Fusion Science and Technology / Volume 46 / Number 1 / July 2004 / Pages 142-151
Technical Paper / Stellarators
Beside quasi-stationary plasma operation, short transient thermal pulses with deposited energy densities on the order of several tens of MJ/m2 are a serious concern for next-step devices, in particular, for tokamak devices such as ITER. The most serious of these transient events are plasma disruptions. Here, a considerable fraction of the plasma energy is deposited on a localized surface area in the divertor strike zone region. The timescale of these events is typically on the order of 1 ms. In spite of the fact that a dense cloud of ablation vapor will form above the strike zone, only partial shielding of the divertor armor from incident plasma particles will occur. As a consequence, thermal shock-induced crack formation, vaporization, surface melting, melt layer ejection, and particle emission induced by brittle destruction processes will limit the lifetime of the components. In addition, dust particles (neutron-activated metals or tritium-enriched carbon) are a serious concern from a safety point of view.
Other transient heat loads that occasionally occur in magnetic confinement experiments such as instabilities in the plasma positioning (vertical displacement events) also may cause irreversible damage to plasma-facing components (PFCs), particularly to metals such as beryllium and tungsten. Other serious damage to PFCs is due to intense fluxes of 14-MeV neutrons in D-T burning plasma devices. Integrated neutron fluence of several tens of displacements per atom in future thermonuclear fusion reactors will degrade essential physical properties of the components (e.g., thermal conductivity). Another serious concern is the embrittlement of the heat sink and the plasma-facing materials (PFMs).
To investigate the performance of carbon-based and metallic PFMs under the aforementioned thermal loads, simulation experiments have been performed in highly specialized high-heat-flux test facilities. The neutron-induced degradation of materials and components was investigated on selected test samples that were irradiated in high-flux material test reactors.