Session CO4: AB: Laser-Plasma Sources of Energetic Photons

Nuclear Photonics at the Technische Universit\"{a}t Darmstadt

Nuclear Photonics is a rapidly growing field of basic and applied sciences. It links the physics of ultra-intense lasers and high-energy-density matter with nuclear physics and enables new tools and new insight in a variety of topics. At TU Darmstadt, we have embraced this new field and focused on combined research using the strong expertise in nuclear, plasma, laser and accelerator sciences. Ten faculty members have teamed up with their research groups to address this field, focusing on two primary topics: the generation of a bright, laser-driven, neutron source and the use of intense, mono-energetic, polarized gamma beams. Research in this field makes use of the Darmstadt PHELIX laser system at Helmholtzzentrum f\"{u}r Schwerionenforschung - GSI, and the superconducting energy recovery electron accelerator S-DALINAC at TU Darmstadt, but is ultimately aimed at research at the two ELI pillars NP and BEAMLINES, the OMEGA laser system at LLE, the Z-PW Laser at SNL and future upcoming systems. We report on our strategy, the recent activities, on latest results in developing a laser driven neutron source and prospects for research at TU Darmstadt.   

Superradiant nonlinear Thomson scattering.

Superradiance, a classical concept first introduced by Robert Dicke in 1954, is the anomalous radiance describing coherent photon emission from a collection of light-emitting particles. Initially proposed in the context of atomic physics, superradiance now provides valuable insights into the fields of quantum electrodynamics, quantum communications, astrophysics, being a crucial concept underlying lasing in free electron lasers. Superradiant regimes lead to peak radiation intensities that scale with the number of particles squared. This effect is intuitively expected when the distance between light-emitting particles is much smaller than the photon wavelength. This was the superradiance condition originally proposed in the seminal work by Dicke, and corresponds to the operation regime of free electron lasers. Here we predict a new superradiance effect that holds even in the limit of vanishing number of particles per radiation wavelength. We illustrate the concept in the form of a previously unrecognised superradiant Thomson scattering. This concept may open new superradiant emission regimes in plasma based accelerators, gyrotrons and free electron lasers.   

Impact of ion dynamics on electron acceleration and gamma-ray emission in hollow micro-channel targets

~ Direct laser acceleration in hollow channels has been presented as a promising regime for generation of secondary particle and radiation beams. However, the impact of ion dynamics on the regime still lacks investigation. Our work [1] shows that when ions start to fill the channel, both electron acceleration and photon emission are susceptible to degradation. Ions reshape the electromagnetic field structure and shift the dominant contributor of work from the longitudinal laser electric field to the transverse one. The ion expansion also has a profound impact on gamma-ray emission, making it become volumetrically distributed while reducing the total emitted energy.~We formulate a criterion for the laser pulse duration to minimize the undesired effects from ions and demonstrate its predictive capability through pre-pulse interactions. Our results provide a guideline for future experiments on micro-channel targets at multi-PW laser facilities with ultra-high intensities. [1]~\underline {Tao Wang et al 2019 Plasma Phys. Control. Fusion 61 084004}   

Characterization of short-pulse laser-produced bremsstrahlung spectrum using x-ray radiograph images of mm-diameter metal rods

Determination of an object density via bremsstrahlung x-ray radiography requires understanding of accurate broadband x-ray spectrum. To characterize laser-produced bremsstrahlung and demonstrate bremsstrahlung x-ray radiography of mm-diameter Al rods, we carried out an experiment using a 50TW Leopard laser at the UNR's Nevada Terawatt Facility. Angularly resolved bremsstrahlung was determined by comparing measured x-ray signals from a silver foil with hybrid particle-in-cell simulations. Transmission of the Al rods from the radiograph images is further simulated with a Monte Carlo code. The measured transmission profiles with three different diameters agree with calculations when a simulated x-ray spectrum composed of line emissions and bremsstrahlung is used with a source size of 600 \textpm 200 $\mu $m. Transmission calculations with only 22 keV Ag K$\alpha $ or an exponential x-ray spectrum do not reproduce the measurement. This work suggests that the accurate modeling of the x-ray source spectrum as well as the photon sensitivity of the detector is critical in transmission calculation to infer the density of an object.   

Compact spectral characterization of 10-500 MeV $\gamma $-rays from the Texas Petawatt Laser-Driven Plasma Accelerator

GeV ($\gamma_{\mathrm{e}}$ \textgreater 2000) electron bunches from petawatt-laser-driven plasma accelerators can be converted to tunable, narrowband or to broadband continuum $\gamma $-ray (h$\nu $ \textgreater 10 MeV) pulses by Thomson backscattering (TBS) or bremsstrahlung, respectively. Inserting a plasma mirror (PM) near the accelerator exit converts electrons to $\gamma $-rays compactly and inexpensively, in a TBS/bremsstrahlung mixture determined by PM thickness, material and location. Characterizing the $\gamma $-ray spectra accurately is a challenge, usually addressed with bulky pair production/Compton spectrometers. Here, we spectrally characterize PM-generated TBS/bremsstrahlung $\gamma $-rays from 1-2 GeV Texas-Petawatt-Laser-accelerated electron bunches using a compact stack calorimeter, to record energy-dependent particle showers generated by incoming $\gamma $-rays. An iterative Bayesian algorithm, based on a calorimeter response matrix built from GEANT4 simulations, reconstructs TBS and bremsstrahlung contributions for each shot, as PM and electron parameters vary. ~The method should be widely applicable to plasma-accelerator-based radiation with MeV photon energies.   

