TUOCI01
ANL, Argonne, USA
The characteristics of X-ray free electron laser oscillators (XFELOs) [K.-J. Kim, et. al., Phys. Rev. Lett. 100, 244802 (2008)]–high-stability, high-spectral purity, and high-average brightness–will greatly enrich the era of x-ray FELs begun recently. If combined with a high-gain amplifier possibly with harmonic generation, an XFELO would constitute an ultimate x-ray machine. The drive accelerator can either be a straight SCRF linac or an ERL in energy gain mode. Sub-Angstrom stabilization of the output x-ray pulse spacing may be feasible by locking one of the longitudinal modes to a nuclear resonance such as 57Fe, opening up new opportunities for extreme x-ray metrology, with potential applications to technology and fundamental physics [B. Adams and K.-J. Kim, FEL2012]. A wide range of parameters is possible [R.R. Lindberg, FEL2012], optimized for particular accelerator or applications, such as ultra-low bunch charge for ERL [1], for short cavity length for nuclear resonance metrology, etc. The prospects for an XFELO are encouraging: Diamond as a Bragg reflector has been demonstrated for near 99% reflectivity [2] and high thermal diffusivity at cryogenic temperature [3].
[1] R. Hajima and N. Nishimori, Proc. of the FEL2009; R.R. Lindberg, FEL2012
[2] Y. Shvyd'ko, et al, Nat. Photonics 5, 539 (2011)
[3] S. Stoupin and Y. Shvyd'ko, Phys. Rev. Lett. 104, 085901(2010)
ANL, Argonne, USA
Fluctuations in relativistic electron beams for free-electron lasers (FELs) exhibit both collective and individual particle aspects, similar to that seen in non-relativistic plasmas. We show that the density fluctuations are described by a linear combination of the collective plasma oscillation and the random individual motion of Debye-screened dressed particles. The relative importance of the individual to the collective motion is determined by comparing the fluctuation length scale divided by two pi with the relativistic beam Debye length. Taking into account the fact that the velocity spread is caused by both the energy spread and the angular divergence, we derive a simple formula for the minimum value of the Debye length using a solvable 1-D model. For electron beams used for x-ray self-amplified spontaneous emission (SASE) we find that the Debye length is comparable to the radiation wavelength, and that therefore the collective motion is not relevant.
ANL, Argonne, USA
A scheme is described to length-stabilize the cavity of an x-ray free-electron-laser oscillator (XFELO)* by keeping one of its longitudinal modes in nuclear resonance at 14.4 keV with a sample of 57Fe. The mode spacing corresponding a 100 m XFELO cavity is about 12 neV, which can be resolved with the 5 neV linewidth of 57Fe, even with some inhomogeneous line broadening. With a cavity thus stabilized, a standing-wave pattern can be maintained over hours, to be probed by another sample of 57Fe in a meter-long scan to compare the nuclear-resonant wavelength with a known optical standard. This should improve the relative accuracy of this wavelength from 10-7** to 10-11. Ensemble, or long-time averaging, as used in atomic clocks, can further increase the accuracy. Refining the scheme to other nuclear-resonant species with narrower resonances, such as 181Ta (6.2 keV, 75 peV), will open up precision x-ray metrology for technological and fundamental applications.
* K.-J. Kim, Yu. Shvyd'ko, S. Reiche, Phys. Rev. Lett. 100, 244802
(2008);K.-J. Kim, Yu. Shvyd'ko, Phys. Rev. ST-AB, 030703 (2009)
** M. Lucht, dissertation, Hamburg (2005)