The Institute of Materials, Minerals and Mining 1 Feb 2017

CEA – Tungsten monoblock elements

The WEST project’s success in producing its first plasma allows it to test materials for the International Thermonuclear Experimental Reactor.

A tokamak in France has produced its first plasma, demonstrating the success of modifications made to the fusion reactor. The French Alternative Energies and Atomic Energy Commission (CEA) began the WEST project (Tungsten Environment Steady-state Tokamak) in 2013 to transform the Tore Supra, a tokamak constructed in the 1980s, into a test bed for the International Thermonuclear Experimental Reactor (ITER) – a research and engineering megaproject – to test a plasma-facing divertor made from tungsten. The tokamak is capable of containing plasma, using a strong magnetic field, in a vacuum vessel. The field acts as a container as it runs in both horizontal and vertical directions, created from toroidal and poloidal field coils.

Located on the vacuum chamber’s floor, the divertor’s purpose is to minimise contamination of the plasma, while extracting some of the heat and the ash produced by the fusion reaction. It is a crucial component, undergoing extreme conditions. WEST project scientist Emmanuelle Tsitrone said, ‘In next-step fusion devices, such as ITER, the materials interacting with the plasma will experience extreme heat and particle loads. In addition, these devices, aiming at steady state operation, will run much longer plasma discharges than present day facilities and produce a significant rate of neutrons, putting further constrains on the materials to be used.’

It is therefore necessary that potential materials used in the divertor have a high melting point, first-rate thermo-mechanical properties, resilience to neutrons, are easy to machine and are compatible with plasma operation, as material eroded from the vessel wall may enter the plasma.

‘ITER plans to operate with an actively cooled tungsten divertor. The technology is based on individual tungsten monoblocks bonded to a copper heat sink with water coolant. The main materials to be tested under tokamak operation conditions are tungsten, used as a plasma-facing material coping with large heat and particle fluxes from the plasma, and copper, used as a heat sink,’ Tsitrone explained. The durability of the bonding between these two materials is also being tested.

Tsitrone described how ‘divertor elements [that could be of use in ITER] are planned to be exposed to plasma in WEST during dedicated long pulse experimental campaigns, while their thermal behavior will be monitored through a large number of diagnostics. Regular post-mortem analysis of selected components is also planned to gain more insight on material ageing, involving a large variety of metallography and surface analysis techniques.’

The WEST programme is currently in its first exploitation phase, concentrating on design issues such as improving the monoblock geometry. The second exploitation phase, running from 2019-2020, will address issues associated with long pulse operation.

The WEST tokamak is intended to reduce the risks, such as exceeding costs and deadlines, associated with industrialising the high-tech components of the ITER divertor. The number of tungsten components developed for WEST is thought to be roughly 15% of those needed for ITER, allowing the manufacturing process for ITER to be optimised. The testing of the divertor allows the researchers to potentially improve and exploit its use in ITER.

Speaking of future applications, Tsitrone said, ‘WEST will open new possibilities to demonstrate the sustainability of plasma scenarios over relevant plasma wall equilibrium timescale, as required for steady state operation of future fusion devices. WEST therefore provides an opportunity to anticipate current unknowns on the way to long pulse operation in a metallic plasma-facing component environment before they are encountered in ITER, and identify solutions.’

A tokamak is a machine designed to control and capture the energy of nuclear fusion reactions. An electric current and magnetic field heat and confine plasma in the vessel.