Radiation pressure driven ion acceleration in near critical density targets Oliver Ettlinger John Adams Institute for Accelerator Science Imperial College London JAIFest 10th December 2015 http://www.adams-institute.ac.uk
BNL ion acceleration experiments ! N.P. Dover, G. Hicks, Z. Najmudin I.V. Pogorelsky, O. Tresca, M.N. Polyanskiy Accelerator Test Facility N. Cook, C. Maharjan, P. Shkolnikov Y.Chen, M. H. Helle, A. Ting, JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Benefits of Laser-Plasma Ion Sources • Why laser-plasma ion sources? – High flux, low emittance, short bunch length – Plasmas can support high acceleration gradients ~100GeV -1 - potential for more compact source – lower cost than conventional sources – inherent source flexibility - variable source species and energy • Challenges – unwanted radiation production - neutrons/x-rays – stability/reproducibility (laser technology challenge) – applications require high peak energies/currents with narrow energy spreads JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Radiation Pressure • Can we directly accelerate ions through the radiation pressure of our laser? 100W Light Ti:Saphire CO 2 - ATF at Bulb (at Gemini - RAL BNL 10cm) Intensity 8x10 -2 1x10 21 3x10 16 [Wcm -2 ] ~10 -11 Bar >1TBar >50MBar Pressure JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Acceleration Regime • For radiation pressure driven acceleration (RPA), this pressure must dominate the thermal pressure of the plasma • Thermal Pressure, P Th Balancing the thermal and radiation pressures one can show the regimes of thermal and radiation pressure domination JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Acceleration Regime • What does this actually mean? Radiation pressure Thermal pressure dominant dominates - collisionless shock acceleration? • Need extreme intensities for dense targets (solid density) • Can use much lower intensities for lower densities • Radiation pressure always dominant for n e < 4n c JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Acceleration Regime If P rad > P Th radiation pressure dominates - critical surface • driven into the target accelerating ions If P rad < P Th thermal pressure dominates - the laser can induce • a shock that accelerates ions Radiation pressure leads to ‘hole-boring’ acceleration the • critical surface is spatially driven into the plasma, snowploughing the upstream ions – ions gain twice the hole-boring velocity, v HB JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Collisionless Shock Acceleration If thermal pressure dominates, the incident laser pulse can • launch a shock into the plasma – strong plasma heating – induced density gradients Leads to a potential step through the induced charge • separation “Collisionless” - length scale of shock front is less than the • collisional mean free path of the particles comprising the front Ions are reflected off the shock potential to twice v shock Fiuza F. et al. “Laser-Driven Shock Acceleration of Monoenergetic Ion Beams” PRL 109 (2012) JAIFest 2015 RPA in near critical density targets, O.Ettlinger
A comparison of mechanisms 40 shock 35 crit. surf. Distance from initial pos. (μm) 30 25 20 15 10 5 0 0 0.5 1 Time (s) − − 11 11 x 10 • The shock moves ahead of the hole-boring front – the subsequent energies are theoretically higher from shock acceleration JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Optimising Acceleration • Gas targets offer a number of benefits over their solid counterparts – less secondary radiation (Bremsstrahlung) – higher rep rate – flexibility with ion species • Still need an overdense plasma • Both acceleration mechanisms scale with the density – the lower the bulk density, the higher the ion energies • Blast waves an interesting avenue – allow lower bulk densities – target profile shaping possible? JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Blast Waves • Blast Wave: the pressure and flow resulting from the deposition of a large amount of energy in a small, localised volume i.e. a collisional shock wave • Theory well understood for these shock waves 1 ✓ E 0 ◆ 1 2 η 2 r b 2+ ν 2+ ν − 1 = ✓ E 0 ◆ 2 2+ ν v s = 2 + ν − 1 t 2 r b = η t 2+ ν (2 + ν ) t ρ 1 ρ 1 JAIFest 2015 RPA in near critical density targets, O.Ettlinger
BWs are not only seen in plasmas Dover N. et al. “Optical probing of shocks driven into overdense plasmas by laser hole-boring” JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Blast waves for target shaping • The blast waves also result in a known density spike compared to the background Theory predicts a cavity wall density for the blast wave that scales as γ + 1 γ − 1 n i γ - ratio of the specific heat capacities (at room temp and atmospheric pressure) [2] γ = 5/3 for Helium Ratio = 4 : 1 γ = 1.3981 for Deuterium Ratio = 6 : 1 γ = 1.41 for Hydrogen Ratio = 5.93 : 1 The theory agrees very well with experimental results Fig 1. Tresca O et al. “Controlled shock acceleration of helium ions by [2] – Zel’dovich Y. B. and Raizer Y. P. “Physics of Shock Waves and laser irradiation of hydrodynamicallyshaped gas jets” 2014 High-Temperature Hydrodynamic Phenomena” (Academic Press, New York, 1967) (page 52, eqn. 1.80) JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Current Experiments • Experiments at the Accelerator Test Facility at Brookhaven National Laboratory • Allows investigation of ‘novel’ acceleration regimes currently difficult to achieve with NIR laser systems λ L ≈ 10 µm n c = ✏ 0 m e e 2 ! 2 JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Experimental Work at BNL λ =527 nm Pulse length=10 ps λ =10 µm Pulse length=5 ps Spot size ~ 65 µm Intensity ~ 10 16 W/cm 2 JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Previous Results Palmer et al. experimentally demonstrated ion energy scaling in line with RPA – Peak energy ≈ 1MeV – Narrow energy spread, 𝜏 ≈ 4% 734 7 !"#$#%&'%'"()&*+',- 236 235 23. 234 232. 2328 2325 2329 2326 232: ./0%1 Palmer C. et al. PRL 106 (2011) JAIFest 2015 RPA in near critical density targets, O.Ettlinger
Previous Results a) x10 10 b) 1.4 10 9 1.1 Parameter space for He + 0.9 ion generation Number He + /Mev/sr Detect limit 0.7 1.2 0.5 E pp (J) 10 8 1 no pre-pulse 0.3 0.8 10 7 0.1 0 0 3 1.1 1.3 1.5 1 2 Energy (MeV) Main pulse a 0 ideal pre-pulse a) x10 10 b) ~200mJ 1.4 10 9 1.1 He + 0.9 Number He + /Mev/sr Detect limit 0.7 1.2 0.5 E pp (J) Typical Spectrum 10 8 1 pre-pulse too 0.3 large ~1J 0.8 10 7 0.1 0 0 1 2 3 1.1 1.3 1.5 Energy (MeV) Main pulse a 0 Tresca O et al. PRL 115 (2015) JAIFest 2015 RPA in near critical density targets, O.Ettlinger
2015 Results • Work conducted in collaboration with colleagues at the Naval Research Laboratory • Alternative scheme for generating the blast wave used JAIFest 2015 RPA in near critical density targets, O.Ettlinger
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