Abstract
Free electrons are to be distinguished from electrons which are bound in atoms or molecules. The electrons in a Free Electron Laser (FEL) have the form of an electron beam in a vacuum, for example, like as the beam in Braun tube of TV. Electrons bound in atoms and molecules vibrate only at specific frequencies. Thus, the laser light from conventional lasers, which make use of bound electrons, appears only at these specific frequencies. On the other hand, the electrons in FEL are forced to vibrate by their passage through an alternating magnetic field. Thus, the vibration frequency can be adjusted by altering the construction of the magnetic field or by changing the speed of the electrons passing through the magnetic field. This changes the laser frequency or, equivalently, the wavelength. The broad tunability of FEL from the far infrared to the visible and beyond was the origin of the great interest in these lasers. More recently it has been recognized that free electron lasers have unique advantages for operation at high average power levels, and this has made them attractive for medical and industrial applications.
