Survivability of First-Wall Materials in Fusion Devices: An Experimental Study of Material Exposure to Pulsed Energetic Ions

Abstract

The confining walls in future fusion power plants will be subjected to an intense energetic bombardment from X-rays, ions, and neutrons. This is true for both direct-drive inertial fusion energy (IFE) and magnetic fusion energy (MFE) designs. We focus in this paper on the threat spectra presented by energetic ions. X-rays are predicted to present a less significant threat in direct-drive IFE, and neutron effects cannot be readily simulated in current experimental facilities. For the experimental results presented herein, the energetic ions are generated in the Repetitive High-Energy Pulsed Power 1 (RHEPP-1) facility at Sandia National Laboratories. Depending upon whether the ion pulses are of nitrogen (previous database) or helium (this paper), the pulse width varies from 100 ns to as much as 500 ns, respectively. While this is short compared to ˜500-μs transient events anticipated in MFE operation, data from both IFE and MFE experiments for tungsten exposure are shown to exhibit similar fluence thresholds when thermal diffusion is taken into account by use of the heat flux parameter H = Power density × t1/2, where t is the characteristic event time duration.Long-term exposure of tungsten to RHEPP-1 nitrogen pulses indicates that above a level of ˜1 J · cm−2 /pulse, polycrystalline tungsten roughens severely, the cause of which appears to be thermomechanical distress, with loosening of grains near the surface the primary result. This roughening is correlated with unacceptable mass loss. While this occurs below melting temperatures, allowing the surface to melt by raising the per-pulse fluence does not appear to be a viable approach to smoothing the surface. Oriented grain material such as ITER-specified tungsten performs significantly better than polycrystalline tungsten, but under helium exposure it appears to suffer additional surface deterioration that appears to be connected to helium pore and bubble formation at absorbed implantation levels of mid-1015 He/cm2. This level is below previously reported levels of concern for helium retention effects and well short of required survival duration. Experiments with three-dimensional "needle" geometries, designed to increase the effective surface area for heat absorption and reduce helium implantation in depth, show promising results that need further investigation to confirm long-term survival.