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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

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

BNL ion acceleration experiments

N.P. Dover, G. Hicks, Z. Najmudin I.V. Pogorelsky, O. Tresca, M.N. Polyanskiy

• N. Cook, C. Maharjan,
• P. Shkolnikov

!

Accelerator Test Facility 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

• Can we directly accelerate ions through the radiation

pressure of our laser?

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Radiation Pressure

100W Light Bulb (at 10cm) Ti:Saphire Gemini - RAL CO2 - ATF at BNL Intensity [Wcm-2]

8x10-2 1x1021 3x1016

Pressure

~10-11Bar >1TBar >50MBar

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, PTh

Balancing the thermal and radiation pressures

• ne 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?
• Need extreme intensities for dense targets (solid

density)

• Can use much lower intensities for lower densities
• Radiation pressure always dominant for ne < 4nc

Radiation pressure dominant Thermal pressure dominates - collisionless shock acceleration?

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Acceleration Regime

• If Prad > PTh radiation pressure dominates - critical surface

driven into the target accelerating ions

• If Prad < PTh 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, vHB

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 vshock

Fiuza F. et al. “Laser-Driven Shock Acceleration

• f Monoenergetic Ion Beams” PRL 109 (2012)

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

A comparison of mechanisms

• The shock moves ahead of the hole-boring front

– the subsequent energies are theoretically higher from shock acceleration

0.5 x 10

− 11

1

− 11

5 10 15 20 25 30 35 40

shock

• crit. surf.

Distance from initial pos. (μm) Time (s)

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

rb = η ✓E0 ρ1 ◆

1 2+ν

t

2 2+ν

vs = 2η 2 + ν − 1 ✓E0 ρ1 ◆

1 2+ν

t

2 2+ν −1 =

2rb (2 + ν) t

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 − 1ni

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 laser irradiation of hydrodynamicallyshaped gas jets” 2014 [2] – Zel’dovich Y. B. and Raizer Y. P. “Physics of Shock Waves and 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

λL ≈ 10µm nc = ✏0me e2 !2

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Experimental Work at BNL

λ=10 µm Pulse length=5 ps Spot size ~ 65 µm Intensity ~ 1016 W/cm2 λ=527 nm Pulse length=10 ps

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Previous Results

234 23. 235 236 7 734 232. 2328 2325 2329 2326 232: !"#$#%&'%'"()&*+',- ./0%1

Palmer et al. experimentally demonstrated ion energy scaling in line with RPA

– Peak energy ≈ 1MeV – Narrow energy spread, 𝜏 ≈ 4%

Palmer C. et al. PRL 106 (2011)

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Previous Results

no pre-pulse ideal pre-pulse ~200mJ pre-pulse too large ~1J

Tresca O et al. PRL 115 (2015)

Main pulse a0 Epp (J) 0.1 0.3 0.5 1.1 1.1 1.3 1.5 Energy (MeV) Number He+/Mev/sr a) b) 0.8 1 1.2 1.4 x1010 0.7 0.9 He+ Detect limit 109 108 107

1 2 3 Main pulse a0 Epp (J) 0.1 0.3 0.5 1.1 1.1 1.3 1.5 Energy (MeV) Number He+/Mev/sr a) b) 0.8 1 1.2 1.4 x1010 0.7 0.9 He+ Detect limit 109 108 107

1 2 3

Parameter space for ion generation Typical Spectrum

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

2015 Results - YAG Driven Blast Waves

Longitudinal focus scan - along laser axis

Nozzle Outer Edge Nozzle Inner Edge

CO2

Move this direction

High Energy Bunch Low Energy Thermal Tail

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Mono-energetic Beams

• High energy mono-

energetic beams were

• bserved.
• Energies as high as

~3MeV observed

• Mono-energetic

bunches due to radiation pressure driven process

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Proton Beam Filamentation

• Early runs demonstrated poor shot to shot

reproducibility - even for apparently very similar shots

• Magnetic spectrometer shots determined proton beam

structure Zero point Structured beam

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Proton Beam Filamentation

• CR-39 data also demonstrated this beam structure
• Cause of this structure is as yet unknown

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Other Studies

• Initial work on colliding blast waves - can we make

thin gas targets for enhanced ion acceleration?

• Optically driven blast waves - issues with new pre-

pulse system meant low ion energies – energy and timing fixed – blast wave expansion not optimised

JAIFest 2015 RPA in near critical density targets, O.Ettlinger

Conclusions

• Optimisation of the gas density profile is key to effective

ion acceleration higher ion energies than previously

• bserved
• Underlying physics of these interactions still not completely

understood - what causes the ion beam structure? – work to determine this ongoing

• New system for pre-pulse driven blast waves required to

allow optimal conditions to be met – new pre-pulse system currently being tested

• Optical shaping of gas targets is still essential to access

shock acceleration regime for ions