Session BO04: Beams: Relativistic Laser-Plasma Interactions and X-Ray Sources

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At the BELLA Center an electron beamline was designed and built to deliver electrons generated by a laser plasma accelerator into a 4 meter long undulator. In addition to the usual dipole magnets found in an undulator, a series of permanent magnets are embedded along the whole length of the undulator to produce a focusing channel based on standard quadrupole focusing and FODO lattice concepts. This channel ensures the electron beam transverse size is kept small along the full 4 m propagation length, which has significant advantages in the context of generating FEL radiation. This focusing channel naturally requires the incoming beam to be properly matched to ensure stable propagation. It is therefore critical to have a well characterized transport line preceding the undulator. We discuss here salient aspects of the beamline design as well as some preliminary measurements and characterization of our electron beamline.   

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Thomson scattering of laser pulses from relativistic electron beams can produce MeV photon beams with narrow energy spread. Applications are foreseen in nuclear nonproliferation, medicine and other fields. We discuss performance, stability and alignment procedures for multibeam Thomson experiments using a Ti:Sapphire based 100 TW class laser system with two arms, dedicated to light source development. Both laser arms are independently tunable and compressible multipass-amplifier arrays (2.8 J and 0.7 J on target, down to 40 fs pulse duration, repetition rate of 5 Hz) with the seed-beam split off just after the common CPA stretcher. An independently tunable 40 fs probe beam is available in various configurations. Stable electron beams and Thomson Scattering have been established with further development ongoing. This flexible facility is open to users through LaserNetUS and successful user experiments have been conducted.   

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Compact, narrow bandwidth, femtosecond-pulsed, MeV photon sources have the potential to offer advanced source parameters to benefit a number of fields, including nuclear nonproliferation, medicine, industrial CT scan, and photon nuclear spectroscopy. We produce such sources through Thomson scattering of~a~separately controlled `scatter' laser pulse from the electron beam of a laser-plasma accelerator. The~``scatter'' line has independent compression to enable high flux with controlled photon bandwidth and to enable future control of pulse shape. A high flux photon source with tunable energy and quasi-monoenergetic spread was achieved using this independent control of pulse shape and~a~well-developed alignment technique. The presentation will include techniques required for spatial and temporal overlap between focused beams, beam stability studies, and the MeV gamma ray diagnostics employed in the research.~   

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Thomson scattering is a laser-particle interaction mechanism that leads to the emission of radiation due to the oscillations of the particles in the laser field. The emitted radiation is composed of harmonics of laser frequency, but high harmonics at the x-ray frequency usually require ultra-intense lasers (a0\textgreater \textgreater 1). By using a plasma mirror to separate the laser and particle beams we are able to induce a huge deceleration in the particle beam at the vacuum-plasma surface interface as the laser gets reflected. In this case, the emitted radiation pulse can be as short as the skin depth of the plasma divided by the Lorentz factor of the radiating particles, squared, and the pulse-length does not depend on the laser intensity. This result opens an unprecedented pathway to produce ultra-short, high-frequency radiation. Using typical solid density plasma could then lead to broad-band x-ray emission with low-intensity lasers (a0\textless \textless 1). In this work, we explore the fundamental physics behind this scenario using the recently developed radiation diagnostic for OSIRIS (RaDiO), that captures the spatiotemporal properties of the radiation emitted by charged particles with built-in spatial and temporal coherence and its integration in the standard PIC algorithm.   

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We show using particle-in-cell simulations and theoretical analysis that a high-quality electron beam whose density is modulated at angstrom scale can be generated directly in density downramp injection in plasma-based acceleration. When two counter-propagating linearly polarized frequency degenerate laser pulses interfere inside a plasma downramp, a plasma density modulation at the one-half the optical wavelength is created driven by the ponderomotive force of the standing wave pattern and the restoring force of the plasma ions. The density modulation can turn on and off the injection of electrons at the modulation wavelength when an intense driver excites a wake across the downramp. Due to the unique longitudinal mapping between the electrons' initial positions and their final trapped positions inside the wake, an electron beam with density modulation at the X-ray wavelength can be generated in this scheme. Such a high quality, modulated beam can produce fully coherent, stable, hundreds of GW X-rays by going through a resonant undulator.   

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We reconstruct spectra of secondary x-rays from a tunable 250-350 MeV laser wakefield electron accelerator from single-shot x-ray depth-dose measurements in a compact (10 x 15 cm), modular x-ray calorimeter made of alternating layers of absorbing materials and imaging plates. X-rays range from few-keV betatron to few-MeV inverse Compton to \textgreater 100 MeV bremsstrahlung emission and are characterized both individually and in mixtures. Geant4 simulations of energy deposition of single-energy x-rays in the stack generate an energy-vs-depth response matrix for a given stack configuration. An iterative reconstruction algorithm based on analytic models of betatron, inverse Compton and bremsstrahlung photon energy distributions then unfolds x-ray spectra, typically within a minute. We discuss uncertainties, limitations and extensions of both measurement and reconstruction methods.   

