Experiments and simulations of tunnel-ionized plasmas

Abstract

The tunneling-ionization model predicts that fully ionized plasmas with controllable perpendicular (${\mathit{T}}_{\mathrm{\ensuremath{\perp}}}$) and negligible longitudinal temperature (${\mathit{T}}_{\mathrm{\ensuremath{\parallel}}}$) can be produced. The validity of these predictions has been studied through experiments and supporting theory and simulations. Emission of odd harmonics of the laser frequency, indicative of a stepwise ionization process, has been observed. X-ray measurements show that the plasma temperature is higher for a circularly polarized laser-produced plasma compared to when linear polarization is used. Analytically we find that the growth of the stimulated Raman (SRS) and Compton scattering (SCS) instabilities are suppressed during the ionization phase. A higher ${\mathit{T}}_{\mathrm{\ensuremath{\parallel}}}$ than expected from the single-particle-tunneling model was observed after the ionization phase through SCS fluctuation spectra. The maximum achievable plasma density is found to be limited by ionization induced refraction. One-dimensional (1D) simulations show that, after the ionization phase, the initial ${\mathit{T}}_{\mathrm{\ensuremath{\parallel}}}$ is low as expected from the single particle model and SRS density fluctuations grow to large values. In 2D simulations, however, ${\mathit{T}}_{\mathrm{\ensuremath{\parallel}}}$ at the end of the ionization phase is already much higher and only SCS is seen to grow. The simulations indicate that stochastic heating and the Weibel instability play an important role in plasma heating in all directions and in making the plasma isotropic. Two-dimensional simulations also confirm that refraction plays a crucial role in determining the maximum electron density that can be obtained in such plasmas.