Post on 22-Dec-2015
John T. CostelloNational Centre for Plasma Science & Technology (NCPST) and School of Physical Sciences, Dublin City Universitywww.physics.dcu.ie/~jtc & john.costello@dcu.ie
VUV Photoabsorption Imaging
QuAMP - Open University -September 8th 2003
Outline
1. ‘Centre for Laser Plasma Research’ /NCPST
2. VUV Photoabsorption/ionization Imaging Principle
3. ‘VPIF’ - VUV Photoabsorption Imaging Facility
4. Charge & State Selected Plasma Specie Images
5. Time Resolved Column Density Maps (Ba+)
6. Conclusions and Current/Proposed Applications
NCPST/CLPR Who are we ?What do we do ?
NCPST What is it ?1. NCPST established with Government/Benefactor funding
(Euro 8M) in 1999. Now EU Training Site.
2. Consortium of new and existing laboratories in plasma
physics, chemistry and engineering
3. Fundamental and Applied Scientific Goals
Staff: John T. Costello, Eugene T. Kennedy, Jean-Paul Mosnier andPaul van Kampen
PDs: John Hirsch , D Kilbane (post Xmas)PGs: Kevin Kavanangh & Adrian Murphy (JC),
Jonathan Mullen (PVK), Alan McKiernan & Mark Stapleton (JPM), Eoin O’Leary & Pat Yeates (ETK)
MCFs: Jaoine Burghexta (Navarra) and Nely Paravanova (Sofia)
Vacancies: PDRA-1: XUV FEL Experiments (ETK)PDRA-2: Pulsed Laser Deposition (JPM)PhD: Dual Laser Plasma Experiments (PVK/JC)
The CLPR node comprises 5 (soon to be 6) laboratories focussed on PLD & photoabsorption spectroscopy/ imaging
NCPST/ CLPR - What do we do ?DCUPico/Nanosecond Laser Plasma Light SourcesVUV, XUV & (X-ray) Photoabsorption SpectroscopyVUV Photoabsorpion ImagingVUV LIPS for Analytical PurposesICCD Imaging and Spectroscopy of PLD Plumes
Orsay/Berkeley SynchrotronsPhotoion and Photoelectron Spectroscopy
Hamburg - FELFemtosecond IR+XUV Facility Development
What’s a Laser-Plasma ?
How do you make a laser plasma ?
Target
Lens
Laser Pulse- 1 J/ 10 ns
Spot Size = 100 m (typ. Diam.)
> 1011 W.cm-2
Te = 100 eV (~106 K)
Ne = 1021 cm-3
Vexpansion 106 cm.s-1
Emitted -Atoms,Ions,
Electrons,Clusters,
IR - X-ray Radiation
PlasmaAssisted
Chemistry
Vacuum orBackground Gas
What does a Laser Plasma look like ?
PLASMAGENERATION
PLASMAEXPANSION
FILMGROWTH
Target
IncidentLaserbeam
Expanding PlasmaPlume
Substrate
Intense Laser Plasma Interaction
S Elizer, “The Interaction of High Power Lasers with Plasmas”, IOP Series in Plasma Physics (2002)
Part II - VUV Photoabsorption Imaging
John Hirsch et al, Rev.Sci. Instrum. 74, 2992 (2003)
POSTER P45 - Kevin Kavanagh
VUV Photoabsorption Imaging Principle
Pass a collimated VUV beam through the plasma sample and measure the spatial distribution of the absorption.
Io(x,y,,t)
Sample
I(x,y,,t)
VUVCCD
€
I =I0e−σ n(l )dl∫
John Hirsch et al, J.Appl.Phys. 88, 4953 (2000)
Laser Plasma VUV/XUV Continua
P K Carroll et al., Opt.Lett 2, 72 (1978)
E T Kennedy et al., Opt.Eng 33, 3894 (1994)
Motivations1. To add to the DCU Laboratory a new diagnostic to work alongside the existing spectroscopic systems
2. Pulsed Laser Deposition (PLD) and Dual Laser Plasma (DLP) photoabsortion expeiments require increasingly detailed knowledge of the spatio-temporal characteristics of plasma plumes
3. Lots of photoionization cross sections due (Aarhus/ALS)
Limitations of existing imaging methods1. Direct imaging of light emitted by a plasma using gated array detectors (e.g., ICCD) provides information on excited species only
2. Probing plasma plumes using tuneable lasers provides information on non-emitting species but is limited to wavelengths > 200 nm or so
Why a pulsed, tuneable and collimated beam ?
