Machine Design
Addressing the scientific Grand Challenges of the future will require new tools for photon science, complementing the existing and planned synchrotron radiation facilities. To address these needs, we suggest a new user facility equipped with an array of task-designated FELs, wherein each FEL may be configured to operate in a different mode, independently of the other FELs. Each FEL will have independent control of wavelength and polarization, and optical manipulations of the electron beam will be used to produce seeded x-ray pulses with control of pulse duration, offering flexibility and versatility to many experiments simultaneously.
High repetition rate and high average photon flux are essential to many experimental techniques. To meet these needs, we envision a facility comprised of a high bunch repetition rate (~MHz), low-emittance and low energy spread RF photocathode electron gun, and a low-energy (~2 GeV) superconducting linac, feeding an array of approximately ten FELs through an elaborate beam switchyard. Each FEL operates independently at a repetition rate of ~100 kHz. Photon energies would span approximately 10 eV to 1 keV, with the possibility to reach higher photon energy at the expense of reduced photon flux. A variety of seeded and SASE FELs provide the above described output radiation with a peak power from a few hundred megawatts to a few gigawatts. The temporal coherence available in the seeded FEL allows close to transform-limited x-ray pulses, resulting in a narrow bandwidth signal that could possibly be utilized in experiments without a monochromator. Techniques have also been developed to use optical manipulations of the electron bunch to produce x-ray pulses of a few hundred attosecond duration.
Figure (upper left) shows a schematic of a multi-user FEL facility concept. The major components are: (1) a low-emittance, low energy spread RF photocathode electron gun operating in CW mode at ~60 MHz, providing electron bunches at up to MHz repetition rate, (2) hardware for manipulating the electron-beam emittance in preparation for the FEL process, (3) a CW superconducting RF linac, (4) a beam-switching system, (5) multiple independent FELs and beamlines, and (6) lasers for the photocathode gun, FEL seeding, pump-probe experiments, and timing and synchronization. A low-energy linac is used to minimize costs. The electron beam is dumped at the end of each FEL as we do not currently believe the added cost and complexity of electron beam recirculation and energy recovery is worthwhile for a machine of modest electron beam power.
