Properties of ultraviolet and near-infrared laser induced air plasmas and their application for spark ignition

The present contribution describes a novel means of forming laser induced plasmas in air based on a multiple pulse preionization scheme. The method makes use of a nanosecond duration ultraviolet (UV) 266 nm pulse to generate a weakly ionized plasma kernel. This is followed ∼15 ns later by an overlapped Near-Infrared (NIR) 1064 nm pulse for controlled energy addition into the gas. The energy added by the second pulse is due to the inverse bremsstrahlung process in which the free electrons generated during the UV pre-ionization pulse absorb the NIR laser energy and, through collisions with gas molecules, increase the temperature of the gas. The approach allows tailoring of the plasma temperature and potentially size by varying the pulse conditions. One application is to generate weakly ionized, low temperature plasmas in the range of ∼2000–3000 K which can then serve as ignition sources for combustion. We report plasma energy absorption and resulting luminosity for the two pulses (wavelengths) acting individually. We show that the UV pulse does not exhibit a threshold behavior, but rather the fraction of energy absorbed inside the plasma changes continuously with pulse energy. This is in contrast with the NIR pulse where there is a discontinuous jump to plasma onset as the pulse energy is increased. Both beams exhibit a stochastic nature of the plasma formation process, i.e. for fixed pulse energy, the fractional energy absorbed inside the plasma varies from shot-to-shot. Rayleigh scattering thermometry is used to measure the plasma temperature for both the UV and NIR pulse as well as for the two pulses combined. Initial gas heating is possible due to the UV pulse (only). A temperature as high as T=1500 K was measured for a UV pulse energy of 20 mJ at 1μs after the end of the pulse. For the NIR, much higher temperatures (>10,000 K) are observed at early times due to the full breakdown of the plasma. Energy absorption and thermometry results for the dual pulse configuration are also presented, along with demonstration of ignition of a propane air mixture by the dual pulse plasma for a total combined pulse energy of ∼30 mJ which is lower than the pulse energy needed for either wavelength acting on its own.