Authors: Jérôme Paméla, Michael Watkins, Alain Lioure, Shakeib Arshad and Andrea Murari

In 2005 EFDA-JET embarked on a major new enhancements programme, aimed at supporting critical needs of ITER. The main elements of this programme are a new ITER-like first wall, a power upgrade, a new pellet injector and a set of new or upgraded diagnostics. The first three of these were described in previous issues of the EFDA Newsletter. Here we discuss diagnostics enhancements.

Diagnostics for ITER scenario development on JET

Plasma scenarios foreseen for ITER include the ‘ELMy H-mode’, the ‘Improved H-mode’ (often called the ‘ Hybrid mode’) and ‘Advanced Scenarios’. The first two are characterised by the formation of a transport barrier at the plasma edge which results in improved confinement, although this is accompanied by edge instabilities known as Edge Localised Modes (ELMs, see box).

In Advanced Scenarios, transport barriers occur deeper in the plasma. A major attraction of these scenarios is the possibility of steady state operation, with the plasma current being generated largely by the transport barriers (through the bootstrap effect) rather than by conventional pulsed transformer action. ITER scenario development on JET consists of exploring and optimising these scenarios in the most ITER-relevant conditions achievable, with the goal of sustaining high performance with acceptable power loading of the wall.

Diagnostic enhancements to support this effort concentrate on improved profile measurements as well as real-time control of profiles. The spectral coverage of the ECE system, which provides the highest quality temperature profile data on internal transport barriers and the edge pedestal, will be extended to access higher density regimes which are of increasing interest. An upgrade of the lithium beam system will allow edge current density measurements with high spatial and temporal resolution, and is expected to give access to key ELM physics. ELM diagnosis will be further enhanced with an upgrade of the edge LIDAR system, which will be able to resolve the steep edge gradients in the pedestal region. In addition, the real-time control infrastructure will be extended to execute additional and more sophisticated real-time analyses, and to include additional diagnostics desired by Task Forces.

Diagnostics related to the other major new JET projects

The ITER first wall, comprising beryllium in the main chamber, carbon fibre composites (CFC) at the divertor strike points and tungsten elsewhere in the divertor, is designed to minimise the tritium inventory (by minimising carbon surfaces) while maintaining adequate power handling capability and plasma performance.

The first ever test of an ITER-like wall in a tokamak is planned on JET, with installation of the wall in a shutdown in 2008/09. The test will focus on erosion/deposition processes and mass flow in the bulk and edge plasmas, ultimately aiming at demonstrating compatibility between high performance plasmas and the ITER-like wall. A set of new or upgraded diagnostics will be dedicated to this test. This will include additional quartz microbalances for erosion/ deposition studies, a new high resolution camera for infrared thermography to study transient heat loading in the divertor and an upgrade of spectroscopic diagnostics for routine coverage of helium, beryllium and tungsten lines.

A new high frequency pellet injector will also be installed on JET, to study ELM control by pacemaking. A new fast camera is foreseen in support of this study, to observe pellet ablation and edge turbulence.

Prototype diagnostics for ITER

To support the sustained DT phase planned for ITER, a set of reliable burning plasma diagnostics will be required. Owing to its ITER-like plasmas, its ability to create high energy ICRF-accelerated alpha particles in DD discharges and its unique DT capability, JET offers an unrivalled environment for the development and testing of such diagnostics. Priorities to be addressed on JET include radiation-hard compact detectors for reliable neutron measurements in the spaceconstrained ITER environment; improved electronics for gamma and neutron detection, aiming at achieving a time resolution, signal-tonoise ratio and a neutron/gamma discrimination level suitable for ITER, with the possibility of real-time measurement for burn control applications; a neutron filter (e.g. LiH), aiming at eliminating the neutron background from gamma measurements to yield information on fast alpha particles; an ultra-thin detector for neutral particle analysis, to extend the diagnosed energy range to below 1 MeV, and to improve the discrimination of different ion species; a fast wave reflectometer to measure the isotope ratio in the core; and radiation-hard Hall probes aiming at complementing ITER magnetic diagnostics with steady state ex-vessel measurements.

These diagnostics will be installed progressively, with some becoming available in 2007, and the rest becoming available at the end of the shutdown for the installation of the ITERlike wall, in 2008/09.