arXiv subjects

John T. Morrison

Publications and source records attributed to John T. Morrison.

8 recordsLinked to original sources

arXiv2020

Optimizing Laser-Plasma Interactions for Ion Acceleration using Particle-in-Cell Simulations and Evolutionary Algorithms

The development of ultra-intense laser-based sources of high energy ions is an important goal, with a variety of potential applications. One of the barriers to achieving this goal is the need to maximize the conversion efficiency from laser energy to ion energy. We apply a new approach to this problem, in which we use an evolutionary algorithm to optimize conversion efficiency by exploring variations of the target density profile with thousands of one-dimensional particle-in-cell (PIC) simulations. We then compare this "optimal" target identified by the one-dimensional PIC simulations to more conventional choices, such as with an exponential scale length pre-plasma, with fully three-dimensional PIC simulations. The optimal target outperforms the conventional targets in terms of maximum ion energy by 20% and show a significant enhancement of conversion efficiency to high energy ions. This target geometry enhances laser coupling to the electrons, while still allowing the laser to strongly reflect from an effectively thin target. These results underscore the potential for this statistics-driven approach to guide research into optimizing laser-plasma simulations and experiments.

physics.plasm-ph↗

arXiv2019

Experimental observation and computational modeling of radial Weibel instability in high intensity laser-plasma interactions

When a relativistic intensity laser interacts with the surface of a solid density target, suprathermal electron currents are subject to Weibel instability filamentation when propagating through the thermal population of the bulk target. We present time resolved shadowgraphy of radial ionization front expansion and Weibel instability filamentation within a thin, sub-micron, sheet initiated by irradiation with a short pulse, high intensity laser. High temporal (100 fs) and spatial (1 $μ$m) resolution shadowgraphy of the interaction reveals a relativistic expansion of the ionization front within a 120 $μ$m diameter region surrounding the laser-target interaction, corroborated by simulations to expand at $0.77c$, where $c$ is the speed of light. Filamentation within the patch persists for several picoseconds and seeds the eventual recombination and heating dynamics on the nanosecond timescale. Particle-in-cell simulations were conducted to elucidate the electron dynamics leading to the radial expansion of the critical surface. Computational results report the ionization expansion is due to field ionization of the expanding hot electron population. Filamentation within the expansion is due to the Weibel instability which is supported by the magnetic fields present.

physics.plasm-ph↗

arXiv2019

Particle-in-Cell Simulations of Density Peak Formation and Ion Acceleration from Short Pulse Laser-Driven Ponderomotive Steepening

We use particle-in-cell (PIC) simulations and simple analytic models to investigate the laser-plasma interaction known as ponderomotive steepening. When normally incident laser light reflects at the critical surface of a plasma, the resulting standing electromagnetic wave modifies the electron density profile via the ponderomotive force, which creates peaks in the electron density separated by approximately half of the laser wavelength. What is less well studied is how this charge imbalance accelerates ions towards the electron density peaks, modifying the ion density profile of the plasma. Idealized PIC simulations with an extended underdense plasma shelf are used to isolate the dynamics of ion density peak growth for a 42 fs pulse from an 800 nm laser with an intensity of 10$^{18}$ W cm$^{-2}$. These simulations exhibit sustained longitudinal electric fields of 200 GV m$^{-1}$, which produce counter-steaming populations of ions reaching a few keV in energy. We compare these simulations to theoretical models, and we explore how ion energy depends on factors such as the plasma density and the laser wavelength, pulse duration, and intensity. We also provide relations for the strength of longitudinal electric fields and an approximate timescale for the density peaks to develop. These conclusions may be useful investigating the phenomenon of ponderomotive steepening as advances in laser technology allow shorter and more intense pulses to be produced at various wavelengths. We also discuss the parallels with other work studying the interference from two counter-propagating laser pulses.

physics.plasm-ph↗

arXiv2017

Relativistic electron acceleration by mJ-class kHz lasers normally incident on liquid targets

We report observation of kHz-pulsed-laser-accelerated electron energies up to 3 MeV in the -$k_\text{laser}$ (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 $\cdot$ 10$^{18}$ 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 $\geq$kHz-repetition, fs-duration electron-beam applications.

