A.P.L.Robinson

CLF

Proton Beams for Fast Ignition:Control of the Energy Spectrum

A.P.L.Robinson1

D.Neely1, P.McKenna2, R.G.Evans1,4,C-G.Wahlström3,F.Linau3, O.Lundh3

1Central Laser Facility, Rutherford-Appleton Laboratory, UK2 University of Strathclyde, Glasgow, UK

3Lund Laser Institute, Sweden4Imperial College London, UK

A.P.L.Robinson

CLF

Spectral Modification for Proton FI

• Quasi-monoenergetic experimental results in 2006.(Hegelich et al. Nature 439 (2006))

• This may help proton driven Fast Ignition.(Temporal et al.,PoP 9 (2002))

• Spectral Modification w/o “Energy Slicing” + all optical approach.

• We studied whether the use of multiple high-intensity laser pulses can produce useful spectral modification.

• Carried out Vlasov and PIC simulations.

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Is there a theoretical basis for this?

Grismayer and Mora, PoP 13 (2006)•Single species•Hybrid•Exponential density profile•Zero velocity in pre-expanded tailbut …•Wave-breaking•Transient features in spectra

Equal pulses: Why shouldn’tthis just be very similar to onepulse?

1.Max. Energy reduced.2.Some part of the spectrumIs enhanced.

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Vlasov simulations• 1D1P Eulerian Upwind method.• Large plasma near solid density (186μm); 40nm contamination layer• Contains a “Set-up” pulse of electrons + a “Main drive” pulse of

electrons.• Sub-question: Does this work within TNSA alone?• 2 ion species; 2 temperature

SUP = Set-Up PulseMDP = Main Drive Pulse

C4+CH2

40nm

186μm = rear surf.

SUPMDP

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Results

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•See a reduction in Emax.•Fewer high energy protons.•Interestingly we have produceda spectral peak.•Simulations run for 750fs.•Proton source layer close to solid density and has 66% H.

Where did these peaks come from?

Seen at certain values of the set-up pulse temperature only (250-750keV) for standard conditions used.

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Two-stage Mechanism

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As the hotter MDP arrives → surge in protons across carbonFront → “wave breaking” + peak in proton density

186μm = rear surf.

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CLF

The Second stagePeak in proton density → “E-field alterations” (spike) →

accumulation of protons in phase space.

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186μm = rear surf.

A.P.L.Robinson

CLF

SensitivityAlso tried reducing density of SUP to 5 x 1026 and 2.5 x 1026m-3

Peaks still occur. Not a “chaotic” sensitivity to initial conditions

A.P.L.Robinson

CLF

PIC simulations

• 1D3P EM PIC code used.• 400nm foil at 4 x 1028 H & 4 x 1028 heavy ion (80ncrit)• Foil placed at 60μm• 40fs sin2 pulses MDP a0 = 4• Comparison run with MDP only.

MDP

SUP

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PIC Results

SUP a0 = 2: Red SUP a0 = 1:Magenta Single Pulse Ref.: Black

We obtain similar results to Vlasov simulations

Reduction inEmax

Spectral Peaks

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CLF

But is this the same mechanism?

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Consider run II (SUP a0 = 2)

So the first stage is v.similar, but no `wave-breaking’

Still see 2-stagemechansim. Protondensity spike produced

Protons

Heavy ions

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But is this the same mechanism? (II)

2 pulse

Second Stage

1 pulse

E-field

Proton /Ion Density

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CLF

But is this the same mechanism?(III)

Second stage is then very similar in run II

protons heavy ions

This feature gives the peak.

•‘wave-breaking’ eventprobably incidental tosome initial conditions.•Appears to be the same mechanism.

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CLF

…and some issues (I).Simulations at reduced density (10ncrit) result in over-optimistic predictions.

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…and some issues (II).Heavy ions aren’t critical to this process:

This is not a target composition effect!

Red = Pure proton target. Black = Proton + Heavy Ion.

A.P.L.Robinson

CLF

Conclusions

• Can multiple 10-100fs ultraintense laser pulses result in a modified proton spectrum?

• Yes, on the basis of our Vlasov and PIC simulations which both agree on this question.

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•Level of modification may be interesting to proton FI•Optical Approach:More practical + better for high rep. rate?•This is the start of an investigation that requires more work. •Future work needs more realism, but also a better understanding so that we can control this.