PowerPointPrsentation

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Electron sheath dynamics and structure in
intense laser driven ion acceleration   S.
Ter-Avetisyan, M. Schnürer, T. Sokollik, P.V.
Nickles, W. Sandner MBI, Max-Born-Straße 2a,
D-12489 Berlin, Germany MBI - TiSa HFL-Laser
M.Kalashnikov, E.Risse J. Stein DLR,Pfaffenwaldri
ng 38, D-70569 Stuttgart, Germany D. Habs LMU
München, Am Coulombwall 1, D-85748, Garching,
Germany T. Nakamura, K. Mima ILE, 2-6 Yamadaoka,
Suita, Osaka 565-0871, Japan Supported by
German Research Foundation DFG Research Programm
Transregio 18
ULIS 2007, Bordeaux, October 01 05, 2007
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Outline

• Problem and motivation
• Experimental techniques
• Pointing of proton beams and the hypothesis of
  source movement
• Source characteristics in dependence on laser
  absorption processes
• -
  consequences for the electron sheath and ion
  front
• Theoretical description and simulation
• - time
  dependent source evolution
• Summary of observed phenomena and future
  prospects

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Problem and motivation
How do specifics in electron beam transport act
back on sheath
formation and laser driven ion beams in TNSA ?
motivated by systematic observation of trace
deviations in recorded ion spectra with Thomson
Spectrometer
Important for broad energy band ion beams (cf. M.
Amin Time-resolved proton probing
Knowledge about
electron and sheath dynamics
over large temporal windows
and spatial extensions
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Experimental techniques Imaging Thomson
spectrometer
Laser TiSa 40 fs , 0.7 J on target, 1 .
2 x 1019 W/cm2 Targets Al foil, mylar foil
(3-20 micron)
target pinhole 4.7cm pinhole MCP
70cm magnification 15
Target
MCP
ions
30µm pinhole
Laser p-pol. at 45
Spectral Dimension
Spatial dimension
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Setup Thomson spectrometer with high spatial
resolution Combination of spectrometer and
pinhole camera
The inversion problem of this Thomson
spectrometer Beam 3 unknown parameters
(xsource,ysource,E) Detection parabola 2
measured coordinates xtrace,ytrace
detector-plane
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Pointing of laser-accelerated proton beams
B-field deflection
Calculation of source position in dependence on
emitted proton energy
E-field deflection
15-fold spatial magnification

• Beam pointing changes due to
• ion source movement ?
• Have to proof
• Consequence of electron beam and
• lateral electron transport processes
• need additional diagnostic
• indirect conclusion
• proton beam shows
• extreme low longitudinal emittance
• emission ordered in time chirped

of protons /MeV/Wph/30µm
target y-coordinate µm
J. Schreiber et al., Physics of Plasmas 13,
033111 (2006)
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Experimental techniques Cerenkov Diagnostic
Cerenkov and Ion diagnostic have been used in the
same experimental run
J. Stein, PhD http//edoc.ub.uni-muenchen.de/arc
hive/00004588/ J. Stein et al., Laser Particle
Beams 22, 315 (2004)
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Source characteristics in dependence on laser
absorption processes
Steering laser pulse parameter
absorption process electron beam
formation electron sheath
dynamics ion source front
evolution
This prediction is compared to simulation for our
laser parameter
temporal laser contrast (10-7 10-8)
intermediate low high
electron acceleration. resonant vs.ponderomotive
laser
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2D PIC simulation of the experiment confirms
electron beam and ion source dependences
High contrast Ponderomotive acceleration
dominates Ion front structured
Low contrast Resonantly driven acceleration
dominates Ion front smooth
S. Ter-Avetisyan et al. submitted
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Correlation of electron beam distribution forming
the sheath and ion emission characteristic
Cerenkov diagnostic
pinhole ion spectrometer Al foil, different
thickness (M15x)
15 µm
6 µm
3 µm
2 electron beams cause source modification and
are probably the reason for observed ion source
splitting
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Verification of the energy dependent pointing of
the proton beam with a double pinhole setup
sheath 1 highest energies are emitted along axis
perpendicular to target
surface sheath 2 position shifts due to changes
in electron source beam
one pinhole is passed by different angle
- trace is shifted / splitted
- two pinholes block different angles
magnet
proton energy MeV
laser p-pol. at 45
pinhole 2
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Time dependent source evolution
1 start localized small electron sheath
most energetic ions generated 2 20 fs later
sheath grows and shifts more ions but less
energetic appear from shifted position 3
40 fs later sheath splits, field further
reduced ions appear separated
Experiment Ion emission - Thomson spectrometer
1
2
3
2
3
Simulation
1
0.5
1
2
3
proton energy MeV
T. Nakamura et al., submitted
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Proton imaging indicates field distribution
features which may be related to ion front dips
as visible in simulation of ion energy density
2 D deflection pictures of a laser driven foil
proton beam (2 2.5) MeV
Laser
cold target
(0 150) ps
(200 150) ps
(400 150) ps
3 mm
proton number low
high
deflection dip
further details cf. talk M. Amin Time-resolved
proton probing
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Examples of strong electron sheath perturbation
enhanced number of electron beam filaments has
been found in Cerenkov pictures with old ATLAS 10
at MPQ (160 fs , 1019 W/cm2) J. Stein PhD-Thesis
2005
Thomson Ion Parabolas with 15x magnification
Cerenkov picture with 10x magnification
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Examples of ion beam emission from
unperturbed electron sheath
ion spectra nearly point-like source
geometry emission fom 20 µm droplets

• ion spectra
• reduced influence of source movement ?
• apparent if
• laser irradiation at normal incidence
• but still visible (cf. talk M. Amin)
• here emission fom 400 nm Al-foil
• with gt 1010 temporal contrast
• future investigation of ion beam
• and source characteristics

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Summary of observed phenomena and future
prospects Ion emission measured via Thomson
Spectrometer (15 x spatial magnification) and
comparison to Cerenkov pictures effect
appearance 0.1 . 0.5

• laser at 45 deg. incidence
• 40 fs , 1019 W/cm2,
• contrast 10-7 10-8
• trace deviation splitting
• due to source evolution
• different electron components
• dependent on laser absorption

• beam-emittance degrades
• for lower proton energies
• emission close to cutoff
• mainly undisturbed low emittance
• background of rapid oscillatory like
• beam fluctuation needs further study