Microwave resonator configuration for 3 GHz pulsed microwave operation with shadowgraph illumination. Components include (a) 25 kW peak microwave generator, (b) circulator, (c) three-stub tuner, (d) WR284 to WR430 waveguide transition, (e) WR430 test section with gas flow access and optical access, (f) 2× Nd:YAG shadowgraph illumination, (g) femtosecond/picosecond seed ionization laser beam, (h) sliding short, (i) femtosecond/picosecond laser focal volume, (j) fuel/air mixture gases.
Homodyne detection system with a 100 GHz oscillator. Radiation passes through (a) the splitter, is emitted by (b) horn, collected, and passed through a (c) directional coupler. This scattered signal is (d) preamplified, (e) mixed with the oscillator, (f) output as 1 GHz signal proportional to the number of scatterers. Microwave polarization is orthogonal to the laser focal volume.
Schlieren images with nanosecond illumination captured at 0, 0.5, 1, 2, 4, 6, and in quiescent air. The top row shows slight heating and a very weak acoustic wave propagating from the , 200 fs laser pulse at 800 nm seed laser. The bottom row shows the effect of a , 50 mJ microwave pulse for heating of the same seed laser pulse. The microwave pulse has ended by image 4 and a weak shock wave with speed equal to Mach 1 is observed propagating away from the heated gas column.
Shadowgraph with nanosecond illumination showing initial heating from microwave for a seed laser pulse of and 200 fs in a methane/air mixture of with a flowrate of 70 cm/s. Early formation of the flame kernel and the propagation of the flame kernel through the ms time scale is depicted. All energy deposition has occurred prior to the second image, at .
Shadowgraph with nanosecond illumination showing much stronger interaction with a , 200 fs laser pulse and an identical 50 mJ microwave pulse in a methane/air mixture of with a flowrate of 70 cm/s. The increased ionization in the seed laser focal volume leads to a much greater volume of interaction with the clear formation of a flame front along the entire microwave-heated region.
Multipoint ignition was demonstrated using a single 75 mJ microwave pulse with two synchronous 7 mJ, 200 ps laser pulses in a methane/air mixture of with a flowrate of 70 cm/s. The flame kernel present at 3 ms indicates successful ignition. Arrows indicate the laser focal volume. Images from left to right depict ignition for cases with (a) no MW, (b) point 1 ignition only, (c) point 2 ignition only, and (d) two-point ignition with MW.
Shadowgraph with nanosecond illumination showing the ignition kernel size at 1, 2, and 3 ms following the ignition event. The top row of images depicts the natural growth in a methane/air of with a flowrate of 70 cm/s. The bottom set of images shows the effect of additional microwave pulses applied at 1 and 2 ms. The effect on kernel growth is evident in the increased kernel size at 2.0 and 3.0 ms.
100 GHz microwave scattering signals for two heating pulses. The seed fs laser arrives at time zero. Pulse energies and peak powers are, respectively, 45 mJ; 25 kW and 42 mJ; 23.3 kW. A significant increase in delay (900 ns compared to 350 ns) from the seed laser to the initial scattering signal is evident for the low power case.
Centerline gas temperature for an initial ionization fraction of for applied electric field values of 0.25, 0.50, and 0.75 times the breakdown electric field for air.
Centerline number densities for electrons and positive ions for 0.75, 0.50, and 0.25 of the breakdown electric field for an initial ionization fraction of .
Evolution of electron (solid blue), position ion (dashed red), and negative ion (dashed black) number densities along the centerline for 50% of the breakdown electric field and for initial ionization fractions of , , and .
Centerline gas temperature for initial ionization fraction of and applied electric field of 0.25 times the breakdown threshold for air. The negligible shielding effect for a long aspect ratio column is evident in the small change in gas temperature as compared to the nonshielded infinite aspect ratio. A stronger shielding effect is shown for a lower aspect ratio .
Radial profiles for the gas temperature and gas pressure are shown evolving in time. The case depicted shows moderate heating for an initial ionization fraction of and an applied electric field of . Initial heating and corresponding pressure rise at results in a propagating weak shock with sonic speed.
For an initial ionization fraction of and two applied electric fields of 53% and 51.5% of the breakdown field, the delay time is reduced by a factor of 2. The delay values are 250 ns and 500 ns, respectively.
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