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.
Q ui ck Ti m e ™ an d a T I FF ( U nc om p r es se d) de co m pr e ss or ar e n ee de d t o se e t hi s p i ct ur e .
100TW at LCLS
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
Parameter
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!
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Components
What & Why
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
A MERCURY-like Pump
Pump Wavelength [Ti:S]
3.5 µs
Macrobunch Energy
500 J
2000
(1 in 5 RF buckets)
Beam Energy [LCLS]
250 MeV
Peak Current [LCLS]
500 A
Undulator Period
5 cm
Undulator Parameter
Undulator length
(Un-optimized; depends on seed)
FEL efficiency
Abstract
490 nm
Macrobunch Length [Ti:S]
Microbunches
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?)
Value
Optical energy per pulse
High Energy Laser Applications
≈ 20 m
5%
12.5 mJ
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Matched to Gain Medium:
Wavelength
Bandwidth
Time structure
Size
And:
Stable
Efficient
Low cost per watt
Not a lot of pumps to choose from…
Conclusions
A comparison of existing laser pump sources with the FEL based pump. The FEL is suited to
high energy and short wavelength applications.
Pump Source
Flashlamp
Diode
Laser
FEL
Avg. Energy
Very high
High
Low
High
Peak Energy
Medium
Low
High
Very High
Heat Load
High
Low
Low
Very Low
Wavelength
VIS
IR-VIS
IR-UV
IR-UV
Can an FEL do this?
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
So, yes, you can do it.
Why use an FEL for this?
Ideal Pump Source
High-field physics
Nuclear physics
Fusion sciences
Proton beam generation
Radiography
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
2.5
High average-power free-electron lasers may be useful for pumping high peak-power solid-state laseramplifiers. 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.
What is MERCURY?
References
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, acceleratorbased 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).
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
Parameter
Pump Wavelength [Yb:S-FAP]
905 nm
Macrobunch Length [Yb:S-FAP]
1.1 ms
Macrobunch Energy
20 k J
Microbunches
300,000
(1 in 10 RF
buckets)
Beam Energy
60 MeV
Peak Current
500 A
Undulator Period
2 cm
Undulator Parameter
Undulator length
(Un-optimized; depends on
seed)
FEL efficiency
Optical energy per pulse
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/
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Work supported by DOE BES grant DE-FG03-98ER45693
Work supported by ONR grant N00014-02-1-0911
Value
1
< 20 m
5%
3 mJ