Characterizing laser-based MeV radiographic capability at NIF-ARC.

X-ray radiography of high-areal density objects is desirable for many applications. During high-energy laser-solid interactions (\textgreater 1 x 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}})$, a population of hot-electrons are accelerated to mega-electron volt energies. Injecting this population into a high-Z, high-density converter creates a similarly high-energy x-ray beam via bremsstrahlung. These x-rays have been shown to have a small source size (100s $\mu $m), high dose (several Rad) and high temperature (\textgreater 1 MeV)[1]. We've performed experiments on NIF-ARC to benchmark the photon spectra. Diagnosing the x-ray spectrum in this regime is difficult due to low interaction cross sections. We have used a suite of complimentary diagnostics, such as photo-nuclear activation and absorption spectrometers, to reconstruct the spectrum. These experiments at NIF ARC have shown that the temperature of these x-rays is on the order of several MeV.   

Generation of MeV photons with kJ multi-ps laser pulses

Studying high energy density (HED) matter is expanding our understanding of 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 passively probe/radiograph such matter. To radiograph high areal density objects, broad band x-ray sources require a large flux (10\textasciicircum 13-10\textasciicircum 14 photons/steradian/MeV) of \textgreater 2 MeV photons. In this work we generate a high yield x-ray source through bremsstrahlung by irradiating a high-Z target with kJ, ps laser pulses. We control the laser to target coupling by changing the plasma scale length, the laser intensity and by using a range of advanced targets, such as compound parabolic concentrator plasma optics to increase the laser intensity for large F/{\#} laser systems. Modeling suggests that these targets can increase the laser intensity by as much as 100x making them very attractive for high F/{\#} systems such as NIF ARC and PETAL at LMJ.   

MeV radiography source development using NIF-ARC and CPC targets

Kilojoule energy, picosecond duration lasers with intensity \textasciitilde 1x10$^{\mathrm{19}}$ W/cm$^{\mathrm{-2}}$ are promising drivers for MeV x-ray radiography [1]. Recent experiments using the NIF Advanced Radiographic Capability (ARC) laser and compound parabolic concentrator (CPC) targets [2] have reached effective intensities \textgreater 1x10$^{\mathrm{19}}$ W/cm$^{\mathrm{-2}}$, as inferred by measured hot electron temperatures, which is a \textgreater 10x enhancement compared to flat targets. The x-ray source from 0.5mm thick Au foils has been systematically studied over a series of ARC shots, and its size, flux and spectrum characterized. The results and applications to MeV radiography will be discussed, and a planned demonstration of MeV radiography with ARC and CPC targets will be presented. [1] Courtois et al., PoP 2013 [2] MacPhee et al., submitted Optics Express 2019   

Laser-Driven, tunable, high-yield x-ray source from a hybrid laser plasma accelerator used for radiography

A broadband (10 keV to MeV), high yield (\textgreater 10\textasciicircum 10 photons/keV/Sr), small source size (\textless 20-100 um) x-ray source has been developed using a hybrid laser plasma accelerator on the Titan laser (700 fs, 140 J). The hybrid laser plasma accelerator uses a combination of self-modulated laser wakefield acceleration and direct laser acceleration to generate a high energy (\textgreater 200 MeV) low divergence (\textless 100 mrad), high-charge (70nC) beam of electrons. The electrons are used to generate x-rays using a combination of three mechanisms: betatron radiation, inverse Compton scattering, and bremsstrahlung radiation. The combination of these x-ray generation mechanisms and control over the electron beam provides a method for tuning the emitted x-ray energy spectra, source size, and yield. In this work we optimize and characterize the x-ray source to radiograph several high and low-Z objects with several spatial resolutions.   

Intense Laser-Driven Multi-MeV Photon Source for Radiography

Intense photon sources with energy $> 1$ MeV are of significant interest for flash radiography applications in research, industry and defense. Such applications include tomographic non-destructive evaluation and dynamic experiments. A small source size enables a commensurately high image resolution not limited by detector pixel size by using point-projection radiography with magnification. Generation of $> 1$ MeV photons by bremsstrahlung of laser-driven relativistic electrons has been investigated on the ALEPH laser at Colorado State Univ. Initial experiments with tantalum foil targets (0.5 mm thickness) have used a high-contrast laser pulse delivering a 8 -- 10 J at 400 nm in 45 fs, with a peak intensity of $4 \times 10^{21}$ W/cm$^2$. Measurements indicate a high-energy spectrum with multi-MeV average photon energy, with $> 10^{11}$ photons/sr (a dose of $\approx 0.05$ Rad) delivered in a cone of $\approx 0.1$ sr. The source size is very small, which has allowed us to resolve features in thick tungsten objects as small as 65 microns. Measurements in progress with more advanced targets, as well as 800 nm wavelength will also be discussed and compared to prior results with sub-ps laser pulses.   

