine the direction of magnetic field lines even inside the hot plasma! The technique, called Motional Stark Effect Spectroscopy (Fig. 4), is quite challenging but vital as the plasma confinement depends so much on the exact topology of the magnetic field, and the topology depends on electric currents in the plasma... At the moment we are planning to feedback data from these measurements to the control systems of JET, so that we can correct the magnetic topology in real time by changing currents in JET's external coils and/or the plasma heating parameters. Similar feedbacks from other diagnostic systems (eg Polarimetry) have already resulted in an improvement of JET plasma confinement.
Normally a light ray cannot be seen unless it hits your eye, but with dust or mist in the air, one can spot it from the side too by the scattering of light. When an intense laser beam is sent into plasma, its light will get scattered on free electrons. The analysis of the scattered laser light is essential in determining local density and temperature of plasma electrons. This diagnostic technique is used worldwide. However, at JET it has been combined with the principle of radar, and this approach is also the best candidate for future reactor designs. To learn more please visit our LIDAR page.
A single vertical cross-section of the plasma is sufficient to learn about
the state of the whole plasma volume as the cross-section does not vary
significantly around the tokamak, in its toroidal direction. As a matter of fact,
any local disturbance is immediately spread along the magnetic field lines -
plasma particles move freely in this direction. Consequently, only very few fast
diagnostic systems (eg magnetic diagnostics) monitor the toroidal irregularities (Fig. 5). On the
contrary, it is essential to measure the plasma's vertical cross-section in as
much detail as possible, to determine plasma profiles in the direction
perpendicular to the toroidal magnetic field. The limiting factor in this is
the number and position of available ports (windows into the plasma). Due to this limitation, a number of diagnostics have very similar geometrical set-ups e.g. the
JET gamma-ray profile monitor, the soft X-ray diagnostics and the JET main
bolometer system that measures total plasma radiated power.
The neutron and gamma-ray profile monitor represents just one of tens of passive diagnostic methods applied at JET. The monitor has two cameras that allow observations of plasma radiation from ten horizontal and nine vertical directions. In this way we can localise the source of the radiation, in this case the neutrons produced by fusion or gamma-rays produced by nuclear reactions. The latter can serve us to trace the presence of fast-ions, in particular helium nuclei (alpha particles).
With increasing confidence in the plasma stability control, fusion research can concentrate on another stepping stone: the power exhaust and plasma-wall interactions (ie interactions of plasma with the vessel's inner surface). Consequently a lot of effort is invested in plasma edge diagnostics. Besides traditionally efficient tools like electrical (Langmuir) probes, neutral particle analysers, infrared cameras and dedicated spectral measurements, new methods are being developed and used at JET. Among the most innovative and successful are the quartz microbalance monitors (Fig. 6) which permit the measurement of the minute erosion and deposition on wall materials. Thanks to these measurements we can rapidly progress towards better plasma configurations to produce lower wall erosion.
At JET, signals from all diagnostic systems are digitised and stored in a central database. The sampling frequencies depend on the requirements of each diagnostic and vary from a few measurements per second up to about one million per second. In total, more than one billion readings of diagnostic data are recorded per JET pulse, each reading with 12 or 16 bits. In other words, every JET pulse (Fig. 7) produces almost 2 GBytes of raw diagnostics data, so that as much as 50 GBytes are stored daily. Most of the data need further processing - this is done automatically where possible by dedicated computer codes, but in many cases human intervention and/or data validation is required. The processed data are stored separately from raw data. All data are accessible to all scientists on the JET site and, moreover, any scientist from any EFDA Association can work with the data from her/his home institute via the technique of Remote Access. Many Associations and Contractors continue to develop new diagnostics for JET or upgrade the present ones. At the same time, JET serves as a unique test bed for the development of diagnostics for the future fusion reactor machine, ITER.
Diagnosing fusion plasmas involves many of the most advanced measurement techniques of physics and electronic engineering. There are more than fifty different approaches (Fig. 8) applied at JET and this explains why hundreds of scientists worldwide are so passionate about the performance of JET diagnostics. Nuclear fusion in general and JET in particular are the main driving forces behind the development of specific measuring techniques like fast neutron/gamma spectrometry and high energy active spectroscopy. Moreover, notice that the diagnostics of a fusion plasma operate on a quite realistic scale. Therefore, these measuring techniques can be relevant for practical applications and can potentially create interesting spin-offs. For example, a method originally developed for laser measurements of plasma flux velocity is under tests in precision wind anemometry, needed for the analysis of wind turbines. |
Fig. 1 Methods of plasma observation
Fig. 2 Charge exchange spectroscopy
Fig. 3 Principles of charge exchange
Fig. 4 Local magnetic field measurements
by Motional Stark Splitting
Fig. 5 Neutron / gamma profile monitor
Fig. 6 Quartz microbalance
Fig. 7 Staff preparing for the next JET Pulse in JET's control room (1996)
Fig. 8 Overview of JET diagnostics
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