Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session CI01: Particles, Beams, and Coherent RadiationLive Streamed
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Chair: Thomas Schenkel, Lawrence Berkeley National Laboratory Room: Ballroom 100 A |
Monday, October 17, 2022 2:00PM - 2:30PM |
CI01.00001: Direct Electron Acceleration and Radiation Generation in Space-Time Structured Laser Pulses Invited Speaker: Dillon W Ramsey By manipulating the space–time focusing of a laser field, recent experiments have created novel laser pulses with properties that promise to revolutionize a wide range of laser–plasma applications. These "flying focus" pulses feature an intensity peak that travels with a near-constant profile at an arbitrary, tunable velocity over distances far greater than a Rayleigh range. The velocity control offered by flying-focus pulses enables unique configurations of light–matter interactions and introduces new pathways to optimization. In particular, a flying focus pulse can be structured so that its intensity peak travels in the opposite direction as its phase fronts—a configuration that is impossible with conventional laser fields. Using newly derived, exact solutions to Maxwell's equations for flying-focus fields, we demonstrate that a backward-travelling intensity peak can ponderomotively accelerate electrons to relativistic momenta in the backwards direction, providing unprecedented control over the electron trajectory in nonlinear Thomson scattering (NLTS). With sufficient amplitude, a backward-travelling, subluminal intensity peak can impart enough momentum to an electron that it outruns the intensity peak and gains net energy. A backward-travelling intensity peak with insufficient amplitude or superluminal velocity can accelerate electrons against the phase fronts for an extended distance. This acceleration compensates the ponderomotive deceleration that diminishes the scaling of the radiation properties of NLTS in traditional laser pulses. By appropriately setting the velocity of the intensity peak, a flying focus pulse can increase the power radiated by orders of magnitude and allow for operation at significantly lower electron energies or intensities. |
Monday, October 17, 2022 2:30PM - 3:00PM |
CI01.00002: Positron acceleration in plasma wakefield accelerators using plasma columns Invited Speaker: Severin Diederichs Plasma accelerators produce extremely large fields, enabling compact accelerators. Owing to these ultra-high accelerating gradients, application of plasma accelerators to the next generation of linear electron-positron colliders has attracted considerable interest. The ability to accelerate and transport high-quality positron beams is critical to realizing this application. In this invited talk, a novel concept for positron acceleration in beam-driven plasma wakefield accelerators (PWFAs) will be presented. This concept relies on a finite radius plasma, or plasma column, to control the wakefields driven by a charged-particle beam, generating a region with focusing and accelerating fields for positron beams [1]. It has been demonstrated, via theory and particle-in-cell simulations, that low emittance and energy spread beams can be accelerated with optimal beam loading providing a path to collider-relevant positron beam parameters [2]. Critical to this application is the stability against misalignment and asymmetries of the drive electron beam and the trailing positron beam propagating along the plasma column. Recent work has demonstrated the intrinsic stability of a drive [3] and trailing [4] beams propagating in the plasma channel, and the stabilization mechanisms will be discussed in this talk. The effect of a non-zero plasma temperature and realistic, non-ideal plasma column profiles will be shown. Finally, experimental implementations of this concept at beam test facilities will also be discussed. |
Monday, October 17, 2022 3:00PM - 3:30PM |
CI01.00003: Femtosecond dynamics of relativistic electron heating in a high-intensity laser-produced solid-density plasma Invited Speaker: Hiroshi Sawada Relativistic electrons generated by a high-intensity short-pulse laser have been studied as a source for creating high-energy-density matter.[1] The transport of the electrons rapidly heats and ionizes a thin metal foil to warm dense matter (WDM) before it hydrodynamically expands. The underlying physics of the relativistic electron isochoric heating has been experimentally confirmed with time-integrated monochromatic x-ray imaging.[2] However, diagnosing transient material conditions has been limited in spatiotemporal resolutions. The recent advent of a high-intensity laser combined with an x-ray free electron laser (XFEL) has enabled ultrafast pump-probe experiments to investigate the interior conditions of solid and high-density matter. Here, we capture femtosecond dynamics of plasma formation driven by relativistic electrons in solid metal for the first time. We demonstrate a diagnostic with femtoseconds and micron-scale resolution using SACLA XFEL pulses and visualize the propagation of the ionization front in a solid copper as a signature of the plasma creation.[3] The novel x-ray transmission imaging with x-ray wavelengths tuned to near the Cu K-edge provides information on the target’s temperatures and ionization states from a smeared K-edge profile and the evolution of the electron-impacted area. Our result reveals that the electron-driven ionization wave produces strongly coupled Fermi degenerate matter. Information on the non-equilibrium WDM could be used to validate quantum molecular dynamics and plasma atomic physics calculations, such as ionization potential depression. |
Monday, October 17, 2022 3:30PM - 4:00PM |
