About this Research Topic

Background

Linear accelerator-based free-electron lasers (FELs) provide fully coherent hard X-rays with ultra-high peak powers and femtosecond-duration pulse lengths. The X-ray peak brightness is approximately one billion times greater than that is available at synchrotron radiation facilities. This enables high-resolution analysis of the atomic structure of crystalline matter, opening up new frontiers across many fields of science and motivating a vast amount of interdisciplinary applications.

Self-amplified spontaneous emission (SASE) is the most common operational mechanism for FELs. While proving excellent performance, the SASE FEL is rapidly approaching towards its next-generation for delivering intense SASE radiation at a MHz-level repetition-rate in a continuous-wave (CW) mode. The MHz-level CW FEL can significantly extend the capability of existing FELs by allowing for more flexibility in the photon beam time patterns and an increased average brightness.

The pulse repetition rate is determined by the acceleration technology, and it is much higher in superconducting facilities. Confronting challenges in developing MHz-level CW X-ray FELs, several world-class FEL machines are put into practice, the Linac Coherent Light Source II in the USA, the Shanghai high repetition rate XFEL and extreme light facility in China, and the European X-ray Free-Electron Laser in Germany.

Realization of MHz-level CW X-ray FELs calls for global efforts in physical design and technical development. This does not only consider necessary modifications of relevant sub-systems in an existing FEL facility, but also start-to-end beam physics simulation of the developed accelerator and photon beam lines. R&D programs are demanded in many aspects including electron sources, superconducting RF (SRF) accelerator cryomodules, cryogenics, cavities and RF power sources. A suitable electron source should demonstrate stable performance at a MHz-level repetition rate and deliver high-quality beams; Producing accelerating cavities with high Q values and high gradients remains its crucial role and motivates studies via nitrogen doping and infusion; Operating SRF modules in the CW mode poses a stringent requirement on bringing the cryogenic load and the capacity of the cryogenics systems in line with each other; Development of CW RF power sources and couplers should be conducted for enabling the overall RF system.

The goal of this research topic is to summarize the obtained achievements and report on the development status of MHz-level CW X-ray FELs covering the aforementioned aspects. It is worth emphasizing that another focus of the call lies on the photon beam line development and the proposals for potential user experiments using the CW FELs.

This Research Topic aims to provide a state-of-the-art summary of the efforts being spent on the global developments towards MHz-level CW X-ray FELs. It is devoted to discussing theoretical and experimental progresses on the main theme. Authors are welcomed to contribute submissions of multiple article types offered by Frontiers including Original Research and Reviews but not limited to:

- Electron Sources
- Beam Dynamics & Diagnostics
- Cryomodules & Cryogenics
- Superconducting RF & Structures
- RF Power Sources & Couplers
- Low Level RF
- FEL Concepts & Technologies
- Photon Beamline Instrumentation
- User Experiments

Keywords: free-electron laser, continuous-wave, future light source, instrumentation, accelerator physics, photon science, large-scale facilities for science, interdisciplinary competencies, innovative applications, technical and physical design

Important note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.

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