Development of robust machine-learning (ML) based surrogates for particle accelerators can significantly benefit the modeling, design, optimization, monitoring and control of such accelerators. It is desirable that the surrogate models embed fundamental physical constraints to the interaction and dynamics of the beams, for which an accelerator must be designed to operate upon. We implement and train a class of phase space structure-preserving neural networks - Henon Neural Networks (HenonNets) [1], for nonlinear beam dynamics problems. It is demonstrated that the trained HenonNet model predicts the beam transfer matrix to a reasonably good accuracy while strongly maintaining the symplecticity. To explore such model's applicability and flexibility for high brightness or intensity beams, we further test it with beam dynamics in the presence of electrostatic and radiative collective effects. Our results indicate that HenonNet may be used as a base ML model for the surrogate of complex beam dynamics, thus opening up a wide range of applications.

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

The self-consistent nonlinear dynamics of a relativistic charged particle beam interacting with its complete self-fields is a fundamental problem underpinning many of the accelerator design issues in high brightness beam applications, as well as the development of advanced accelerators. Particularly, synchrotron radiation induced effects in a magnetic dispersive beamline element can lead to collective beam instabilities and emittance growth. A novel beam dynamics code is developed based on a Lagrangian method for the calculation of the particles' radiation near-fields using wavefront/wavelet meshes via the Green's function of the Maxwell equations. These fields are then interpolated onto a moving mesh for dynamic update of the beam. This method allows radiation co-propagation and self-consistent interaction with the beam in 2D/3D simulations at greatly reduced numerical errors. Multiple levels of parallelisms are inherent in this method and implemented in our code CoSyR to enable at-scale simulations of nonlinear beam dynamics on modern computing platforms using MPI, multi-threading, and GPUs. The current 2D implementation of CoSyR has been used to evaluate the transverse and longitudinal coherent radiation effects on the beam and to investigate beam optics designs proposed for mitigation of beam brightness degradation in a magnetic bunch compressor. In this paper, the design of CoSyR, as well as the benchmark with other coherent synchrotron radiation models, are described and discussed. Extension of the core algorithms to 3D is possible and planned.

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. As recently reported, a burning plasma state, where the alpha-heating in the plasma is the primary source of heating, was achieved in laboratory experiments. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ``burn propagation'' into surrounding cold fuel, enabling the possibility of high energy gain. While ``scientific breakeven'' (i.e. unity target gain) has not yet been achieved, this talk reports the first controlled fusion experiment on the National Ignition Facility to produce capsule gain greater than unity (here 5.8) and reach ignition by many different formulations of the Lawson criterion. In the talk, we will discuss some key basic physics inertial confinement fusion (ICF) principles behind the burning plasma and ignition results as well as discuss future challenges.

Coherent and incoherent effects induced by synchrotron radiation, such as the collective beam microbunching instability and phase space diffusion caused by shot-noise in a dispersive beam optics element, are fundamental problems underpinning many of the accelerator design issues in high brightness beam applications and the development of advanced accelerators. To accurately and efficiently model the beam dynamics in the presence of synchrotron radiation, we investigate several existing methods for the calculation of the radiation near fields, including the finite difference method, the Jefimenko method and a near-field method recently validated where a Lagrangian adaptive mesh and a global mesh are employed. 1 This particle mesh method allows radiation propagation on the mesh with a greatly reduced error. We compare the accuracy and efficiency of these methods in both 1D and multi-dimensions, for the steady-state and dynamical beam trajectories, for the radiation field and space charge field, as well as for the coherent field and the incoherent field. We also discuss the multiple levels of parallelisms inherent in this particle mesh framework, which can be implemented on modern computing platforms using MPI, multi-threading, and GPUs.

