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 TiS Match the FEL wavelength to the
absorption peak of TiS
Goal Produce a high peak power laser using the
LCLS front end
What is MERCURY?
Parameter Value
Pump Wavelength TiS 490 nm
Macrobunch Length TiS 3.5 µs
Macrobunch Energy 500 J
Microbunches 2000(1 in 5 RF buckets)
Beam Energy LCLS 250 MeV
Peak Current LCLS 500 A
Undulator Period 5 cm
Undulator Parameter 2.5
Undulator length(Un-optimized depends on seed) 20 m
FEL efficiency 5
Optical energy per pulse 12.5 mJ

• 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!
• YbS-FAP disks are the final amplifier
• Hypersonic gas cooling of crystals

• Consider a high power FEL that pumps a TiS
  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 gt 10J at lt 100fs (gt 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

• gt10J or gt100TW 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 YbS-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

Parameter Value
Pump Wavelength YbS-FAP 905 nm
Macrobunch Length YbS-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 1
Undulator length(Un-optimized depends on seed) lt 20 m
FEL efficiency 5
Optical energy per pulse 3 mJ
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 materials 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
• Thats on the order of the FEL
• 6000 diodes cost about 3M
• Diodes only work for one arrangement
• Diodes have 108 shot lifetime.
• Thats 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
References
Conclusions
A comparison of existing laser pump sources with
the FEL based pump. The FEL is suited to high
energy and short wavelength applications.
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).
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).
Pump Source Pump Source Pump Source 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
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