IAEA

Chengdu , Oct 2006

Andrew MacKinnon

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.

Studies of isochoric heating by electrons and protons

Rapport: Papers IF/1 -R2b and 1F/1 - R2c

Plus

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 and Advanced concept exploration collaboration between LLNL, General Atomics, UC Davis, Ohio State, UCSD and LLE

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

• Synergy within LLNL, through ‘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

Fast Ignition entails assembly of compressed fuel followed by fast heating by MeV particles

Ignition driver: short pulse laserCompression

driver: Laser

Step 1. Compress fuel

critical surface

Step 2. Ignite fuel

Initial concept utilized kJ source of MeV electrons in picosecond pulse to ignite imploded capsule ( M. Tabak et al., Phys Plasmas 1, 1626, 1994)

MeV particles

Success in fast ignition requires a very large flux of MeV particles to be deposited in 10-20ps

Electron Fast Ignition1

Cone protects ignitor pulse from coronal plasma Laser conversion to fast electrons ~ 30% ~ 60 kJ laser energy required Electron transport most important issue

Proton Fast Ignition2

Currently laser conversion into protons ~10% ~180 kJ laser energy required Improving proton conversion efficiency most important issue

(1)Atzeni et al.,PoP (1999)(2) Roth et al.,86,436 PRL 2000, Atzeni et al., 2002; Temporal et al., PoP 9,3102 (2002)

• FI (1,2) requirements: heat 300 g/cc, (R~2.5 g/cm2) with 18-20 kJ particles at MeV energy in 10 ps over 30-40 m dia.

PW laser

Laser

•DT fuel at 300g/cc• 35 m ignition spot

Coronal Plasma

Cone•DT fuel at 300g/cc•40 m ignition spot

MeV electrons

MeV protons

Laser

Laser

Curved proton rich target

There is wide-ranging research in high intensity physics related to Fast Ignition in institutions around the world

15%coupling

30%coupling

Kodama,et.al,Nature 412(2001)798 and 418(2002)933.

Results from Institute for Laser Engineering, Osaka have shown promise of cone focused scheme

• Existing experimental Facilities: United Kingdom,France, LLNL(Titan), SNL, Universities,…• Theory: US, UK, France, Germany, Italy, Japan, China, India• Upcoming Facilities: Omega EP(Rochester), NIF ARC(LLNL), FIREX I(Japan)• Proposed facilities : HIPER(Europe), FIREX II(Japan), China

Implosion beams300 TW laser

“Cone” implosions

Cone wire targets are being used to study electron transport at FI relevant laser intensities

1m 10 m

256 XUV • Cone/Wire represents conductivity channel in FI scheme: test-bed for existing electron transport models

• K emission images show 100m exponential scalength

• Bell 1D analytic model* gives similar scale-length to experiment

• Peak temperature measured by XUV imaging to be 350 eV with 100 m scale length ~ 1.2% of laser energy

• 1D numerical model injecting 1.2% of laser energy as hot electrons matches observed K

•1D transport scaling gives 20% coupling at 40 m diameter - This would be viable for FI

1.E+08

1.E+09

1.E+10

1.E+11

0 500 1000

Micron

Kalp

ha p

ho

t/m

icro

n

Experimental data

Bell 1D Ohmic modelNe arb units Transport model

* A.R.Bell et al., Cont Plasma 1997

Target 10mCu wire /Al cone

500 µm

Cu K

1 mm 1mm

RAL PW laser Vulcan

Laser 500 J, 0.8 ps

Al+4

Hot e

C+6Thermal e

H+

Fra

ctio

n o

f In

ject

ed E

ner

gy

Refluxing hot e5m Al substrate

0.1m CH4 layer

Electrons

Proton FI: Proton conversion efficiency optimized by reducing target thickness and increasing hydrogen fraction on surface

Refluxing hot electrons continuously lose energy to thermal ions and electrons in substrate Contaminant layer containing hydrogen ionized & accelerated by MeV/m electric field Reducing target thickness increase conversion efficiency as hot electron pressure increases Increasing proportion of H+ in layer increases conversion eff -> hydrides or pure H2 should provide highest conversion efficiency

Varying Z of hydride layer could yield factor 2-3 enhancement in proton conversion efficiency

0

10

20

30

40

Hydrides

BC

Ho

t el

ectr

on

co

nve

rsio

n e

ffic

ien

cy (

%)

H LiH CHn

MgH2

CaH2

CsH ErH3

UH3

CH4

CH2

CH

HZ

ZHn

Thot=880keV5 m Au + 1000 ZH

n

Current experiments with contaminant layers

Cryogenic H2

• Experiments planned for mid FY07

Efficiency simulations for hydrides

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, size of proton spot and extent of heated region

Proton focusing: Mesh imaging of the proton beam provides great deal of information on focusing mechanism

