Does Laser Ablation Vapor Simulate Impact Vapor?

Introduction: Many laser experiments have been done to simulate hypervelocity impact vaporization [e.g., 15]. Laser simulation has a number of advantages. It can create high-temperature plumes with high repeatability and a high repetition cycle at a relatively low cost. Furthermore, because laser light can penetrate through a glass window (if chosen properly), the target material and vaporization products have virtually no risk of contamination. This is an ideal condition for a chemical analysis. However, there is a critical disadvantage in laser simulation; we do not know if a laser irradiation really simulates a hypervelocity impact. The purpose of this study is to address this problem quantitatively. Similarities and Differences: Vapor plumes generated by both laser and hypervelocity impact have very high initial temperatures and pressures. However, there are significant differences, too. First, impact heating is purely thermal, and condition immediately after shock heating is considered to be in thermal equilibrium. However, laser plume may not be in thermal equilibrium, particularly when a UV laser is used. Photonic energy tends to excite certain transitions preferentially and create disequilibrium. Nevertheless, if the initial plume density and mass are large, such disequilibrium is compensated before it expands significantly. Second, laser produces a much lower pressures than an impact. An impact vapor plume whose peak-shock temperature is 10 K goes through a very different adiabatic decompression path that a laser plume with the initial temperature of 10 K does. This difference in thermodynamic path comes from that in entropy. The former vapor has much higher entropy than that of the latter. However, if the entropies of a laser-induced vapor and that of an impact vapor are matched, their adiabatic decompression paths coincide. Thus, the key for laser simulation of impact vaporization phenomena is to match the entropies of the two kinds of vapor. In the following, we discuss a method to match the entropies of laserand impact-induced vapor plumes. Entropy of Impact-Induced Vapor: The increase in entropy due to hypervelocity impact can be calculated by Rankine-Hugoniot relations and thermodynamic relations. Here, we assume a linear velocity relation, which holds for a variety of materials over a wide range of impact velocity [e.g., 6]: Vs = Co + s Up, (1) where Vs, Co, Up, and s are shock velocity, bulk sound velocity, particle velocity, and a constant, respectively. Using equation (1), the differential form of Rankine-Hugoniot relations, and thermodynamic relations, we obtain: dS dUp = sU p 2