Session ZO03: Beams: High-Energy Photon Generation

Live

Relativistic high harmonic generation (RHHG) from plasma mirrors can generate high flux, coherent radiation spanning from infrared to soft x-ray wavelengths. Recent theoretical work and particle-in-cell (PIC) simulations of RHHG have extended the Coherent Synchrotron Emission (CSE) model to account for the finite duration of the emitting electron bunch, showing that the efficiency of RHHG is often limited by the electron bunch width rather than the Lorentz factor of the emitting electrons. However, the PIC simulations typically used to model RHHG do not correctly model the effects of collisions, leading to the appearance of unphysically narrow, high-density electron bunches. Here, we study the effects of adding relativistic binary collisions to the FDTD-based PIC scheme (EPOCH code). The main effects of collisions are to broaden the emitting electron bunch and to increase the number of emitting electrons within the bunch. This modification to the emitting electron bunch has important implications for the ultimate efficiency of RHHG in the relativistic limit. Namely, the extended duration of the emitting electron bunch results in an earlier bunch width dominated spectral cut-off.   

Live

Plasma optics can manipulate high power lasers at far higher fluences than solid-state components. The difference in index of refraction between air and plasma provides one method to control high intensity light. We use the spatial beat wave between 50-fs beams crossed at an angle to create a volumetric ionization grating in air; the intensity modulation produces a one-dimensional grating with alternating regions of plasma and neutral gas. We experimentally demonstrate redirection of millijoule-scale 40 fs pulses by an ionization grating and characterize the effects of spatial and temporal overlap of both pump beams and the probe. Direct interferometry measurements capture the evolution of plasma density in the grating during its sub-picosecond formation and subsequent exponential decay over tens of picoseconds. Three-dimensional simulations reproduce observed diffraction patterns and efficiencies. Control of light at intensities above the damage threshold of solid optics suggests a path towards smaller beam diameters for high power lasers.   

Live

The laser interaction with an electron-positron-ion mixed plasma is studied, from a perspective of the associated high-order harmonic generation. The harmonic spectrum is shown to be significantly changed after an electron-positron pair plasma is produced. A dense positron beam accelerated by the laser at the target front excites strong counterpropagating plasma waves and triggers inverse two-plasmon decay. Moreover, prominent and well-defined signals at harmonics of the plasma frequency in the high-order harmonic spectrum are efficiently produced. Particle-in-cell simulations with OSIRIS show that significant radiation at twice the plasma frequency can be observed for a pair density as low as $\sim 10^{-5}$ of the electron plasma density. For higher pair densities, the radiation at this characteristic frequency after the pair production can be above $5$ orders of magnitude higher than the counterpart radiation from the plasma without pair production.   

Live

Recently developed spatiotemporal pulse-shaping techniques allow the position of maximum intensity in a focused laser beam to propagate at any velocity over long distances. Ionization fronts that move at the velocity of such a ``flying focus'' have been demonstrated when the instantaneous intensity is above the ionization threshold of a background material. Previous simulation results have shown that photon acceleration in such an ionization front can efficiently and coherently shift the frequency of a witness laser pulse from the visible into the extreme ultraviolet (EUV, $\lambda $ \textless 100 nm) in \textasciitilde 1 cm of interaction length. Here we present simulations showing that shorter wavelengths can be achieved over shorter accelerator lengths. Combining multiple transverse spatial modes produces a dynamic guiding structure with steepened accelerating gradients. This eliminates refraction of the witness pulse without the need to shape the neutral density of the target and yields larger frequency shifts over the same accelerator length. Additional spatiotemporal shaping can ``trajectory lock'' an accelerated ionization front to the changing group velocity of the witness pulse, further increasing the frequency shift. This material is based upon work supported by the Department of Energy grant DE-SC0019135 and the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Superradiance is at the core of today's most advanced light sources. Superradiant emission is highly desirable because it is characterised by the emission of temporally coherent radiation, where the intensity grows with the number of light emitting particles squared. Known superradiant emission mechanisms require temporal bunching of a particle beam at the emitted radiation frequency. We have recently showed that this bunching criteria can be strongly relaxed when looking at emission off-axis, where radiation is broadband and leads to ultra-short radiation bursts. We have thus found that coherent emission can be achieved by introducing a transverse modulation of the bunch at the frequency of the first emission harmonic. For off-axis emission, and if many harmonics are emitted, this means that modulating the particle bunch at an optical wavelength (e.g. 1 micron) could produce coherent radiation in the x-ray domain for example. Here, we explore this generalised superradiant emission process in the context spontaneous undulator radiation theory and discuss mechanisms to observe generalised superradiance in experiments.   

