High-Intensity CO2 Laser Breakdown of Low-Pressure Gas

We have performed CO2 interferometry measurements to characterize the high-intensity (1 ¿ 1.0 × 1014 W/cm2) CO2 laser breakdown in low-pressure (0.4 torr < P < 2.0 torr) dry air and demonstrate that this plasma can be used for beat wave accelerator studies. The plasma has a diameter and axial length of 0.2 mm and 2.0 mm, respectively, slightly greater than the focal diameter and depth of focus of the focusing optics. Peak electron density corresponds to partial ionization ( = 3, ne = 1.0 × 1017 cm-3) at low pressure (P = 0.4-0.5 torr) increasing to full ionization (Z = 7.2) at P > 1.0 torr. A theoretical model of laser breakdown provides insight into the time evolution of the electron density.