CI01.00004: Investigation of Boosted Proton Energies through Proton Radiography of Target Normal Sheath Acceleration Fields in the Multi-ps Regime Invited Speaker: Raspberry A Simpson Multi-kilojoule, multi-picosecond short-pulse lasers, such as National Ignition Facility-Advanced Radiographic Capability (NIF-ARC) laser and the OMEGA-Extended Performance (EP) laser, which have been constructed over the last decade enable exciting opportunities to produce high-brightness, high-energy laser-driven particle sources for applications in high-energy-density (HED) science like proton fast ignition for inertial fusion energy. Recent results on these platforms have demonstrated enhanced accelerated proton energies and electron temperatures when compared to established scaling laws. Recent work has developed a new scaling for proton TNSA in the multi-ps regime. However, this new physics in the multi-ps regime motivates the need to understand the origin of this enhancement in proton energies. Towards this goal, this work presents the first measurements of the TNSA accelerating sheath field in the multi-ps regime for pulse durations from 1ps-30ps. This measurement was achieved by using a separate TNSA proton source to radiograph the spatio-temporal profile of the accelerating sheath that is responsible for proton acceleration. Use of stacked radiochromic film detectors allows for a discrete time profile of the radiographs thus enabling the measurement of a ``movie'' of accelerating sheath fields evolution. In performing this measurement, we extract quantities such as the sheath strength as a function of time and pulse duration. In addition, this work presents a novel preliminary methodology based on representation learning to integrate heterogeneous data, such as proton and electron spectra, to constrain parameters that are not directly measurable such as the spatio-temporal evolution of the accelerating sheath field. |
Monday, October 17, 2022 4:00PM - 4:30PM |
CI01.00005: Transfer learning and multi-fidelity modeling of laser-driven ion acceleration Invited Speaker: Blagoje Z Djordjevic Modeling of intense, laser-driven ion acceleration requires expensive particle-in-cell (PIC) simulations that may struggle to capture all the multi-scale, multi-dimensional physics involved at reasonable costs. Explored here is an approach to ameliorate this deficiency using a multi-fidelity model that can incorporate physical trends and phenomena at different levels. As the base framework for this study, an ensemble of approximately 10,000 1D PIC simulations was generated to buttress separate ensembles of hundreds of higher fidelity 1D and 2D simulations. Using transfer learning with deep neural networks, one can reproduce the results of more complex physics at a much smaller cost. The networks trained in this fashion can in turn act as surrogate models for the simulations themselves, allowing for quick and efficient exploration of the parameter space of interest. Standard figures-of-merit were used such as the hot electron temperature, peak ion energy, conversion efficiency, etc. These surrogate models are also useful for incorporating more complex particle acceleration schemes, such as laser pulse shaping where the simulation input parameter space is greatly expanded and standard parameterization of laser pulses (pulse length, intensity, etc.) are no longer descriptive. We can rapidly identify and explore under what conditions dimensionality becomes an important effect and search for optima in feature space. A description of the ensemble simulation and machine learning methodology will be presented along with multi-dimensional parameter space maps and optimizations for short-pulse, laser-driven particle sources found through this work. |
Monday, October 17, 2022 4:30PM - 5:00PM |
CI01.00006: Electron emission models for two-dimensional and topological materials Invited Speaker: Ricky L. K Ang Electron emission from electrode into a diode is a fundamental process for beam physics, plasma science and technology. Depending on the energy used for electron emission, it can be broadly characterized into 3 different processes known as thermionic emission TE (by thermal energy), field emission FE (by quantum tunneling) and photoemission PE (by absorption of photons or optical tunneling). At high current regime, it becomes the space charge limited current (SCLC). The classical models for these processes (TE, PE, PE, SCLE) have been formulated many decades ago, known as the Richardson-Dushman (RD) law, Child-Langmuir (CL) law, Fowler-Nordheim (FN) law, and the Keldysh model, etc. With the development and fabrication of two-dimensional (2D) atomic scale and topological materials, the above-mentioned classical laws may require revisions to account for their unique material properties if such materials are used as electron emitters. This invited talk is focused on thermionc-field emission and will explain why the traditional RD and FN laws are no longer valid for these new quantum materials. The presentation will share some recent models to address the unique properties of the materials and the emission characterisitics. New scalings as a function of temperature and electric field will be shown for various classes of quantum electron emitters. A quick overview of recent experimental results from field emission from such materials will be presented to show its potential as compact electron emitter. The similarity of the emission physics to the charge injection in electrical contact will be discussed. Finally some unsolved questions will be suggested for future studies in order to have a better understanding and applications for the beam and plasma community. These electron emission laws will be useful PIC simulation or gun codes used widely in plasma physics and high power coherent radiation sources. |
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