Gas-Breakdown Dependence on Beam Size and Pulse Duration with 10.6-μ Wavelength Radiation

Gas-breakdown threshold measurements with 10.6-μ wavelength radiation have been made in air, argon, and helium. The threshold was determined with 50- and 200-nsec-duration pulses and was found to depend on the peak intensity of the pulse, showing that the breakdown process is a balance between the rate of energy absorption and some rate of energy loss. The threshold also decreased as the focal spot size was increased from 10−2 to 10−1 cm, showing that the loss process is reduced for larger beam sizes. The threshold varied inversely with gas pressure from 0.1 to 4 atm, which is evidence that the loss mechanism is not electron diffusion but must be some other process which exhibits a beam-size dependence and is apparently weakly dependent on gas pressure.