Fig. 1 The ITER like antenna during installation in JET

From April to November 2007 the JET tokamak was in a phase of shutdown for enhancement and refurbishment. The nature of the tasks to be performed required the JET vacuum vessel (which, during experiments, is pumped by turbomolecular and cryogenic pumps) to be at ambient pressure and temperature. In particular, the extensive use of the remote handling system (a unique feature of JET with an articulated boom) required big ports at two octants of the machine to be opened.

In addition to a significant number of inspections, maintenance tasks and substantial remedial work on key equipments, this shutdown was mainly devoted to the installation of a number of new systems. The most extensive of these systems is the ITER-like Ion Cyclotron Resonance Heating (ICRH) antenna (see fig. 1). ICRH is an established plasma heating technology using electromagnetic waves in a frequency range comparable to the cyclotron frequency of ions (30 – 55MHz in JET’s magnetic fields) to increase the energy of the plasma ions. The ITER-like

ICRH antenna is a further development of the existing ICRH systems towards more ITER-relevant conditions. The baseline operational scenario for ITER is the High Confinement Mode, which is, in most cases, accompanied by Edge Localised Modes (ELMs). ELMs are disruptive events at the plasma edge, which are fast and strong variations in the equivalent loading impedance that the plasma presents to the antenna, leading to a reflection of the radio frequency (RF) energy back to the generator and, hence, to a limitation in the amount of energy which can be coupled to the plasma.

The ITER-like antenna has been constructed in an ELM- or load-resilient way, meaning that, although the plasma may vary strongly, this does not translate into large impedance variations, thus allowing the generators to couple RF power to the plasma despite ELMs. As the walls of a future fusion reactor will have to be covered to a high fraction by a blanket (which will use fusion produced neutrons to breed tritium as fuel to the reactor and produce high grade heat for power production) and only a small area will be available for systems like antennas, it is important that any heating system features a high power density. The ITER-like antenna at JET will be capable of delivering 8MW/m2 to plasmas with ELMs at a relatively large distance between antenna and plasma, which, in turn, is another challenging condition for the ITER ICRH system.

Another enhancement was the High Frequency Pellet Injector, which is a device capable of shooting small cubes of deuterium ice into the plasma. The motivation for this device is to have a direct fuelling method for the plasma core and to mitigate ELMs.

A second array of Toroidal Alfven Eigenmode antennas has also been installed (see fig. 2). The purpose of these antennas is to excite Toroidal Alfven Eigenmodes (TAEs, a type of instability considered to pose a risk for loss of confinement in ITER) in order to study their impact on the plasma. Other enhancements had to do with upgrading some diagnostic equipment in order to improve the quality of measurements during experiments.

Following the completion of the Shutdown activities JET entered a so-called restart phase. This involves powering up all the electrical systems and pumping down the JET vessel, followed by a period of leak testing, inner-wall conditioning and commissioning the various systems, in particular, the power supplies feeding the toroidal and poloidal field coils and the plasma heating systems. The first technical plasma was obtained on 5 February and, since then, further commissioning of the many systems in readiness for the next Experimental Campaigns is in progress, which started on schedule in April 2008.

Fig. 2 Toroidal Alfven Eigenmode antenna array installed in JET

The process of elaborating the Experimental Programme of JET for the forthcoming Campaigns has been revised for 2008 to further improve the coherence of the Programme. The aim has been to ensure a common understanding of the main scientific issues and to achieve the highest level of cross-field integration of expertise and scientific goals in the execution. The week beginning 19 November 2007 150 scientists from across Europe met at JET for a General Planning Meeting. They identified the activities which should form the basis of the 2008 Programme, agreed on the main experimental elements required for a focused scientific programme and developed them in sufficient detail that a draft experimental timeline could be established immediately following the meeting. The meeting also identified those scientists who could elaborate further the experiments, identified the modelling needs in support of the Programme and consolidated the programme of High Level Commissioning of the ITER-like ICRH antenna. The outcome of the General Planning Meeting allowed a detailed timeline to be established for the Experimental Campaigns C20-C25 of the JET 2008 Experimental Programme. These Experimental Campaigns will run from April to December 2008.

ISSN 1818-5355

For more information see the websites:

EFDA Close Support Unit – Garching
Boltzmannstr. 2
D-85748 Garching / Munich – Germany

phone: +49-89-3299-4263

fax: +49-89-3299-4197


editors: Örs Benedekfi

layout: Stefan Kolmsperger

© Jérôme Paméla (EFDA Leader) 2008.

This internal newsletter or parts of it may not be reproduced without permission.

Text, pictures and layout, except where noted, courtesy of the EFDA Parties.

The EFDA Parties are the European Commission and the Associates of the European Fusion Programme which is co-ordinated and managed by the Commission.

Neither the Commission, the Associates nor anyone acting on their behalf is responsible for any damage resulting from the use of information contained in this publication.