Direct-Drive Inertial Confinement Fusion:Status and Future

P. B. RadhaUniversity of RochesterLaboratory for Laser Energetics

AAAS Annual MeetingWashington, DC

17–21 February 2005

Collaborators

R. Betti, T. R. Boehly, T. J. B. Collins, R. S. Craxton, J. A. Delettrez, D. Edgell,R. Epstein, V. Yu. Glebov, V. N. Goncharov, D. R. Harding, R. L. Keck, J. P. Knauer,

S. J. Loucks, J. Marciante, J. A. Marozas, F. J. Marshall, A. Maximov,R. L. McCrory, P. W. McKenty, D. D. Meyerhofer, J. Myatt, S. P. Regan,

T. C. Sangster, W. Seka, S. Skupsky, V. A. Smalyuk, J. M. Soures,C. Stoeckl, B. Yaakobi, and J. D. Zuegel

Laboratory for Laser Energetics, University of Rochester250 East River Road, Rochester, NY 14623-1299

C. K. Li, R. D. Petrasso, F. H. Séguin, and J. A. Frenje

Plasma Science and Fusion Center, MITBoston, MA, USA

S. Paladino, C. Freeman, and K. Fletcher

State University of New York at GeneseoGeneseo, NY, USA

Direct-drive holds great promise for ignition on the National Ignition Facility (NIF)

TC6890

• Two paths to direct-drive ignition on the NIF have been identifi ed—symmetric and polar.

• Good agreement between predictive simulations and ignition-scaled cryogenic implosions is obtained on the OMEGA laser for symmetric drive.

• Polar direct drive may allow for ignition on the NIF in its x-ray drive confi guration.

• A new high-energy petawatt capability at OMEGA (OMEGA-EP) will provide the ability to image core distortions in cryogenic implosions and test fast-ignition concepts.

Summary

Outline

TC6891

• Brief introduction to direct-drive

• Symmetric drive

• Polar direct drive

• Fast ignition

Ablation is used to generate the extreme pressures requiredto compress a fusion capsule to ignition conditions

S5g

“Hot-spot” ignition requires the core temperature to be at least10 keV and the core fuel areal density to exceed ~300 mg/cm2.

2. Compression

Implodingpellet

Expandingblowoff

1. Irradiation

Laser DT fuel–filledpellet

3. Thermonuclear ignition

Compressed pellet

Tem

pera

ture

Radius

Mass d

en

sit

y

Hot spot

Burnwave

TC5576b

The NIF direct-drive point design is a thickDT-ice layer enclosed by a thin CH shell

1.69 mm

1.35 mm

CH

DT ice

DT gas

Symmetric Drive

Gain : 45fusion energy out

laser energy in( )

Absorption fraction: 63%

1.5 MJ

1.69 mm

Z23.5°

44.5°

77.45°

77.45°

45°23.5°

Symmetric Drive

103

102

101

100

0 2 4 6 8 10

Time (ns)

Po

we

r (T

W)

A number of key physics issues associated with capsuleimplosions are being investigated at LLE

E6426i

Direct-drive target X-ray-drive target

Capsule

Laser beams Hohlraum usinga cylindrical high-Z case

Laserbeams

• Energy coupling• Drive uniformity• Hydrodynamic instabilities

Key issues:

The Rayleigh–Taylor instability

can reduce target performance

TC2800b

Classical

Hydrodynamic Instabilities

Light

g

Heavy

Ablation-surface

“outside” instability

Deceleration

“inside”instability

a

Light

g

Heavy

Main fuel

Hotspota

Main fuel

Hotspot

a

a

Time

TC6610a

There are four sources of perturbations a direct-drivecapsule must tolerate to ignite and burn

Control of target and irradiation nonuniformity and subsequent instabilitygrowth provides the greatest challenge to direct-drive ignition.

Ice roughness

Capsulefinish

Driveasymmetry

Impr

int

DTgas

Shell

TC5587g

Example of laser nonuniformity: application of single-beamsmoothing* is necessary for ignition

2-D simulation of Direct Drive Capsule at the end of Acceleration

Z (μm)

0

200

400

600

800

1000

R(μ

m)

–500 –250 5002500

No smoothing

Z (μm)

–500 –250 5002500

With single beam smoothing

Z (μm)

–500 –250 5002500

Spherically symmetric implosion

The NIF direct-drive point design ignites with a gain of 30** whennonuniformities are included in the simulations.

*S. Skupsky et al., J. Appl. Phys. 66, 3456 (1989).**P. McKenty et al., Phys. Plasmas 8, 2315 (2001).

7.5

3.5

0.5

ρ (g/cc)

7.5

3.5

0.5

ρ (g/cc)7.5

3.5

0.5

ρ (g/cc)

TC2998v

The OMEGA laser is designed to achieve high uniformitywith flexible pulse-shaping capability

• 60 beams

• >30 kJ UV on target

• 1% – 2% irradiation nonuniformity with1 THz 2-D SSD, polarization smoothingphase plates, power balance

• Flexible pulse shaping

• Short shot cycle (1 h)

• A wide range of implosion diagnostics

80 m

OMEGA cryogenic targets are energy scaled fromthe NIF symmetric direct-drive point design

E11251k

Energy ~ radius3;

power ~ radius2;

time ~ radius

103

102

101

100

0 2 4 6 8 1010–1

Gain (1-D) = 45

1.69 mm

~3 μm CHNIF: 1.5 MJ

1.35 mm

DT ice

DT gas

~4 μm CH

0.36 mm

0.46 mm

OMEGA: 30 kJ

D2 ice

D2gas

Time (ns)

Po

we

r (T

W)

E12062c

The life cycle of a cryogenic target is an engineeringtour de force

Permeationcryostat

DTcompressor

Existingtritium

equipment

Moving cryostattransfer cart (MCTC)

Tritium Facility

1 m

Targetchamber

Characterizationstation

Shroudretractor

C-mount

26477 28900

Shadowgraph X-ray pinhole camera

MCTC

• Target positioning accuracy is more challenging with cryogenic targets.

