Modern day RF particle accelerators were born in 1940s and 50s following World War II, stimulated by W. Hansen’s invention of the MARK I electron linear accelerator at Stanford utilizing RF resonant cavities and powered by the klystron that had recently been invented by the Varian brothers. In the ensuing 75 years accelerators have become an engine of scientific discovery leading to experimental verification of the Standard Model for particle physics; to x-ray free-electron lasers for ultra-fast chemical and materials science; and to applications such as radiation oncology, industrial processing, and national security. The tremendous development of accelerator technology over the last half-century has even capitalized on developments such as superconducting RF technology to enable very high average power operation. We now find ourselves at the point where the current technology can no longer deliver the desired performance required in the future at a cost that is acceptable and within the desired size and power constraints. Understanding the coupling of particle beams, electromagnetic fields, materials properties, and plasmas underpins the future of accelerator science and technology. Exciting R&D is underway in numerous areas that has the promise of greatly increasing the science reach of high-energy physics accelerators as well as enabling a broad range of applications for smaller and even compact accelerators in areas that include industrial processing, medical, security, and others. Advancements are occurring in the understanding of accelerator materials properties, advanced electrodynamic accelerating structure topology, operation in new frequency regimes up to millimeter-wave/THz, high efficiency RF power generation, and advanced manufacturing that could dramatically improve the cost/capability curve for these systems. Unconventional approaches are also being considered that utilize accelerators powered by lasers and high energy electron beams to generate intense accelerating fields in silicon microstructures and plasmas. These technology developments and novel approaches seek to achieve breakthroughs in accelerating gradient, cost, and efficiency leading to a new era of discovery science and applications.

The evolution of computational physics over the last 30 years has been frighteningly fast, and the change has affected every aspect of the trade, including education, hardware, operating systems, languages, methodologies, standards, available libraries, information sharing, teaming, management, and specialization. This talk will concentrate on those changes to computational plasma and beam physics and engineering that result from the increasing specialization of the field, for which computationalists now typically concentrate on the computer science aspects, on the applied math issues. on the science implementation, or on science discovery.

Several computational discoveries are described. Laser-plasma acceleration has come to depend heavily on simulation to identify the physical mechanisms for variation of the beam emittance in the direction of and transverse to laser polarization. Many ideas are also first tested by computation, which can be done more rapidly than experiments can be reconfigured. Similarly, propagation of radiation in magnetized plasmas has been analyzed computationally; the computations show that propagation of electromagnetic energy into plasma is difficult for frequencies somewhat above the gyrofrequency because of the many nonlinear decay channels.

Improvements include algorithms that preserve the properties of the underlying system, are efficient on distributed memory computers, and take advantage of vector parallelism of GPUs and Advanced Vector Instructions. Regarding the former, we discuss structure-preserving algorithms that maintain properties of the underlying equations (that is, volume preservation) and hence allow computation of sensible equilibria even though the solutions are only second-order accurate. For numerical efficiency, new multi-step algorithms allow better use of cache and lead to factors of several in speedup.

Computation has become sufficiently complex that practitioners must have a workflow that allows problem setup with an easy-to-use GUI, transition of the problem to a supercomputer, and capture of a provenance to replicate the results. A preferred workflow from GUI setup to supercomputer usage and data analysis will be described.

Finally, the rapid evolution has affected education, as it is no longer possible, within the graduate school training period, for a student to learn everything from the impact of CPU/GPU hierarchy on implementations through how to set up simulations to answer questions. To this end, the University of Colorado now offers multiple courses, including applied math courses on parallel algorithms to physics courses on how to make effective use of simulations. The latter will be described.

High-energy-density (HED) physics has emerged as one of the frontiers of plasma science for the 21^st century. While this area of study dates back to the early years of the nuclear weapons program, the cessation of underground nuclear weapons testing and the initiation of Science Based Stockpile Stewardship in the early 1990’s launched an era in which focused laboratory experimentation is challenging the fidelity of our theoretical understanding of the interaction of matter and radiation in some of the most extreme regimes ever produced in the laboratory. As a result, this area of research has been thrust to the forefront of scientific discovery. The ability to create, control, and diagnose such extreme states of matter is not only vital to the Stockpile Stewardship Program but is also advancing our fundamental understanding of the visible universe. In this presentation I will discuss what I see as some of the most compelling and challenging questions in HED science and the pursuit of inertial fusion, and how the NNSA is preparing to meet these challenges.

The interactions of non-equilibrium plasmas with a liquid state represent a growing interdisciplinary area of research in the field of plasma science and technology. Plasma–liquid interactions are important in many applications ranging from environmental remediation to material science and health care. Different types of discharge plasmas generated either directly in the liquid, in the gas phase over and in contact with a liquid, or in the multiphase environments such a as discharges in bubbles inside liquids or discharges contacting liquid sprays or foams are used. Much of the work has focused on plasmas in contact with aqueous liquids (primarily water). Depending on the type of discharge, its energy, and the chemical composition of the surrounding environment various types of physical processes (ultraviolet radiation, overpressure shock waves) and plasma-chemical reactions can be initiated. Various short and long-lived chemical species are produced by plasma in the liquid either directly, or transferred from the gas phase discharge plasma being in contact with the liquid. Among these processes, the oxidative properties of reactive oxygen species (OH radical, atomic oxygen, O₃, H₂O₂) and nitrogen species (NO, NO₂ radical) are generally accepted to play central role in the chemical and biological effects of plasma produced in gas-liquid environments. These species can react at or penetrate through the plasma-gas/liquid interface and dissolve into the bulk liquid, and can also initiate secondary chemical processes in the liquid. In addition to the use for chemical decontamination processes, plasma in liquid can be used for material processing and synthesis (e.g., production of H₂ and polymers from liquid hydrocarbons, or production of nanoparticles). There also novel developments in the understanding of electrical breakdown phenomena in liquid produced in nanosecond pulse duration range. In this talk an overview of basic principles of electrical discharge plasmas in liquids will be presented with particular emphasis on elementary physical and chemical processes induced by non-equilibrium plasma in aqueous liquids and their applications.

