The excellent maintenance work performed by UKAEA leads to a good performance and availability of all systems. This has made it possible to investigate a wide range of physics and technology issues of relevance to ITER. For the longer term the operation of the JET Facilities beyond 2014 is being considered.

It is 8 pm on this cold November evening. In the control room of JET, where a number of physicists and engineers have been on duty since 3 pm, attention is high. No surprise, the next experiment is imminent. Maximos Tsalas, Leader of today’s experimental session, is staring at his computer screen where, after the countdown, a video image of the running plasma discharge will soon be visible. Breaking the silence a loud voice starts narrowing down the time to the exciting moment: 8, 7, 6… and finally 0! A few seconds later Maximos’s face is resplendent, his catching smile tells it all: the experiment was a success.

Michael Watkins, Head of Programme Deparetment.

A day like this is not extraordinary at JET. In 2008 there has been a large number of successful experiments. One of the reasons for this success is the very good performance of the various systems forming part of the JET Facilities, such as the heating systems, the power supply systems and more than 80 different diagnostic systems. This level is the result of the excellent maintenance work performed by UKAEA, who operates the JET Facilities. The performance and availability of the systems have made it possible to investigate a wide range of physics and technology issues of relevance to ITER. Michael Watkins, Head of the Programme Department, summarizes the main aim of the scientific work: “The JET programme is devoted to the consolidation of ITER design choices and the qualification of the ITER regimes of operation.”

The JET programme is devoted to the consolidation of ITER design choices and the qualification of the ITER regimes of operation

A few examples might help to understand the close relationship between JET and ITER. In a tokamak fusion device the fuel is confined in the vessel by magnetic fields largely generated by purpose-built powerful coils. The coils are placed around the ringshaped vacuum vessel, the torus, just like pearls on a necklace. For the field they generate to be constant along the torus direction and simultaneously allow access to the reaction chamber, their size would need to be much larger than the size of the plasma. In practice, the size of coils chosen for a particular device reflects a compromise between cost, scientific capability and access for heating, fuelling and diagnostic systems. However, the finite number of coils implies a certain modulation of the magnetic field, named ripple. Such a modulation has an adverse effect on the confinement of the alpha particles produced in fusion reactions. However, its effect on the confinement of thermal particles has not been fully appreciated until now.

Thanks to the flexible way JET’s magnetic field coils can be powered, it has been possible to simulate in experiments the level of ripple expected in ITER, thereby investigating its impact on a number of issues such as energy and particle confinement. These studies have resulted in recommendations to the ITER Organisation regarding the level of magnetic field ripple that should be allowed in the ITER device.

A view into JET's control room. Maximos Tsalas is pictured leading the experimental session.

A considerable fraction of the experimental time has been devoted to commissioning and exploiting the newly installed ITERlike Antenna (see October 2008 issue of JETInsight). This antenna features a similar degree of complexity as the one foreseen on ITER. It has been designed to deliver high power densities to the plasma under conditions of varying particle density in front of the antenna where conventional antennas typically fail.

Progress achieved so far places the ITER-like Antenna half way to its objective as moderate levels of heating power have been reliably delivered to plasmas independently of conditions at the front of the antenna.

Due to the expected large amount of energy stored in ITER plasmas and future fusion power plants, plasma scenarios must be compatible with power load limits imposed by first wall materials. Transient power loads are, for instance, induced on plasma facing components by a phenomenon which occurs at the edge of the plasma: Edge Localised Modes (ELM). In 2008 JET experiments have investigated different active techniques for reducing ELM induced power loads on plasma facing components. These techniques are based on applying a perturbation to the plasma, which results in an increase of the frequency of the ELM event and a decrease of the induced power loads on the plasma facing components.

Thanks to the flexible way in which JET was designed, the experiment can always be adapted to meet changing requirements. Michael Watkins comments: ”JET continues to make major contributions to the development of fusion as a safe and environmentally benign energy option. Its programme of upgrades to 2014 and beyond will allow JET to help consolidate the design, define auxiliary systems and optimise operations for ITER. And this will be accomplished with a truly international environment which is prototypical of not only the European Research Area, but also ITER.”

Richard Kamendje, Petra Nieckchen