Laser-initiated channels for ion transport: CO2-laser absorption and heating of NH3 and C2H4 gases

Abstract

Initiation and guiding of an electrical discharge by CO₂‐laser heating of a molecular gas can provide a channel suitable for the transport of a light ion beam for inertial‐confinement fusion. We report absorption measurements for a CO₂ laser tuned to the molecular vibrations of NH₃ and C₂H₄ as a function of gas pressure, laser frequency, and laser energy density. The SATUR laser‐gas‐interaction code models the details of the absorption saturation process for NH₃, calculating gas temperature in the process. The calculation is normalized to absorption data at 0.04 J/cm² and tested with transmission measurements up to 15 J/cm² into a 50‐cm cell. These calculations are the basis for understanding the electrical breakdown and discharge evolution as observed and simulated in the following report. Of particular importance is the calculation of gas temperature of 1900–2100 °K for 15–35‐J/cm² incident laser energy density.