The Road to a Fully Coherent X-ray Free-Electron Laser – from RAFEL to XFELO

Thursday, August 1, 2019 - 3:00pm to 4:00pm

Speaker:  Dinh Nguyen, LANL

Program Description:

With the construction of the LCLS-II superconducting accelerators at SLAC, a fully coherent X-ray free-electron laser oscillator (XFELO) delivering transform-limited X-ray pulses with unprecedented brilliance is becoming a real possibility in the near future [1]. The requirements for an XFELO, however, are rather stringent: low-loss, high-quality Bragg crystals with high reflectivity over a very narrow range of angle forming a very long optical cavity with micron dimension accuracy and nanoradian angular stability; and high-energy electron bunches with relatively long bunch length and high charge delivered at MHz bunch repetition rate. Numerical simulations predict the X-ray pulses inside an XFELO optical cavity will need to recirculate a few hundred passes before they reach steady-state and achieve narrow linewidth [2]. An intermediate step on the path to XFELO is the Regenerative Amplifier FEL (RAFEL) which requires an optical feedback cavity with lower reflectivity and less stringent angular stability compared to the XFELO.  The RAFEL has been demonstrated in the infrared by myself and my colleagues [3] and analyzed by Zhirong Huang et al. [4] and Gabriel Marcus et al. [5] as a means to generate fully coherent X-rays on the LCLS. The RAFEL can have very high single-pass gain, thereby reducing the number of passes to reach saturation and spectral narrowing. In this talk, I will cover a possible research program that involves: 1) a near-term RAFEL demonstration at 10-keV X-ray energy using two to three electron bunches from the LCLS copper linac; 2) a transition from RAFEL to XFELO at 4.3 keV using the same optical cavity but with higher optical feedback and longer separation between the two electron bunches from the copper linac; and 3) the ultimate demonstration of the 4.3-keV XFELO driven by the LCLS-II superconducting linac.  I will also discuss an exciting possibility of using the RAFEL-generated third harmonic to seed a fully coherent 30-keV hard X-ray FEL.  Finally, I will briefly mention the proposed collaborative research program between SLAC and ANL to build and test a cavity-based X-ray FEL, to fabricate high-quality diamond drumheads as hard X-ray mirrors, and to develop a user program employing the fully coherent, narrow-linewidth X-ray FEL.

[1]  K.-J. Kim, Y. V. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100 244802 (2008)

[2]  R. Lindberg, 2017 IPAM Beam Dynamics Workshop, UCLA, January 23-27, 2017.

[3]  D. C. Nguyen et al., Nucl. Instr. Meth. Phys. Res. A429 125 (1999)

[4]  Z. Huang and R. D. Ruth, Phys. Rev. Lett. 96 144801 (2006)

[5]  G. Marcus et al., Proceedings of the FEL2017 Conference, paper MOP061 (2017)



The Road to a Fully Coherent X-ray Free-Electron Laser – from RAFEL to XFELO
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