MeV photon source development based on Thomson scattering using compact laser-plasma accelerators

Compact, narrow bandwidth, femtosecond-pulsed, MeV gamma ray sources have the potential to offer important advances across a number of fields, including nuclear nonproliferation, chemistry, medicine, and photon nuclear activation. The BELLA Center aims to produce such sources through Thomson scattering of a laser from the electron beam of a laser-plasma accelerator (LPA). A recently completed 100 TW laser system is delivering 2.8 J and 38 fs pulses at 5 Hz repetition rate on target to consistently produce 120 MeV, 50 pC stable LPA electron beams. A newly commissioned designated ``scatter'' line will deliver 0.7 J, 38 fs pulses on target. Through independent control of pulse shape and laser guiding, high flux and narrow energy spread can be achieved. The presentation will focus on progress toward this goal with particular focus on the techniques required for spatial and temporal overlap between focused beams, beam stability studies, and the MeV gamma ray diagnostics employed in the research.   

Update on BELLA Center laser system development towards a compact Thomson photon source

Thomson scattering of laser pulses from relativistic electron beams can produce bright, narrow energy-spread MeV photon beams, with relevance to many applications including nuclear nonproliferation. Here we present a 5 Hz, 40 fs, 100 TW class laser system at the BELLA Center, dedicated to this light source development. The system consists of two independently tunable and compressible Ti:Sapphire multipass-amplifier arrays (2.8 J and 0.7 J on target, respectively) with the seed-beam split off just after the common CPA stretcher. Additionally, an independently tunable 40-fs probe beam is available in various configurations. We discuss the system layout, commissioning, performance, stability and alignment procedures to enable stable day-to-day operation and multi-beam experiments. Stable electron beams have been established via ionization injection with further source development and Thomson scattering experiments underway.   

Progress towards BELLA Center's Laser-Plasma Accelerator based Free Electron Laser

Soft X-rays are a highly desired tool for novel experiments in the biological, chemical, and physical sciences. At the BELLA Center, we are pursuing the technology for a Laser-Plasma Accelerator (LPA) driven free-electron laser (FEL). A new dedicated 100TW-class laser system now delivers pulses of 2.5J and 40 fs duration (at 5 Hz repetition). After an upgrade with a deformable mirror, we are now routinely producing electron beams at the 100-200 MeV level. In this presentation we will describe our LPA FEL facility, including the advanced electron beam transport line to the 4-meter long strong-focusing VISA undulator. Transport and manipulation devices include a permanent quadrupole triplet, several steering magnets, an electro-magnetic triplet, a magnetic chicane to decompress the electron beam, a mid-line magnetic spectrometer, and a diagnostic chamber. Our simulations indicate that FEL gain should be observed by decompressing the few-femtesecond few-{\%} energy spread beams with the chicane. The FEL diagnostics and recent results will be presented.   

Development of High Fluence X-Ray Sources on the NIF Using Laser Heated Novel Nano-Wire Metal Foams

High fluence K-shell x-ray sources are being developed for high energy density physics experiments. The sources are produced by laser heating free standing pure Ag nano-wire foams in the shape of cylinders nominally 4 mm in diameter, 4 mm tall. The manufacture of robust low density foams (6 - 12 mg/cm$^{3}$) is now possible through a new technique of freeze drying an aqueous suspension of nano-wires. X-ray conversion efficiency from these laser heated underdense nano-wire foams have been measured to be $\sim$0.6\% which is about twice that observed in more conventional laser heated cavity x-ray sources. 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$950 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.   

Shaped Laser Short-Pulses for Manipulating Time-Dependent Particle Acceleration

Laser pulse shaping at the nanosecond level has enabled lasers to become one of the best tools available for studying plasmas and high-energy-density (HED) systems. Typical short-pulse laser experiments deliver sub-picosecond laser pulses that are often not well characterized and assumed to be Gaussian-like. In this work we examine the possibilities of laser pulse-shaping at the sub-ps level to precisely influence time-dependent laser-particle acceleration. Modeling suggests that the use of a shaped short-pulse could enhance laser coupling to MeV electrons and manipulate ion acceleration physics to boost maximum ion energies [J. Kim, et al., PoP 25, 083109 (2018)]. Pseudo shaped short-pulses can be delivered by combining separate short-pulse beams. Experiments conducted at the Omega EP facility demonstrated a concept similar to the simulated case and showed a significant enhancement in laser coupling to 1$+$ MeV electrons and an increase in maximum proton energy compared to single pulses. The experimental data will be presented and compared with modeling in order to elucidate the time-dependent nature of the particle acceleration physics.