EFDA Associate Leader for JET, Francesco Romanelli, welcomes the guests at the celebration of the 25th Anniversary of JET’s first plasma.

With the year coming to an end JET has many reasons to be looking back in delight. To start with, 2008 marks an anniversary year for the JET Experiment. Indeed, on the 25 June 1983 the first plasma was achieved. The continuous success of the Experiment since then made it possible to gather together many of the key players from day one for a joyful celebration exactly 25 years later. The 25 June 2008 went down in history as a day when JET’s present young scientists had the opportunity to meet and exchange with many of those who contributed to the milestones set by JET in international fusion research.

On the other hand, it was also an appropriate occasion to look into the future with a certain degree of confidence. In fact, at its meeting in Ljubljana back in March, the EFDA Steering Committee unanimously approved a resolution that  recognised “the scientific need for full exploitation of the JET Enhancement Programme 2 and for tritium operation” and requested the Commission “to investigate the possibility of making adequate resources available within the fusion programme budget”. This will entail exploiting JET beyond the currently foreseen horizon of 2010, up to 2014/15. Moreover, later in the year, the Facilities Review Panel (see our other article in this issue) recognised JET as “the most relevant device for support to ITER until new devices with improved capabilities become available” and concluded that “JET needs to operate until 2014/15 at least and would benefit from an early installation of an Electron Cyclotron Resonance Heating System. Depending on the JT60SA schedule JET operation for a few further years should be foreseen.”

The remarkable scientific capabilities of JET, recognised by these positive recommendations, have been extensively exploited during 2008 in a scientific programme devoted to the consolidation of ITER design choices and the qualification of ITER regimes of operation. These regimes of operation, referred to as plasma scenarios, are the sequence of operational events applied to prepare and then initiate the plasma, raise the plasma current to the required value, apply the auxiliary heating and current drive during the burning phase and finally extinguish the plasma discharge safely.

A tokamak, like JET and eventually ITER, uses a transformer such that the secondary current (the plasma current) is driven inductively by continuously increasing/ decreasing the current in the primary circuit. This feature effectively limits the pulse length (it ends when the poloidal field coils have reached their maximum achievable currents). For this reason, in ITER, the baseline plasma scenario (referred to as ELMy H-mode) is envisaged to operate for a duration not exceeding 500s at a plasma current of 15 million Ampere.

A view into the JET vacuum vessel using a wide-angle infrared camera system. The system allows the observation and measuring of the temperature increase of plasma facing components following transient power loads induced, for instance, by Edge Localised Modes such as in this picture.

During this year’s experimental campaigns a substantial part of the ITER plasma scenarios development activities at JET has been dealing with the development of the tokamak concept towards steady-state operation, based on “advanced tokamak” (AT) plasma scenarios. The objective of the AT research  is to provide a candidate plasma scenario applicable for continuous operation in fusion power plants. In ITER, fully non-inductive operation (i.e. without transformer flux consumption) is envisaged for up to 3000s at a reduced plasma current of 9 million Ampere (compared to the 15 million Ampere current capability). To compensate for the reduction in energy confinement associated with reducing the plasma current, the ITER steady-state scenario must achieve improved energy confinement by a factor of 1.5 compared to the standard confinement projection for the baseline scenario. Crucially, JET results have demonstrated that substantially improved confinement is achievable on a large tokamak (so far up to a factor 1.4), thereby increasing confidence of the successfully operation of this scenario on ITER.

However, resolving the issues associated with the performance of plasma scenarios is not the entire story. In reality, 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. This is currently a very active field of research in which JET has been at the forefront of integrating ITER scenarios with relevant 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 Localized 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 a frequency increase of the ELM events and a decrease of the induced power loads on the plasma facing components.

These are just a few examples of the many achievements made by JET in 2008. Work is still ongoing to complete the comprehensive set of experiments planned before and in preparation for the future ITER-like wall which will build the very core of the JET programme in support of ITER and whose installation will start during the Summer of 2009.

Richard Kamendje & Petra Nieckchen

ISSN 1818-5355
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editor: Örs Benedekfi

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© Jérôme Paméla (EFDA Leader) 2008.

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