Nonneutral plasmas are plasma systems in which there is no overall charge neutrality, including the limit of systems that are fully unneutralized in which there are particles of only a single sign of charge.1,2 Examples of nonneutral plasmas include charged-particle beams,3 pure electron plasmas,4,5 pure positron plasmas,6 and pure-ion plasmas consisting of a variety of ion charge states in a single trap.7 A key feature of nonneutral plasmas which distinguishes them from quasineutral plasmas is that their self-electric and self-magnetic fields can play a dominant role in the behavior of the system. Moreover, single-component plasmas can be confined in states of global thermal equilibrium,8 enabling detailed theoretical and experimental studies of fundamental plasma phenomena and precise testing of models.
The body of knowledge developed through the study of nonneutral plasmas finds application in many areas. Consider charged-particle beams as one example. Beams can be thought of as nonneutral plasmas with a significant average velocity, with a suitably small spread in axial and transverse velocities, with respect to an observer's frame of reference. Many modern practical uses of beams—such as for free-electron lasers, accelerators for high-energy physics, nuclear physics, medical isotope generation, spallation neutron sources, high-energy-density physics, and heavy-ion-driven inertial fusion energy—demand ever-increasing amounts of charge in the beam bunch. The need to understand the behavior of intense charged-particle beams drives the fields of plasma physics and beam physics closer together.
Professor Ronald Crosby Davidson (Figs. 1 and 2), who passed away on May 19, 2016, made many contributions to both the fields of plasma physics and beam physics. Born on July 3, 1941, in Norwich, Ontario, he grew up on a family dairy farm, where he drove a tractor by age 11. He attended a one-room school house and met his future wife, Jean, in high school. After obtaining his B.Sc. in physics from McMaster University in 1963, Ron moved to Princeton, where he completed his Ph.D. in 1966. After a postdoctoral appointment at the University of California, Berkeley, Ron served as an Assistant Professor, an Associate Professor, and a Full Professor at the University of Maryland until 1978. Ron then went to MIT, where he was a Professor of Physics at the Plasma Science and Fusion Center from 1978 to 1991 (Director from 1978 to 1988). In 1991, Ron returned to Princeton and served as the Director of the Princeton Plasma Physics Laboratory from 1991 to 1996, until he stepped down to continue his research and teaching activities. During his career, Ron also served as the Assistant Director for Applied Plasma Physics DOE-OFES (1976 to 1978). Among Ron's many accolades, notable ones include the 1986 DOE Distinguished Associate Award, the 1986 Fusion Power Associates Leadership Award, the 1993 Kaul Foundation Award for Excellence, the 2005 IEEE Particle Accelerator Science and Technology Award, the 2008 James Clerk Maxwell Prize in Plasma Physics, the 2009 DOE Science Outstanding Mentor Award, and the 2016 Fusion Power Associates Distinguished Career Award.
Ron worked in many areas, including basic plasma instabilities, Weibel instabilities, lower-hybrid-drift instabilities, two-stream instabilities, free-electron lasers, magnetic fusion energy, and ion-beam-driven inertial fusion energy. A consistent theme in Ron's research was to apply, extend, and develop the concepts and techniques of plasma theory to systems where its utility and power had not been fully appreciated. Examples include stable equilibria classification, nonlinear dynamics and stability theorems, instability growth and saturation, and inclusion of self-fields in the lowest order. He applied these ideas in areas such as Vlasov and averaged models, nonneutral plasmas, radiation generation from electron beams, intense charged particle beams and accelerators, and magnetic field geometry—uniform, toroidal, and helical.
Ron's first publication resulted from work he carried out while a graduate student at Princeton, “Higher-Order Corrections to the Chew-Goldberger-Low Theory” (1966),9 in collaboration with Edward Frieman and Bruce Langdon. Subsequent publications included many noteworthy papers in the field of nonneutral plasmas including “Vlasov Description of an Electron Gas in a Magnetic Field” (1969),10 “Vlasov Equilibria and Stability of an Electron Gas” (1970),11 “Electrostatic Shielding of a Test Charge in a Non-Neutral Plasma” (1971),12 “Self-Consistent Vlasov Description of Relativistic Electron Rings” (1972),13 “Nonlinear Development of Electromagnetic Instabilities in Anisotropic Plasmas” (1972),14 “Self-Consistent Theory of Cyclotron Maser Instability for Intense Hollow Electron Beams” (1978),15 “Influence of Untrapped Electrons on the Sideband Instability in a Helical Wiggler Free Electron Laser” (1987),16 “Nonlinear Stability Theorem for High-Intensity Charged Particle Beams” (1998),17 “Kinetic Description of High Intensity Beam Propagation Through a Periodic Focusing Field Based on the Nonlinear Vlasov-Maxwell Equations” (1998),18 and “A Paul Trap Configuration to Simulate Intense Non-Neutral Beam Propagation Over Large Distances Through a Periodic Focusing Quadrupole Magnetic Field” (2000).19 The last publication of Ron's is “Generalized Kapchinskij-Vladimirskij Distribution and Beam Matrix for Phase-Space Manipulations of High-Intensity Beams” (2016),20 jointly with Moses Chung and Hong Qin. Ron published approximately five hundred research papers in his long and prolific career.