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Laser wakefield accelerators (LWFA) are a promising alternative for generating bright radiation sources at a fraction of the size and cost of conventional synchrotron-like facilities. The X-ray bursts emitted from a LWFA have sub-micron size, femto-second duration and low beam divergence, thus making them suitable for imaging small-scale dynamic phenomena. In this work we will image the evolution of hydrodynamic shock waves produced by the interaction of a long laser pulse with a stream of water. By taking advantage of the unique properties of plasma-based accelerators, the X-ray pulses will capture the full dynamic evolution of the propagating shock. We have made preliminary measurements and simulations of electron beam and X-ray characteristics, are developing a continuous carbon-free (water) target, and have performed radiograph hydrodynamic simulations of the laser-target interaction using CRASH software.   

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Interaction of high-intensity lasers with solid targets can result in a high yield of energetic electrons. Experimental and computational research shows that structured targets can absorb more of the incident laser pulse energy, enhancing the acceleration of electrons. However, the details of the electron acceleration mechanism during the laser-target interaction is not yet understood. In this work, we study a relativistic laser interaction with a grated target both numerically and analytically, and reveal the physics processes governing the electron acceleration. The laser-target interaction is simulated with the relativistic particle-in-cell code EPOCH. Simulation results show that, during the laser-target interaction, quasi-static electric and magnetic fields are developed in the gratings due to the laser-induced electron extraction from the solid. These quasi-static fields promote electron acceleration in the laser field beyond the ponderomotive energy scaling. We employ a 3/2-dimensional Hamiltonian approach to describe the electron dynamics in both laser and quasi-static electromagnetic fields and show that the electrons can be stochastically accelerated in these fields to high energies. We verify our analysis with numerical simulation.   

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Coupling laser energy with plasma is of great interest in context of ICF, hot electron generation, ion acceleration etc. With advancement in laser pulse technology where intensity goes as high as $10^{ 21 } W/cm^2$ , high electromagnetic fields of laser can bring out non-linear response of plasma in couple of femto-seconds of interaction. Therefore, a comprehensive analysis of plasma behaviour in that time scale becomes crucial. In this study, we demonstrate the spontaneous generation of magnetic vortices which get formed after intense laser pulse has interacted with an overdense plasma. We observe that these structures entrap EM fields in them and can propagate in the denser regions of plasma. We present a detailed study on these coherent structures and how their characteristic features makes them possible candidate for energy transport.   

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Ni K shell emission from near-solid density nanowire arrays and solid density foils was spectrally and temporally resolved using an x-ray streak camera with sub-picosecond temporal resolution coupled to a Von Hamos crystal spectrometer. The targets were 100nm diameter Ni arrays with various fractions of solid density (7{\%}, 15{\%} and 24{\%}) and were irradiated with high contrast (\textgreater 10$^{\mathrm{12}})$,~ \begin{figure}[htbp] \centerline{\includegraphics[width=0.08in,height=0.17in]{290620201.eps}} \label{fig1} \end{figure} $=400$nm, 45fs laser pulses focused to intensities \textgreater 10$^{\mathrm{21}}$Wcm$^{\mathrm{-2}}$. These results were compared to the emission from solid density Ni foils. The duration of the Ni He- \begin{figure}[htbp] \centerline{\includegraphics[width=0.09in,height=0.17in]{290620202.eps}} \label{fig2} \end{figure} line was measured to decrease from 21 ps for an initial electron density of 1.6x10$^{\mathrm{23}}$cm$^{\mathrm{-3}}$ to 5 ps for a solid foil with an initial electron density of 2.4x10$^{\mathrm{24}}$cm$^{\mathrm{-3}}$. The increased time duration of the x-ray emission from the lower density nanowire arrays is shown to be a consequence of the increased volumetric heating of the plasma which delays the hydrodynamic expansion. These results are in good agreement with hybrid three-dimensional particle-in-cell/radiative hydrodynamic simulations.~   

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Laser irradiated nanowire structures can create extreme plasma conditions [1] and substantially increase the laser energy to x-ray conversion efficiency [2]. While the increased laser energy absorption (LEA) by the structured targets [1] is a main contributing factor, there have been no experimental LEA measurements in the relativistic intensity regime (\textgreater 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}})$. The LEA by nanowire targets is measured using the frequency doubled COMET laser at the Jupiter Laser Facility. The results show that the laser energy absorption of 0.7 ps frequency doubled (527 nm) pulses focused to an intensity of 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}}$ on Au nanowire targets varies with nanowire parameters and reaches up to 71{\%} of the incident energy, greatly exceeding that from foil targets. The absorption is expected to further increase for femtosecond pulses. [1] M. A. Purvis et al., Nature Photonics \textbf{7}, 796 (2013) [2] R. Hollinger et al., Optica \textbf{4}, 1344 (2017)   

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While much of high-intensity short-pulse laser-driven particle acceleration experiments typically focus on increasing energy through increased intensity, only limited attention has been paid to the time-dependent intensity profile. Real short-pulses contain familiar structures such as pre-pulses and pedestals, however, the same laser technology used to make the primary pulse Gaussian-like can also be used to modify the temporal intensity profile. While research with multiple short-pulses has shown significant benefits to proton acceleration, this work extends this concept to more complex intensity profiles with proof-of-principle experiments. Techniques for generating multiple short pulses or shaping at the femtosecond level were used to drive MeV particle sources from solid targets at the CSU ALEPH facility. The effect of such pulses on electron and proton acceleration will be discussed with comparisons to new many-simulation techniques for exploring this vast parameter space and examining the time-dependent particle acceleration.