Fusion Science and Technology / Volume 47 / Number 1 / January 2005 / Pages 27-45
This paper considers the physical processes and material removal mechanisms associated with the energy deposition in an inertial fusion energy liquid wall from the prompt X-ray spectrum of an indirect-drive inertial fusion target. These are important as the ablated material could generate aerosol in the chamber, which without adequate chamber clearing could result in a chamber environment unsuitable for driver propagation and/or target injection. Simple computations were used to identify and characterize the important material removal mechanisms relevant to the energy deposition regime under consideration. Explosive boiling was found to be the most relevant thermal response mechanism due to the high heating rate from the X-ray photon energy deposition. Investigation showed that explosive boiling occurs when the material temperature approaches the critical temperature and has a threshold value that can be derived from the material equation of state or the rate of homogeneous nucleation. Another important mechanism is mechanical spall that can result when shock wave-induced local tensile stresses exceed the spall strength of the material. Both explosive boiling and mechanical spall occur upon crossing the thermodynamic stability border (spinodal curve) either through rapid heating or through overexpansion of the material.
Relevant material properties of the candidate liquid wall materials needed to perform the present assessment are compiled, derived, and presented. A simple energy deposition volumetric analysis is used to estimate both thermally ablated and mechanically spalled regions of the liquid wall material. The choice of liquid/wall combination is found to play an important role in reducing or eliminating the occurrence of spall in the liquid wall.