Rayleigh Scattering

It is possible to measure gas temperature using the Rayleigh scattering signal8,9. The intensity of elastically scattered photons (i.e. Rayleigh signal) is in direct proportion to the density of scattering gas. In case of ideal gas this density is inversely proportional to gas temperature, via ideal gas law $p = nk_BT_g$ , where $p$ is the given constant pressure, n is the number density of gas and $T_g$ is the temperature of the gas. So we can say that Rayleigh signal is related to the gas temperature via $I_{Rayleigh} \sim n_g=p/(k_B T_g )$ . To find the absolute value of $T_g$ we must perform a reference measurement at a known temperature $T_{ref}$ so that $I_{Rayleigh} \sim n_g=p/(k_B T_g )$ . When gas employed in the reference measurement is identical to the gas in the experiment (or at least has a similar differential cross section for Rayleigh scattering), as are the experimental setup and pressure we can infer that $T_g = (I_{ref}/I_{Rayleigh})T_{ref}$ . We assume that major scatter will be the background gas and the addition of species ablated from the target or electrodes will have a negligible contribution due to low densities, when compared to the background gas. A convenient way of performing Rayleigh temperature measurements is with 532 nm wavelength laser pulse, supplied by Continuum SLIII laser. The 532 nm light is easily separated from the plasma background with widely available laser line optical filter. Planar geometry of the laser beam can also be used, thus allowing to capture a 2D temperature distribution (in axial and radial directions), with iCCD camera (Andor iStar).