FEL vs. Optical Laser

Describes the similarities and differences between a Free Electron Laser and a "conventional" optical laser

Radiation from a Free Electron Laser (FEL) has much in common with radiation from a conventional optical laser, such as high power, narrow bandwidth and diffraction limited beam propagation. One of the main differences between the two lasers is the gain medium: In a conventional LASER (Light Amplification by Stimulated Emission of Radiation) the amplification comes from the stimulated emission of electrons bound to atoms, either in a crystal, liquid dye or a gas, whereas the amplification medium of the FEL are "free" (unbound) electrons. The free electrons have been stripped from atoms in an electron gun and are then accelerated to relativistic velocities.

While the electrons are propagating through a long, periodic magnetic dipole array - a so called undulator - the interaction with an electromagnetic radiation field leads to an exponential growth of the radiation emitted by the electrons. This amplification of radiation is initiated by an increasingly pronounced longitudinal density modulation of the electron bunch. The initial radiation field can be an external one, e.g. a seed laser, or an "internal" field, i.e. the spontaneous emission of the undulator. In the latter case it is called a SASE(Self Amplified Spontaneous Emission) FEL[9, 10]. Since the electrons in the FEL are not bound to atoms and thus not limited to specific transitions, the wavelength of the FEL is tunable over a wide range depending on accelerator energy and undulator parameters.

For IR, visible and UV FELs, light amplification can be reached in a multi-pass setup, i.e. by using an optical cavity with mirrors on both sides and the electrons passing the undulator as the gain medium in between. With such an arrangement, which - apart from its normally much larger size - exhibits a certain resemblance to optical laser setups, the light from many successive electron bunches is stored and amplified. For VUV and X-ray FELs, mirrors can no longer be applied due to their low reflectivities in normal incidence geometry at these wavelengths and potential mirror deformation/damage due to the high absorbed powers. Since a SASE FEL operates in the high-gain regime, it does not require an optical cavity and it can hence be used to deliver light in the VUV and X-ray regime. In such a "single pass" SASE FEL the full radiation power builds up from spontaneous emission when an electron beam with high phase space density passes a long undulator just once. While FELs in the visible and UV range can also be realized in synchrotron radiation storage rings, there is a consensus that - due to the higher demands on the electron beam properties - one needs a linear accelerator to generate FEL radiation in the VUV and X-ray range. The most promising approach is the setup of a single pass SASE FEL at a state of the art linear accelerator in combination with a high-performance radio frequency photo-cathode electron gun and longitudinal bunch compression to achieve the required peak current of several kA. Details on the parameters of such an electron gun, the linear accelerator, bunch compressors, etc. can be found in Part II of this report, describing the machine setup for the TESLA project.