Experimental measurements of multiphoton enhanced air breakdown by a subthreshold intensity excimer laser

Abstract

This work presents density, spectroscopic temperature, and shockwave measurements of laser induced breakdown plasma in atmospheric air by subthreshold intensity ͑5.5ϫ 10 9 W / cm 2 ͒ 193 nm laser radiation. Using molecular spectroscopy and two-wavelength interferometry, it is shown that substantial ionization ͑Ͼ10 16 cm −3 ͒ occurs that is not predicted by collisional cascade ͑CC͒ breakdown theory. While the focused laser irradiance is three orders of magnitude below the theoretical collisional breakdown threshold, the substantial photon energy at 193 nm ͑6.42 eV/ photon͒ compared with the ionization potential of air ͑15.6 eV͒ significantly increases the probability of multiphoton ionization effects. By spectroscopically monitoring the intensity of the N 2 + first negative system ͑B 2 ⌺ u + − X 2 ⌺ g + ͒ vibrational bandhead ͑vЈ =0,vЉ =0͒ at low pressure ͑20 Torr͒ where multiphoton effects are dominant, it is shown that two photon excitation, resonant enhanced multiphoton ionization is the primary mechanism for quantized ionization of N 2 to the N 2 + ͑B 2 ⌺ u + ͒ state. This multiphoton effect then serves to amplify the collisional breakdown process at higher pressures by electron seeding, thereby reducing the threshold intensity from that required via CC processes for breakdown and producing high density laser formed plasmas.

Focused excimer laser initiated, radio frequency sustained high pressure air plasmas

“…A 4 Torr increase over RF only start-up pressures represents a 22% pressure range improvement in RF air plasma formation with only a 0.1% energy increased contribution from the laser pulse. The sustainment of the laser formed plasma above 20 Torr also validates that REMPI processes in air at these pressures (as seen in earlier UV laser works) 16,17 do occur and dominate the seed ionization process in this range at laser power fluxes well below (10 À3 ) the classical collisional cascade threshold flux of 1 TW=cm 2 . Our plasma volumes are nearly double our antenna volume at 300 cm 3 .…”

“…17 These quantum effects significantly lower the power fluxes needed for air breakdown compared with classical processes by a factor of 1000 for our laser in air. 16,18 Previous studies of these laser focus plasmas with an 18 cm focal length lens producing 5.5 GW=cm 2 fluxes at atmospheric pressure have revealed very high electron densities ($10 16 =cm 3 ), far in excess of the densities of the laser TMAE plasmas ($10 13 =cm 3 ) albeit at much smaller volumes. 16 Nevertheless, it is believed that these high density, small volume laser sparks can contribute significantly to high pressure breakdown initiation and can assist in producing large volume, RF sustained air plasmas.…”

“…Through the use of a high transmissivity focusing compound microscope system or single parabolic lens the laser beam can be concentrated to very high fluxes (10 9 to 10 12 W=cm 2 ). 16,17 The very high photon population causes ionization in air through intense electric fields via collisional cascade (classical effect) and resonance multi-photon ionization (quantum process). The ionization energies of nitrogen and oxygen are 15.6 eV and 12.1 eV, respectively.…”

Spectroscopic temperature measurements of air breakdown plasma using a 110 GHz megawatt gyrotron beam

“…20 Agreement between the code's predicted spectra and spectra from low temperature air plasmas has already been reported, and the code has been used for non-equilibrium plasmas from nearly room temperature to several eV. 11,21 SPECAIR models the finite resolution of the spectrometer by using a user defined slit function to calculate the resulting line shapes of the transitions. In addition, SPECAIR can model the optical thickness of the plasma.…”

Quantitative Measurements of Electron Number Density and Threshold for Laser Induced Breakdown in Air

“…[16,17] The intermediate/excited states of atoms or molecules provide the intermediate energy levels for photon absorption access and enhanced photon ionization. Previously, the resonant-and avalanche-ionization of laserinduced plasma in air have been studied at low pressure conditions by using coherent microwave Rayleigh scattering from REMPI process.…”