Many applications such as compact accelerators and electron microscopy demand high brightness electron beams with small beam size and ultra-low emittance. Electric-field-assisted diamond emitters manufactured from semiconductor processes has been recognized as a leading candidate for such compact sources. The micro-scale pyramid structure of the emitter has the desirable attribute of significant electric field enhancement at the sharp interfaces (apex and edges) to facilitate electron emission. We investigate the dependence of field enhancement on the geometric shape. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Combining the results from the Monte Carlo and the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics.1 The electron beam formation and dynamics in a 1D diode setup are simulated with a particle-in-cell code to obtain the macroscopic observables such as the beam energy, voltage, and divergence. The physical characteristics and parametric dependence of the emitted beam are compared with experiments and understood through the analysis of particle trajectory in a model field configuration.2 We further develop an effective mass based theoretical model accounting for the conduction band quantization in a high aspect ratio semiconductor nanostructure and the corresponding Monte Carlo implementation to describe electron transport and subsequent electron emission from the nanotip of the emitter. The effects of level quantization, electron scattering due to the nanotip diameter variation, and electron-phonon scattering on the nanotip emission properties are identified and compared with the case of a bulk slab. 3

Fuel-ion species dynamics in hydrodynamiclike shock-driven DT^{3}He-filled inertial confinement fusion implosion is quantitatively assessed for the first time using simultaneously measured D^{3}He and DT reaction histories. These reaction histories are measured with the particle x-ray temporal diagnostic, which captures the relative timing between different nuclear burns with unprecedented precision (∼10  ps). The observed 50±10  ps earlier D^{3}He reaction history timing (relative to DT) cannot be explained by average-ion hydrodynamic simulations and is attributed to fuel-ion species separation between the D, T, and ^{3}He ions during shock convergence and rebound. At the onset of the shock burn, inferred ^{3}He/T fuel ratio in the burn region using the measured reaction histories is much higher as compared to the initial gas-filled ratio. As T and ^{3}He have the same mass but different charge, these results indicate that the charge-to-mass ratio plays an important role in driving fuel-ion species separation during strong shock propagation even for these hydrodynamiclike plasmas.

