John T. Morrison

Design of and data reduction from compact Thomson parabola spectrometers

Thomson parabola spectrometers are used to characterize MeV ion beams produced in high intensity laser interactions. These spectrometers disperse multiple ion species according to their charge to mass ratio through the use of parallel electric and magnetic fields. Analytical solutions for ion deflection in electric and magnetic fields have been used to extract ion spectra with the assumption that fringing effects are negligible. Experimental space restrictions and dynamic range requirements necessitate designs that stress the analytical assumptions. Depending on design parameters, the error in the analytical assumption can be comparable to the energy resolution. Estimates are provided to approximate the error on the total ion deflection. A method for modeling ion trajectories including fringing effects is presented using software freely available or in common use. The magnetostatic fields are modeled in 3D, including material properties of nearby magnetic materials using RADIA. Electrostatic fields are modeled in 2D for a spectrometer implementing angled plates using the partial differential equation toolbox in MATLAB(®). Using these models to calculate the ion trajectory allows for analysis of a Thomson parabola spectrometer with an arbitrary field configuration.

Selective deuteron production using target normal sheath acceleration

We report on the first successful demonstration of selective deuteron acceleration by the target normal sheath acceleration mechanism in which the normally overwhelming proton and carbon ion contaminant signals are suppressed by orders of magnitude relative to the deuteron signal. The deuterium ions originated from a layer of heavy ice that was deposited on to the rear surface of a 500 nm thick membrane of Si3N4 and Al. Our data show that the measured spectrum of ions produced by heavy ice targets is comprised of ∼99% deuterium ions. With a laser pulse of approximately 0.5 J, 120 fs duration, and ∼5×1018Wcm-2 mean intensity, the maximum recorded deuterium ion energy and yield normal to the target rear surface were 3.5 MeV and 1.2×1012sr−1, respectively.

Development of an in situ peak intensity measurement method for ultraintense single shot laser-plasma experiments at the Sandia Z petawatt facility

Using the physical process of ultraintense field ionization of high charge states of inert gas ions, we have developed a method of peak intensity measurement at the focus of high energy short pulse lasers operating in single shot mode. The technique involves detecting ionization products created from a low pressure gas target at the laser focus via time of flight detector. The observation of high ion charge states collected by the detector yields peak intensity at the focus when compared with the results obtained from well established tunnel ionization models. An initial peak intensity measurement of 5×1016Wcm−2 was obtained for a 1.053μm center wavelength, 0.4J pulse with 1ps pulse duration focused with an f∕5.5 off-axis parabola. Experiments with multijoule level, 500fs laser pulses are on the way.

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...

Fast neutron production from lithium converters and laser driven protons

Experiments to generate neutrons from the 7Li(p,n)7Be reaction with 60 J, 180 fs laser pulses have been performed at the Texas Petawatt Laser Facility at the University of Texas at Austin. The protons were accelerated from the rear surface of a thin target membrane using the target-normal-sheath-acceleration mechanism. The neutrons were generated in nuclear reactions caused by the subsequent proton bombardment of a pure lithium foil of natural isotopic abundance. The neutron energy ranged up to 2.9 MeV. The total yield was estimated to be 1.6 × 107 neutrons per steradian. An extreme ultra-violet light camera, used to image the target rear surface, correlated variations in the proton yield and peak energy to target rear surface ablation. Calculations using the hydrodynamics code FLASH indicated that the ablation resulted from a laser pre-pulse of prolonged intensity. The ablation severely limited the proton acceleration and neutron yield.

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...

Characterization of heavy-ice deposition-thickness on flat metal targets for deuteron ion acceleration

Summary form only given. Heavy ice constitutes a source of deuteron ions that are accelerated by the target-normal-sheath-acceleration mechanism to energies in excess of 1 MeV1. The thickness of cryogenically-cooled, heavy-ice deposits on the rear surface of laser-irradiated flat metal foils is inferred by a method of ratios. The relationship between the ice thickness and the injected heavy water volume has been established. This relationship is used to determine the injection volumes that are required for sub-micron layering. Known volumes of heavy water at vapor pressure (~17.5 torr at 20 degrees) are injected into the vacuum chamber, nominally held at 10-5 torr, and onto the foil through a 3mm diameter nozzle. Liquid nitrogen lines, non-rigidly coupled to the foil, cool the foil down to around -150 °C. The heavy ice deposition is observed in real time using a Mitutoyo 10× infinity corrected microscope objective that provides around 2 um of spatial resolution. Experiments to characterize the spectrum and brightness of ions accelerated from the heavy ice will be carried out using the GHOST laser facility at the University of Texas at Austin.

Observation of energetic deuteron ions accelerated from the rear surface of laser-irradiated flat-foil targets

Deuteron ions accelerated from the rear-surface of laser-irradiated flat-foil targets have been observed with energies up to 7 MeV. The observations were made using a compact-design Thomson Parabola Spectrometer with a static magnetic field strength of around 0.8 T and an electric field strength of around 3 kV. The ions were accelerated from the rear surface by the Target-Normal-Sheath-Acceleration mechanism1. The targets were 500×500×3 µm3 silicon substrates, etched from rigid wafers, coated with 50 nm of Al. The laser was incident into the etched-out cavity at the front of the wafer. The rear surface of the wafer was coated with a solution of deuterated plastic (Cambridge isotopes CD 98%) dissolved in toluene in the ratio of 1∶150. Upon evaporation of the toluene, a residue of CD with a thickness of about1 µm constituted the deuteron source and the target final layer. The experiments were performed on the GHOST laser facility at the University of Texas at Austin2. The laser, which is focused to a radius of around 6 µm, provided up to 2 J of energy in a pulse of duration 115 fs corresponding to a mean intensity exceeding 1019 Wcm−2.