Studies of proton generation and focusing for fast ignition applications

Fast Ignition WorkshopNov 4th 2006

Andrew Mackinnon

Lawrence Livermore National Laboratory

This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

Co-authors and acknowledgements

K. Akli, F. Beg, M.H. Chen, H-K Chung, M Foord, K. Fournier, R.R. Freeman, J. S. Green, P. Gu, J. Gregori, H. Habara, S.P. Hatchett, D. Hey, J.M. Hill

J.A. King, M.H. Key, R. Kodama, J.A. Koch, M Koenig, S. Le Pape, K. Lancaster, B.F.Lasinski, B. Langdon, S.J. Moon, C.D. Murphy,, P.A. Norreys, N. Patel, P.K Patel, H_S.Park, J. Pasley , R.A. Snavely, R.B. Stephens, C Stoeckl, M Tabak,

W. Theobold, K. Tanaka, R.P. Town, S.C. Wilks, T. Yabuuchi, B Zhang,

• This work is from a US Fusion Energy Program Concept Exploration collaboration between LLNL, General Atomics, UC Davis, Ohio State and UCSD

• International collaborations at RAL,LULI and ILE have enabled most of the experiments

• Synergy with an LLNL ‘Short Pulse’ S&T Initiative has helped the work

• US collaboration in FI has recently expanded in a new Fusion Science Centrelinking 6 Universities and GA with LLNL and LLE and a new Advanced Concept Exploration project between LLNL,LLE,GA, UC Davis , Ohio State and UCSD

The power and flux requirements for proton fast ignition are similar to the original electron scheme

Proton-FI (1) requirements: heat 300 g/cc with 18 kJ protons at 3 MeV in 10 ps over 30-40 m dia. (R~2.5 g/cm2)

Proton foil to fuel distance, interaction with plasma (~ 1 mm)

Requires ~180 kJ laser energy if laser conversion into protons = 10% Requires proton spot size 30-40 m

(by focusing)

Imploded Fuel

Laser Protons

(1)) Roth et al.,86,436 PRL 2000, Atzeni et al., 2002; Temporal et al., PoP 9,3102 (2002)

Outstanding questions: Can laser conversion efficiency be increased from 10% to 15-20%? Can sheath uniformity be improved to give 30-40m spot with 1mm spherical focusing target?

1mm

* For work on improving electron coupling, see B. Lasinski, K.Tanaka

3.8MeV 6.5MeV 11MeV 14MeV 17MeV 20MeV

Target: 15µm AuN = 1.4 E12 protonsT = 3.0 MeVE = 670 mJ= 2% laser energyDivergence 1-20deg

Proton beam from Titan laser

Laser driven proton beams: Extreme hot electron pressure,nhTh , drives sheath ion acceleration mechanism

Debye Sheath

Proton beam

Laser RCF film

Eacc ~ TH/d = TH /(TH / nH)1/2

Eacc ~ (nH TH)1/2 ~ MeV/m

E= 37J0.7ps5x1019Wcm-2

e-

At relativistic laser intensities, Lorenz force accelerates electrons in forward direction Escaping MeV electrons set up Debye Sheath Trapped electrons reflux through target transferring energy to ions and thermal plasma Sheath field accelerates protons from contaminant layers on target surface

e-

MeV electron

Titan data shows good proton beam at p = 10ps and strong dependence on target thickness

• Good proton beam obtained at 10ps (but intensity reduced to 5x1018 Wcm-2)• If proton beam scales as Emax(I)0.5 then @10ps Emax~ 40MeV at 1x1020Wcm-2

• Rapid decrease in peak proton energy vs target thickness (L)-0.4

y = 21.843x-

0.3141

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Emax vs laser pulselength

Laser pulselength (ps)

Pea

k P

roto

n E

ner

gy,

Ep (

Me

V)

Ep (I)0.5 scaling

Titan data

Titan Emax vs target thickness

0

5

10

15

20

25

30

35

40

0 100 200 300

y = 89.779x-0.4275R2 = 0.8833

Target thickness (m)P

eak

Pro

ton

En

ergy

, E

p (

Me

V)

Best fit to data

1x1020 Wcm-2

5x1018 Wcm-2

Titan data

Maximum conversion efficiency obtained to date is 10% using PW class systems from CH targets

0.1

1

10

100

0.1 1 10 100

JanUSP , 10J,100fs

Nova PW , 400J, 0.8 ps

Vulcan PW, 300J, 0.8 ps

Energy J / thickness micron

Eff

icie

ncy

> 3

MeV

%

= 10% : Nova (1999), 500J, 0.5ps, 55m CH

= 2% : Titan (2006), 35J, 0.7ps, 15m Au

Hybrid PIC simulations (LSP1) are being used to study methods to optimize proton conversion efficiency and focusing

1D. R. Welch, et al, Nucl. Inst. Meth. Phys. Res. A 242, 134 (2001).

