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Francesco Romanelli, Director of JET

15 Dec 2008 - Francesco Romanelli, Joint European Torus

The Joint European Torus (JET) is the machine whose parameters are the closest to those of ITER by virtue of its large size (~100 m³ plasma volume), ITER-like geometry and large plasma current (up to 5 Mega Ampere).

The JET design, which was started in 1973, introduced bold new concepts such as D-shaped plasmas in large tokamaks, a closed-loop tritium plant, and the use of beryllium as a first-wall material. It implied increasing by two orders of magnitude the plasma volume and the heating power compared to the standard at the time. One of the most striking novelties was that the machine was conceived from the beginning as a nuclear machine, allowing for tritium operation with full remote handling capability. The basic principles guiding the design of JET were simplicity and sturdiness. This philosophy was expressed in the JET founding document as follows: "First we must construct a simple apparatus with a very high reliability. Second, it must be possible to dismantle the apparatus from the outside using remote handling techniques."

Situated in Culham, UK, JET was built from 1979 to 1983 (First Plasma was achieved on 25 June 1983) with the essential objective to obtain and study plasmas in conditions and dimensions approaching those needed in a thermonuclear reactor. The studies were aimed at defining the parameters, the size and the working conditions of a tokamak reactor. The realization of these objectives involves four main areas of work:

(i) The scaling of plasma behaviour as parameters approach the reactor range;

(ii) The plasma-wall interaction in these conditions;

(iii) The study of plasma heating;

(iv) And the study of alpha-particle production, confinement and consequent plasma heating.

Throughout the years of JET exploitation, most of the design parameters were exceeded and after achieving all its initial objectives, JET was upgraded and modified to investigate the most promising regimes of operations and to perform comprehensive studies of heat exhaust techniques and plasma-wall interaction. JET holds all records in fusion power (16 MW) and energy (22 Mega Joule) and has allowed a unique experience in deuterium-tritium operation to be gained. The basic layout of ITER follows closely the one ultimately implemented on JET, and JET contributed the data points closest to ITER in the scaling laws used for its dimensioning.

JET today remains the only tokamak of its class that can use tritium and beryllium. On the basis of these assets, it is being prepared to make further essential contributions to ITER with regard to the aforementioned four main areas, i.e., qualifying ITER scenarios, consolidating the ITER design choice for plasma-facing components and heating systems, developing control tools and techniques, and providing a basic understanding of plasma dynamics.

Within the JET program in support of ITER, a set of enhancements of the JET facilities is underway with completion foreseen for mid-2010. An ITER-like wall and divertor will be inserted with the combination of metallic plasma-facing materials foreseen for ITER: beryllium wall and tungsten divertor. Experiments with the ITER-like wall and divertor should deliver answers to urgent plasma surface interaction questions for ITER, such as tritium retention, and provide operational experience in steady and transient conditions with ITER wall materials under relevant geometry and relevant plasma conditions. A high-frequency pellet injector system to mitigate the impact of transient power loads on first-wall components and divertor and to enable deep plasma fuelling was already installed in 2007. This system is currently being tested in JET experiments and the JET results and experience will give confidence to the foreseen use of such a system on ITER.

Also for ITER, JET is currently testing new matching tools for radio-frequency plasma heating in the ion cyclotron range with, for instance, an ITER-like antenna that has been designed to feature resilience to varying loads at the plasma edge due to transient instability events (edge-localized modes). Furthermore, the neutral beam power will be upgraded to ~ 35 MW and the pulse-length capability to 20 s. This will help progressing operating scenarios for ITER, in particular, the hybrid scenario and the advanced scenarios, which require full or partial current profile control, thereby making use of new dedicated diagnostics. After the testing of the ITER-like wall, a deuterium-tritium experiment is envisaged to allow extrapolation of the scenarios to ITER-relevant conditions. JET can bring its performance closer to breakeven (Q= fusion power/input power = 1) in stationary conditions, and such an achievement could be the best starting point for successful development of ITER operations.

In the coming years, until the upgrade of JT60-U into JT60-SA is completed, JET will be the largest tokamak of its class in operation; this would make it the ideal machine where scientists collaborating on ITER could start working together to develop a common strategy on ITER scenarios, where natural leaderships could emerge, and where researchers could be trained to operate a multi-Mega Ampere device. With these goals in mind, JET is being further opened up to the participation of all the other ITER Members.

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