Tour of JET and MAST
With Dr David Keeling
The Joint European Torus (JET), located at Culham Centre for Fusion Energy, is the world’s largest and most powerful tokamak and the focal point of the European fusion research programme. Designed to study fusion in conditions approaching those needed for a power plant, it is the only device currently operating that can use the deuterium-tritium fuel mix that will be used for commercial fusion power.
In the core of the machine is the vacuum vessel where the fusion plasma is confined by means of strong magnetic fields and plasma currents (up to 4 tesla and 5 mega amperes). In the current configuration the major and minor radii of the plasma torus are 3 metres and 0.9 metres respectively, and the total plasma volume is 200 cubic metres. A divertor at the bottom of the vacuum vessel allows escaping heat and gas to be exhausted in a controlled way. Since 2011 the first wall of the vacuum vessel has been made of beryllium and tungsten, mirroring the material choices of ITER. Other important features of JET are:
1) A flexible and powerful plasma auxiliary heating system, consisting of Neutral Beam Injection (34 megawatts), Ion Cyclotron Resonance Heating (10 megawatts) and Lower Hybrid Current Drive (7 megawatts)
2) An extensive diagnostic suite of around 100 individual instruments capturing up to 18 gigabytes of raw data per plasma pulse.
3) A high frequency pellet injector for plasma refuelling and for ELM pacing studies.
4) A massive gas injection valve for plasma disruption studies.
5) Capabilities to operate with tritium fuel – unique among today’s tokamaks
6) Beryllium handling facilities, allowing use of beryllium plasma-facing components. Beryllium gives a clean vacuum.
7) Remote handling facilities that allow advanced engineering work to be performed inside the vacuum vessel without the need for manned access.
Our tour began with Dr Keeling using pictures and a model to explain how JET works. He first talked about Bremsstrahlung radiation.
Bremsstrahlung radiation is produced by the acceleration of charged particles. In a tokamak, by far the largest source of Bremsstrahlung is the continuous deflection of the electron trajectories by the electrostatic fields of the ions. As the number of collisions increases with pressure, bremsstrahlung losses present an upper limit on the density possible for effective energy confinement in a plasma.
The silvery tiles you can see in the picture below are a combination of beryllium and tungsten tiles which are used as the plasma facing material. This has led to a reduction of fuel retention (i.e. hydrogen isotopes, co-deposited together with carbon, beryllium and other elements present in-vessel) on these tiles by a factor of twenty on the previous Carbon Fibre Composite (CFC) tiles. Beryllium has the advantage of acting to keep the vacuum at a high level by gettering oxygen. Gettering is the removal of impurities or defects with a getter and a getter is a deposit of reactive material that is placed inside a vacuum system, for the purpose of completing and maintaining the vacuum.
Tungsten with its 74 protons and electrons has advantages and disadvantages. The metal withstands high temperatures and it does not absorb the fusion fuels tritium and deuterium, as other potential wall materials do. But if tungsten debris enters the plasma centre, most of the 74 electrons get excited and take up immense plasma energy. At the same time, tungsten is very brittle and brakes easily once fractured. It is one of JET’s tasks to test wall compositions containing tungsten in order to develop suitable modes of operation for ITER. It is also testing methods of flushing tungsten away.
Beryllium evaporator inside the JET vessel, next to a microwave antenna.
The above picture shows a tungsten tile.
The above picture is a model cross sectional model of JET.
The above picture shows a cross section of the JET diverter coil.
Originally the main purpose of a divertor was to separate plasma from the first wall and improve the performance of the tokamak. It provided magnetic fields to confine and control the geometry of the plasma and ensured it remained stable (radially and vertically) within the vessel. However the main function now is to remove most of the α-particle (fusion reaction ash), unburnt fuel, and eroded particles from the reactor. Beryllium can’t be used in the diverter.
Heating by radio-frequency wave
Waves can propagate inside a plasma and this is a large field in plasma physics, since there is a huge wealth of possibilities depending on the nature of the wave (its frequency, polarisation and on the plasma properties (density, temperature,…). Waves are generally speaking ranked by family according to frequency and propagation direction relative to the magnetic field (parallel or perpendicular). Depending on the latter, the waves may either be propagative or evanescent, may be reflected or change polarisation, may change amplitude in the course of time or, quite the contrary, may transfer energy to the plasma. It is the latter that is of interest, and it is this property that can be used to heat the plasma with electromagnetic waves with specifically selected characteristics.
The JET Control room
The laser room
Far infra-red (wavelength ~ 0.2 to 1 mm). FIR lasers are used to measure the magnetic field (by Faraday rotation) and plasma density.
The laser is split in half and one half sent through the torus before being reunited with the other half. Comparing of the two beams gives information about the density of the plasma, a vital parameter.
The above picture shows the JET device in 2007. Operating since 1983, JET is the world’s largest and most powerful magnetic fusion experiment.
The closest we could get to JET
JET wasn’t running when we visited so we could get closer than normal.
The reason for the distance is that there is some radioactivity present even when it is not running. Staff working on JET have to wear radiation badges to monitor how much they have been exposed to radiation.
With Dr Scott Allan
MAST (Mega Amp Spherical Tokamak) is the UK’s fusion energy experiment, based at Culham Centre for Fusion Energy. Along with NSTX – a complementary experiment at Princeton in the USA – MAST is one of the world’s two leading spherical tokamaks (STs).
The above picture is of a model of MAST
Part of a transformer coil. Its job is to induce current in the plasma to heat it up.
The above picture shows (From left to right): High voltage cable cross section, JET saddle coil cross section, JET toroidal field coil cross section.
The picture below left shows a MAST toroidal field coil limb (left). This is the corner joint of one of the 12 toroidal magnetic field coils and is designed to carry up to 100kA of current for approximately 1 second. Cable section (right).
The above right picture shows a MAST graphite centre column protection tile with Langmuir probes and cables.
The above picture shows a 33kV fuse.
The picture above shows the MAST control room