Novel approach to laser ignition using a gas pre-ionization technique
A new laser ignition concept is demonstrated through the development of a dual-pulse gas pre-ionization technique. The approach uses an ultraviolet (UV, =266 nm) laser beam to achieve initial gas ionization, followed nanoseconds later by an overlapped near-infrared (NIR, =1064 nm) pulse for energy addition. The role of the NIR pulse is to heat the pre-ionized gas through inverse Brembtrahlung absorption of radiation. Temporally resolved absorption profiles are obtained for the two beams using a photodiode placed at the chamber output. The results show that a 20 mJ NIR pulse can be almost completely absorbed inside the pre-ionized gas generated by a 10 mJ UV beam. Furthermore, gas heating is confirmed through the use of a Rayleigh scattering diagnostic technique and temperatures in exceb of 1200 K were measured at 50 s after the end of the second laser pulse. Finally, the dual-pulse laser heating technique is contrasted with the "clabical" laser induced spark approach. By attempting the ignition of a propane-Air mixture (=1.0, p0=1 bar, T0=300 K), it is shown that the dual-pulse leads to a significant decrease in the minimum ignition energy. Succebful ignition was obtained using only 30 mJ of laser energy (EUV=10 mJ and ENIR=20 mJ). This represents leb than half the amount of energy required if ignition is attempted using a NIR pulse alone (75 mJ).