• Pulsed1. Automatic time resolution: the VUV pulse ~ laser pulse duration (~15 ns)
2. By varying the delay between the lasers the plasma can be probed at
different times after its creation
• Tuneable3. One can access all resonance lines of all atoms and moderately charged
ions with resonances between 30 nm and 100 nm
• Collimated4. Light path identical for all rays: can derive the eqn of radiative transfer
5. The detector can be located far away from the sample plasma, reducing
the ‘sample’ plasma signal on the detector, and improving SNR
1. VUV light can probe the higher (electron) density regimes not accessible in visible absorption experiments
2. The refraction of the VUV beam in a plasma is reduced compared to visible light with deviation angles scaling as
3. The images analysis is not complicated by interference patterns since the VUVcontiuum source has a small coherence length (ms)
4. VUV light can be used to photoionize atoms and ions - this process simplifies greatly the equation of radiative transfer (no bound states).
5. Fluorescence to electron emission branching ratio for many inner shell transitions can be 10-4 or even smaller, almost all photons are converted to electrons
Q. Anything Else ?A. Yes, it’s a VUV beam
VUV Photoabsorption Imaging Facility-‘V-P-I-F’
Monochromator
Grating
Exit slit
Entrance slit
FocussingToroidal Mirror
Plasma source
Collimating Toroidal Mirror
Sample Plasma CCD
VUV Bandpass Filter
The obligatory picture !!
Another one !
VUV Monochromator
Mirror Chambers
LPLS Chamber
Sample Plasma Chamber
VUV-CCD
VPIF - Design Considerations & Measured Characteristics
Parameter Focusing Toroid
Collimating Toroid
Entrance arm 400 mm 400 mm
Exit arm 400 mm -
Tangential radius 4590 mm 9180 mm
Sagittal radius 34.9 mm 63.5 mm
Incidence angle 85 degrees 85 degrees
Coating Gold Gold
Mirror size 60 20 mm 60 20 mm
Angle of acceptance 10 10 mrad 10 10 mrad
Final Design Parameters
VUV Photoabsorption Imaging Facility-Ray Tracing with ‘Light Path Simulation’
Computed point spread distributions at entrance slit for various apertures.
Ray Tracing with ‘Light Path Simulation’Beam Footprints
Computed and measured VUV beam footprints (A) 0.5m & (B) 1.0 m
NOTE LOW DIVERGENCE !!
Wavelength (nm)
Res
olut
ion
He, 1s - 2p line50m/50m slits
R>1000
Wavelength (nm)
Spectral Resolution at 54 nm
‘LPS’
Iint (
Arb
. Uni
ts)
Spatial Resolution (100m/100m slits & = 50 nm)
Horizontal Plane (120 m) Vertical Plane (150 m)
VPIF Specifications
Time resolution: ~20 ns (200 ps with new EKSPLA)
Inter-plasma delay range: 0 - 10 sec
Delay time jitter: ± 1ns
Monochromator: Acton™ VM510 (f/12, f=1.0 m)
VUV photon energy range: 10 - 35 eV
VUV bandwidth: 0.025 eV @25 eV (50m/50m slits)
~0.05 nm @ 50 nm
Detector: Andor™ BN-CCD,
1024 x 2048/13 m x 13 m pixels
Spatial resolution: ~120 m (H) x 150 m (V)
VUV Photoabsorption Imaging Principle
Pass a collimated VUV beam through the plasma sample and measure the spatial distribution of the absorption.
Io(x,y,,t)
Sample
I(x,y,,t)
VUVCCD
€
I =I0e−σ n(l )dl∫
What do we extract from I and Io images ?
