Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets

PERSISTENT SURVEILLANCE FOR PIPELINE PROTECTION AND THREAT INTERDICTION

Richard B. Stephens General Atomics

P.Patel

M. Roth et al

9 th International Fast Ignition Workshop Cambridge, MA 3 November 2006

Contributors from a large collaboration

M Mauldin, E Giraldez,C Shearer M Foord, A J MacKinnon, P Patel, R A Snavely, S C Wilks, K Akli, F Beg, S Chen, H-K Chung, D J Clark, K Fournier, R R Freeman, J S Green, C D Gregory, P-M Gu, G Gregori, H Habara, S P Hatchett, D Hey, K Highbarger, J M Hill, J A King, R Kodama, J A Koch, K L Lancaster, C D Murphy , , H Nakamura, M Nakatsutsumi, P A Norreys, N Patel, J Pasley , H-S Park, C Stoeckl, M Storm, M Tabak, M Tampo, W Theobold, K Tanaka, R Town, M S Wei, L van Woerkom, R Weber, 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 have enabled the experiments

•

Synergy with an LLNL ‘Short Pulse’ S&T Initiative has helped the work ICFT/P2006-054

Proton ignition concept has evolved

•

Initial concept avoided complexity

–

External focusing surface

–

Simple proton transport

• • •

Velocity spread cause problems

–

Energy must be delivered in short time

Roth et al., Phys. Rev. Lett. 86, 436 (2001)

70 Simple solutions …

– 

Reduce energy spread Reduce separation

(M. Hegelich, LANL)

60 50 40 30 20 10 d = 4 mm d = 2 mm d = 1 mm Introduce new problems



Protection from the imploding shell 0 0 5 10 15 T p (MeV) 20

Atzeni et al., Nucl Fusion 42, L1 (2002)

25 ICFT/P2006-054

Use a reentrant cone for protection



Protects proton source from coronal plasma



Limits accelerating surface Laser



Causes focusing edge effects



Scatters proton beam ICFT/P2006-054

Tested concept by making prototype

•

Cone dimensions same as for electrons

– 30° full cone opening •

Focusing surface same as for hemi tests

(existing focal length data) – r c = 170 m m – d focus ~290 m m  Limits accelerating area (125 m m dia) •

Target Cu foil - 32

m

m thick (29 mg/cm 2 )

– Stops < 4 MeV protons

ICFT/P2006-054

Proton source area depends on energy

•

Accelerating electrons cool off as they travel to the edge

Patel et al.,

Phys. Rev. Lett.

91

, 125004 (2003) Hybrid PIC LSP simulation M. Foord - LLNL 100 fs, 50 m m FWHM Gaussian beam 45 J beam

ICFT/P2006-054



200

m

m dia includes most useful protons (flat foil data) Our source will have limited energy output

Low energy protons are most important to ignition Protons must deliver energy in short time for ignition 40 Fusion Emission Proton Deposition 300 30



limits useful proton energy range Sim parameters: Proton spectrum: Tp = 3 MeV, dn/de



sqrt(

e

)e -

e

/Tp Total proton energy = 26 kJ Proton beam radius = 10

m

m Useful for ignition Source distance = 4 mm Target density = 400 g/cc 20 10 45 65 85 t [ps] 105 125

Temporal et al.,

Phys of Plasma

9

3098 (2002)

200 100 ICFT/P2006-054

Protons are not easily scattered



The cone tip can be far from the compressed core

5 m m Au 1-5° •

Scattering angle



E -2

 3 Mev Protons ~ 5°  15 Mev Protons ~ 1° 15° •

Broadens spot 5-10

m

m

200 m m

End wall scattering is insignificant ICFT/P2006-054

Prototype proton focusing cone was constructed

ICFT/P2006-054 Construction is feasible

Initial tests show moderate proton focusing and heating ICFT/P2006-054 K



imager

2500 2000 1500 K  1000 500 0 back surface front surface

HOPG

K  7.8

8 8.2

8.4

8.6

8.8

Energy (KeV) 9 9.2

160

m

m

500 0 -500 -1000 600 650 700 750 800 850 900 950 1000 Intensity (a. u.)

Proton heating is reasonable for conditions

• • •

Ratio of HOPG intensities gives slope temp 1-4 MeV for protons K



spots have 10 6 counts - to be compared to equivalent shots using full hemi Focal spot is rather large - 160

m

m

– Could be consequence of side walls changing the proton focus.

ICFT/P2006-054

Measure focus changes by radiographing grids

Put grids in flat washers for simpler construction

• • •

Send proton beam through grid and detect with RCF stack Magnification determines focus position, fuzziness of grid shows focus size, number of grids show source area These experiments are in preparation ICFT/P2006-054

ICFT/P2006-054

Will use data to design integrated experiments for Omega EP Omega EP hydro simulations (S. Hatchett)

Backlit radiograph (8 keV) at imploded max

r

R

Conversion to protons, focusing/ heating?

40 µm PW laser 55 * beams, pulse-shape “26” CD 2 vacuum more compact?

• What is signature of heating, increased emission? Ka fluorescence? X-ray scattering? neutron production? Abs spectroscopy?

improve eff’y?

Hi-Z mix?

Blob

r

R ~ 0.44 g cm -2 <

r

> ~ 120 g cm -3 ~ 0.4 keV Total Energy in blob ~ 0.6 kJ

Laser spot size influences proton focus

• The proton focal spot radius reduces as laser focal

spot increases

10 um spot z=50 m m (long axis) 55 m m 60 m m 50 um spot z=50 m m 55 m m 60 m m • Trade-off between fully illuminating surface, and building edge effect

ICFT/P2006-054

Tight laser spot gives ‘aberrated’ proton focus

320

m

m Al shell Laser X-ray phc image Gekko PW data Protons Cu K



image Cu K



image Cu K



image Proton heating data 20

m

m heated spot ICFT/P2006-054