Plasma Resonance in the X-Ray Emission from Gaseous Laser Targets

I report the first observation of C${\mathrm{O}}_2$-laser-induced x-ray emission from gaseous targets. By varying the gas pressure, the plasma density of the target was tuned through resonance at the critical density. The x-ray emission rose quickly from zero at the critical pressure, and reached a peak when the plasma was slightly overdense. Because of the resonant enhancement, a C${\mathrm{O}}_2$-laser pulse of only 0.1 J was sufficient for this experiment.