€
A=log10(I0(x,y,t,λ)dλ∫I (x,y,t,λ)dλ∫ )Absorbance:
€
WE = [1−e−σ (λ)NL]∫
€
WE =Δλ[I0 −I ]dλ∫I 0dλ∫
⎛
⎝ ⎜
⎞
⎠ ⎟
EquivalentWidth:
d
WνIν(0)Io
Equivalent Width (nm)
W
1 - exp[-NL] = 1 -I/Io = 1 -T
Some Preliminary Results:
Tune system to 3 unique resonances
Ca: 3p64s2 (1S) - 3p54s23d (1P)
Ca+: 3p64s (2S) - 3p54s23d (2P)
Ca2+: 3p6 (1S) - 3p53d (1P)
Time resolved W maps of Ca plume species
VUV Absorption Spectra of Ca Plasma Plumes
Maps of equivalent width of atomic calcium using the 3p-3d resonance at 39.48 nm (31.4 eV)
Maps of equivalent width of singly ionized calcium using the 3p-3d resonance at 37.34 nm (33.2 eV)
Maps of equivalent width of doubly ionized calcium using the 3p-3d resonance at 35.73 nm (34.7 eV)
Plume Expansion Profile of Singly Charged Calcium Ions
Ca+ plasma plume velocityexperiment: 1.1 x 106 cms-1
simulation: 9 x 105 cms-1
Ba+ plasma plume velocityexperiment: 5.7 x 105 cms-1
simulation: 5.4 x 105 cms-1
Delay (ns)
Plu
me
CO
G P
ositi
on (
cm)
Extracting maps of column density,e.g.,Barium
We measure resonant photoionization, e.g., Ba+(5p66s 2S)+h Ba+*(5p56s6d 2P) Ba2+ (5p6 1S)+e-
h = 26.54 eV (46.7 nm)
ANDThe ABSOLUTE VUV photoionization cross-section for Ba+ has been measured,Lyon et al., J.Phys.B 19, 4137 (1986)
Ergo ! We should be able to extract maps of column density -
'NL' = ∫n(l)dl
Maps of equivalent width of singly ionized Barium using the 5p-6d resonance at 46.7 nm
dl
Convert from WE to NLCompute WE for a range of NL and fit a function f(NL) to a plot of NL .vs. WE
Apply pixel by pixel
€
WE = [1−e−σ (λ)NL]∫ d
Result - Column Density [NL] Maps
(A) 100 ns (B) 150 ns(C) 200 ns(D) 300 ns(E) 400 ns(F) 500 ns
VPIF - Provides pulsed, collimated and tuneable VUV beamfor probing dynamic and static samples
Spectral, spatial, divergence etc. all in excellent agreement with ray tracing
Recorded time and space resolved maps of equivalent width of Ca and Ba plasma species
Extracted time and space resolved maps of column density for various time delays
Measured plume velocity profiles which compare quite well with simple simulations based on self similar expansion
Summary
Space Resolved Thin Film VUV Transmission and Reflectance Spectroscopy - PVK
‘Colliding-Plasma’ Plume Imaging
Combining ICCD Imaging/Spectroscopy & PI Photoion Spectroscopy of Ion Beams ?
Non-Resonant Photoionization Imaging
Lots of new measurements from Aarhus & ALS
Current & Future Applications
Collaborators - VPIF
DCUJohn Hirsch
Kevin KavanaghEugene Kennedy
Univ. PaduaGiorgio Nicolosi
Luca Poletto
Collaborators - Proof of Principle @ RAL
DCUJohn Hirsch et al
QUBCiaran LewisAndy McPheeR O’Rourke
RALGraeme Hirst
Waseem Shaikh
Ideally we would like a VUV/ XUV source
with lots of photons to do these experiments !!
And there is one in Germany !(and coming to the UK and US)
X-VUV FELs + Femtosecond OPAs- The Ultimate Photoionization Setup ?
Tuneable: NOW! 80 - 110 nm (20 - 60 nm in 2004)Ultrafast: 100 fs pulse durationHigh PRF: 1 - 10 bunch trains/sec with up to 11315pulses/bunchEnergy: Up to 1 mJ/bunchIntense: 100 J (single pulse) /100 fs /1 m => 1017 W.cm-2
•Moving to XUV (2005) and X-ray (2010):•Need a Linac + insertion devices => Fraction of a GigaEuro !!
Project Title:‘Pump-Probe’ with DESY-VUV-FEL (EU-RTD)Aim: FEL + OPA synchronisation with sub ps jitter URL: http://tesla.desy.de/new_pages/TDR_CD/start.htmlPersonnel: MBI, DESY, CLPR-DCU, LURE, LLC, BESSY
Femtosecond X-VUV + IR Pump-Probe Facility,Hasylab, DESY
DESY, MBI, LURE, BESSY, LLC & NCPST-DCU