D. W. Schumacher

On the origin of super-hot electrons from intense laser interactions with solid targets having moderate scale length preformed plasmas

We use particle-in-cell modeling to identify the acceleration mechanism responsible for the observed generation of super-hot electrons in ultra-intense laser-plasma interactions with solid targets with pre-formed plasma. We identify several features of direct laser acceleration that drive the generation of super-hot electrons. We find that, in this regime, electrons that become super-hot are primarily injected by a looping mechanism that we call loop-injected direct acceleration.

Effects of target charging and ion emission on the energy spectrum of emitted electrons

We present numerical simulations of the energy spectrum of electrons escaping from a target struck by an ultra-intense laser pulse using 2D implicit hybrid particle in cell code LSP (large scale plasma) [D. R. Welch et al., Phys. Plasmas 13, 063105 (2006)] and simple 1D capacitor model. The simulated energy spectrum as recorded by an electron spectrometer is found to differ significantly from the spectrum computed within the target. Analysis of the LSP simulations suggests two major mechanisms are responsible for this phenomenon: (1) The emitted electron energy spectrum is heavily influenced by the self-consistent electric fields generated along the target surface as the electrons escape and (2) these fields are themselves substantially modified by the simultaneous departure of accelerated surface ions. For electrons with internal energy greater than 4 MeV, both models predict a good correlation between the slope temperature of the input electron spectrum and that measured in a vacuum. We discuss the appl...

Coupling of laser energy into hot-electrons in high-contrast relativistic laser-plasma interactions

We use particle-in-cell simulations to explain the mechanisms responsible for the coupling of laser energy into relativistic electrons for the case of sharp interface, solid density metal targets free of pre-plasma. For perfectly flat interfaces, the accelerated electron trajectories are dominated by the standing-wave (SW) field structure formed by interference between incident and reflected pulses. We find that quasi-static magnetic fields that develop near the interface play only a minor role in perturbing the relativistic electron trajectories but can contribute to enhanced absorption. Target surfaces that are structured exhibit enhanced absorption, and the acceleration mechanism deviates from the clean standing-wave acceleration mechanism leading to more stochastic electron heating and larger divergence angles.

How well do time-integrated Kα images represent hot electron spatial distributions?

A computational study is described, which addresses how well spatially resolved time-integrated Kα images recorded in intense laser-plasma experiments correlate with the distribution of “hot” (>1 MeV) electrons as they propagate through the target. The hot electron angular distribution leaving the laser-plasma region is critically important for many applications such as Fast Ignition or laser based x-ray sources; and Kα images are commonly used as a diagnostic. It is found that Kα images can easily mislead due to refluxing and other effects. Using the particle-in-cell code LSP, it is shown that a Kα image is not solely determined by the initial population of forward directed hot electrons, but rather also depends upon “delayed” hot electrons, and in fact continues to evolve long after the end of the laser interaction. Of particular note, there is a population of hot electrons created during the laser-plasma interaction that acquire a velocity direction opposite that of the laser and subsequently reflux of...

Experiment and simulation of novel liquid crystal plasma mirrors for high contrast, intense laser pulses.

We describe the first demonstration of plasma mirrors made using freely suspended, ultra-thin films formed dynamically and in-situ. We also present novel particle-in-cell simulations that for the first time incorporate multiphoton ionization and dielectric models that are necessary for describing plasma mirrors. Dielectric plasma mirrors are a crucial component for high intensity laser applications such as ion acceleration and solid target high harmonic generation because they greatly improve pulse contrast. We use the liquid crystal 8CB and introduce an innovative dynamic film formation device that can tune the film thickness so that it acts as its own antireflection coating. Films can be formed at a prolonged, high repetition rate without the need for subsequent realignment. High intensity reflectance above 75% and low-field reflectance below 0.2% are demonstrated, as well as initial ion acceleration experimental results that demonstrate increased ion energy and yield on shots cleaned with these plasma mirrors.

Moderate repetition rate ultra-intense laser targets and optics using variable thickness liquid crystal films

Liquid crystal films are variable thickness, planar targets for ultra-intense laser matter experiments such as ion acceleration. Their target qualities also make them ideal for high-power laser optics such as plasma mirrors and waveplates. By controlling parameters of film formation, thickness can be varied on-demand from 10 nm to above 50 μm, enabling real-time optimization of laser interactions. Presented here are results using a device that draws films from a bulk liquid crystal source volume with any thickness in the aforementioned range. Films form within 2 μm of the same location each time, well within the Rayleigh range of even tight F/# systems, thus removing the necessity for realignment between shots. The repetition rate of the device exceeds 0.1 Hz for sub-100 nm films, facilitating higher repetition rate operation of modern laser facilities.

Hot electron generation and transport using Kα emission

We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40μm diameter wire emulating a 40μm fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of pre-pulse level inside the cone by a factor of 50 reduces coupling by a factor of 3.

Experimental capabilities of 04 PW, 1 shot/min Scarlet laser facility for high energy density science

We report on the recently completed 400 TW upgrade to the Scarlet laser at The Ohio State University. Scarlet is a Ti:sapphire-based ultrashort pulse system that delivers >10  J in 30 fs pulses to a 2 μm full width at half-maximum focal spot, resulting in intensities exceeding 5×1021  W/cm2. The laser fires at a repetition rate of once per minute and is equipped with a suite of on-demand and on-shot diagnostics detailed here, allowing for rapid collection of experimental statistics. As part of the upgrade, the entire laser system has been redesigned to facilitate consistent, characterized high intensity data collection at high repetition rates. The design and functionality of the laser and target chambers are described along with initial data from commissioning experimental shots.

Using time-integrated Kα images to study refluxing and the extent of pre-plasmas in intense laser-plasma experiment

We report the results of an experimental and numerical modeling study of the formation of time-integrated Kα images by electrons excited during an intense laser-plasma interaction. We report the use of the spatial structure of time-integrated Kα images to quantitatively characterize the pre-plasma profile near the critical surface and to verify the near elimination of back-surface refluxing from targets when a thick layer of a low-Z material is attached to the back. The time integrated Kα images are found to be sensitive to the relative separation between the critical surface and the bulk target, permitting a single parameter exponential pre-plasma scale length to be determined by fitting to experimental results. The refluxed electrons affect different parts of the Kα images in a manner that varies depending on the location of the refluxing. We use these properties to characterize refluxing also by fitting to experimental results. Experiments were performed using the Titan laser at the Lawrence Livermore ...

On specular reflectivity measurements in high and low-contrast relativistic laser-plasma interactions

Using both experiment and 2D3V particle-in-cell (PIC) simulations, we describe the use of specular reflectivity measurements to study relativistic (Iλ2 > 1018 W/cm2⋅μm2) laser-plasma interactions for both high and low-contrast 527 nm laser pulses on initially solid density aluminum targets. In the context of hot-electron generation, studies typically rely on diagnostics which, more-often-than-not, represent indirect processes driven by fast electrons transiting through solid density materials. Specular reflectivity measurements, however, can provide a direct measure of the interaction that is highly sensitive to how the EM fields and plasma profiles, critical input parameters for modeling of hot-electron generation, evolve near the interaction region. While the fields of interest occur near the relativistic critical electron density, experimental reflectivity measurements are obtained centimeters away from the interaction region, well after diffraction has fully manifested itself. Using a combination of P...