Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session CI01: Invited: X-rays and High Energy Density PhysicsLive
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Chair: Cameron Geddes, LBNL |
Monday, November 9, 2020 2:00PM - 2:30PM Live |
CI01.00001: Single-shot multi-keV X-ray absorption spectroscopy using an ultrashort laser wakefield accelerator source Invited Speaker: Brendan Kettle X-ray absorption spectroscopy can provide a wealth of information regarding the structure and state of a sample. Techniques such as XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) provide a simultaneous measurement of the temperature and structure of both the electronic and ionic distributions. Making these measurements using a single ultrashort probe pulse provides a powerful tool for investigating, for example, laboratory-based high energy-density samples. These states are notoriously difficult to probe due to their extreme conditions, transient nature, and often limited shot rate. We present high-resolution single-shot K-edge XANES measurements of a cold titanium sample from a recent experiment using a laser wakefield accelerator source at the Gemini laser facility [1]. $1.2\pm0.2\times10^6$ photons/eV where generated in the 5 keV region with a smooth broadband spectrum, a signal-to-noise ratio of approximately $300:1$, and a few femtosecond pulse duration. We demonstrate that this source is capable of single-shot simultaneous measurements of both the electron and ion distributions in matter heated to eV temperatures by comparison with density functional theory simulations. The unique combination of a high-flux, large bandwidth, few femtosecond duration x-ray pulse synchronized to a high-power laser will enable key advances in the study of ultrafast energetic processes such as electron-ion equilibration and non-thermal phase transitions.\\\\ References:\\ 1. B. Kettle et al. Phys. Rev. Lett. 123, 254801 (2019) [Preview Abstract] |
Monday, November 9, 2020 2:30PM - 3:00PM Live |
CI01.00002: Hot Electron Generation and Laser--Plasma Instabilities in Shock Ignition Relevant Experiments Invited Speaker: Shu Zhang Shock ignition (SI) is an alternative inertial confinement fusion scheme, which uses a strong convergent shock generated by a $\sim$10$^{16}$ W/cm$^2$ spike laser pulse to ignite a pre-compressed fusion capsule. Understanding nonlinear laser--plasma instabilities (LPIs) and hot electron generation is critical for SI. LPIs can reduce energy coupling through scattering but can also accelerate electrons that assist in shock generation. We have conducted a series of experiments on the OMEGA EP and OMEGA-60 laser facilities demonstrating that stimulated Brillouin scattering (SBS) can deplete the laser energy nearly 100\% during the first $\sim$0.5 ns of a 10$^{16}$ W/cm$^2$ ultraviolet laser in an SI-relevant plasma ($L_n \sim 260-330$~$\mu$m). The pump-depletion starts from the $\sim$0.02 critical density ($n_{\rm c}$) region and progresses to the 0.1--0.2$n_{\rm c}$ region, which is evidenced by the shape of the laser-generated blast wave and the time-resolved stimulated Raman backscattering spectra. This dynamic pump-depletion is consistent with an ion-acoustic wave-breaking SBS saturation model. Strong SBS is also observed in our large-scale particle-in-cell modeling. Although the pump-depletion would inhibit the collisional laser absorption, LPIs convert 2--6\% of the laser energy into hot electrons with $T_{\rm hot} \sim 45-90$ keV, inferred from the bremsstrahlung x-ray spectra and Cu K-shell fluorescence from a target tracer. Overlapping beams doubled the energy conversion compared with the single beam configuration. Analytical models suggest that these hot electrons are suitable to generate the required 300 Mbar shock for SI in a megajoule laser facility. [Preview Abstract] |
Monday, November 9, 2020 3:00PM - 3:30PM Live |
CI01.00003: Axial Proton Radiography of Electric and Magnetic Fields Inside Laser-Driven Coils Invited Speaker: Jonathan Peebles In a laser-driven coil (LDC) a laser ejects electrons from a plate, which then draws a current through an advantageously shaped loop to generate a magnetic field. B-dot probes, Faraday rotation, and transverse (perpendicular to the coil axis) proton radiography have been used in previous LDC experiments, where fields of 600 T or more are commonly cited, but cannot directly probe inside the coils and are subject to confounding factors, often delivering conflicting measurements on the same experiment. Here we report the first detailed measurements of both magnetic and electric fields inside an LDC using axial proton radiography with a grid fiducial. Protons traveling down the axis of a coil are rotated by the radial magnetic field due to coil current and are displaced radially by the radial electric field due to coil charging. In conjunction with probing at multiple proton energies, axial radiography can unambiguously break the degeneracy between magnetic and electric fields. OMEGA EP experiments were carried out using axial radiography of double- and single-plate LDC's. Detailed reconstructions of radiographs over several proton energies were performed using current and charge distributions to reproduce the proton grid distortions. In single-plate coils axial magnetic fields at the center of up to 65 +/- 15 T were inferred, but with nonuniform currents around the coils. In double-plate coils magnetic fields were below the detection threshold of 15 T. Significant radial electric fields due to electron ejection from the coils were present in both configurations. Laser probe measurements of plasma expansion on the plates indicate that x-ray drive of the second plate led to the lack of a detectable current in double-plate coils. [Preview Abstract] |