Ron's books on nonneutral plasmas, nonlinear plasma physics, and high-intensity beam physics are well-known and well-regarded: “Methods in Nonlinear Plasma Theory” (1972),21 “Theory of Nonneutral Plasmas” (1974),1 “Physics of Nonneutral Plasmas” (1990),2 and “Physics of Intense Charged Particle Beams in High Energy Accelerators” (2001).22
Physicists around the world knew Ron as the founding Editor-in-Chief of Physics of Plasmas, a role he served for 25 years from 1989 to 2015. The leadership and vision for Physics of Plasmas, as exemplified by Ron, received high praise from the international plasma physics community. He often reminded us that “everybody has his/her own version of Landau damping,” which reflected his enthusiasm for new ideas in plasma physics and his optimism for the future of plasma physics.
People who worked with Ron respected his tireless work ethic. He was known for crossing off items from his “to-do” list as quickly as possible. Ron was a kind and supportive, yet firm, teacher, mentor, manager, and leader. We are grateful that, while preparing this preface, many members of the community communicated stories and anecdotes to us along these lines. Many fondly recalled classes taught and lectures given by Ron. Others shared the common experience of looking forward to receiving extensive, careful, and thoughtful comments and suggestions, hand-written in red ink, on drafts of manuscripts. Ron's dedication to education prompted him to write several books: “Mathematical Preparation for General Physics” (1972),23 “Mathematical Preparation for General Physics with Calculus” (1973),24 “Mathematical Review for the Physical Sciences” (1974),25 “Mathematical Methods for Introductory Physics with Calculus” (1994).26
This Special Topic issue follows a session at the 2016 58th Annual Meeting of the APS Division of Plasma Physics, “Non-neutral Plasmas, Fusion, and Beams: The Legacy of Ron Davidson” in San Jose, California. In this Special Topic, we draw attention to his work on collective effects in particle beams and nonneutral plasmas. Ron was one of the early contributors to, and shapers of, the field of nonneutral plasmas. Ron's efforts on plasma instabilities and beam dynamics while at the University of Maryland helped draw the attention and support of the Office of Naval Research to help launch, and then sustain, the field.
The following papers constitute this Special Topic issue:
“Current flow instability and nonlinear structures in dissipative two-fluid plasmas” by Koshkarov et al. discusses the physics leading to highly nonlinear quasi-coherent structures resembling breathing mode oscillations in Hall thrusters.
“Excitation of a global plasma mode by an intense electron beam in a dc discharge” by Sydorenko et al. discusses numerical results showing two regimes of the two-stream instability that is saturated by energy transfer to suprathermal electrons.
“Nonlinear structures and anomalous transport in partially magnetized E × B plasmas” by Janhunen et al. discusses the nonlinear dynamics of the electron-cyclotron instability.
“Weak turbulence theory for beam-plasma interaction” by Yoon discusses the various stages of the beam-plasma instability.
“Dynamic stabilization of filamentation instability” by Kawata et al. discusses stabilization of electron beams passing through a plasma.
“Control of the diocotron instability of a hollow electron beam with periodic dipole magnets” by Jo et al. discusses two-dimensional particle-in-cell simulations that show stabilization of the diocotron mode when the frequency of the dipole magnets is optimally chosen.
“Generalized Parametrization Methods for Centroid and Envelope Dynamics of Charged Particle Beams in Coupled Lattices” by Chung and Qin discusses an extension of the Qin-Davidson method.
“Generation of forerunner electron beam during interaction of ion beam pulse with plasma” by Hara et al. discusses long-time evolution of the two-stream instability of a cold ion beam propagating through a plasma.
“Low Magnetic Field Cooling of Lepton Plasmas via Cyclotron-Cavity Resonance” by Hunter et al. discusses methods for enhanced cooling rates that serve as useful experimental tools.
“Amplification due to Two-Stream Instability of Self-Electric and Magnetic Fields of an Ion Beam Propagating in Background Plasma” by Tokluoglu et al.