demonstration of high repetition rate soft x ......demonstration of high repetition rate soft x-ray...
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DEMONSTRATION OF HIGH REPETITION RATE SOFT X-RAY LASERS AT WAVELENGTHS NEAR 30 NM IN NE-LIKE IONS
David AlessiColorado State UniversityElectrical and Computer Engineering Dept.December 8, 2006
Masters Thesis:
Outline• Introduction
– Extreme Ultraviolet and Soft X-Ray Light– Comparison of Soft X-Ray Laser Sources– Collisional Lasers– Ne-Like vs. Ni-Like– Previous Work of Collisional Lasers near 30 nm
• Experimental Configuration: – Grazing Incidence Pumping Geometry– Ti:Sapphire Pump Laser
• Simulation of the 32.6 nm Ne-Like Ti Laser• Experimental Method• Results
– On Axis Spectra– 5Hz Operation– Laser Gain Measurement– Pump Beam Grazing Incidence Angle– Optimization of Pre-Pulse Sequence– Dependence on Time Delay– Pump Laser Pulsewidth– Spatial Coherence Measurement
• Summary
Extreme ultraviolet (EUV) radiation and soft x-ray (SXR) light
Span ~0.1nm to ~100 nm• Relatively unexploited region of the spectrum
– Commercial sources not yet available• Can be used for a wide range of applications
– Short wavelength allows: nanoscale imaging and metrology, nanopatterning, dense plasma diagnostics
– High photon energy allows: atomic/molecular spectroscopy
Figure from: D. T. Attwood, Soft X-Rays and Extreme Ultraviolet Radiation: Principles and Applications(Cambridge University Press, Cambridge, England, 1999)
Comparison of Soft X-Ray Sources• Coherent SXR radiation from 3rd gen
synchrotron– Large facilities– High repetition rate– Wavelength tunable– Low energy per pulse– Requires GeV electrons
• Laser pumped collisional SXR lasers– Table-top size– 10 Hz repetition rate– Not wavelength tunable– High energy per pulse– High peak spectral brightness
• Higher Harmonic Generation– Table-top size– High repetition rate– Wavelength tunable– Low energy per pulse– Low temporal coherence
Advanced Light Source (LBL)
Table-top Soft X-Ray Laser (CSU)
Figure from Wikipedia
Collisional LasersCollisional Lasers
Singly ionized Ar ion, Kr ion lasers in the visible spectral region
Highly ionized (8-25 times) in the EUV/SXR spectral region
Plasma requirements:Te ~ 5 eVNe ~ 1x1014 cm-3
Laser created plasma
Discharge created plasma
2ZEh ∝∆=ν
Ar+
Ar
35eV
514 nm laser
e
e
Ar+
Ar
35eV
514 nm laser
e
e
Cd+20
>5000 eV
13.2 nm laser
e
Ionize 20 times
Ne x Te increases by 108-109
Te ~ 300 - 1000 eVNe ~ 2x10 20 cm-3
Ne-Like vs. Ni-Like
• Collisional lasing has been demonstrated in both Ne-like and Ni-like ions
• Ni-like ions are more efficient
• Ne-like ions provide access to lasers near 30 nm
• Solar corona studies
– HeII and FeXI-XVI lines
– EUV normal incidence spectrometer (NASA SOLAR-B mission)
• Wavelength specific spectroscopy
• Future applications that may require it
Why 30 nm ?
