Development of High Current Thermionic Injector for an Energy Recovery Linac
Free-electron lasers (FELs) generate electromagnetic radiation through the interaction of a high quality, high energy electron beam with a periodic wiggler magnetic field. Typically, an rf linear accelerator (rf linac) provides the electron beam to drive the interaction, and for high average power operation, the overall device efficiency becomes increasingly important, since the efficiency of the FEL interaction itself may be rather low. Energy recovery linacs (ERLs) are designed to recover most of the remaining energy of the electron beam by decelerating the electrons to generate some of the rf that will be used to accelerate later bunches of electrons. The electron injector is a critical component of an ERL system, since it must deliver the average beam current required by the linac, while at the same time satisfying a variety of constraints on the quality of the electron beam. Most FEL systems use injectors based on laser-excited photocathodes to produce the required high quality electron beams, but they have difficulty operating at high average current. For that reason, thermionic injectors may be more suitable for some high current applications, provided that they can generate beams of sufficiently high quality. This lecture will present an overview of ERL FEL systems, and discuss some of the basic principles of rf acceleration and thermionic electron gun design. Following this, it will focus on a program to develop a high-current electron injector based on a gridded thermionic electron gun.
Study of Multipactor in Dielectric-Loaded Accelerating Structures
Multipactor is an electron avalanche phenomenon that occurs in many systems that combine rf electric fields, metal or dielectric surfaces, and vacuum conditions. The electron avalanche extracts energy from the rf fields, and can lead to localized heating or rf breakdown. For these reasons, it must be controlled or suppressed in practical systems, including both rf sources and rf accelerating structures. In the case of rf windows, multipactor is a non-resonant single-surface phenomenon that typically saturates at low levels, and can generally be suppressed by the use of surface coatings with low secondary emission characteristics; while in the case of cavities with conductive walls, multipactor is a resonant phenomenon that involves electrons transit between two surfaces in synchronism with the rf fields. Two-surface multipactor can generally be avoided by careful design of rf structures. However, in the case of dielectric-lined metal accelerating structures, a new and particularly harmful type of multipactor occurs that involves resonant motion of electrons emitted from and returning to the same dielectric surface. This lecture will discuss the various regimes of multipactor, and then focus on the problem of suppressing multipactor in dielectric-loaded accelerating structures.
Steven H. Gold retired in 2016 from his position as Senior Scientist for Radiation Generation Physics in the Beam Physics Branch of the Plasma Physics Division at the U.S. Naval Research Laboratory (NRL), where he had worked since receiving his Ph.D. in physics from the University of Maryland in 1978, and is currently working as a consultant in the areas of plasma science and accelerator physics. In recent years, his research has concentrated on i) high power microwave source development, ii) applications of high power microwaves to accelerators and industrial processes, including advanced accelerator research, iii) free-electron lasers. From 1992 through 2014, he was the principal investigator on a series of research projects funded by the Office of High Energy Physics of the US Department of Energy under its advanced accelerator R&D program. Under this program, he developed a 25 MW X-band magnicon amplifier, and used it as part of a facility to study externally driven dielectric-loaded accelerating structures, focusing on multipactor and breakdown phenomena, and also active microwave pulse compressors. In recent years, he also served as co-PI and lead experimentalist for a program to develop a new injector for the Navy high average power infrared free-electron laser program based on a gridded thermionic electron gun. Dr. Gold has published 70 articles in refereed journals, 115 papers in conference proceedings, and 47 technical reports He also holds 6 US patents and 2 Statutory Invention Registrations, and has presented 24 invited papers at professional conferences and workshops.
Dr. Gold has been involved with the activities of the Executive Committee of the NPSS Plasma Science and Applications Committee since 1990, including three 3-year terms as an elected member, two years as Vice-Chair, and the past sixteen years as Secretary. He was an Associate Editor of the IEEE Trans. Plasma Sci. for twenty years (1988–2007), with responsibility for its biennial special issues on High Power Microwave Generation. He has served on the program committee of the IEEE International Conference on Plasma Science (ICOPS) on a number of occasions, as Technical Program Chair of ICOPS 2011 in Chicago, and as Treasurer of ICOPS/BEAMS 2014 in Washington, DC. He is currently serving his third 4-year terms on the NPSS Administrative Committee (AdCom), and chairs its Chapters and Local Activities Committee. He is a Fellow of the IEEE (1996) and the American Physical Society (1998), and served for four years on the IEEE Fellow Committee (2008–2011). In recognition of his activities, he received the 2008 NPSS Richard F. Shea Distinguished Member Award “for outstanding contributions to the IEEE Nuclear and Plasma Sciences Society and its Plasma Science and Applications Technical Committee.”