Live

An accurate description of excited atomic states in high-energy-density matter remains an experimental and theoretical challenge. Using the ALEPH laser at Colorado State University, we perform high-resolution X-ray spectroscopy of exotic matter produced via laser-solid interactions at ultra-relativistic intensities (I $\sim 10^{21}$ W/cm$^2$). We examine the origin of anomalous X-ray emission from copper foil, foam and buried layer targets through precise measurements of K-shell fluorescence and hot electron emission, as well as spectroscopy of XUV plasma emission. These measurements also elucidate the generation and propagation of hot electrons under self-generated electric and magnetic fields, providing crucial constitutive data for HED matter in an ultra-high intensity regime.   

Live

We present radiographic data and modulation transfer function (MTF) analyses of a multi-component test object probed at an effective backlighter energy \textgreater 40 keV using a 5 $\mu $m-thin dysprosium foil driven by the NIF-ARC short pulse laser (\textasciitilde 2 kJ, 10 ps). The thin edge of the foil acts as a bright line source of hard x-rays which is placed in a point-projection configuration with the test object and a filtered and shielded image plate detector stack. The system demonstrates superior spatial and temporal resolution when compared to an existing long-pulse-driven backlighter now used routinely at NIF for dynamic strength experiments, using only a small fraction of the laser energy and fewer beamlines.   

Live

A series of experiments have been performed to improve the brightness of the Si He$_{\mathrm{\alpha }}$x-ray emission at 1865 eV from a high-energy, short-pulse, laser-driven backlighter target at pulse durations of 20 ps and energies of up to 1 kJ. High backligher brightness is important to maximize the number of photons registered in the detector in radiography experiments and to minimize the background for plasma objects with high self-emission. In this study the emission from low-density foam targets, the effects of a laser prepulse and V-shaped targets were compared to solid-density, flat Si targets. The V-shaped targets showed the best performance with an approx. 5x improvement in time-integrated emission and an x-ray pulse duration of 25 ps with no measurable spectral shift of the Si emission line.   

Live

A broadband hard x-ray source is necessary for radiography of high areal density objects in high energy density and inertial confinement fusion experiments. We have studied characterization of short-pulse laser-produced broadband x rays and demonstrated modeling of laser-based x-ray radiography. Fast electrons produced by a UNR's 50-TW Leopard laser were characterized by modeling measured bremsstrahlung signals with a hybrid particle-in-cell code, LSP, to calculate angularly resolved x-ray spectra. Radiographic images of a spark plug object were then simulated using a Monte Carlo code incorporating an LSP-calculated spectrum, realistic 3D CAD-like spark plug model, an x-ray attenuation filter, and an IP detector into a photon transport calculation. Simulated transmission profiles of the test object agree with the experiments for various filter materials and source targets, suggesting that the inferred x-ray spectrum from the bremsstrahlung analysis is consistent with one used for the radiography. The simulations reproduce a high-quality radiographic image recorded through a plastic filter compared to aluminum or brass filters. Details of the experiment and comparisons will be presented. This material is based upon work supported by the National Science Foundation under Grant No. 1707357.   

Live

Intense photon sources with energy \textgreater 1 MeV are of significant interest for radiography of dense objects in research, industry and defense. One important application is point-projection imaging in tomographic non-destructive evaluation. Irradiation of a high-Z foil with an intense laser drives a large population of relativistic electrons that in turn generate a copious directed emission of high-energy Bremsstrahlung photons. We have reported on such a source of \textgreater 1 MeV photons driven by the ALEPH laser at Colorado State Univ., featuring a source size well below 0.1 mm. Small source size enables commensurately high image resolution in magnified point-projection radiography, not limited by detector-pixel size. We have exploited the high repetition rate of ALEPH to demonstrate high resolution 2D radiography (10 line-pairs/mm) and a tomographic image (71 views) of a complex object, which shows feasibility for tomography with that photon source. (The softer x-ray part of the spectrum was used for tomography due to the low areal density of the object.) We present the image reconstruction and further characterization of the source, such as the photon spectrum which has been adjusted by varying the laser energy and target thickness.   

