On 25th September 2006, experiments resumed on JET, with the start of Experimental Campaign C16 of the EFDA-JET 2006
Workprogramme. After successful completion of this Campaign on 13th October 2006, Campaign C17 began on 23rd October 2006, and was completed on 15th December 2006.

In these Campaigns a strong focus was maintained on the preparation of the ITER detailed design and ITER exploitation, including preparation for the planned change of the wall/divertor materials on JET.

In the ELMy H-mode the effect of the X-point position on the L-H transition threshold was studied with the new divertor. Studies of the L-H transition concentrated on the location of the X-point, and measurement of the radial electric fi eld and poloidal velocity with the upgraded charge exchange recombination spectroscopy diagnostic, see Figure 1. Plasma-wall compatibility on ITER may require the ITER plasma to be surrounded by a radiating zone, which could be produced by impurity injection. The plasma current at which production of such a zone has been studied with nitrogen injection, was extended to 3 MA. It is important that impurities injected for this purpose should not accumulate to high levels in the plasma core and degrade core confi nement. To understand impurity transport, systematic studies were conducted with injection of low to high Z impurities within the same discharge. These experiments will also be useful to understand transport of ITER fi rst-wall materials into the plasma.

Studies of material erosion, migration and retention constituted another major theme. A fuel balance and retention study was conducted with cryopump regeneration before and after a series of RF-heated H-mode plasmas. Beryllium and carbon erosion and migration were studied in experiments with dedicated beryllium evaporations, using spectroscopy and quartz microbalances, and the effect of nitrogen seeding on carbon deposition was assessed in H– and L-mode in a range of plasma confi gurations. Other studies in the ELMy H-mode included exploration of small (convective) Edge Localised Modes (ELMs) in wider parameter space (high and low triangularity, and q95 between 3.5 and 5.5); the effect of changing the q profi le on density peaking, at ITER-relevant collisionality; and momentum transport with modulated Neutral Beam (NB) injection, to improve predictions of the rotation profi le on ITER and its effect on transport.

Work on the Hybrid Mode was aimed at porting this scenario closer to ITER (towards an ITER-like edge safety factor q, higher confi nement, higher beta, higher density and smaller ELMs). This mode of operation was also compared with the ELMy H-mode, the baseline scenario for ITER, including the impact of a radiative edge on confi nement.

Advanced Tokamak (AT) Scenarios were studied at high triangularity, which can be reached with the new divertor. These studies employed high power (31MW) and ELM control by controlling edge radiation using neon injection. In addition, high beta AT discharges were studied in a quasi-double-null confi guration where ELMs are naturally mild, as on ASDEX Upgrade. Signifi cant effort was also devoted to integrated control of the current and pressure profi les, for a robust AT regime with high bootstrap current and a strong Internal Transport Barrier (ITB). The damping mechanisms of the Resistive Wall Mode (RWM) in high beta AT discharges were studied by measuring the response of the plasma to applied n=1 and n=2 magnetic perturbations. RWMs are unstable at high beta and low plasma rotation, conditions which are expected in ITER Advanced Scenarios. Insights from the new JET data may allow improved predictions of RWM behaviour in ITER AT Scenarios, and are expected to be valuable for the assessment/development of RWM control systems for ITER.

The potential for mitigating Type I ELMs with externally applied magnetic perturbations is also under study on JET in the ELMy H-mode and AT Scenarios. The technique was shown to be effective on DIII-D, without degradation of pedestal profi les or core confinement, when a resonant n=3 magnetic perturbation was applied. On JET the Error Field Correction Coils (see Figure 2), capable of producing an n=1 or n=2 magnetic perturbation, were used in similar studies, in the ELMy H-mode and AT scenarios.

Several new diagnostics for burning plasma physics were also tested successfully. These include Extreme Ultraviolet (XUV) spectroscopy for the detection of alpha particles at energies below 500keV, a new set of Toroidal Alfvén Eigemmode (TAE) antennas for measurement of TAE damping rates, and a scintillator probe and Faraday cups for measurement of fast particle losses.

In ITER a relatively large gap will be required between the plasma and fi rst-wall components. This poses a problem for coupling of Lower Hybrid (LH) and Ion Cyclotron Resonance Heating (ICRH), across a plasma-antenna gap foreseen at ~15cm. During Campaigns C16 and C17 studies were conducted on JET in which an ITER-like separation was maintained between the plasma and the LH and ICRH antennas, and gas was injected close to the antennas in order to improve plasma-antenna coupling.