Transcript Future Light Sources 3-12

Optics for VUV and soft x-ray FEL Oscillators
Michelle Shinn & Steve Benson
Future Light Sources
Jefferson Lab
March 5, 2012
Work supported by the U.S. Dept. of Energy under contract DE-AC05-06OR23177 and the
Commonwealth of Virginia
Introduction
• Past DOE Basic Energy Science (DOE-BES) workshops have stressed
the desire to have a soft x-ray FEL operating at 1keV (1.24nm) .
• While great progress is being made to achieve this energy, it remains
an elusive goal.
• This presentation proposes a path forward to achieve this energy
– Use an FEL oscillator’s harmonics to provide the harmonics, and
modulation to seed a radiator undulator.
• Must consider the challenging environment for the cavity optics.
The flat HR optical resonator
• Last year we published a design* for an FEL oscillator operating in the
VUV (12-150nm).
• In the process of designing the optical resonator, we came up with an
entirely new architecture dubbed, “flat HR”
• This high gain, very low Q resonator makes use of the low divergence
beam to avoid diffraction at the wiggler.
• Strong optical guiding permits this architecture, even though the cold
cavity is unstable.
• Such a cavity architecture should also work at higher energies, where
the 3rd or 5th harmonic output can be used as a seed for a radiator,
producing coherent output at ~ 1keV.
– Goal is to produce fundamental output in the GW level, so 3rd &
5th harmonic’s powers are of order 1MW.
• To keep the resonator length reasonable, rep rate will be in the range
2.5 – 5 MHz.
* Benson et al JMO 58 p1431 (2011)
Flat HR FELO schematic and transverse profiles @ 12.4nm
Wiggler and e beam parameters
• Cold cavity parameters calculated using Paraxia-Plus (Sciopt Inc)
• 3D FEL simulations were done using Genesis/OPC
Wiggler period (cm)
2.5
Number of periods
240
Wiggler gap (cm)
0.7
Krms
0.600
Emittance (microns)
2
Energy spread (%)
0.15
Peak current (kA)
1.44
Cavity length (m)
32.04196
Mirror radii (cm)
1.27
High reflector mirror radius of curvature (m)
flat
Output coupler mirror radius of curvature (m)
23.5
Hole radius (cm)
0.03
Mirror reflectivity (%)
70
Mirror microroughness (nm rms)
≤0.5
Nominal pulse bandwidth (FWHM)
0.1%
VUV FEL output
• Very high gain g = 6 @ 12.4nm, g=21 @ 20nm
– Saturated gains > 1, imply intracavity power much lower than output.
9
10
8
10
Intracavity Power (Watts)
7
10
20 nm
12.4 nm
6
10
5
10
4
10
3
10
2
10
1
10
0
10
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Number of passes
Advantages
• Relative to a two curved-mirror resonator, the flat HR resonator
– Does not have evidence of mode-hopping or shift in position in an
attempt to avoid the hole.
– Is less sensitive to mirror steering (e.g. vibration).
– To changes in mirror figure
• It’s easier to maintain flatness of the HR.
• Loading on OC is minimized due to the majority of power
transported through the hole.
VUV FELO @ 200eV
• Goal is to seed downstream radiator with 5th harmonic
– Must not induce too much energy spread on the exhaust e beam.
Wiggler period (cm)
4.0
• 2GeV, 100 pC e beam
Number of periods
150
• Minimum rep rate of 4.68 MHz Wiggler gap (cm)
1.4
• Genesis/OPC single slice
K
1.928
rms
*G. Neil private communication
Based on discussions with FOM-IPP staff
Emittance (microns)
0.7
Energy spread (%)
0.05
Peak current (kA)
2.0
Cavity length (m)
32.04196
Mirror radii (cm)
1.27
High reflector mirror radius of curvature (m)
flat
Output coupler mirror radius of curvature (m)
23.5
Hole radius (cm)
0.05
Mirror reflectivity (%)
65*
Mirror microroughness (nm rms)
≤0.5
Nominal pulse bandwidth (FWHM)
0.1%
Output characteristics at 200eV
• Lasing efficiency = 0.225%  P = 2.1 kW (!) E = 0.45mJ
Far-field output
net gain ~ 100% sat gain ~ 13
Wiggler exit
HR mirror
Wiggler input
SXFELO @ 340eV
• Goal is to seed downstream radiator with 3rd harmonic
– Relative to 200eV case:
Wiggler period (cm)
• Same e beam parameters
Number of periods
• Wiggler gap larger
Wiggler gap (cm)
• Outcoupling hole ½ as large K
Emittance (microns)
• Reflectivity lower
Energy spread (%)
• Genesis/OPC single slice
rms
* C. Montcalm et al Appl. Opt. 35 (1996)
4.0
150
1.87
1.928
0.7
0.05
Peak current (kA)
2.0
Cavity length (m)
32.04196
Mirror radii (cm)
1.27
High reflector mirror radius of curvature (m)
flat
Output coupler mirror radius of curvature (m)
23.5
Hole radius (cm)
0.025
Mirror reflectivity (%)
55*
Mirror microroughness (nm rms)
≤0.5
Nominal pulse bandwidth (FWHM)
0.1%
Output characteristics at 340eV
• Lasing efficiency = 0.076%  P = 714W E = 0.15mJ
Far-field output
net gain ~ 30% sat gain ~ 10
Wiggler exit
HR mirror
Wiggler input
Optics considerations
• Scattering
– While scattering does not distort optics, it competes with the reflectivity, so it
must be considered.
• Hole quality
– Requires ion milling to achieve desired shape and smoothness.
• Laser damage
– A survey of the literature shows that laser-induced damage is dominantly
thermal in nature.
– Damage most likely to occur at the HR mirror.
– Fluence estimated to be 480mJ/cm2 @ 200eV and 160mJ/cm2 @ 340eV
– For comparison, Ethr = 45mJ/cm2 @ 92eV* & many 100s/mJ/cm2 @ 830eV**
– Suggests we need to consider a longer resonator.
– Cryogenically cool the mirrors – known to raise damage threshold.
* A.R. Khorsand et al Opt. Ex. 18 (2010)
** S. P. Hau-Riege et al Opt. Ex. 18 (2010)
Conclusions
• We’ve presented conceptual designs for FEL oscillators at 200eV and
340eV.
– Based on the flat HR architecture.
• Lasing induces modulation at harmonic’s frequencies and the
harmonics seed a radiator to produce output in the soft x-ray region (~
1keV)
• Mirror reflectivity's at higher energies continue falling, and become
too low for energies above ~ 500eV.
• Appears to be a viable alternative to laser-seeded amplifiers in the 0.1
– 2keV energy range.
• Laser damage must be managed –
– Resonator length will do this, of order 60m.
• Future plans
– Optimize oscillator e.g., OC ROC, hole size, etc.
– Use 4D Genesis/OPC and Medusa/OPC to predict performance of
complete system  oscillator + radiator
Acknowledgements
• Anne Watson (JLab/NC State) & Peter van der Slot
– Genesis/OPC software development and discussions.
• Gwyn Williams – font of information on VUV (and beyond) optical
properties.
• George Neil – for goading us to think beyond near-concentric
resonators.