Live

The production of hot-electrons from high-intensity laser interactions is the key to the development of high energy particle and photon sources. The acceleration of the hot-electron population is proportional to the incident laser intensity. The highest intensities are often achieved via a final short focal length focusing optic. However, the development of miniature targetry and 3D printing has opened the door to a cheap and effective alternative. Cone targets can therefore be fabricated such they operate as a plasma optic. We on compound parabolic concentrator (CPC) [1] targets that geometrically increase the intensity on target. Experimental measurements were made at the Texas Petawatt laser facility with a short-pulse (150 fs) high-intensity (10$^{\mathrm{18}}$ W/cm2) and long focal length (F/40). We report a hot-electron temperature enhancement of approximately a factor of 7 from the CPC target when compared to planar target. Using PIC simulations, we describe this hot-electron enhancement from a purely geometric intensity enhancement and existing temperature-intensity scaling laws.   

Live

High intensity laser pulses can be used to produce MeV energy x-rays which can radiograph high energy density plasmas, providing important insight into a range of problems including inertial confinement fusion and laboratory astrophysics. Our team has experimentally demonstrated using the NIF-ARC laser that compound parabolic concentrator (CPC) targets dramatically improve x-ray production compared to a flat target [MacPhee et al. \textit{Optica} \textbf{7}, 129-130 (2020)]. CPCs increase energy concentration at the cone tip and reduce lateral plasma expansion at the cone tip, creating a long scale-length pre-plasma, which can enhance laser absorption. We will present RZ and 3D HYDRA simulations of plasma expansion due to pre-pulse and how this affects the interaction of the main high energy pulse. This is relevant for understanding single beam interactions and also when used as a multiple pulse target which could potentially produce MeV x-rays over many nanoseconds.   

Live

High-energy x-rays have the ability to passively probe/radiograph HED matter providing essential information to inertial confinement fusion, astrophysical systems, planetary interiors and fundamental plasma physics. Laser produced x-rays are key tools in this exploration due to their ability to produce broad band x-ray sources with large flux (\textasciitilde 10$^{\mathrm{13}}$-10$^{\mathrm{14}}$ photons/steradian/MeV) of \textgreater 2 MeV photons that are necessary to radiograph high-areal density objects. In this work we generate a high yield x-ray source through bremsstrahlung by irradiating a high-Z target with a kJ, ps laser. We increased the laser to target coupling by using advanced targets, such as compound parabolic concentrators and capillary structured targets.   

Live

High fluence K-shell and L-shell x-ray sources are being developed for high energy density physics experiments. Sources have been produced by laser heating Ag and Au nano-wire foams in the shape of cylinders nominally 4 mm in diameter, 4 mm tall. The manufacture of robust low density foams (6 - 15 mg/cm$^{3}$) is now possible through a new technique of freeze casting an aqueous suspension of nano-wires. X-ray conversion efficiency from these laser heated underdense nano-wire foams have been measured to be $\sim$1.0\%. 192 laser beams from NIF are used to heat the foams with $\sim$400 TW of 3$\omega$ laser light in a 2.5 ns square pulse in time depositing $\sim$1000 kJ into each foam. Experimental results and comparisons with simulations will be presented. This work was done under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.   

On Demand

Recently we reported a novel idea of generating a localized bunch of electrons oscillating in-phase, named plasma dipole oscillation (PDO), by colliding two detuned laser pulses in a plasma. From a series of two-dimensional particle-in-cell (PIC) simulations and theoretical analysis, we verified that PDO oscillates with the local plasma frequency and emits a strong dipole radiation at the same frequency. Such a property of PDO enables it to be used as a light source in terahertz band (Kwon et al., Sci. Rep. 2018) and also as a novel diagnostic method of reconstructing non-uniform plasma densities (Kylychbekov et al., PSST 2020). In this paper, we present our new calculation, where we find that the PDO is not just a linear harmonic oscillator as described in the slab-model of the plasma oscillation, but has a high nonlinearity, which yields high harmonic radiations. The second harmonic radiation emitted from the nonlinear PDO can be a new model of 2fp radio-burst from solar plasmas, which is conventionally explained by two-plasmon merger. Since the harmonic radiations are strongly dependent on the shape of PDO, the high harmonics of PDO can be controlled by manipulation of frequency chirp and profiles of the driving laser pulses.