Page 107 - High Power Laser Handbook

CHAPTER 4

High-Power

Free-Electron

Lasers

George R. Neil
Associate Director, Thomas Jefferson National Accelerator
Facility, Newport News, Virginia

4.1 Introduction
The development of high-average-power free-electron lasers (FELs)
has been underway for more than 30 years. And yet it has only been
in the recent era that significant progress to high power has been
achieved. This progress has been primarily due to the technical
status of the available driver accelerator technology, especially the
crucial electron injector, though other components have also played
a limiting role. This chapter reviews the physics of FELs, as well as
the technical approaches to high-power FELs, and discusses some of
the applications of this technology.
4.2 FEL Physics

4.2.1 Physical Mechanism
Lasing of an FEL can be understood to result from the interaction of
electromagnetic fields on a relativistic electron beam. In the simplest
arrangement, a relativistic electron bunch is sent through a sinusoidal
magnetic field produced by alternating magnets in a device called a
wiggler. This causes the electrons to oscillate transversely. From the
perspective of the electrons the wavelength of the wiggler (also called
an undulator) is shortened by a Lorentz contraction of (1 + b)g, where
b is v/c, or the electrons’ velocity along the axis divided by the speed
of light, and g is 1 plus the ratio of the electron’s kinetic energy to its

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