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? 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)

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 10µm100µm

Physical Principle 1.Imagine a hot spot being created in a uniform plasma or fluid 2.Overpressure region cavitates region and creates a thin shell of dense plasma at edge 3.Cavity expands as compression wave 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 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 4. Produce a thin shell of dense plasma at far side. 5ps 8ps12ps

Results 1 Use simple hydrodynamic calculation to obtain a density profile. Conclusion : Can produce a potentially suitable target. Density x-y Plot Density Line-out

Scheme 2 1.Use small ball of cryogenic H 10 micron diameter 2.Use short pulses to create hot spots in cold plasma on both sides 3. Drives 2 compression waves into surrounding plasma 4. Compression Waves collide to create a thin dense region.

Results 2 Use 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 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 Wcm -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 n c ) 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, 5x10 18 Wcm -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 x-y plot Density line-out

Experimental Motivation

Experimental Motivation 2 Can probe the motion of the critical surface by looking at Doppler shift of a probe beam. Extracts from Mondal et al. PRL 2010

Experimental Motivation 3 ExperimentSimulation (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 AspectProgressFuture Multi-ps Hydrodynamics Basic 2D hydrodynamic simulations Better EOS + transport or VFP code Energy Deposition/Heating 1D collisional PIC2D collisional PIC Suitability for RPA1D/2D PICComprehensive study ExperimentsMondal et al. PRL 2010 Extend using same method

Summary A HEDP technique for generating pure H targets for RPA.