Pisces ELM Laser Simulation System

Full text of publication follows: One of the largest temperature gradients we know is present near the wall of fusion tokamaks. This great temperature difference calls for the use of special materials to withstand the harsh environment in steady-state operation. Off-normal events such as edge-localized modes (ELMs) can also occur in these devices at varying frequency and amplitude levels. When an ELM occurs, up to 30% of the pedestal energy can be deposited on the plasma facing boundary of the tokamak in the form of a heat and particle load. There are two main consequences of these ELMs. First, is the heating and material loss from the divertor and first wall due to sublimation of CFCs, or evaporation and melt splashing of metals. Following this event, the expansion of the ejected material into the outer region of the plasma, known as the scrape off layer (SOL), and even penetration to core plasma, can occur. Simulation of the heat pulse and particle load of such ELMs in laboratory devices is desirable to determine the processes that occur and the effect on plasma-facing materials and the plasma. A moderate energy laser can be used to develop diagnostics necessary to study these phenomena and determine the heat and particle sources necessary to simulate the effect of ELMs in the laboratory environment. We are integrating a laser system into the existing plasma-facing materials research program in PISCES, a laboratory facility capable of reproducing plasma materials interactions expected during normal operation of large tokamaks. A Q-switched Nd:YAG laser capable of delivering up to 750 mJ of energy over 5 nsec at a wavelength of 1064 nm is used for the experiments. A study of laser-material interaction was conducted in vacuum and in a low-pressure gas environment. We then studied the effect of laser heat pulses on PFCs that have been irradiated under different plasma conditions (Te, ne, Γ, etc.). Initial results indicate that materials behave very differently while exposed to plasma and simultaneously experiencing a heat pulse similar to that of an ELM. Materials that have been irradiated with plasma for times sufficient to saturate the near-surface layer with hydrogen experience an explosive release of material, where only melting occurs in other non-hydrogen saturated samples. We are currently acquiring an additional laser system with a much longer pulse length so as to better simulate the temporal heating conditions during an ELM. This work was supported by grant DE-FG03 - 95ER-54301 from the US DoE. (authors)