physics.plasm-ph↗

arXiv2017

Particle-in-Cell Simulations of Electron Beam Production from Infrared Ultra-intense Laser Interactions

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 explore intense mid-IR laser interactions with dense targets. We present the results of thirty PIC simulations over a wide range of intensities (0.03 < $a_0$ < 39) and wavelengths (λ =780 nm, 3 μm, and 10 μm). Earlier studies, limited to λ =780 nm and $a_0 \sim$ 1 [1, 2], 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 intensity is not highly relativistic ($a_0 \lesssim 1$) and that the pre-plasma scale length is similar to or longer than the laser wavelength. Under these conditions, ejected electron angular and energy distributions are similar to expectations from an analytic model used in [2]. We also find that, for $a_0 \sim 1$, the mid-IR simulations exhibit a classic ponderomotive steepening pattern with multiple peaks in the ion and electron density distribution. Experimental validation of this basic laser-plasma interaction process will be possible in the near future using mid-IR laser technology and interferometry.

physics.plasm-ph↗

arXiv2016

3D PIC simulations of electron beams created via reflection of intense laser light from a water target

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 (AFRL) using the PIC code LSP. This is the first time PIC simulations have been performed in 3D for this experiment which involves an ultra-intense, short-pulse (30 fs) laser interacting with a water jet target at normal incidence. These 3D PIC simulation results are compared to results from 2D(3$v$) PIC simulations for both $5.4~\cdot~10^{17}$ W cm$^{-2}$ and $3~\cdot~10^{17}$ W cm$^{-2}$ intensities. Comparing the 2D(3$v$) and 3D simulation results, the laser-energy-to-ejected-electron-energy conversion efficiencies were comparable, but the angular distribution of ejected electrons show interesting differences with qualitative differences at higher intensity. An analytic plane-wave model is discussed which provides some explanation for the angular distribution and energies of ejected electrons in the 2D(3$v$) simulations. We also performed a 3D simulation with circularly polarized light and found a significantly higher conversion efficiency and peak electron energy, which is promising for future experiments.

physics.plasm-ph↗

arXiv2015

Backward-propagating MeV electrons from $10^{18}$ W/cm$^2$ laser interactions with water

We present an experimental study of the generation of $\sim$MeV electrons opposite to the direction of laser propagation following the relativistic interaction at normal incidence of a $\sim$3 mJ, $10^{18}$ W/cm$^2$ short pulse laser with a flowing 30 $μ$m diameter water column target. Faraday cup measurements record hundreds of pC charge accelerated to energies exceeding 120 keV, and energy-resolved measurements of secondary x-ray emissions reveal an x-ray spectrum peaking above 800 keV, which is significantly higher energy than previous studies with similar experimental conditions and more than five times the $\sim$110 keV 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 pre-pulse. These results are supported by 2D Particle-in-Cell (PIC) simulations of the laser-plasma interaction that exhibit beam-like backward-propagating MeV electrons.

physics.plasm-ph↗

arXiv2015

Backward-Propagating MeV Electrons in Ultra-Intense Laser Interactions: Standing Wave Acceleration and Coupling to the Reflected Laser Pulse

Laser-accelerated electron beams have been created at a kHz repetition rate from the {\it reflection} of intense ($\sim10^{18}$ W/cm$^2$), $\sim$40 fs laser pulses focused on a continuous water-jet in an experiment at the Air Force Research Laboratory. This paper investigates Particle-in-Cell (PIC) simulations of the laser-target interaction to identify the physical mechanisms of electron acceleration in this experiment. We find that the standing-wave pattern created by the overlap of the incident and reflected laser is particularly important because this standing wave can "inject" electrons into the reflected laser pulse where the electrons are further accelerated. We identify two regimes of standing wave acceleration: a highly relativistic case ($a_0~\geq~1$), and a moderately relativistic case ($a_0~\sim~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 investigating electron acceleration in the {\it specular (back reflection) direction} and over a wide range of intensities ($10^{17}-10^{19}$ W cm$^{-2}$).

physics.plasm-ph↗