1. V. I. Parvulescu, M. Magureanu, P. Lukes, “Plasma Chemistry and Catalysis in Gases and Liquids”, Wiley-VCH: Weinheim, 2012.
2. P. J. Bruggeman, M. J. Kushner, B. R. Locke, et al., “Plasma-Liquid Interactions: A Review and Roadmap”, Plasma Sources Sci. Technol., 25, 2016, 053002.

I have been privileged to experience a reasonably long and successful career in plasma science and applications. Most importantly, I have been gifted by a vast number of wonderful collaborations and collaborators who have generously shared their wisdom in the pursuit of answers to perplexing questions, and the privilege to share the reward of learning and discovery with over three decades of students and colleagues.

During this time, we have discovered how a nonlinear microwave driving force can accelerate high temperature solid-state reactions, how periodic magnetic fields can stably focus sheet electron beams, how microwave fields interact with heterogeneous biological tissues, how electromagnetic metasurfaces can enable rapid, large-area plasma breakdown, how surface imperfections alter the absorption of millimeter- and submillimeter-wave radiation, and numerous other discoveries. This talk will give a bit of a whimsical perspective about how the simple, yet universal truths of the book All I Really Need to Know I Learned in Kindergarten¹ have played a crucial role in this journey of discovery and shared learning.

1. R. Fulghum, “All I Really Need to Know I Learned in Kindergarten: Uncommon Thoughts on Common Things,” Ballantine Books (2004).

Physical science and engineering practice have been, over most of history, advanced through experimentation and observation, along with analysis, calculations, and analog simulation. The use of digital computer simulations is a relatively recent development, but has matured rapidly and is now a full peer to the classical approaches.

In the fields of plasmas and particle beams, such simulations have been especially effective. Here, the underlying physical processes are largely understood, and equations describing the fundamental dynamics to good approximation (e.g., the Vlasov-Maxwell set) are known. However, first-principles treatments are (more often than not) intractable. Thus, considerable innovation of descriptive models and the methods of their solution has taken place over recent decades. The development of efficient computer models embodying a suitable level of description has thus been a central element of computational plasma and beam physics. The speed of at least some computations has benefited more from algorithmic innovation than from computer hardware advances¹.

The author has had the privilege of participating in this enterprise, and in this talk seeks to illustrate how topics arose and interacted over the course of his career. Themes such as the nature of collective behaviors, avoidance of numerical instability, appropriately accurate single-particle motion, and the value of user-programmable software appeared repeatedly.

The presentation will touch on simulations of field-reversed ion rings, implicit techniques, laser raytracing for inertial fusion, beam physics for heavy-ion inertial fusion (using the Warp code, and reduced models), and other applications^2-6.

1. “Software Progress Beats Moore’s Law,” New York Times, March 7, 2011, and comments thereto.
2. A. Friedman, A. B. Langdon, and B. I. Cohen, “A Direct Method for Implicit Particle-in-Cell Simulation,” Comments on Plasma Phys. and Controlled Fusion 6, 225 (1981).
3. A. Friedman, R. N. Sudan, and J. Denavit, “Stability of Field Reversed Ion Rings,” Phys. Fluids 29, 3317 (1986).
4. A. Friedman and S. P. Auerbach, “Numerically Induced Stochasticity,” J. Comput. Phys. 93, 171 (1991) et seq.
5. A. Friedman, et al., “Beam dynamics of the Neutralized Drift Compression Experiment-II, a novel pulse-compressing ion accelerator,” Phys. Plasmas 17, 056704 (2010).
6. A. Friedman, et al., “Computational Methods in the Warp Code Framework for Kinetic Simulations of Particle Beams and Plasmas,” IEEE Trans. Plasma Phys. 42, 1321 (2014).

Relativistic electrons can oscillate above the Earth trapped in the radiation belts (known as the Van Allen Belts). These electrons, which can originate from the solar wind or a high-altitude nuclear explosion, have the potential to damage satellites in low-Earth orbit. For example, in 1962, the US detonated the Starfish warhead at an altitude of about 400 km. The unexpected resulting enhancement of the radiation belts disabled several satellites within a few months and energetic electrons remained in the radiation belts for up to several years. In order to address this potential vulnerability, schemes have been proposed to drain electrons from the radiation belts, with the most promising based on using high-power very-low frequency (VLF) waves to change the transverse energy of the electrons, allowing them to precipitate into the Earth’s atmosphere. This talk will provide an overview of the radiation belts and their electron distribution as well as approaches to VLF wave belt remediation including the use of either antennas or relativistic electrons beams in space to generate the VLF waves.