Suitability of tunneling ionization produced plasmas for the plasma beat wave accelerator

Tunneling ionization can be thought of as the high-intensity, low-frequency limit of multi-photon ionization (MPI). Extremely uniform plasmas were produced by the latter process for beat wave excitation experiments using a 0.5- mu m laser. Plasmas with 100% ionization were produced with densities exceeding 10/sup 17/ cm/sup -3/. The experiment described uses a CO/sub 2/ laser (I/sub max/ approximately=5*10/sup 14/ W/cm/sup 2/) which allows the formation of plasmas via the tunneling process. Plasmas with densities in the range of 5 to 10*10/sup 16/ cm/sup -3/ are required. Thomson scattering was used to explore the density and temperature regime of tunneling-ionization-produced plasmas. It is found that plasmas with densities up to 10/sup 16/ cm/sup -3/ can indeed be produced and that these plasmas are hot. Beyond this density, strong refraction of laser radiation occurs due to the radial profile of the plasma. Implications of this work for the beat wave accelerator program are discussed.<>