ITER and future fusion power plants will need plasma heating systems with much higher power density than the present fusion devices are working with. To match this and other required values the ITER-like Antenna was designed, manufactured and installed in the JET fusion device. The knowledge gained by JET engineers and physicists will serve as a basis for developments on future devices such as ITER.

To produce fusion power efficiently in ITER and in a power plant it is necessary to raise the fuel temperature to more than 100 million degrees Celsius. To reach these extreme temperatures there are different systems available to heat the fuel. One of them is Ion Cyclotron Resonance Heating (ICRH).

It has been an essential learning exercise for the European ICRH fusion community.

Mark Nightingale

JET has tested several antenna systems to couple ICRH power to the plasma. In 2000, a specific Enhancement Project was started at JET in order to test a prototype for ITER, the ITER-like Antenna.

ICRH works like a microwave oven: the plasma ions are heated by the absorption of radio frequency waves at the characteristic frequency of the ion motion around magnetic field lines. The devices which are necessary to build up the system are a generator, transmission lines and an antenna. A generator produces highpower radio frequency waves that are carried along a transmission line to an antenna, located in the reaction chamber that couples the waves to the plasma. The ITER-like Antenna consists of eight paired conductors, with each pair fed by a separate generator.

While generators and transmission lines are used in radio technology and therefore are commercially available, the antenna needs to be specially designed in order to couple the required power to the plasma because of a phenomenon called Edge Localised Mode (ELM). ELMs produce abrupt variations in the density in the edge of tokamak plasmas, leading to a change of the antenna characteristics such that it reflects a significant amount of power back into the generator. The result: on average the radio frequency power coupled into the plasma is much lower than the power produced by the generator. In view of the use of ICRH in ITER and in future fusion power plants, it is mandatory to develop antennas which are resilient to these disturbances.

Since JET is the machine closest to ITER in terms of edge plasma conditions, in 2000 it was decided to design, built, install and test an ITER-like ICRH Antenna at JET. The ITER-like Antenna project is carried out under the European Fusion Development Agreement (EFDA) and by leading fusion laboratories in Europe and the United States. The implementation of such a new piece of sophisticated technology presented many challenges.

An example is the undesirable interference between conductors: a major feature of the ITER-like Antenna developed is its ability to work at high power density. The antenna therefore had to be built in a more compact way. To achieve this, the antenna conductors were built much shorter and located very close together.

Mark Nightingale, one of the two project leaders and Head of the Wave Heating & Current Drive Systems Group at the UKAEA.

Without practical experience of the new system the engineers and designers couldn’t be sure about the level of undesirable interference between the different conductors. Mark Nightingale, one of the two project leaders and Head of the Wave Heating & Current Drive Systems Group at the UKAEA, comments: “Initially, there was some concern as to whether this antenna control system could operate in the presence interference, but we were successful in overcoming this problem by demonstrating that we could fire power into the plasma. It wasn’t easy, but it has been possible”.

The European industry was deeply involved in the manufacturing process. In general, the components of a fusion device need a very high degree of accuracy. In most cases – such as for the ITER-like Antenna – the first task is to develop prototypes. The involvement of industry at this stage helps establish the technical feasibility of the entire project. Mark says “We have learnt that industry should be involved in the design phase”.

He summarises in his own words the development of the ITER-like Antenna: “It has been an essential learning exercise for the European ICRH fusion community. The antenna has been quite complex to test. It took quite some time to learn how to fully use it, but in the end we have been successful”.

Remote Handling

The ITER-like ICRH Antenna was fully installed during August 2007. With the remote-handling facilities developed at JET the 250 kilogram load was moved into exactly the right position inside the plasma vessel without causing any damage. The required accuracy was less than one millimetre.

JET was built as a nuclear device and remote-handling was incorporated from the beginning of the design. Special manipulators were developed at JET to extend the operators’ own arms. These manipulators provide the operator with a sense of touch and feel and, together with the associated Closed Circuit TV system, create a sense of being inside the Torus.

Frederic Durodie, Co-Leader of the project, recalls his experiences: “This has been a very challenging project for me where we have been able, for the first time, to use and validate new design tools. The complexity of the technical and managerial issues was fully recognised only as the project evolved.” He remembers in a very positive manner the cooperation during the project: “It has been an honour to work closely with the colleagues on site and the many, many others across Europe and the USA.”

The commissioning of the ITER-like Antenna started in May. The knowledge gained by JET engineers and physicists will provide a firm basis for developments for future devices such as ITER.

Petra Nieckchen