The phenomenon of synchrotron radiation (SR) from electrons is at the core of modern accelerator based light sources. While SR in the far field has been well characterized, the near-field SR and its impacts on self-consistent electron beam dynamics remain an ongoing topic. Since it is difficult to experimentally characterize the near fields, it is desirable to develop accurate and efficient numerical methods for the design of these light sources. Here, we investigate a novel method, originally proposed by Shintake and which potentially has both high efficiency and accuracy. We focus on the field calculation of this method and show that the original idea has missed the important terms of fields due to electron acceleration and therefore only applies to a linear motion. To correct this limitation we developed a modified algorithm that gives consistent fields with direct calculations using the Liénard-Wiechert equation. Some basic signatures of the near-field SR fields are also drawn for a cyclotron motion by using this modified approach. NEAR-FIELD SYNCHROTRON RADIATION AND ITS MODELING Synchrotron radiation (SR) in the far field has been well characterized and routinely used in synchrotron beamlines worldwide for advanced applications such as x-ray spectroscopy and structural imaging [1]. Meanwhile, the nearfield SR and its impacts on self-consistent electron beam dynamics have only received increasing attention in recent years. The continuing quest for coherent x-ray free electron lasers [2] and advanced accelerators [3] require electron beams of ultra-high brightness. The power of SR grows nonlinearly with the beam brightness or energy, and therefore nonlinear beam dynamics inevitably arises due to the strong near-field SR. For instance, the coherent SR fields may cause collective beam instabilities such as longitudinal energy modulation, and increase the beam emittance; the incoherent fields may generate random shot noises and phase space diffusion, leading to beam quality degradation. Different from the far-field SR, it remains a challenge to directly characterize the near-field SR in experiments. Several simulation models have therefore been considered [4]. Standard beam design tools mostly treat the Liénard-Wiechert (LW) potential and adopt the steady-state assumption. This approach is generally not self-consistent by ignoring the temporal dependence of the emission, and hence is only suitable for describing the linear stage of the instability growth. On the other hand, the particle-mesh models via discretization ∗ Work supported by the LDRD program at LANL. † fyli@lanl.gov of the full-wave Maxwell equations (e.g. Finite Difference Time Domain, FDTD) are self-consistent for the coherent effects, but their accuracy is severely limited by numerical errors due to numerical dispersion and numerical Cherenkov instability. In this study, we investigate a novel near-field method that can potentially overcome the above issues. In this original idea proposed by Shintake [5], one calculates radiation fields at the current position and then propagates them outwards to obtain real-time fields at nearby locations. By mapping the fields onto a co-moving mesh, it allows for greatly reduced propagation errors in comparison to the FDTD method. Most importantly, it allows for real-time selection of the temporal information that is only relevant to the current beam-radiation interaction. This can be much more efficient than the LW method where complete emission history has to be kept for reconstructing fields at the present time. So far, the Shintake’s near-field (SNF) method has been mainly used to construct field patterns in nearby zones. The calculation of the fields remains to be verified. As a first step towards building a comprehensive framework, we provide a validation of the field calculation by applying it to a fixed observation point near the electron trajectory. We discovered that the original idea of Shintake missed the important term of the acceleration field and applied only to linear electron motion. To correct this limitation we come up with complete steps that can accurately determine the fields at arbitrary positions. As we shall see, the modified algorithm gives consistent field calculations with the LW equation. Some basic signatures of the near-field SR due to cyclotron motion are also explored with the updated method. SHINTAKE’S NEAR-FIELD METHOD In the original idea [5], a moving electron emits wavelets of electromagnetic fields which form a set of outgoing spherical waves in free space by following the wave equation. Once emitted, the spherical wavefronts will expand outwards at the speed of light. The centers of these spheres emitted at different times, however, shift in positions due to the electron motion. The wavelet propagation direction is related to the electron instantaneous velocity at the time of emission, and is given by the Lorentz transformation of the unit propagation vector ® k (essentially a displacement vector) from the electron to the lab frame: kx = (cos θ ′ + β)/(1 + β cos θ ), ky = sin θ /γ(1 + β cos θ ), where θ ′ is the propagation angle in the electron frame relative to the electron velocity ® β (normalized by c), and γ = 1/ √ 1 − β2 is the electron’s Lorentz factor. 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPGW116 MC5: Beam Dynamics and EM Fields D05 Coherent and Incoherent Instabilities Theory, Simulations, Code Developments MOPGW116 397 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I

This paper reports the results of a divergence study and emittance measurements for an electron beam emitted from a single diamond pyramid in a sparse diamond field-emitter array (DFEA) cathode. DFEA cathodes are arrays of micrometer scale diamond pyramids with nanometer scale tips. A single diamond pyramid produces currents as high as 10 μA from a small surface area. DFEA cathodes are expected to be a good beam source for compact dielectric laser accelerators. For the electron beam divergence experiment, we have designed and assembled a test stand consisting of a DFEA cathode, a mesh anode, and a screen. We measured current and the size of the beam on the screen as functions of the distance between the cathode and the anode, the size of a pyramid’s base, and applied voltage on the cathode. In order to perform beam emittance measurements, we designed and fabricated a magnetic lens to be placed between the mesh and the screen. By measuring the size of the beam for different distances between the lens and the screen, we estimate the beam’s emittance. We also simulated the electron beam dynamics with Computer Simulation Technology Studio and General Particle Tracer codes. Experimentally measured divergence angles and normalized transverse emittance show good agreement with simulations.

