Free-Electron Lasers as Pumps for High-Energy Solid-State Lasers

1. Free-Electron Lasers as Pumps for High-Energy Solid-State Lasers • G. Travish1, J. K. Crane2 and A. Tremaine2 • (1) UCLA Dept. of Physics & Astronomy, Los Angeles, CA 90095. USA. (2) Lawrence Livermore National Laboratory, Livermore, CA 94551. USA. 100TW at LCLS A MERCURY-like Pump The Concept Consider a MERCURY-class pump that can deliver 1 KJ of 905 nm light in 1.1 ms: Use a high average power FEL to pump a conventional laser Match the macrobunch length to the florescence lifetime of Ti:S Match the FEL wavelength to the absorption peak of Ti:S Goal: Produce a high peak power laser using the LCLS front end What is MERCURY? • State of the art diode pumped solid state laser (DPSSL) • Designed as a scalable direct drive fusion laser • Goals: 100J, 10% efficiency, 10Hz • Pulse length is 5ns, but compressible • System uses over 6000 diodes producing 60kW peak! • Yb:S-FAP disks are the final amplifier • Hypersonic gas cooling of crystals • Consider a high power FEL that pumps a Ti:S amplifier: • Use front end of LCLS • Assume a multibunch photoinjector (i.e. TTF or AFEL) • Compress beam in BC-1 • Send all but head and tail bunches to long tapered undulator • Produce 25J or 490nm pump light over 3µs • Obtain > 10J at < 100fs (> 100 TW) of 800 nm light • Can do this at 120 Hz! • Challenges: • Prove high efficiency for visible FEL • Beam loading compensation (more linac sections?) • Syncrhonization of light to x-ray due to BC-2, etc (use head pulse to measure phase error?) • High energy seed laser (Multiple diode pumped YLF? OPA?) Components What & Why Can an FEL do this? • High brightness injector • High average power accelerator • Compressor • Seed laser • Long tapered undulator • Conventional laser amplifier • >10J or >100TW laser hard to make • Pumps only available for some wavelengths • Large diode array only good for 1 laser • Can use existing FEL facility • Can synchronize big laser to beam & FEL • New materials, new power formats 100J is a lot, but Yb:S-FAB has a 1.1 ms florescence time! • Superconducting linac is selected to take advantage of the long fluorescence-time. • Assume a TTF based linac • Need 3x105 bunches of 1 nC each • Filling 1 in 10 RF buckets • Run at about 60 MeV • FEL-wavelength is long: • RF thermionic-gun with a compression alpha-magnet may work • Though a long undulator (≈20 m) • A 5%-efficient FEL. • Each pulse is 3 mJ of optical energy • Yielding 1 KJ of optical power Abstract High average-power free-electron lasers may be useful for pumping high peak-power solid-state laser-amplifiers. At very high peak-powers, the pump source for solid-state lasers is non-trivial: flash lamps produce thermal problems and are unsuitable for materials with short florescence times, while diodes can be expensive and are only available at select wavelengths. FELs can provide pulse trains of light tuned to a laser material’s absorption peak, and florescence lifetime. An FEL pump can thus minimize thermal effects and potentially allow for new laser materials to be used. This paper examines the design of a high average-power, efficient high-gain FEL for use as pump source. Specifically, the case of a 100 J class pump for a 100 TW class laser is considered. FEL design goals, laser-material selection-guidelines, and specific examples are discussed. The modification and use of planned fourth-generation light-source infrastructure to also act as high-energy pumps is considered. So, yes, you can do it. Why use an FEL for this? High Energy Laser Applications Ideal Pump Source • Problem with diodes: • 100J class laser costs about $10M • That’s on the order of the FEL • 6000+ diodes cost about $3M • Diodes only work for one arrangement • Diodes have 108 shot lifetime. • That’s 1 year at 10Hz • Advantages of FEL • Can pump many different lasers • Can run at much more than 10Hz • Optically superior — easier to couple to crystal • Matched to Gain Medium: • Wavelength • Bandwidth • Time structure • Size • And: • Stable • Efficient • Low cost per watt • High-field physics • Nuclear physics • Fusion sciences • Proton beam generation • Radiography Not a lot of pumps to choose from… Conclusions References A comparison of existing laser pump sources with the FEL based pump. The FEL is suited to high energy and short wavelength applications. The use of an FEL as a pump for a solid-state lasers may find application in existing facilities as well as purpose built machines. A high energy, high-efficiency FEL has yet to be demonstrated experimentally, but appears achievable. Ultimately, the practicality of such a system may be an economic decision as diodes become more affordable. However, the flexibility of the FEL to pump at multiple wavelengths and to act as a useful source in its own right may prevail over a simple cost-analysis. Work remains to find materials better suited to the FEL based pump-source. Optimization of the FEL design as well as a realistic accelerator design also remain to be done. Finally, accelerator-based alternatives to FEL pumping need to be considered such as direct electron-beam excitation of a gain material, optical pumping of laser diodes, and FEL assisted mixing using an optical parametric amplifier (OPA). [1] T. Tajima and J. M. Dawson, Phy. Rev. Lett. 43 267 (1979). [2] M. D. Perry and G. Mourou, Science 264, 917 (1994). [3] T.E. Cowan, et. al., Laser and Particle Beams 17, 773 (1999). [4] M. H. Key, Nature 412, 775 (2001). [5] Y. Sentoku, T. E. Cowan, A. Kemp, and H. Ruhl, Phys. Of Plasmas 10, 2009, (2003). [6] M.D. Perry, et. al., Rev. Sci. Instr. 70, 265-269 part 2, (1999). [7] J. A. Paisner et. al, SPIE Proceedings Series 2633, p. 2, Bellingham, WA (1995). [8] W. Koechner, Solid-State Laser Engineering, Springer (1999), pp312. [9] J. T. Weir, et al., Proc. SPIE 1133, pp.97-101 (1989). [10] A. J. Bayramian, et. al., Proc. Adv. Solid State Photonics 83, 268 (2003). [11] V. Ayvazyan et al., Phys. Ref. Lett. 88 (2002) 104802. [12] J. Lewellen et al., Proc 1998 Linac Conf., ANL-98/28, 863-865 (1999). [13] Linac Coherent Light Source (LCLS), SLAC-R-521, UC-414 (1998). [14] J. Als-Nielsen, Proc. Workshop on 4th Gen. Light Sources, ESRF Report, Grenoble (1996). [15] I. B. Vasserman, et al., Proc. Part. Accel. Conf. (1999). Acknowledgments The authors thank James Rosenzweig, Sven Reiche, Nick Barov, Alex Murokh and Bill Krupke for useful discussions. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. http://pbpl.physics.ucla.edu/ Work supported by DOE BES grant DE-FG03-98ER45693 Work supported by ONR grant N00014-02-1-0911