Q: Where is the JET Tokamak located?
A: the JET machine is at the Culham Science Centre near Abingdon, about 9 miles south from Oxford in the UK.

Q: What output fusion power has been achieved by JET and for what containment time?
A: In 1997 JET produced a peak of 16.1 MW fusion power, with fusion power of over 10 MW sustained for over 0.5 sec.

Q: What is plasma current and what is a JET Pulse?
A: JET (being a tokamak design) acts as a transformer, where electric currents in primary windings produce magnetic fields which induce electric currents into secondary windings. The primary windings are coils on the outside of the plasma vessel. However, instead of a wire secondary winding there is a single 'short-circuit' loop of highly conductive plasma. A large electric current is generated in the plasma for two reasons: Firstly the current produces a powerful magnetic field which, in addition to externally generated fields, confines the plasma within the tokamak vessel. Secondly it also heats the plasma up to high temperatures (just like a current in a wire heats the wire up) so increasing the likelyhood of fusion reactions. The pulse is the short plasma that we generate in any of JET's experiments - it only last a few tens of seconds due to the large amount of energy required to maintain it. We then study it and conduct experiments to try and improve its performance etc.

Q: How long is the longest sustained fusion reaction achieved by JET and elsewhere?
A: JET is the only operational machine to observe fusion from D-T (Deuterium-Tritium) reactions. Such fusion reactions have been maintained on JET for around five seconds. Experiments in a device called TFTR in Princeton, USA also observed fusion neutrons from their plasma but TFTR is no longer operational. The next step tokamak (ITER), which has been designed and should be built in the next ten years or so, will demonstrate much more powerful fusion reactions for 5-10 minutes and will, hopefully, provide the stepping stone to commercial fusion powerplants.

Q: I've read that 10 MW of power could be sustained for only .5 seconds. What is the limitation preventing a longer maintaining of power, and how does the output of power compare to the amount of power invested necessary to produce the 10MW?
A: In fusion power demonstrations there are usually two different goals. One is to produce a steady, sustainable level of power, the other is to reach as high a power as possible, even if this is only possible for a short time.

In the 1997 JET experiments, for instance, a steady fusion power of about 5MW was produced, lasting for around 5s, after which time the plasma heating was turned off and the plasma allowed to cool. In that case the reduction in plasma temperature is a deliberate result of reducing the heating power.

On the other hand, techniques have been developed to improve the insulation properties of the plasma, for a short time, allowing higher fusion power to be produced, but only for that short time. Typically this results in fusion powers in the 15-20MW range produced for a time of around 1 second. Usually the end of such a high performance phase is due to a reduced level of confinement and a fall in temperature. In this case the reduction in plasma temperature is not a deliberate effect, but an illustration that the very high temperatures achieved in these plasmas are not sustainable in steady state.

In both types of fusion power demonstration, heating powers of around 20MW are used. In the steady state plasmas, the fusion power is around 20% of the input power. In the high performance plasmas the fusion power is approximately the same as the applied heating power.

Q: How much electricity has been consumed by JET to this point?
A: I cannot put a number on the amount of electricity consumed by JET I am afraid, although, suffice to say it is a lot!! Each JET pulse uses ~700MW of electricity (used to directly heat the plasma and also to provide the magnetic fields required to keep the plasma confined).

Q: Could you tell me how the magnetic fields contain the plasma and how these fields are set up and powered?
A: As the ions in the plasma are charged (the plasma is so hot all the electrons are stripped off the atoms, leaving them with a positive charge) they respond to magnetic fields. By setting up magnetic field lines toroidally around the interior of the tokamak, the ions and electrons in the plasma are forced to travel tightly around these field lines, preventing them from escaping the vessel. Extra fields help shape the plasma and hold it stable within the tokamak interior.

The magnetic fields are generated by coils which surround the tokamak vessel - these are made of Copper in the case of JET and, if you send large currents through these coils, a magnetic field is generated around them. Careful design of the shape and orientation of the coils ensures the net magnetic field within the tokamak vessel is suitable to confine and control the plasma.

Q: What do the central solenoid, TF, PF and divertor coils do?
A: The hot plasma created in the tokamak (such as JET) needs to be confined and controlled in order for sustained fusion to occur. The solenoid, which is positioned around the plasma, induces a powerful electric current in the plasma, thus heating the plasma up (much as a wire is heated when a current is passed through it). The TF (Toroidal Field), PF (Poloidal Field) and divertor coils all provide magnetic fields used to confine and control the geometry of the plasma. The TF coils provide the main field that traps the plasma particles within the interior of the vessel. The PF and divertor coils shape the plasma and ensure it remains stable (radially and vertically) within the vessel.

