arXiv subjects

Kathleen Weichman

Publications and source records attributed to Kathleen Weichman.

7 recordsLinked to original sources

arXiv2022

Arbitrarily Structured Laser Pulses

Spatiotemporal control refers to a class of optical techniques for structuring a laser pulse with coupled space-time dependent properties, including moving focal points, dynamic spot sizes, and evolving orbital angular momenta. Here we introduce the concept of arbitrarily structured laser (ASTRL) pulses which generalizes these techniques. The ASTRL formalism employs a superposition of prescribed pulses to create a desired electromagnetic field structure. Several examples illustrate the versatility of ASTRL pulses to address a range of laser-based applications.

physics.optics↗

arXiv2021

Spatiotemporal control of laser intensity through cross-phase modulation

Spatiotemporal pulse shaping provides control over the trajectory and range of an intensity peak. While this control can enhance laser-based applications, the optical configurations required for shaping the pulse can constrain the transverse or temporal profile, duration, or orbital angular momentum (OAM). Here we present a novel technique for spatiotemporal control that mitigates these constraints by using a "stencil" pulse to spatiotemporally structure a second, primary pulse through cross-phase modulation (XPM) in a Kerr lens. The temporally shaped stencil pulse induces a time-dependent focusing phase within the primary pulse. This technique, the "flying focus X," allows the primary pulse to have any profile or OAM, expanding the flexibility of spatiotemporal pulse shaping for laser-based applications. As an example, simulations show that the flying focus X can deliver an arbitrary-velocity, variable-duration intensity peak with OAM over distances much longer than a Rayleigh range.

physics.optics↗

arXiv2021

Emission of electromagnetic waves as a stopping mechanism for nonlinear collisionless ionization waves in a high-$β$ regime

A high energy density plasma embedded in a neutral gas is able to launch an outward-propagating nonlinear electrostatic ionization wave that traps energetic electrons. The trapping maintains a strong sheath electric field, enabling rapid and long-lasting wave propagation aided by field ionization. Using 1D3V kinetic simulations, we examine the propagation of the ionization wave in the presence of a transverse MG-level magnetic field with the objective to identify qualitative changes in a regime where the initial thermal pressure of the plasma exceeds the pressure of the magnetic field ($β>1$). Our key finding is that the magnetic field stops the propagation by causing the energetic electrons sustaining the wave to lose their energy by emitting an electromagnetic wave. The emission is accompanied by the magnetic field expulsion from the plasma and an increased electron loss from the trapping wave structure. The described effect provides a mechanism mitigating rapid plasma expansion for those applications that involve an embedded plasma, such as high-flux neutron production from laser-irradiated deuterium gas jets.

physics.plasm-ph↗

arXiv2020

Strong surface magnetic field generation in relativistic short pulse laser-plasma interaction with an applied seed magnetic field

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field $B_\mathrm{seed}$ can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, $B_\mathrm{gen} \sim - 2 πB_\mathrm{seed} Δx/λ_0$, where $Δx$ is the target thickness and $λ_0$ is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a $μ$m-thick laser-irradiated target with a kilotesla-level seed magnetic field.

physics.plasm-ph↗

arXiv2020

Laser reflection as a catalyst for direct laser acceleration in multipicosecond laser-plasma interaction

We demonstrate that laser reflection acts as a catalyst for superponderomotive electron production in the preplasma formed by relativistic multipicosecond lasers incident on solid density targets. In 1D particle-in-cell simulations, high energy electron production proceeds via two stages of direct laser acceleration, an initial stochastic backward stage, and a final non-stochastic forward stage. The initial stochastic stage, driven by the reflected laser pulse, provides the pre-acceleration needed to enable the final stage to be non-stochastic. Energy gain in the electrostatic potential, which has been frequently considered to enhance stochastic heating, is only of secondary importance. The mechanism underlying the production of high energy electrons by laser pulses incident on solid density targets is of direct relevance to applications involving multipicosecond laser-plasma interactions.

physics.plasm-ph↗

arXiv2019

Observation of Plasma Bubble Structures in a GeV Laser-Plasma Accelerator

We measure characteristics of plasma bubbles in GeV-class laser-plasma accelerators (LPAs) using Faraday rotation diagnostics. We extend these techniques, previously demonstrated for LPAs in atmospheric density plasmas (electron density $n_e >10^{19}$ cm$^{-3}$), to LPAs in low-density plasmas ($n_e \approx 5\times10^{17}$ cm$^{-3}$), in which plasma bubbles are $\sim 5$ times larger, and correspondingly easier to visualize in detail. The signals show $\approx 0.5^\circ$ rotation streaks of opposite sign separated by $\sim50$ $μ$m, consistent with bubble diameter; no on-axis rotation; streaks length consistent with transverse probe pulse duration ($180$ $μ$m for $500$ fs pulse length, and $600$ $μ$m for $2$ ps pulse length). We utilized an anamorphic imaging system to obtain a wide longitudinal field of view ($>1$ cm) and a high transverse resolution ($<9$ $μ$m). We also demonstrated that Faraday rotation signals are sensitive to the stages of acceleration processes using extended 2D Finite Difference Time Domain (FDTD) simulation.

physics.plasm-ph↗

arXiv2019

Extreme Nonlinear Dynamics in Vacuum Laser Acceleration with a Crossed Beam Configuration

A relatively simple model problem where a single electron moves in two relativistically-strong obliquely intersecting plane wave-packets is studied using a number of different numerical solvers. It is shown that, in general, even the most advanced solvers are unable to obtain converged solutions for more than about 100 fs in contrast to the single plane-wave problem, and that some basic metrics of the orbit show enormous sensitivity to the initial conditions. At a bare minimum this indicates an unusual degree of non-linearity, and may well indicate that the dynamics of this system are chaotic.

physics.plasm-ph↗