A future X-ray source - FEL oscillator with ERL and optical cavity

Recently, Professor K.-J. Kim (Argonne National Lab., USA) and his colleagues published a very interesting proposal for the world's brightest X-ray source. In most currently on-going X-ray free electron laser (FEL) projects, self-amplified spontaneous emission (SASE) is employed. It is known that SASE-FEL creates extremely brilliant, coherent X-ray pulses of 0.1 ps duration. Due to the low repetition rate, the average brightness is only about 10,000 times compared with existing 3^rd generation synchrotron sources. On the other hand, future X-ray sciences will require other types of X-ray laser source, with an even smaller number of photons in one pulse (to reduce radiation damage to the sample) and with much greater average intensity via a high repetition rate. In Professor Kim's X-ray source based on a FEL oscillator (X-FELO), a pulse of electrons is carried into an undulator as ordinary FEL, but in order to reflect back the generated X-rays into the undulator entrance, there is an optical cavity consisting of two or more Bragg reflectors with low-Z atoms and with low Debye temperature, such as diamond, beryllium oxide and sapphire crystals. In the next step, the X-ray photons connect with the next electron bunch and again travel back along the undulator. This pattern is repeated indefinitely with the X-ray intensity growing each time until equilibrium is reached. As the spectral bandwidth is extremely narrow, at three to four orders of magnitude finer than those produced by SASE-FEL, the intensity of an individual X-ray pulse from an X-FELO is rather low. But the average X-ray intensity is higher than that of SASE-FEL. Over the past 5 years, highly advanced electron beam technologies, which can be used, for example, for a multi-GeV class energy recovery linac (ERL), have become available. One of the key elements of Professor Kim's idea is combination with ERL. This is predicted to produce X-ray pulses with 10⁹ photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps. To gain an understanding of the original concept of X-FELO, see the paper, "Proposal for a free electron laser in the X-ray region", R. Colella and A. Luccio, Optical Commun., 50, 41-44 (1984). For more information on the proposed X-ray source, see the paper, "A Proposal for an X-Ray Free-Electron Laser Oscillator with an Energy-Recovery Linac", K.-J. Kim et al., Phys. Rev. Lett., 100, 244802 (2008).