A 2-D hydrodynamic simulation demonstratesgood agreement in predicting targetperformance for shot 35713 (α ~ 4)

TC6465f

1-D 2-D Expt

Y1n 9.1 × 1010 1.8 × 1010 1.6 × 1010

Y2n 1.7 × 109 2.8 × 108 2.6 × 108

<ρR> (mg/cm2) 117 101 88

Tion (keV) 1.9 1.7 3.0

150

180

ρR(m

g/c

m2)

Azimuth (degrees)

1206000

100

50

Range of measuredρR’s (4 LOS)

2-D DRACO polarlineout of the total ρR

100

75

0

R (μ

m)

50250–25–50

Z (μm)

DRACO 2-D densitynear peak burn,shot 35713α ~ 4, 17.5 kJ

53

24

5

Density(g/cm3)

11

50

25

15μm offset, 4.1 μm ice rms

E13085f

Hydrodynamic simulations are consistent with implosiondata over a wide range of ice roughness and target offset

0.8

0.6

0.4

0.2

0.0

Yie

ld r

ela

tive t

o 1

-Dα ~ 6

0 543

rms ice roughness (μm)modes � = 1 to 16

21

30-μmoffset

33600(28 μm)

35653(38 μm)

34998(28 μm)

33945(23 μm)

DRACO (no offset)

33687(42 μm)

33955(32 μm)

Performance with NIF requirements

Conversion to direct drive requires the addition of optics to the midplane of the NIF target chamber

TC5056c

Indirect Drive Direct Drive

TC6892

Polar direct drive (PDD) enables ignition experimentswhile the NIF is in its x-ray drive configuration

• Refractive losses due to higher angles of incidence at the equatorcan be compensated for by varying pulse shapes.

• Preliminary 2-D simulations of PDD achieve ignition with gain 30.

23.5°

30°, 44.5°

50°, 44.5°

1690 μm

Z

23.5°

44.5°

77.45°

Z

Polar Direct Drive

85°

55°

23.5°

77.45°

45°

23.5°

Symmetric Drive Polar Direct Drive

OMEGALaser Bay

OMEGAtarget chamber

OMEGA EPLaser Bay

Compressionchamber

G5546r

OMEGA EP will be used to backlight cryogenicimplosions and study fast ignition

OMEGA EP: ICF Program

Short-pulseperformance

Short pulse (IR)

IR energyon-target (kJ)

Intensity (W/cm2)

Short-pulsebeam 1

1 to 100 ps

2.6

6 × 1020

Short-pulsebeam 2

35 to 100 ps

2.6

~ 4 × 1018

OMEGA EPtargetchamber

Mainamplifiers

OMEGA EP will becompleted in FY07

TC5944f

Core distortions in OMEGA cryogenic implosions can bediagnosed using backlighting techniques and OMEGA EP

*F. J. Marshall et al., Rev. Sci. Inst. 68, 735 (1997).

ΔXmotion = ν • Δt= 5 × 107 • 5 × 10–12

= 2.5 μm

Monochromatic imager(3-μm resolution)*

P target (~2.3 keV)20-ps pulse for stagnation

LiF target (~900 eV)5-ps pulse for startof deceleration

Monochromaticimager (3-μmresolution)*

200 μ

m

100 μ

m

CompressedEP beam Compressed

EP beam

E12526d

A complementary approach to hot-spot ignition, namelyfast ignition is an active area of research at LLE

OMEGA EP: Fast Ignition

Fast Ignitor*Conventional ICF

Fastinjectionof heat

Fast-heated side spot ignitesa high-density fuel ball

ρhot ≈ ρcold (isochoric)

Low-density central spot ignitesa high-density cold shell

ρThot ≈ ρTcold (isobaric)

Tem

pera

ture

Radius

Key physics issues

• hot electron production• transport to the core• core formation

* M. Tabak et al., Phys. Plasmas 1, 1626 (1994).

Mass d

en

sit

y

Hot spot

Burnwave

Tem

pera

ture

Mass d

en

sit

y

Radius

Burnwave

Fast ignition with cryogenic fuel will be conductedon OMEGA with the high energy petawatt OMEGA EP

E11710g

Holeboring Ignition

10 psLight pressurebores hole in

coronal plasma.~1-MeV electronsheat DT fuel to~10 keV, ~300 mg/cm2.

Channeling Concept* Cone-Focused Concept**

Au cone

Singleignitor

beam: 10 ps

e–

* M. Tabak et al., Phys. Plasmas 1, 1626 (1994).** R. Kodama, Nature 418, 933 (2002).

Key physics issues

• hot electron production• transport to the core• core formation

Direct-drive holds great promise for ignition on the National Ignition Facility (NIF)

TC6890

• Two paths to direct-drive ignition on the NIF have been identifi ed—symmetric and polar.

• Good agreement between predictive simulations and ignition-scaled cryogenic implosions is obtained on the OMEGA laser for symmetric drive.

• Polar direct drive may allow for ignition on the NIF in its x-ray drive confi guration.

• A new high-energy petawatt capability at OMEGA (OMEGA-EP) will provide the ability to image core distortions in cryogenic implosions and test fast-ignition concepts.

Summary/Conclusions