W. M. Roquemore
Air Force Research Laboratory
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Featured researches published by W. M. Roquemore.
Review of Scientific Instruments | 2014
Scott Feister; John A. Nees; John T. Morrison; Kyle D. Frische; Chris Orban; Enam Chowdhury; W. M. Roquemore
Ultra-intense laser-matter interaction experiments (>10(18) W/cm(2)) with dense targets are highly sensitive to the effect of laser noise (in the form of pre-pulses) preceding the main ultra-intense pulse. These system-dependent pre-pulses in the nanosecond and/or picosecond regimes are often intense enough to modify the target significantly by ionizing and forming a plasma layer in front of the target before the arrival of the main pulse. Time resolved interferometry offers a robust way to characterize the expanding plasma during this period. We have developed a novel pump-probe interferometry system for an ultra-intense laser experiment that uses two short-pulse amplifiers synchronized by one ultra-fast seed oscillator to achieve 40-fs time resolution over hundreds of nanoseconds, using a variable delay line and other techniques. The first of these amplifiers acts as the pump and delivers maximal energy to the interaction region. The second amplifier is frequency shifted and then frequency doubled to generate the femtosecond probe pulse. After passing through the laser-target interaction region, the probe pulse is split and recombined in a laterally sheared Michelson interferometer. Importantly, the frequency shift in the probe allows strong plasma self-emission at the second harmonic of the pump to be filtered out, allowing plasma expansion near the critical surface and elsewhere to be clearly visible in the interferograms. To aid in the reconstruction of phase dependent imagery from fringe shifts, three separate 120° phase-shifted (temporally sheared) interferograms are acquired for each probe delay. Three-phase reconstructions of the electron densities are then inferred by Abel inversion. This interferometric system delivers precise measurements of pre-plasma expansion that can identify the condition of the target at the moment that the ultra-intense pulse arrives. Such measurements are indispensable for correlating laser pre-pulse measurements with instantaneous plasma profiles and for enabling realistic Particle-in-Cell simulations of the ultra-intense laser-matter interaction.
Physics of Plasmas | 2015
Chris Orban; John T. Morrison; Enam Chowdhury; John A. Nees; Kyle D. Frische; Scott Feister; W. M. Roquemore
Laser-accelerated electron beams have been created at a kHz repetition rate from the reflection of intense (∼1018u2009W/cm2), ∼40 fs laser pulses focused on a continuous water-jet in an experiment at the Air Force Research Laboratory. This paper investigates Particle-in-Cell simulations of the laser-target interaction to identify the physical mechanisms of electron acceleration in this experiment. We find that the standing-wave pattern created by the overlap of the incident and reflected laser is particularly important because this standing wave can “inject” electrons into the reflected laser pulse where the electrons are further accelerated. We identify two regimes of standing wave acceleration: a highly relativistic case (a0 ≥ 1), and a moderately relativistic case (a0 ∼ 0.5) which operates over a larger fraction of the laser period. In previous studies, other groups have investigated the highly relativistic case for its usefulness in launching electrons in the forward direction. We extend this by investiga...
Physics of Plasmas | 2015
John T. Morrison; Enam Chowdhury; Kyle D. Frische; Scott Feister; V. Ovchinnikov; John A. Nees; Chris Orban; R. R. Freeman; W. M. Roquemore
We present an experimental study of the generation of ∼MeV electrons opposite to the direction of laser propagation following the relativistic interaction at normal incidence of a ∼3u2009mJ, 1018u2009W/cm2 short pulse laser with a flowing 30u2009 μm diameter water column target. Faraday cup measurements record hundreds of pC charge accelerated to energies exceeding 120u2009keV, and energy-resolved measurements of secondary x-ray emissions reveal an x-ray spectrum peaking above 800u2009keV, which is significantly higher energy than previous studies with similar experimental conditions and more than five times the ∼110u2009keV ponderomotive energy scale for the laser. We show that the energetic x-rays generated in the experiment result from backward-going, high-energy electrons interacting with the focusing optic, and vacuum chamber walls with only a small component of x-ray emission emerging from the target itself. We also demonstrate that the high energy radiation can be suppressed through the attenuation of the nanosecond-scale...
