Assembly of Targets for RPA by Compression Waves

A.P.L.RobinsonPlasma 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 Htargets for RPA.

BackgroundRPA = 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?

• Classic Light-Sail analysis shows that momentum or energy per ion defined by

• Energy per nucleon is then 1/A of this energy per ion.• For a given 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.

Robinson et al., NJP (2008)

lItf /

lIt /

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 beingcreated in a uniform plasma

or fluid2.Overpressure region cavitatesregion and creates a thin shell of

dense plasma at edge

3.Cavity expands as compressionwave propagates outward.

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 11.Use small ball of cryogenic H

10 micron diameter

2.Use short pulse to create hot spoton one side

3. Drives compression wave into surrounding plasma

4. Produce a thin shell of dense plasmaat far side.

5ps 8ps 12ps

Results 1Use simple hydrodynamic calculation to obtain a density

profile.

Conclusion : Can produce a potentially suitable target.

Density x-y PlotDensity Line-out

Scheme 2 1.Use small ball of cryogenic H

10 micron diameter

2.Use short pulses to create hot spotsin cold plasma on both sides

3. Drives 2 compression waves into surrounding plasma

4. Compression Waves collide to create a thin dense region.

Results 2Use simple hydrodynamic calculation to obtain a density

profile.

10 microns of low density plasma is problematic.

Density x-y Plot

Density Line-out

Suitability for RPA

Long shelf of plasma at rear (or front) of target is principalconcern.

Output from hydrodynamicsimulation.

Profile considered in PIC sim.

Suitability for RPA 2Still 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 HydroHeating profile based on collisional PIC code resultsproduces similar results to those based on pure assumption.

Density x-y plotDensity line-out

Experimental Motivation

Experimental Motivation 2

Can probe the motion of the critical surface by looking atDoppler shift of a probe beam.

Extracts from Mondal et al. PRL 2010

Experimental Motivation 3

Experiment Simulation (Coll. PIC+Hydro)

Simulation model reproduces experimental measurements well.Motion of crit. surface due to compression wave in simulation.

Extracts from Mondal et al. PRL 2010

Review

Aspect Progress Future

Multi-ps Hydrodynamics

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 Htargets for RPA.