Scott Feister

Liquid crystal films as on-demand, variable thickness (50–5000 nm) targets for intense lasers

We have developed a new type of target for intense laser-matter experiments that offers significant advantages over those currently in use. The targets consist of a liquid crystal film freely suspended within a metal frame. They can be formed rapidly on-demand with thicknesses ranging from nanometers to micrometers, where the particular value is determined by the liquid crystal temperature and initial volume as well as by the frame geometry. The liquid crystal used for this work, 8CB (4′-octyl-4-cyanobiphenyl), has a vapor pressure below 10−6 Torr, so films made at atmospheric pressure maintain their initial thickness after pumping to high vacuum. Additionally, the volume per film is such that each target costs significantly less than one cent to produce. The mechanism of film formation and relevant physics of liquid crystals are described, as well as ion acceleration data from the first shots on liquid crystal film targets at the Ohio State University Scarlet laser facility.

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 reflection of intense (∼1018 W/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...

Backward-propagating MeV electrons from 1018 W/cm2 laser interactions with water

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 ∼3 mJ, 1018 W/cm2 short pulse laser with a flowing 30  μm 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 ∼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...

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.

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 -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.

Particle-in-cell simulations of electron acceleration from relativistic interaction of mid-infrared laser interactions with near solid density matter

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= 780 nm, 3 μm, and 10 μm). Earlier studies [Orban et al., Phys. Plasmas 22, 023110 (2015) and Ngirmang et al., Phys. Plasmas 23, 043111 (2016)], limited to λ= 780 nm 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...

Wavefront Transfer for On-Shot Focal Spot Characterization at the 400 TW SCARLET Laser

Ohio State’s 400 TW SCARLET laser can deliver intensities of 1021 W/cm2. We present work to establish a phase relationship between wavefronts far from target and those near target. The calculated wavefront predicts the on-target focal spot on each shot.