Monday, November 9, 2020 3:30PM - 4:00PM Live |
CI01.00004: Development of Bright MeV X-ray Sources with Compound Parabolic Concentrator Targets on Petawatt Class Lasers Invited Speaker: Shaun Kerr The peak on-target laser intensity is a key parameter for High Energy Density applications such as laser driven particle acceleration and x-ray sources. In most Petawatt class laser systems, high intensities are achieved using low F/# (F/2-F/3) optics. This puts steep requirements on laser beam quality, which leads to increased facility cost and complexity. As an alternative approach, we have implemented miniature Compound Parabolic Concentrator (CPC) targets which act as a non-imaging focusing optic [1], which relax focusing requirements while achieving target performance consistent with high intensity. A series of experiments have been performed on a range of facilities - the Texas Petawatt, Titan, and NIF Advanced Radiographic Capability (ARC) - to characterize and understand the performance of CPC targets. Laser conditions varied from pulse durations of 0.1 – 40 ps, energies from 100 – 2600 J, and F/#’s of 10 – 60. In these experiments the production of MeV electrons and x-rays was measured, and a large enhancement of mean electron energy (4-5x higher) and x-ray brightness (>10x higher for photons > 100keV) was observed with CPC compared to flat targets. These experiments were modeled with 1, 2 and 3D particle in cell simulations, giving results consistent with a hypothesis that the CPC acts as an in-situ focusing plasma mirror that increases the laser intensity at the tip. These simulations further suggest that plasma confinement at the cone tip enhances the efficiency of energetic electron production. This talk will discuss the experimental and simulation results, and their implications for the development of bright MeV x-ray and particle sources on Petawatt class laser systems. [1] MacPhee et al., “Enhanced laser-plasma interactions using non-imaging optical concentrator targets,” Optica, vol. 7 issue 1 (2020). [Preview Abstract] |
Monday, November 9, 2020 4:00PM - 4:30PM Live |
CI01.00005: Coupling of LPA e-beams into Undulators, as a Path Toward LPA-Driven FELs Invited Speaker: Marie Emmanuelle Couprie The recent spectacular development of laser plasma accelerators (LPA) that deliver GeVs electron beams in an extremely short distance make them more and more promising. Applications for free electron laser appear as a way to move from an acceleration concept to an accelerator qualification. Still, the presently achieved divergence and energy spread require some beam manipulations. The electron beam at the plasma exit should be strongly focused. The energy spread, besides specific generation schemes, can be handled by a decompression chicane or a transverse gradient undulator. Various results on the LPA coupling to undulator will be reported, and will be illustrated in more details with the COXINEL test line example. It comprises variable permanent magnet quadrupoles for divergence handling, a magnetic chicane for energy sorting, a second set of quadrupole before an undulator. The transport along the line is controlled. The observed undulator radiation exhibit transverse distributions in agreement with theoretical observation and spatio-spectral moon shape type patterns, with radiation performance close to those achieved with synchrotron radiation light source. Future prospects towards laser Plasma Acceleration based Free Electron Laser will be given. [Preview Abstract] |
Monday, November 9, 2020 4:30PM - 5:00PM On Demand |
CI01.00006: Efficient electron acceleration and pair production using the NIF-ARC laser Invited Speaker: Jackson Williams Relativistic plasmas play an important role in the morphology of energetic astrophysical phenomenon, particularly within the shock formation that results in gamma ray bursts. Developing a laboratory-based platform to study the microphysics of such shocks has been pursed for the past decade using high-energy intense short pulse lasers. Here, we report on bringing up a pair plasma experimental capability at the National Ignition Facility (NIF) using the Advanced Radiographic Capability (ARC) laser. ARC has the largest amount of available short pulse energy in the world (up to 4 kJ) but is delivered at sub-relativistic intensities ($I_L ≤ 10^{18}$ W/cm$^2$), below the established threshold to generate positron-electron pair plasmas. Through a series of experiments within a NIF Discovery Science campaign, this limitation was overcome using novel target-based plasma optics which produced a significant enhancement to the laser-to-target coupling efficiency [1] and resulted in a pair yield increase >10x [2]. Compound parabolic concentrator (CPC) targets have subsequently been used to enhance proton and high-energy x-ray production [3]. With this established platform, we will discuss a proposed experiment to test plasma collective effects inside a relativistic pair plasma that could be performed at the NIF. \\ References: [1] G. J. Williams et al., “Production of Relativistic Electrons at Sub-Relativistic Laser Intensities.” Phys. Rev. E, 101:031201, Mar 2020. [2] G. J. Williams et al., “Increasing intense laser to target coupling efficiency using CPC targets” [To be submitted, 2020]. [3] A. G. MacPhee et al., “Enhanced laser–plasma interactions using non-imaging optical concentrator targets.” Optica, 7(2):129–130, Feb 2020. [Preview Abstract] |
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