John T. Costello

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