Assembly of Targets for RPA
by Compression Waves
A.P.L.Robinson
Plasma Physics Group,
Central Laser Facility,
STFC Rutherford-Appleton Lab.
Acknowledgements
J.Pasley and I.Bush,
University of York, UK
R.Kumar, S.Mondal, and whole TIFR team,
Tata Institute for Fundamental Research,
Mumbai, India
A.R.Bell, P.A.Norreys, D.Symes,
Central Laser Facility, UK
Summary
A HEDP technique for generating pure H
targets for RPA.
Background
RPA = Radiation Pressure Acceleration
Challenge
• Great interest in Radiation Pressure Acceleration.
• Early experimental demonstrations of RPA have
been reported.
• Want to find laser-target configuration that
produces highest energy per nucleon.
• Here we investigate the production of thin pure
hydrogen targets via a laser-driven hydrodynamic
approach
Why Pure H targets?
Robinson et al., NJP (2008)
• Classic Light-Sail analysis shows that momentum or energy
per ion defined by f It / l 
• Energy per nucleon is then 1/A of this energy per ion.
• For a given It / l we are therefore always better off with a
pure H target.
• Mixed targets don’t efficiently accelerate protons.
• Therefore we need suitable pure H targets.
Problem
• Solid H only available under cryogenic conditions
(but a low density form of H).
• Producing sub-micron foils not yet demonstrated.
• Producing foils less than 10 microns not
demonstrated.
• NIF/ ICF targets : 10s of microns of DT ice on solid
material.
1µm
10µm
100µm
Physical Principle
1.Imagine a hot spot being
created in a uniform plasma
or fluid
3.Cavity expands as compression
wave propagates outward.
2.Overpressure region cavitates
region and creates a thin shell of
dense plasma at edge
Key Problems
1. Creation of a hot spot at a
sufficiently high temperature.
2. Hydrodynamic evolution of
compression wave on multi-ps
timescale.
3. Suitability of final structure for RPA.
Scheme 1
1.Use small ball of cryogenic H
10 micron diameter
2.Use short pulse to create hot spot
on one side
3. Drives compression wave into
surrounding plasma
5ps
4. Produce a thin shell of dense plasma
at far side.
8ps
12ps
Results 1
Use simple hydrodynamic calculation to obtain a density
profile.
Density x-y Plot
Density Line-out
Conclusion : Can produce a potentially suitable target.
Scheme 2
1.Use small ball of cryogenic H
10 micron diameter
3. Drives 2 compression waves into
surrounding plasma
2.Use short pulses to create hot spots
in cold plasma on both sides
4. Compression Waves collide to
create a thin dense region.
Results 2
Use simple hydrodynamic calculation to obtain a density
profile.
Density x-y Plot
Density Line-out
10 microns of low
density plasma is
problematic.
Suitability for RPA
Long shelf of plasma at rear (or front) of target is principal
concern.
Output from hydrodynamic
simulation.
Profile considered in PIC sim.
Suitability for RPA 2
Still obtain effective acceleration of narrow bunch to > 100 MeV via RPA
1D PIC result below (80fs pulse at 5 x 1021Wcm-2)
Producing a Hot Spot
• Used a 1D PIC with a simple collision model.
• Done series of simulations for pure H target at cryogenic H
density (40 nc) and step-like density profile.
• Allows us to include effects like resistive heating while still
generating fast electrons fully self-consistently.
• Next step – 2D model
Heating Profiles
• Linearly Polarized, 20fs, 0.5µm, 5x1018Wcm-2
Can produce keV temperatures over a few microns. Long gradient to cooler
region though.
Effect on Hydro
Heating profile based on collisional PIC code results
produces similar results to those based on pure
assumption.
Density line-out
Density x-y plot
Experimental
Motivation
Experimental Motivation
2
Extracts from Mondal et al. PRL 2010
Can probe the motion of the critical surface by looking at
Doppler shift of a probe beam.
Experimental Motivation 3
Extracts from Mondal et al. PRL 2010
Experiment
Simulation (Coll. PIC+Hydro)
Simulation model reproduces experimental measurements well.
Motion of crit. surface due to compression wave in simulation.
Review
Aspect
Multi-ps
Hydrodynamics
Progress
Future
Basic 2D
hydrodynamic
simulations
Better EOS +
transport or VFP
code
Energy
Deposition/Heating
1D collisional PIC
2D collisional PIC
Suitability for RPA
1D/2D PIC
Comprehensive
study
Experiments
Mondal et al. PRL
2010
Extend using same
method
Summary
A HEDP technique for generating pure H
targets for RPA.