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Ionization injection in a plasma wakefield accelerator was investigated experimentally using two lithium plasma sources of different lengths. The ionization of the helium gas, used to confine the lithium, injects electrons in the wake. After acceleration, these injected electrons were observed as a distinct group from the drive beam on the energy spectrometer. They typically have a charge of tens of pC, an energy spread of a few GeV, and a maximum energy of up to 30 GeV. The emittance of this group of electrons can be many times smaller than the initial emittance of the drive beam. The energy scaling for the trapped charge from one plasma length to the other is consistent with the blowout theory of the plasma wakefield.

We report on measurements of the nonlinear index of refraction of air, N₂, and O₂ at a wavelength close to 10 μm by collinear four-wave mixing of a 200 MW CO₂ laser beat-wave. The use of a 200 ps long beat-wave comprising radiation amplified on the 10P20 and 10R16 lines of the CO₂ laser provides a sensitive method to measure the small nonlinearities characteristic of the gas phase in a spectral region where no such data exists.

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We present measurements of the third-order optical nonlinearity of Kr, Xe, N 2 , O 2 , and air at a wavelength near 10 µm by using four-wave mixing of ~ 15 GW/cm 2 , 200 ps (FWHM) CO 2 laser pulses. Measurements in molecular gases resulted in an asymmetric four-wave mixing spectrum indicating that the nonlinear response is strongly affected by the delayed, rotational contribution to the effective nonlinear refractive index. Within the uncertainty of our measurements, we have found that the longwavelength nonlinear refractive indices of these gases are consistent with measurements performed in the near-IR.