UV/X-ray free electron lasers through high-gain single pass amplifiers: basic principles and key issues

We review the basic principles of the high gain free electron laser amplifier for generation of intense, tunable radiation at wavelengths shorter than 1000إ. Two schemes are discussed: For wavelengths between 1000 and 100إ, high gain harmonic generation and amplification of a coherent input radiation can be used. For X-ray wavelengths as short as a fewإ, self-amplified spontaneous emission is the only known free electron laser scheme. We also present a brief introduction to key issues in realizing these schemes, which will be discussed in detail in other papers in these proceedings.