Physics of Plasmas | 2016
Gregory Ngirmang; Chris Orban; Scott Feister; John T. Morrison; Kyle D. Frische; Enam Chowdhury; W. M. Roquemore
We present 3D Particle-in-Cell (PIC) modeling of an ultra-intense laser experiment by the Extreme Light group at the Air Force Research Laboratory using the Large Scale Plasma (LSP) PIC code. This is the first time PIC simulations have been performed in 3D for this experiment which involves an ultra-intense, short-pulse (30u2009fs) laser interacting with a water jet target at normal incidence. The laser-energy-to-ejected-electron-energy conversion efficiency observed in 2D(3v) simulations were comparable to the conversion efficiencies seen in the 3D simulations, but the angular distribution of ejected electrons in the 2D(3v) simulations displayed interesting differences with the 3D simulations angular distribution; the observed differences between the 2D(3v) and 3D simulations were more noticeable for the simulations with higher intensity laser pulses. An analytic plane-wave model is discussed which provides some explanation for the angular distribution and energies of ejected electrons in the 2D(3v) simulat...
Optics Express | 2017
Scott Feister; Drake R. Austin; John T. Morrison; Kyle D. Frische; Chris Orban; Gregory Ngirmang; Abraham Handler; Joseph R. H. Smith; Mark Schillaci; Jay A. LaVerne; Enam Chowdhury; R. R. Freeman; W. M. Roquemore
We report observation of kHz-pulsed-laser-accelerated electron energies up to 3 MeV in the -klaser (backward) direction from a 3 mJ laser interacting at normal incidence with a solid density, flowing-liquid target. The electrons/MeV/s.r. >1 MeV recorded here using a mJ-class laser exceeds or equals that of prior super-ponderomotive electron studies employing lasers at lower repetition-rates and oblique incidence. Focal intensity of the 40-fs-duration laser is 1.5 · 1018 W cm-2, corresponding to only ∼80 keV electron ponderomotive energy. Varying laser intensity confirms electron energies in the laser-reflection direction well above what might be expected from ponderomotive scaling in normal-incidence laser-target geometry. This direct, normal-incidence energy spectrum measurement is made possible by modifying the final focusing off-axis-paraboloid (OAP) mirror with a central hole that allows electrons to pass, and restoring laser intensity through adaptive optics. A Lanex-based, optics-free high-acquisition rate (>100 Hz) magnetic electron-spectrometer was developed for this study to enable shot-to-shot statistical analysis and real-time feedback, which was leveraged in finding optimal pre-plasma conditions. 3D Particle-in-cell simulations of the interaction show qualitative super-ponderomotive spectral agreement with experiment. The demonstration of a high-repetition-rate, high-flux source containing >MeV electrons from a few-mJ, 40 fs laser and a simple liquid target encourages development of future ≥kHz-repetition, fs-duration electron-beam applications.
Physics of Plasmas | 2017
Gregory Ngirmang; Chris Orban; Scott Feister; John T. Morrison; Enam Chowdhury; W. M. Roquemore
Advances in ultra-intense laser technology are enabling, for the first time, relativistic intensities at mid-infrared (mid-IR) wavelengths. Anticipating further experimental research in this domain, we present high-resolution two dimensional Particle-in-Cell (PIC) simulation results using the Large-Scale Plasma (LSP) code that explores intense mid-IR laser interactions with near solid density targets. We present the results of thirty PIC simulations over a wide range of intensities ( 0.03<a0<40) and wavelengths ( λ= 780u2009nm, 3u2009μm, and 10u2009μm). Earlier studies [Orban et al., Phys. Plasmas 22, 023110 (2015) and Ngirmang et al., Phys. Plasmas 23, 043111 (2016)], limited to λ= 780u2009nm and a0∼1, identified super-ponderomotive electron acceleration in the laser specular direction for normal-incidence laser interactions with dense targets. We extend this research to mid-IR wavelengths and find a more general result that normal-incidence super-ponderomotive electron acceleration occurs provided that the laser intens...
Bulletin of the American Physical Society | 2016
Scott Feister; Chris Orban; John T. Morrison; Gregory Ngirmang; Joseph R. H. Smith; Kyle D. Frische; A.C. Peterson; A.J. Klim; Enam Chowdhury; R. R. Freeman; W. M. Roquemore
arXiv: Plasma Physics | 2015
Scott Feister; Drake R. Austin; John T. Morrison; Kyle D. Frische; Chris Orban; Gregory Ngirmang; Abraham Handler; Mark Schillaci; Enam Chowdhury; R. R. Freeman; W. M. Roquemore
New Journal of Physics | 2018
John T. Morrison; Scott Feister; Kyle D. Frische; Drake R. Austin; Gregory Ngirmang; Neil R. Murphy; Chris Orban; Enam Chowdhury; W. M. Roquemore
New Journal of Physics | 2018
John T. Morrison; Scott Feister; Kyle D. Frische; Drake R. Austin; Gregory Ngirmang; Neil R. Murphy; Chris Orban; Enam Chowdhury; W. M. Roquemore