Many applications, such as compact accelerators and electron microscopy, demand high brightness electron beams with small source size and ultralow emittance. Diamond emitters manufactured with semiconductor processes can be employed in such compact beam sources. The micrometer-scale pyramid structure of the emitter allows enhancement of the external field compared to that at the substrate, leading to electron emission with small beam size. We investigate the dependence of the field enhancement on the shape of the emitter and the resulting emission characteristics. The beam formation and dynamics are simulated with the LSP [D. Welch, D. Rose, R. Clark, T. Genoni, and T. Hughes, Comput. Phys. Commun. 164, 183 (2004)] particle-in-cell code to obtain the macroscopic observables. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Using this Monte Carlo emission model and the result from the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics. A comparison between our model and experiments indicates that the beam current is mostly emitted at the apex of the emitter.

An effective mass based model accounting for the conduction band quantization in a high aspect ratio semiconductor nanotip is developed to describe injected electron transport and subsequent electron emission from the nanotip. A transfer matrix formalism is used to treat electron scattering induced by the variation in the tip diameter and in the electron emission. Numerical analysis of the scattering and emission probabilities is performed for the diamond parametrized nanotip model. Our scattering and emission models are further combined with a Monte Carlo (MC) approach to simulate electron transport through the nanotip. The MC simulations, also accounting for the electron-phonon scattering and externally applied electric field, are performed for a minimal nanotip model and an equivalent width diamond slab. An effect of the level quantization, electron scattering due to the nanotip diameter variation, and electron-phonon scattering on the nanotip emission properties are identified and compared with the case of a bulk slab.

Field emission from nanocrystalline diamond and especially from diamond field emitters is known to have an onset at low electric fields of a few MV/m, although the discussion on the agreement of the results with the classical Fowler-Nordheim model is still pending. While measurements of pure photoemission from flat nanocrystalline diamond agree reasonably well with the 3-step photoemission model for the wide bandgap semiconductor with low electron affinity, we are not aware of systematic studies of photoemission in the ~0.1-5 MV/ m range, where the electron emission mechanism is expected to be affected by the Schottky effect and crossover with the field emission. In order to understand applicability of field enhanced photoemission from diamond to generation of bright coherent photo-gated electron beams suitable for dielectric laser accelerators, we have designed a system to measure spectral response (quantum efficiency vs wavelength) of ~mm-sized samples with up to 5 MV/ m electric field in the anode-cathode gap. The system is based on an incoherent Xe lamp-based tunable ultaviolet light source, therefore relatively large and dense diamond field emitter arrays are required for comparative studies of arrays versus flat nanocrystalline diamond samples. We present the results of our original measurements in the range between 195 nm and 270 nm. Potential schemes of laser-triggered photoemission from a single diamond field emitter tip are discussed in view of the obtained results.

In the last five years, large amounts of high quality data on inertial confinement fusion (ICF) experiments were produced at the National Ignition Facility (NIF). From this data we have significantly advanced our scientific understanding of the physics of thermonuclear (TN) ignition and identified critical issues that must be addressed to achieve a burning hotspot, such as implosion energetics, pusher adiabat, tamping effects, and confinement time. In this paper we present a review of recently developed TN ignition and implosion scaling theory (Cheng et al 2013 Phys. Rev. E 88 041101; Cheng et al 2014 Phys. Plasmas 21 10270) that characterizes the thermodynamic properties of the hotspot and the ignition criteria for ICF. We compare our theoretical predictions with NIF data and find good agreement between theory and experiments. We demonstrate the fundamental effects of the pusher adiabat on the energy partition between the cold shell and the hot deuterium–tritium (DT) gas, and thus on the integrated performance of ICF capsules. Theoretical analysis of NIF experiments (Cheng et al 2015 Phys. Plasmas 22 082704; Melvin et al 2015 Phys. Plasmas 22 022708; Cheng et al 2016 Phys. Plasmas 23 120702) and physical explanations of the discrepancies between theory, data, and simulations are presented. It is shown that the true experimental adiabat of the cold DT fuel can be inferred from neutron image data of a capsule implosion. We show that the ablator mix and preheat in the cold fuel can be estimated from the experimentally inferred hotspot mix. Finally, possible paths forward to reach higher yields at NIF implied by the theory are discussed.