Fine mesh w/ element separation = 25m

Laser : spot~50µm

1mm

Focal Plane

70mm

x

Equatorial Plane

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

Mesh image and XUV emission from the proton heated mesh indicates very small proton spot (~30-50m)

• 68eV image shows very bright image and large plume - consistent with high Temp

• 256 eV image also shows ~30-50m heated region

• RCF shows 3-4 mesh elements in 20MeVproton beam - agrees well with 256eV image

• Data is being used to test proton focusing models

256eV XUVLaser

30m

Proton dose (20MeV)

68eV XUV

30m

User experiments will begin in 2008

Integrated experiments are being planned for Omega EP: These will test fast ignition concepts at kJ short pulse energy levels

2 x1.2 kJ per beam line One beam line in FY09 (Option for 13 kJ quad)

Fast ignition experiments can also be carried out onthe NIF using the high energy radiography beams

• Paper IF/1 -2b: R. Kodama et al., “ Plasma photonic devices for Fast Ignition Concept in Laser Fusion Research”

• Paper 1F/1 - 2c: K. Tanaka et al., “Relativistic Electron Generation and its behaviors Relevant to Fast Ignition”

Rapport: Papers IF/1 -R2b and 1F/1 - R2c

• Paper IF/1 -2b: Describes how cones and other guiding devices modify Electron generation in FI

Cones appear to strongly collimate MeV electrons into beams

Integrated experiments show plasma heating by short pulse, consistent with collimated electron beams

Please see poster IF/1-2Rb for full paper

Relativistic Electron Generation and Its Behaviors Relevant to Fast Ignition

K. A. Tanaka1,2, H. Habara1,2, R. Kodama1,2, K. Kondo1,2, G.R.

Kumar1,2,3, A.L. Lei1,2, K. Mima1, K. Nagai1, T. Norimatsu1, Y. Sentoku4, T. Tanimoto1,2, and T. Yabuuchi1,2

1Institute of Laser Engineering, Osaka University,2-6 Yamada-Oka, Suita, Osaka 565-0871 Japan

2Graduate School of Engineering, Osaka University,2-1 Yamada-Oka, Suita, Osaka 565-0871 Japan

3Tata Institute of Fundamental Research,Homi Bahbha Rd., Mumbai 400 004 India

4 Department of Physics, University of Nevada, Reno, Nevada 89521-0042 U.S.A.

IF/1-2Rc

• To increase the heating efficiency of the core plasma, we propose a foam cone-in-shell target design.

Target design improvement: foam cone-in-shell target for increasing the heating efficiency of core plasma

Multiple implosion beams

Relativistic laser

Gold cone with inner tip covering with a foam layer

Fuel shell

Au foam coating enhances laser absorption and hot electron generation

F r o n t X P H C B a c k X P H C

G X I I P W l a s e r

T a r g e t

E l e c t r o n s p e c t r o m e t e r

a

c

d

b

0 2 5 0 0 5 0 0 0

e

F r o n t

F r o n t

B a c k

B a c k

F o a m c o a t i n g

S o l i d c o a t i n g

F o a m c o a t i n g

S o l i d c o a t i n g

•Hot -e yield measurement via the back x-ray emission from the target rear due to the heating from hot –e beams

-weak front x-ray emission from the Au foam-coated target. This is due to the low density of the foam.

-stronger back x-ray emission from the Au foam coated target. This is attributed to higher laser absorption and more hot electrons generated with the foam coated target. Back x-ray emission is caused by the hot –e beam heating of the target rear.

-target is thick so that the front x-ray emission may not be responsible for the enhancement of back x-ray emission with foam coated target. Moreover, if it happens, one would expect weak x-ray emission from the foam coated target rear, contrary to the experimental results.

-narrow band-width x-ray image diagnostics needed to give the relative hot –e yield through assuming Plankian emission from the target rear.

-quantitative models and simulations needed

Surface Acceleration of Fast Electrons with Relativistic Self-Focusing in Preformed Plasma

Hot electron distribution differs for with and without plasmas.

PIC simulation shows surface hot electrons at 1019 W/cm2

• We propose a foam cone-in-shell target design aiming at improving the cone-in-shell target design to increase the laser energy deposition in the dense core plasma.

• Our element experiment results demonstrated increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel .

• Phys. Rev. Lett., A.L.Lei, K.A. Tanaka et al., 96, 255006(2006).

Summary I

Summary II

Surface direction hot electrons observed at oblique incidence UIL

experiment.

Relativistic laser self-focusing increases laser intensity causing

surface hot electrons.

Several tens of MGauss field inferred

H. Habara, K.A. Tanaka et al., Phys. Rev. Lett. 97, 095004(2006).

Please see poster IF/1- 2Rc for more details