Determining plasma parameters from laser-driven THz radiation

In numerical simulations, the creation of a wakefield by a laser pulse propagating through neutral gas results in the radiation of an axially polarized electromagnetic pulse (EMP), which initial calculations indicate may carry off 10-20% of the wakefield energy. Frequencies of 10 - 90 THz have be observed, with peak amplitudes on the order of 10 - 100s MV/m. The VSim/Vorpal computational application has been used to produce full PIC, 2D simulations of neutral gas ionization, axial current generation, and EMP propagation. A reduced model for EMP generation and evolution, driven by the ponderomotive potential, is shown to accurately represent this phenomenon, with significantly shorter run-times than full PIC simulations. Also, the reduced model is noise-free, while PIC simulations require averaging techniques to fully extract the EMP. From this reduced model, we see that the EMP's temporal profile depends on plasma density, ionization radius, and the laser's normalized vector potential. As such, the THz EMP may act as a diagnostic for plasma parameter extraction. A detailed comparison of the EMP's energy with the wakefield's will be presented for a range of plasma densities. In addition, 2D PIC simulations of plasma channel formation will be presented for several plasma densities.