A technique for measuring the ablation threshold and ablation rate of fusion reactor components is being developed. Ablation rates and thresholds are important parameters that can aid in determining a material's suitability for high temperature applications such as diverter plates and other plasma-facing components. In order to quantify these ablation rates and thresholds a plasma armature railgun will be used. The armature reaches high temperatures (a few electron Volts) thus causing ablation of the bore material (rail and insulator) in the barrel. The ablated products are accumulated in the armature, increasing its mass and lowering the acceleration. This increase in armature mass may be characterized by measuring the free-arc speed as a function of rail current.

In general, previous attempts to characterize a material's resistance to arc melting and erosion have been based on assessments of its thermal properties. These methods have only been useful in determining ablation thresholds but not ablation rates. In the railgun community, other techniques to quantify gun wall material erosion include monitoring the insulator surface and determining volume or mass loss after exposure to a known heat flux. These methods are dependent on instrumentation that must meet challenging precision requirements.

The proposed technique is a more sensitive method of measuring ablated mass in that small changes in armature mass due to arc erosion are easily recognized thus enabling the determination of the insulator ablation rate. In addition, the environment in which the measurements are done allow for the duplication of high temperature scenarios similar to actual applications. Operating rail currents of 1.5 MA and associated armature temperatures of several electron volts can be achieved. Previous work in which free-arc tests were done have been successful in demonstrating a direct relationship between the predicted ablation threshold of a material and the velocity at which ablation occurs. Results from recent experiments suggest a relationship between the ablation rates/thresholds and the effective increase in armature mass. Experimental data suggests that the armature mass can be characterized or tested by varying the pressure of the bore backfill gas and measuring the free-arc speed as a function of current.