Q: I recently heard a sort-of scientific urban myth that at some point during its operation the plasma inside the torus touched the top and the whole thing jumped up in the air. Is there any truth to this story?
A: Normally in JET, the plasma is controlled (by powerful magnetic fields) so that it stays away from the walls of the vessel. In this way the plasma can be sustained and heated to fusion temperatures. However, if the magnetic field system is unable to control the movement of the plasma for some reason, the plasma will hit the vessel wall and rapidly extinguish (or disrupt). In this process, the plasma energy is transferred to the vessel structure, heating the vessel, driving currents in the vessel structure and (as you pointed out) moving the vessel a few mm. This is possibly where the urban myth has originated but it is worth remembering that disruptions are pretty rare on JET and scientists are constantly trying to determine when the plasma is losing control and correct the situation before a disruption can occur. Also, the structure of JET has been designed to withstand such transient movements and is in no danger of falling off its supports!!!!

Q: When using neutral beams to heat up the plasma do they not pollute the plasma and cause it to disrupt and how do you accelerate these neutral atoms ?
A: Neutral beams atoms do not pollute the plasma - they will be Deuterium atoms that will actually fuel the fusion reaction and increase it by making the background fusion ions hotter and increase their density (via collisions and ionisation).

The energetic neutral atoms are created by first accelerating a beam of ions (via powerful electric fields) and then neutralising it to produce a beam of energetic atoms that are able to be injected into the plasma (the beam must be neutral to find a way through the magnetic fields that contain the plasma in a tokamak).

Q: Tell me about the temperatures achieved at JET and in fusion reactors.
A: In the JET tokamak plasma is contained in a doughnut shaped vessel and is heated up by (amongst other methods) passing a current through it. Temperatures around 100million degrees C have been achieved and fusion (albeit at a relatively low level) has been observed. An eventual fusion power plant will be hotter (200-300million degrees C or so) and bigger than JET and will produce abundant amounts of fusion energy which will be used to generate electricity.

Q: If the Plasma is 100 million degrees, how do you get all those pictures?
A: The pictures of the plasma are taken with fast framing CCD cameras (the latest ones digitally) that is positioned outside the vessel, looking at the plasma through a transparent window.

Q: What is the current yearly funding for JET ?
A: The present (2003) yearly funding for EFDA-JET is 60 million Euro, spread across the whole of Europe. This supports the operation and maintenance of the tokamak (undertaken by UKAEA on behalf of Europe) and the undertaking of experiments by groups of scientists from EFDA laboratories all over Europe.

Q: What else needs to be done before JET can be fully operational?
A: (written late 2004) JET has been fully operational since 1983, during which time it has undergone many upgrades, culminating in D-T [Deuterium-Tritium] plasma experiments in 1997 which produced 16MW of fusion power. More such experiments are planned. During the current 2004/5 operational shutdown JET has been undergoing further upgrades to plasma diagnostics, neutral beams, and the divertor configuration (bottom of the vacuum vessel). In plasma diagnostics, dozens of new state-of-the-art systems are being installed to observe and measure plasmas. The restart of full operations is scheduled for the second half of 2005. JET is not large enough to produce more fusion power than the power that is needed to heat the plasma. However, results from JET and other tokamaks have led to the design of a new machine, ITER (2-3 times larger than JET) which is planned for construction in 2010-2015 and will produce 10 times more fusion power than the power required to heat the plasma. This will act as a stepping stone towards a commercial fusion powerplant. Actually, the forthcoming JET operation is fully devoted to ITER-relevant studies and most of the current JET upgrades are directly linked to this programme orientation.

Q: Is research at JET still aimed at the ultimate goal of commercial energy production?
A: Yes. Results from JET and other tokamaks around the world have given scientists tremendous confidence that they can control and confine the plasma sufficiently that ITER will deliver sustainable fusion power production. However, work continues on JET and other machines to optimise the plasma confinement etc. - work in the areas of how to exhaust plasma impurities in the divertor of the tokamak and how to reliably induce so-called "transport barriers" in the plasma (and thus improve the insulation of the burning plasma core) are particularly important at the moment.