The upgrade of the JET neutral beam system will allow significant advances in optimising ITER scenarios, such as the ELMy H-mode, the Improved H-mode and Advanced Scenarios. In particular, higher power operation in JET will also contribute to the development of high density scenarios compatible with a metallic wall as foreseen in ITER, and integrated in the overall JET enhancements which includes the installation of a metal wall [“The ITER-like Wall Project at JET”, EFDA Newsletter Vol 2005/3 page 6].

The upgrade of the neutral the Neutral Beam Enhancement Project beam system on JET, planned for installation in 2008, will deliver up to 35 MW of power for up to 20 seconds (compared with the 22MW available for 10 seconds at present), and up to half this power for up to 40 seconds.

With the higher power installed on JET, the ELMy H-mode and Improved H-mode will be taken to higher normalised pressure (beta), allowing further refinement of the scalings for core confinement, ELM energy losses and divertor/wall interactions. In particular, it will be possible to determine the contribution of the pedestal energy to the total plasma energy and to refine the dependence of confinement on beta. At present, dedicated scans carried out on JET and DIII-D show a betaindependent scaling which is more favourable than the ITERH98(y,2) scaling, assumed for the ITER design. Regarding the study of Improved Hmodes, it will be possible to obtain conditions significantly closer to ITER than those achievable today by operating JET at high beta and low normalised larmor radius, “rho-star” (see figure). The long pulse capability of the upgraded neutral beam system will be crucial to progress Advanced Scenarios with full current drive sustained for an entire current diffusion time at ITER- relevant normalised pressure.

In addition, the higher power will allow Neo-classical Tearing Mode and Resistive Wall Mode control techniques to be optimised under conditions closer to ITER. This work will include sawtooth control and further studies of the effect of rotation on plasma stability. Greater demands will be placed on techniques for mitigation against large ELMs and disruptions. This is of particular importance in view of their potential to cause melt damage to the ITER-like first wall. JET will also be able to test mitigation techniques, such as pellet injection for ELM-pacemaking and fast, high pressure impurity injection for disruption mitigation, under edge plasma conditions similar to those expected in ITER.

The neutral beam power will be increased primarily by changing the magnetic configuration of the ion sources from the present “supercusp” configuration to a pure “chequerboard” configuration. The latter produces larger fractions of molecular ions (D2+ and D3+), which are slower after acceleration due to their higher mass, and hence neutralised more efficiently, resulting in a more powerful neutral beam.

As part of the power upgrade several other changes will be made to the NB system: eight of the existing 80kV/60kA High Voltage Power Supply units will be replaced with four new 130kV/130A/20s units; critical components of each beamline, which presently rely on interpulse cooling, will be replaced with actively cooled components, extending the pulse duration from 10s to 20s; and all 16 ion-source/accelerators (PINIs) will be modified to allow a maximum beam current of 65A (at 125 kV acceleration voltage), instead of the current maximum of 60A.