Laser induced sparks in atmospheric helium and helium mixtures, probbed with thomson scattering

Summary form only given. In the plasma community there is widespread interest in atmospheric plasmas as they have various applications such as medicine, surface treatment and lighting for example. Helium is a commonly used gas in such plasmas. Emission spectroscopy is usually used to obtain information about the plasma parameters. However, most of the emission line studies rely on Stark broadening theories where self-absorption of lines can be an issue and result in incorrect values of n_e. A diagnostic such as Thomson scattering can be used to test the reliability of emission based methods. Thomson Scattering is one of the most powerful nonintrusive diagnostic tools for plasma characterization and it has been applied to a variety of circumstances. The interpretation of the Thomson scattering spectrum is relatively simple and renders unambiguous values of the electron temperature and density. However, this technique has not been widely applied in studies of laser induced breakdown in gases. We will present our recent experimental results in this area. We have used Thomson Scattering to study the temporal evolution of laser induced sparks in atmospheric He and He mixtures with (0.2-50)torr of H₂ or N₂. The experiment has been carried out using two Nd:YAG lasers. An Nd:YAG laser (E≈140mJ) operating in the first harmonic regime (1064nm) was used to breakdown gas, and a frequency doubled (532nm) Nd:YAG (E≈70mJ) was used to probe a slice of the plasma spark. The time history of the electron temperature and density (of the orders of 10¹⁴-10¹⁷cm^-3 and 0.1-3eV) for the different gas environments, from 1μs to 25μs after breakdown will be presented.