Previous Work of 30 nm Collisional Lasers
• 1990 – Ne-like Ti – First demonstration of 32.6 nm Ne-like Ti laser– Using GDL laser at LLE (Rochester)
• Single 200 J laser pulse (∆t = 650 ps)– Then confirmed at LLNL with NOVA laser
• Single 550 J laser pulse (∆t = 600 ps)
• 1993 – Ne-like Ti and Cr – First demonstration of 28.6 nm Ne-like Cr laser– Done with NOVA laser
• 6 J pre-pulse (∆t = 600 ps)• 1100 J pump pulse (∆t = 600 ps)
• 1995 – Ne-like V – First demonstration of 30.4 nm Ne-like V laser– Done with Asterix IV laser at Max Planck Institute
• 430 J (pre-pulse and main pulse energy) (∆t = 450ps)
NOVA Laser
Previous Work of 30 nm Collisional Lasers
• 1997 – Ne-like Ti– Transient excitation of collisional SXR laser– In quasi-steady state, upper level population is distributed by collisions– Transient uses preferential excitation of upper level on short time scale– Resulted in much higher gain than quasi-steady-state– At Max Born Institute
• 4-6 J pre-pulse (∆t = 1.5 ns)• 1.5 J pump pulse (∆t = 0.7 ps)
• 1997 – Ne-like Ti– Saturation of the 32.6 nm line – Table-top COMET laser at LLNL
• 6 J pre-pulse (∆t = 800 ps)• 6 J pump pulse (∆t = 1 ps)• 1 shot every 3 min
• 2005 (this work) – Ne-like Ti, V, Cr– First demonstration of these lasers with grazing incidence pumping– Saturation of 32.6 nm (Ti) line and 30.4 nm (V) line – Table-top CPA Ti:Sapphire laser at CSU
• 350 mJ pre-pulse (120 ps)• 1 J pump pulse (1 ps)• 5Hz repetition rate
[D. Alessi et al. Opt. Express 13, 2093 (2005)]
Grazing Incidence Pumping
Grazing Incidence Pumping
NCritical
AbsorptionRegion
PumpPulse
Pre-Pulse
Ne GainRegion• Normal incidence pre-pulse
– 350 mJ, 120 ps• Plasma expansion
– Few hundred ps• Pump pulse at grazing incidence wrt
target– 1J, 8 ps
• Selects density of plasma to deposit laser energy
c
o
nn
=θsin
( )mcme
emn o
c µλωε
2
321
2
2 /1011.1 ×≈=
no = density at perigree
θ
Ti:Sapphire Pump Laser
• Mode locked oscillator – 5 nJ/pulse (∆t = 50 fs)
• Stretcher• 8 pass 1st stage
– 2 mJ/pulse (∆t = 120 ps)• 5 pass 2nd stage
– 150 mJ/pulse• 3 pass 3rd stage
– 2 J/pulse (uncompressed)• 1200 l/mm grating compressor
350 mJ, 120 ps
1 J, 8 ps FigureFrom Y. Wang
Simulation of 32.6 nm Ne-like Ti laserPre-pulse: 300 mJ 120 ps time delay = 0Pump pulse: 1 J 8 ps time delay = 400 ps
1.5D Hydrodynamic/Atomic model with radiation transport code and post processor ray tracing developed by Mark Berrill, Colorado State University
Simulation of 32.6 nm Ne-like Ti laser
go = 86 cm-1
G×L = 19.2E(4mm) = 5 µJ
3p1S0→3s1P1 transition
Simulated Energy vs. Plasma Length
Experimental Method• Pre-pulse
– 350 mJ for Ti– 500 mJ for V and Cr– 30 µm × 4.1 mm FWHM line
focus– 120 ps FWHM pulsewidth
• Pump Pulse– 1 J for Ti– 0.9 J for V and Cr– 30 µm × 4.1 mm FWHM line
focus– 8 ps FWHM pulsewidth
• Grazing incidence grating spectrometer
– Hitachi VLS grating• Laser attenuation
– Wire meshes– Al filter
On Axis SpectraSpectra of 4 mm long plasmas showing:
1. Lasing in Ne-like 3p1S0→3s1P1 : 32.6 nm (Ti), 30.4 nm (V), 28.6 nm (Cr)
2. Lasing in Ne-like 3d1P1→3p1P1 : 30.1 nm (Ti)
Ne-like Ti Energy Levels
3p 1So
3d 1P1
2p 1So
3s 1P1
3p 1P1
3s
3d
30.1 nm
32.6 nm
Ti+12 ground state
2.33 nm
Figure similar to that by Nilsen et al. Phys. Rev. A 55, 3271 (1997)
