ASDEX Upgrade

The tokamak fusion experiment ASDEX Upgrade (AUG) is a medium size divertor tokamak (major radius R=1.65 m, minor radius a=0.5 m) with an ITER-like configuration, high shaping capability (single null and double-null divertor, elongation up to 1.8, triangularity δ up to 0.5) and a versatile heating system. The design combines the successful divertor concept with the requirements of a next step fusion reactor, in particular the need for an elongated plasma shape and poloidal magnetic field coils outside the toroidal magnetic field coils. AUG is close to ITER in its magnetic and divertor geometry and in particular the relative length of both divertor legs compared to the plasma dimensions. The installed heating power of up to 28 MW ensures that the energy fluxes through the plasma boundary are equivalent to those in ITER. The scientific programme gives priority to the preparation of the design (heating, fuelling, first wall materials), physics basis and discharge scenarios of ITER and the exploration of regimes beyond the ITER baseline scenario.

The studies were guided by four Task Force (TF) groups which consisted of

• Confinement and performance of the ITER base-line scenario, the ELMy H-mode, and the advanced scenario of the "improved H-mode" leading to enhanced performance and longer inductive pulse lengths by non-inductive current drive,
• H-mode pedestal physics and ELM mitigation and control,
• Magnetohydrodynamic (MHD) stability, active stabilization of β limiting instabilities as well as avoidance and mitigation of disruptions,
• Scrape-off layer and divertor physics with the aim of optimizing power exhaust and particle control (ash removal) and optimization of first wall material with emphasis on tungsten.

The similarity of ASDEX Upgrade to ITER makes it particularly suited to testing control strategies for shape, plasma performance and MHD modes. The similarity in cross-section to other divertor tokamaks is important in determining size scalings for core and edge physics. In particular, the 2006 physics programme was based on our conclusions of the last years, persisting ITER requirements and tokamak concept improvement. Our programme largely covers the "High Priority Physics Research Areas" provided by the ITPA Co-ordinating Committee. Again, several items have been investigated in joint experiments at all major tokamaks as proposed by the ITPA Topical Groups. In summary, the AUG programme in close collaboration within the EU fusion programme is embedded in a framework of national and international collaborations.

The AUG Programme Committee established in 2001 enables the Associations to take responsibility for our programme. This body defines the Task Forces responsible for the different elements of our programme, and approves the experimental programme. Furthermore, the bodies that work out the programme proposals are open to external participants, and remote participation in the meetings is used. For the 2006 campaign 160 proposals were received including 60 proposals from outside IPP. With this structure, we have achieved a compromise between the increased international participation and the flexibility that has so far been typical for the AUG programme.

The flexible heating systems consist first of the neutral beam heating (NBI) with powers up to 20 MW. The ion cyclotron resonance system (ICRH) is capable of routinely coupling up to 7 MW in ELMing H-mode discharges. The present electron cyclotron system was kept available up to a coupled power of 1.2 MW allowing pure electron heating (ECRH) and current drive (ECCD). These versatile heating methods allow the effects of heat, particle and momentum deposition on energy and particle transport, MHD stability and fast particle physics to be separated. In 2006 the coverage of the AUG vessel interior with tungsten was further extended up to 85% (36 m² with <5μm W) with the coating of the poloidal limiters at the low field side which receive the highest load in the main chamber. The W concentration could still be kept at an acceptable level over a broad range of discharge scenarios. The complete W coverage is being done in the present shut-down, namely the 200 μm coating of the lower LFS divertor targets.

Stationary discharges with up to 10 s flat-top allow steady state investigations not only on the transport and MHD time scales but also for up to 10 current diffusion times. The new fast integrated control and data acquisition system (CODAC) with a reduced cycle time was fully commissioned. It is specially adapted to ITER needs with its machine-independent design, its integrated discharge scenario control and protection functions and the large number of real-time diagnostics. This extremely flexible system is now in routine use at ASDEX Upgrade.