LSP Electrons injected at front of target

M Foord et al.

LSP proton cut off vs target thickness

• LSP: Hot electrons injected with appropriate kThot (ponderomtive or “Beg”) scaling with laser intensity• LSP shows decrease in conversion efficiency (& max proton energy ) with increasing target thickness as experimentally observed

3

4

5

6

7

8

9

0 10 20 30 40 50 60

Peak Proton Energy

Foil Thickness ( )m

2- D LSP

Au foil60 m laser dia500 , 150 fs J laser50 , 1 J MeV hot electrons= 700 t fs

1- :D Mora scaling≈( ( )^.5)^2Emax ln nehot

I = 1x1019 Wcm-2

LSP has reproduced the essential features from JanUSP (Callisto) laser: 10J, 100fs, 1x1020 Wcm-2

Z (µm)

R (µm)

Radial distribution proton acceleration from 5µm Au foil

• 10J, 100fs, 1x1020Wcm-2 interaction with 5m gold with 12Å layer of CH• 2D LSP: 0.5J electrons injected kThot = Edrift = 0.9MeV Maxwellian • LSP Matches experimentally observed proton flux, Emax (cut off) and Ep

Data

Proton spectrum LSP vs JanUSP

Emax

Ep = 1.7MeV

Al+4

Hot e

C+6Thermal eF

ract

ion

of

Inje

cted

En

ergy

Refluxing hot e

H+

5m Al substrate

0.1m CH4 layerElectrons

5m Au substrate0.1m CHO layerElectrons

LSP show proton conversion can be improved using low Z substrates and using hydrogen rich targets

Reduce Thermal energy ( use Low Z substrate, Al instead of Au) Increase hot electron pressure: increase kThot

Use CH4 instead of CH Cryogenic hydrogen should provide highest conversion efficiency

50%

6%

kThot = 0.9MeV kThot = 2.5MeV

• Solid Methane target cell

Solid CH4 and H2 targets could be tested using cryo target cell

LaserSP

EC

CH4 or H2

5m Gold substrate100nm CH4 layer

• Layer uniformity and thickness monitor

50mm

“LULI show no beam degradationup to 100 nm CH coating at the rear of Au foils”M. Roth et al., (PRST-AB, 5, 061301 (2002))

~7MeV

Metal hydrides could present a simpler solution than cryogenic methane or hydrogen layers

• Hydrogen density in hydride can be higher than liquid H2

1D simulations predict that the atomic weight of hydride appears to be an important factor in efficiency

0

10

20

30

40

Hydrides

BC

H LiH CHn

MgH2

CaH2

CsH ErH3

UH3

CH4

CH2

CH

HZ

ZHn

Thot=880keV5 + 1000 m Au Å ZH

n

Fraction of energy in heavy ion

Fraction of energy in H+

• Heavy ions are left behind at back surface during ion separation

Current experiments with contaminant layers

Ho

t el

ectr

on

to

pro

ton

co

nve

rsio

n e

ff (

%)

LSP simulations predict that Erbium and Uranium hydride have high electron to proton conversion efficiency

• Assumed 1000 Å layer of Mg+10, Er+10, U+10 on 5 m Au foil.• Hot electron temperature, kThot = Edrift= 880 keV• Heavy ions are left behind at back surface during ion separation.

0

2000

4000

6000

8000

1 104

1.2 104

0 0.2 0.4 0.6 0.8 1 1.2

Time (ps)

UH3

ErH3

MgH2 H

U

H

Er

Mg

H

30

20

10

Pure H

Kinetic Beam Energy

Erbium Hydride has practical advantages for near term proton efficiency studies

1. It is not Uranium!

** M. Allen, P. K. Patel, et al., PRL 93 265004 (2004)

• Surface contaminants and barrier layers will be removed by ion sputtering**

• Films 100nm thick have been manufactured by reactive sputtering*

• Oxide and hydrogen barriers may be necessary to maximize hydrogen content

ErH2 and ErH3

10-15 umgold layer

~1 um Eror U layer

10-30 nm Pd oxidationprotective layer

Laser* Sandia National lab

Proton focusing appears promising - but scaling studies are required

P. Patel et al., PRL 91 125004 (2003)

• Hemisphere focuses protons to < 50m spot • Planar foil Te = 4eV, Hemisphere heating, Te = 20eV• Emittance allows for much smaller spot (< 1m)• Problem is mapping of divergent flow onto hemisphere • Improvements required in sheath toplogy• Simplest solution - increase laser spot size

High intensity on small focal region causes bell shaped sheath with complex laminar flow and ‘aberrated’ focus