5Hz Operation
• Sequential laser shot intensities• Obtained by moving target at constant speed• STD = 17% of mean• Average laser energy = 530 nJ• Average Power = 2.6 µW
32.6 nm Ne-like Ti
Laser Gain Measurement32.6 nm Ne-like Ti30.4 nm Ne-like V
go = 72 cm-1
G×L = 21.7El,max = 590 nJ
go = 57 cm-1
G×L = 18.4El,max = 790 nJ
( )( )( ) ( )( ) 2/10
2/30
01
LG
LG
oavLeG
eII −=
( ) LgIILG o
s
av =+ 20 Solid line is fit of experimental data to expression of ASE laser taking into accountthe saturation. [Tallents et al. AIP Conf. Proc. 641, 291 (2002)]
Pump Beam Grazing Incidence Angle
17º → ne = 1.5×1020 cm-3
20º → ne = 2.0×1020 cm-3
23º → ne = 2.6×1020 cm-3
The density of the laserHeated region is controlledBy the angle
Optimization of Pre-Pulse Sequence
• Add an additional pre-pulse arriving 5 ns before main pre-pulse
• Diverting 5% into this pulse can increase the output laser intensity by a factor of 5
~5x
32.6 nm Ne-like Ti laser
30.1 nm vs. 32.6 nm Lasers in Ne-like Ti
Spectra at 420 ps delay
Spectra at 620 ps delay
Delay between the main pre-pulse and the pump pulse
Dependence on Time Delay
Ne-like V
32.6 nm Ne-Like Ti laser Dependence on Pump Laser Pulsewidth
2 ps 4 ps
6 ps 9 ps
•Longer pulse durationreduces transient effect
•Shorter pulse durationreduces ionization
Spatial Coherence Measurement
Measurement made in collaboration with Y. Liu, University of California Berkeley
Young’s Double Slit Experiment• 4mm × 5 µm slits• 30 µm, 50 µm, 75 µm separation• placed where the beam is 500
µm in diameter• Measure fringe visibility for each
slit separation
Spatial Coherence Measurement
Modulation with 30 µm slit separation CCD image
72.016.1116.01
minmax
minmax =+−
=+−
=IIIIV
Imin
Visibility:
Modulation with 50 µm slit separation CCD image
64.022.1122.01
minmax
minmax =+−
=+−
=IIIIV
Imin
Spatial Coherence Measurement
Modulation with 75 µm slit separation CCD image
Spatial Coherence Measurement
26.059.1159.01
minmax
minmax =+−
=+−
=IIIIV
Imin
Fit to gaussian source• Rc = 47 µm
Recall beam size• Rb = 250 µm
Laser is only moderately coherent
Spatial Coherence Measurement
The spatial coherence can be improved significantly by seeding the amplifier with a high harmonic generated pulse. [Y. Wang PRL 97, 123901 (2006)]
Summary
• Ne-like soft x-ray lasers near 30 nm were demonstrated with grazing incidence pumping geometry using a 1 J table-top pump laser– 32.6 nm Ne-like Ti laser
• Saturated operation demonstrated (GxL = 18)• 2.6 µW average power while operating at 5Hz repetition rate• 790 nJ best shot energy• Strong lasing over a wide range of parameters• Moderate spatial coherence• Strong lasing also demonstrated at 30.4 nm in Ne-like V, 30.1 nm in
Ne-like Ti, and 28.6 nm in Ne-like Cr
Acknowledgments
• Advisor– Dr. Jorge Rocca
• Committee Members– Dr. Carmen Menoni– Dr. Chiao-Yao She (physics)
• This work would not be possible without: Yong Wang, Miguel Larotonda, Brad Luther, Mark Berrill, Mario Marconi, Ann Dummer, Dinesh Patel
• Fellow ERC students who have helped me at one point: Jorge Filevich, Mike Grisham, Fernando Brizuela, Scott Heinbuch, Brendan Reagan, Georgiy Vaschenko, Dale Martz, Federico Furch, Jonathan Grava, and Courtney Brewer
• National Science Foundation
Questions