The Status of the Energy Recovery Linac Source of Coherent Hard X-rays at Cornell University

Synchrotron radiation research as a field is still in a growing phase because newer and more capable X-ray sources are presently under development. Advances in storage ring technology, as explained in the introductory article to this special New Source issue by Kwang-Je Kim, is rapidly approaching the point of diminishing returns. It is expected that within a few years storage ring technology will have matured to the point where it will be become exceptionally difficult and expensive to make further significant improvements to the X-ray beam quality. In contrast, linac-based sources such as energy recovery linacs (ERLs) and X-ray free electron lasers (XFELs) are at an early stage of development, and provide a clear path for dramatically improving X-ray beam qualities that is likely to continue for many years. ERLs promise to generate bright electron beams, and thus X-rays, with dramatically smaller emittances and pulse durations than those available from storage rings, the present workhorse technology for all existing hard X-ray synchrotron radiation sources. As described in the companion X-ray Free Electron Laser (XFEL) article, new opportunities also exist in a complimentary direction, namely, that of making a lower duty factor X-ray laser that will also generate exciting new science. Though both ERL1 and the XFEL2 technologies have considerable promise as future X-ray sources, much development work and learning in both accelerator and X-ray technology will be required to realize their potential. Cornell University is designing a coherent hard X-ray light source based on an ERL upgrade to the CESR storage ring. Here, we review the principle of an ERL, the status of various other ERL synchrotron light projects, the scientific applications of high brightness, coherent, and short pulse X-rays, and some specifics of the Cornell project. The transverse emittance of the electron bunches used to generate X-rays determines the spectral brilliance and transverse coherence of the X-ray beams. Ideally, the emittance should be small enough to produce full transverse coherence of the X-rays—i.e. to produce a diffraction-limited X-ray beam. Very low emittance electron beams can be generated from a laser-driven photocathode, and accelerated to high (GeV) energies without substantial emittance growth in linear accelerators. High brightness, highly coherent X-ray beams can then be generated from these electrons. Since the electron beam carries several hundred MW of beam power, this is feasible only if the electron beam energy is recovered after the X-rays are generated. Using a superconducting (SC) linac, essentially all the electron energy may be recovered by passing the beam through the linac a second time, 180° out of phase with the accelerated beam, as shown in Figure 1; hence, the name Energy Recovery Linac. The ERL idea was suggested many years ago by Maury Tigner3, but only became practical for X-ray generation in the mid-1990s, following advances in superconducting linacs and photoemission electron sources4. Energy recovery (ER) was first demonstrated with a low average current pulsed electron beam at the superconducting linac at Stanford5. It was subsequently demonstrated with higher average current CW beam during injector development for the CEBAF accelerator. ER has also been demonstrated with room temperature linacs6, but this is