U.S. Particle Accelerator School

High Brightness Electron Injectors for FEL Light Sources course

Sponsoring University:

Massachusetts Institute of Technology (MIT)


High Brightness Electron Injectors for FEL Light Sources


David H. Dowell, (Ret)

Purpose and Audience
This course is an introduction to the fundamental physics of electron sources and the technology of high-brightness injectors for FEL-based light sources. The course is suitable for students with an undergraduate or higher degree in physics, electrical engineering or a related field. It is also appropriate for practitioners of accelerator physics wishing to increase their understanding of the fundamentals of modern electron injectors.

The student should have knowledge of undergraduate and beginning graduate level classical mechanics, electromagnetism and solid state physics courses plus a basic understanding of accelerator physics. Basic knowledge of calculus and matrix algebra is also required. Some problems will require the use of a computer for computing and plotting derived formulae.

It is the responsibility of the student to ensure that he or she meets the course prerequisites or has equivalent experience.

This course will concentrate on the fundamentals of electron sources and injectors for free electron laser applications. The basic concepts will be explained with an emphasis on an analytic description of injectors from the cathode through the low energy injector. Computer codes available for numerical simulations will be reviewed however they will not be used during this course. Upon completion of this course the student should understand the underlying physics of modern electron sources and injectors, how these concepts are implemented and how basic beam measurement techniques are performed. The student will also have an understanding of the current limits of beam brightness and the research necessary for future advances.

Instructional Method
The course consists of three 1 hour morning lectures and two 1 hour afternoon sessions for the first four days of the one week course. Homework will be assigned and explained during the last afternoon session. There will be a final exam the last day of the class.

Course Content
The development of the modern RF photo-injector is one of the principal enabling technologies for 4th generation light sources and other free electron lasers. This course begins with a brief history and motivation for research of electron injectors used to drive free electron lasers. The basic elements of the electron injector are defined. The fundamentals of electron emission for thermionic, photo-electric and field emission cathodes are described in detail. The thermal (cathode) emittance is derived and examples given for the three emission processes. The theory of space charge limited emission and the effects of image charge forces on electron dynamics very close to the cathode surface is presented along with experimental examples. The dynamics of acceleration from the cathode is described by comparing DC and RF acceleration in conjunction with longitudinal and transverse focusing dynamics, and the unequal partitioning of the longitudinal and transverse emittances or temperatures of the rapidly accelerated beam. The concept of slice or time-dependent emittance is introduced for bunched beams and the fundamental principle of emittance compensation is presented first for the RF gun and then developed as a more general principle to control emittance growth during RF compression and for beam transport such as ERL beam merger systems. System design and the available simulation codes will be described. The diagnostics used to measure the beam properties are also discussed. The course concludes with a discussion of how these ideas are implemented into current state-of-the-art photocathode and thermionic injectors, and presents areas of future research to advance the brightness of electron beams for the next generation of injectors.

Reading Requirements
(to be provided by the USPAS) “Theory and Design of Charged Particle Beams” 2nd edition by Martin Reiser, (1994) John Wiley & Sons, Inc. Additional lecture notes and other reading material will be provided in electronic format.

Credit Requirements
Students will be evaluated based on performance: final exam (60%) and homework (40%).

IU/USPAS course number P671