U.S. Particle Accelerator School

RF Linac for High-Gain FEL course

Sponsoring University:

University of New Mexico


RF Linac for High-Gain FEL


Dinh Nguyen, John Lewellen and Leanne Duffy, Los Alamos National Lab

Purpose and Audience
This course is an introduction to the physics and technology of high-brightness electron beam generation, acceleration and bunch compression in a linear accelerator (linac) driving a high-gain free electron laser (FEL). Starting from the photocathode, we will cover the fundamental mechanisms of electron beam production using a photoinjector (aka laser-gated electron gun), acceleration in RF linac cavities, electron bunch compression, as well as beam conditioning processes to generate the high-peak-current, low-emittance electron beams for modern FELs and the 4th Generation Light Sources. The specific RF linac designs to be covered are pulsed normal-conducting copper for low-duty-factor FELs and CW superconducting niobium cavities for high-duty-factor FELs. This course is geared toward students and scientists who are interested in the practical aspects of high-brightness relativistic electron beam generation.

Upper division undergraduate courses in classical mechanics and in electromagnetism (at the level of "Introduction to Electrodynamics" by David J. Griffiths).

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

The goals of this course are 1) to present the basic working principles of three major components, namely electron injector, linac, and bunch compressor, of a modern RF linac driving a high-gain X-ray FEL, 2) to elucidate single-particle and collective-effect beam dynamics that may significantly impact the FEL performance, and 3) to guide the students in the design of an RF linac that can generate the high-current, low-emittance electron beams.

Instructional Method
The course consists of lecture sessions in the morning (3 hrs. per session), and afternoon computer simulation sessions (2 hrs. per session). Particle tracking codes such as PARMELA and Elegant will be used to model a representative RF linac driving a high-gain X-ray FEL. The afternoon sessions can also be extended to discuss homework assignments.

Course Content
    Introduction: Introductory beam physics (transverse and longitudinal phase space, emittance, Twiss parameters, space charge, radiation); electron beam requirements for FEL (energy, current, emittance and energy spread).

    Photoinjectors: Physics of photoelectron emission in both semiconductor and metal photocathodes, various photoinjector designs (DC guns, full-wave RF guns, quarter-wave RF guns), emittance growth, emittance compensation, envelope equation and beam matching.

    RF Linac: Fundamentals of RF acceleration, introduction to RF resonant structures (pillbox, re-entrant, elliptical, half-wave, quarter-wave), basics of RF superconductivity, wakefield, and beam breakup instabilities.

    Bunch compressors: Ballistic compression, velocity compression, magnetic compression (e.g., chicane), incoherent and coherent synchrotron radiation, longitudinal space charge and microbunching instabilities, laser heater.

Reading Requirements
(to be provided by USPAS) “RF Linear Accelerators” second, completely revised and enlarged edition (Wiley 2008), by Thomas P. Wangler . Instructors will provide lecture notes.

Credit Requirements
Students will be evaluated based on performance as follows: final exam (40% of final grade), homework assignments (30%) and computer class (30%).

IU/USPAS course number P671