X-

20m heated spot

PW laser

Laser

Proton heating

Cu K image

Gekko PW data

320 m Al shell

Protons

X-ray phc image Cu K image

X-ray phc image

Cu K image • Divergence of electrons from small laser spot leads to non uniform sheath• Analogous to spherical aberration• Protons focus in different planes along hemisphere axis• Best focus not at geometric center of hemisphere D/R ≠ 1

Laser

Sheath

Best focus

R

D

Increasing laser spot size is a simple way of reducing proton spot size

50 m-dia1J, 100 fs laser pulse

.88 ps 1.6 ps1.2 ps

H H H

.88 ps 1.2 ps

H

H

10 m-dia1J, 100 fs laser pulse

Z=50 m

Z=60 m

Z=70 m (best focus)

Z=80 m

Z=90 m

10 um spot 50 um spot

Improved sheath planarity reduces proton spot and depth of focus

Z=50 m

Z=60 m

Z=70 m

Z=80 m (best focus)

Z=90 m

• Proton radial focusing for 3MeV proton energy• Best focus is at 1.4-1.6 x hemisphere radius (D/R = 1.4 - 1.6)• Larger spot improves focusing from 5 m to 2.5 m diameter• Self similar scaling for 50 m spot would give proton focus of 25 m for 1000m Hemisphere

Shot No:060622_s1:

20µm thick, 350µm Diameter Al hemi-shell with 25µmx25µm Cu mesh at 1mm spacing

RCF pack for measuring proton dose

This technique allows simultaneous determination of location of proton focus,D, size of proton spot and extent of heated region

A new mesh imaging technique is being developed to investigate proton focusing

Fine mesh w/ element separation = 25m

Laser : spot~50µm

1mm

Focal Plane

70mm

x

D

Oblique view XUV Imagers at 68 and 256eV to measure size of heated region

Side view

d = 250m

Laser view

mesh

mesh

LaserView of xuv

hemisphere

R

Titan laserpulse

Protonbeam

400µm

600µm

1000lpi mesh

350µm diameterhemisphere

• 68eV XUV image showing plasma emission from mesh heated by focused proton beam• RCF shows same spot size as XUV image• Measured mesh magnification gives location of proton focus D/R~1.9

Proton heated spot correlates well with RCF image of proton beam

400µm

RCF at 20MeV68eV XUV image

Strong heating was observed with mesh placed close to geometric focus

• Mesh at +50m from

geometric focus

• 68eV image consistent with high Temperature

• RCF image @ 20MeV

agrees well with ~ 30m

diam 256eV image

• Brightness consistent

with 100-200eV plasma

• Proton source d/R ~1.8

256eV XUV

TitanLaser

25m

Proton dose (20MeV)

68eV XUV

25m

Conclusions

• Proton fast ignition is an attractive alternative to electron ignition • Required proton temperature can be achieved for available laser irradiance - but need higher proton energy density • Conversion efficiency: Require 15-25% > 3MeV

• Maximize hot electron production • Determine optimum pre-pulse level for electron production + proton conversion• Maximize energy into protons - CH4, H or hydride targets

• Proton focus: • Require 30-40m spot with 1000m radius spherical target

• Understand sheath topology effects• Tailor target shape (aspherical)• Tailor laser irradiance pattern (multiple spots may help)

• Environment: Design required that mitigates against radiation/plasma/prepulse effects known to disrupt proton beam

The Fast Ignition Concept

Conceptual full scale proton fast ignition* must satisfy stringent criteria

XUV

20m heated spot

PW laser

Laser

Proton heating

Cu K image

150m

Laser 100kJ,10 ps~1020 Wcm-2

50kJ electrons

(le~ 0.5)

kT = 3 MeV

20 kJ protons

(ep~ 0.4)

kT = 3 MeV4x1016 protons !

• Cone protects source foil from shock & x-rays• Molieré scattering limits Z, distance and

thickness of cone tip

• DT fuel at 300g/cc• R ~ 2-3g/cm-2• 33 m ignition spot

* Roth et al.,86,436 PRL 2000, Atzeni et al., 2002; Temporal et al., PoP 9,3102 (2002)

1mm

• Acceleration occurs during hot electron lifetime - Debye sheath moves forward • Edge effects limit depth uniformity and thus focal spot quality

• Thick proton source foil protects rear surface from pre-pulse - thickness limits conv. efficiency

RAL PW data show =45 m focus in 256 eV image - quite close to scaled LSP model for small laser spot (=43 m)

68 eV XUV streak

10 ns

Proton heating

256eV XUV image

Imploded shell

45 m

Narrow peak of proton heating

Imploded shell

Protons

=360m Cu hemi, 608J( x0.65), 0.6 ps , 30CD/1Al/8kapton m foil

1213043

Hemi shell

Foil

M. Allen Thesis

Residual gas analysis of vacuum chamber

Ion sputtering gun - details

M. Allen Thesis

Sputtering Geometry

M. Allen Thesis