Executive Summary
The intent of this White Paper is to outline a focused research and development plan in accelerator science and technologies that is essential for the realization of future light sources with capabilities significantly beyond existing, planned, and under-construction facilities. This R&D plan is necessarily driven by a careful consideration of major scientific challenges and opportunities, and is guided by a quantification of light source characteristics required to meet those scientific challenges. We outline a vision of an ambitious soft x-ray free-electron laser (FEL) that pushes the performance envelope in ways that complement the unique scientific capabilities of the Linac Coherent Light Source (LCLS). An intensive 5-year R&D program is described to provide the technological advances necessary for both next-generation FELs as well as related future light sources such as energy recovery linacs (ERLs). The estimated cost of this R&D program is $32M.
Scientific Challenges.
The underlying theme of the grand challenges now being posed for physics, chemistry, and materials science is to understand, predict, and ultimately control the properties of matter. The “emergent” properties of complex systems are of particular interest. Here, correlated interactions among charge carriers, and between charge carriers and constituent atoms, give rise to new properties and functionality with tremendous potential for practical applications. These same correlated interactions also challenge our understanding of complex systems in that they defy conventional paradigms based on the Born-Oppenheimer approximation, single-electron band structure models, Fermi liquid theory, etc. These problems cry out for tools that are sharper than those currently available. To meet these challenges we must answer fundamental questions about the coupling between the correlated motion of electrons and the motion of atoms. The intrinsic time scales of those motions, differing as they do by three orders of magnitude, require both femtosecond and attosecond time resolutions. The need to directly probe electronic structure and dynamics demand a focus on the VUV and soft x-ray regions, and the creation of experimental facilities that complement those being constructed with hard x-ray capabilities.
A Path to a New Class of Light Source.
A machine that is responsive to the scientific needs expressed above would be a seeded, second-generation, FEL (free electron laser)-based light source. We envision a 1–2 GeV superconducting linac feeding an array of ten FELs each independently operating at a repetition rate of 100 kHz. This would therefore be a true user facility. The wavelength range would be 200–1 nm (photon-energy range 25–1200 eV). A very attractive feature of the proposed machine is that it can simultaneously support complementary beamlines offering: (1) short x-ray pulses (20–100 fs); (2) high energy resolution with longer pulse length (500-1000 fs); (3) attosecond x-ray pulses (0.1 fs).
Critical R&D Needs.
Unlike third-generation SR facilities, next-generation light sources will need extensive R&D to be conducted to define the final optimum configuration. A comprehensive five-year R&D program is proposed to address the challenges of implementing a high-repetition-rate seeded-FEL facility, while also advancing capabilities and technologies for other facility concepts such as ERLs (energy recovery linacs) with common needs. By developing advanced technologies of high-repetition-rate (of order MHz), low-emittance electron injectors; CW superconducting-RF linacs; and optical manipulations, the second generation of FEL facility will be able to open up new areas of research, complementing the FEL facilities currently being built or planned, as well as enhancing the technology base for other accelerator-based light sources.
Injector
For high-repetition-rate FEL applications, the optimum injector-technology choice cannot presently be made based on available experimental and theoretical work. Some of the fundamental issues to be resolved can be addressed by working with the leading groups in each of the relevant fields. In addition, several core areas of R&D need to be established where the goal is to explore the limits of accelerator technology in areas of high potential. In order to establish the optimum solution, ultimately we need to explore systems with their full operational characteristics. In order to make substantial progress, we need to test complete injector systems capable of operating under realistic conditions with high rep rates, high bunch charges, and with an energy high enough to make precise beam-quality evaluations.
Electron beam manipulation.
Electron beam manipulation of increasing sophistication can be used in the future, from emittance exchange for improvement of transverse emittance to optical manipulation to enhance peak current or to produce attosecond photon pulses. While some of these techniques can be experimentally tested on existing accelerators, what is required is a program to develop and validate the methodologies in a dedicated development accelerator designed for the purpose.
Superconducting linacs.
While much is being done already in superconducting-linac design, the major outstanding issues are to do with achieving operation that is reliable enough for a major user facility while at the same time pressing the accelerating gradient to higher values in order to obtain a compact and economic solution.
Laser system engineering.
Although very recent developments in laser materials and techniques have now given us amplifiers with the pulse energy needed for operation at high repetition rate, the sophistication of the applications in temporal and spatial distributions, in synchronization, and in harmonic generation needs significant development. In addition, these research systems need to be developed to a level where they operate with a robustness commensurate with a user facility. What is needed is an R&D program based on a state-of-the-art laser with a high repetition rate and a high pulse energy for development of the techniques discussed above.
Simulation and validation of FEL design and diagnostics.
X-ray FEL performance is extremely sensitive to the details of the electron beam’s phase-space distribution, and design using state-of-the–art simulation tools is required for the next generation of x-ray FELs. Multi-physics computer modeling tools need to be developed to allow the full start-to-end simulations required when designing nm-wavelength FELs, and diagnostics need to be developed to allow realistic comparisons between measurements and simulations. A vigorous experimental program is required in order to validate the detailed physics of seeding, and this will require substantial collaboration with the FERMI @Elettra and BESSY FEL projects.
