James B. Michael

Bio: James B. Michael is an academic researcher from Iowa State University. The author has contributed to research in topics: Laser & Microwave. The author has an hindex of 13, co-authored 74 publications receiving 905 citations. Previous affiliations of James B. Michael include Princeton University.

High-gain backward lasing in air.

Abstract: The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air

Abstract: Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

Subcritical microwave coupling to femtosecond and picosecond laser ionization for localized, multipoint ignition of methane/air mixtures

Abstract: Ignition in methane/air mixtures has been achieved using low energy seed laser pulses and an overlapping subcritical microwave pulse. It is shown that the extremely weak ionization of the laser localizes the microwave energy deposition—leading to rapid heating, high temperatures, and ignition. Multiple simultaneous localized regions of ignition are also achieved using the same microwave pulse. Interactions of the seed laser pulse and microwave heating pulse were observed using schlieren and shadowgraph to record the intensity of heating, the scale of the interaction, and for confirmation of ignition. In addition, a coupled one-dimensional gasdynamic-plasma dynamic model has been developed to follow the rapidly evolving plasma properties and the gas properties achieved through this interaction.

Single-shot ultrafast coherent anti-Stokes Raman scattering of vibrational/rotational nonequilibrium

Abstract: The study of internal molecular energy transfer is important for a variety of nonequilibrium and nonthermal environments, including plasma-based manufacturing and materials treatment, medical device treatment applications, and plasma-assisted combustion In the current work, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering spectroscopy is demonstrated for simultaneous, single-shot measurement of pure-rotational and rovibrational energy distributions in the highly nonequilibrium environment of a dielectric barrier discharge plasma Detailed spatial distributions and shot-to-shot fluctuations of rotational temperatures spanning 325–450 K and corresponding vibrational temperatures of 1200–5000 K are recorded across the plasma and surrounding flow with high precision and accuracy This approach allows concise measurements of vibrational/rotational energy distributions in nonequilibrium environments at kilohertz rates that are free of nonresonant background and minimize interference from molecular collisions

PIVlab – Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB

Abstract: Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.

Atomization and Sprays

Abstract: Preface 1.General Considerations 2.Basic Processes in Atomization 3.Drop Size Distributions of Sprays 4.Atomizers 5.Flow in Atomizers 6.Atomizer Performance 7.External Spray Charcteristics 8.Drop Evaporation 9.Drop Sizing Methods Index

High-gain backward lasing in air.

Abstract: The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.