Issues and Opportunities for IFE Based on Fast Ignition

HAPL Progress Meeting

13-14 November 2001

Pleasanton, CAPreviously presented at Second International Conference on

Inertial Fusion Science and Applications

9-14 September 2001, Kyoto, Japan

W. Meier, S. Hatchett, W. Hogan, M. Key, J. Latkowski, L. J. Perkins, S. Reyes, M. Tabak

Lawrence Livermore National Laboratory*

D. Goodin, R. Stephens, General Atomics

K. A. Tanaka, Institute for Laser Engineering, Osaka University

* Work performed under the auspices of the U. S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract W-7405-Eng-48

UCRL-PRES-143227

IFSA-2001 WRM 2

Co-authors have several related talks/posters at this meeting

• M. Tabak“Issues for Fast Ignition”• L. J. Perkins et al., “Inertial Fusion Energy with Advanced

Fusion Fuels” • J. F. Latkowski et al., “Opportunities to Expand the IFE

Chamber Parameter Space with Fast Ignition” • D. Goodin and R. Stephens, “Target Injection and Fabrication

Possibilities for Fast Ignitor IFE”• M. Key et al., “Studies of Energy Transport by Relativistic

Electrons in the Context of Fast Ignition” • R. Stephens and S. Hatchett, “Optimizing the Structure of Fast

Ignition Targets”• K.A. Tanaka et al., “Review of Fast Ignition Related Research

at ILE Osaka”

IFSA-2001 WRM 3

Fast ignition offers potential advantages for IFE

• High gain (> 100) at low driver energy (< 1 MJ) reduces the recirculating power for the driver, driver cost and the cost of electricity (COE).

– Possibility for smaller-scale, lower-cost development steps

– Competitive COE with smaller plant sizes (< 1000 MWe)

• High gain with two-sided, indirect-drive for fuel compression

– This would allow use of thick-liquid-wall chambers giving long-life structures

• Less demanding requirements for fuel compression

– Perfectly symmetric implosion not required as in hot spot ignition

– Lower overall density (~100-300 g/cm3 vs. 500-600 g/cm3)

IFSA-2001 WRM 4

Fast ignition offers potential advantages for IFE (cont.)

• Relaxed fuel compression requirements lead to other benefits

– Relaxed drive symmetry (indirect-drive, possibly even one-sided?)

– Relaxed target fabrication requirements

– Targets less susceptible to thermal and mechanical damage during injection

• Possibility of igniting advanced fuels with reasonable driver energies

– Tritium self-breeding in target would allow use of non-Li bearing liquids in thick-liquid-wall chambers

– High charged particle output compatible with direct conversion

– Advanced fuels, however, ignite at a higher temperature, so will require more compression.  Achieving that extra compression will require tighter specs, perhaps offsetting gains listed in the first bullet.

IFSA-2001 WRM 5

To achieve the promise of FI, several issues must be resolved

• Target physics (not the subject of this talk)

• Target fabrication – Added features such as high-z cone for ignitor beam complicate fabrication

• Target injection and tracking – How do requirements differ from direct drive?

• Beam propagation through blow-off plasma from compression beams

• Beam focusing to small spot size (typically < 50 m diameter) and timing to < 100 ps (compressed core dwell time)

• Size, location, and protection of final optics

• Impact of chamber conditions (e.g., residual vapor) on propagation, focusing of ignitor beams

IFSA-2001 WRM 6

Fast ignition delivers high gain at lowdriver energy

Direct-drive, hot spot ignition, = 1 (NRL pt.)G = 135 at E = 1.2 MJ

EL =I2 =

=

IFSA-2001 WRM 7

FI leads to significantly lower COEand smaller optimal driver energy

Emin COE COE(MJ) (¢/kWh) (%)

2.4 8.6 37 1.4 7.0 120.8 6.3 0

Assumes:Net power = 1000 MWeSombrero cost scalingDriver efficiency = 7%Driver total cost = $500/JFI Laser adds 20% to driver cost (e.g., $100M at Ed = 1 MJ)

0 1 2 3 4 50

2

4

6

8

10

DD hot spot ignitionDD hot spot ignitionFast ignition

Driver Energy, MJ

CO

E, c

ents

,kW

eh

= 2 = 1

IFSA-2001 WRM 8

0 1 2 3 4 50

2

4

6

8

10

Direct-drive hot spot ignitionDirect-drive hot spot ignitionFast ignitionRep-rate for DD HS ignitionRep-rate for DD HS ignitionRep-rate for fast ignition

Driver Energy, MJ

Rep-rate limit could lead to off-optimal operation and higher COE

E5HZ COE COE(MJ) (¢/kWh) (%)

4.7 9.9 46 2.5 7.8 151.4 6.8 0

Example for 5 Hz:

= 2

= 2 = 1

= 1

IFSA-2001 WRM 9

0

2

4

6

8

10

12

14

0 250 500 750 1000 1250 1500

Net Power, MWe

CO

E,

cen

ts/k

Weh

DD HSI, = 2

DD HSI, = 1

Fast Ignition

With fast ignition, a given COE can be met at lower net power

Example:For COE = 8 ¢/kWehFI needs < 600 MWevs. 750-1250 MWe for hot spot ignition

• Small plant components(e.g., HTS) could have lower development cost• Multi-unit plants could take advantage of small unit sizes

Max rep-rate = 10 Hz

IFSA-2001 WRM 10

Target fabrication considerations

• Opportunities– May be able to stamp hard-frozen DT hemispheres and “weld” together– DT might serve as ablator so no capsule or overcoat would be needed.

This could significantly reduce T inventory in target factory due to fast fuel layer formation (eliminates diffusion fill step).

– Perfect surface finish not required• Issues

– Cone-focus approach requires insertion of high-z cone – Is it compatible (possible) with existing layering techniques?

– Cone will add debris to target output spectrum and likely directed toward ignitor beams

• Proposed work– Implosion calculations with rough, solid DT shell to see if a dense core

can be assembled as a first step to better define target fab requirements/ options

– See talk by D. Goodin and R. Stephens

Example - Stamp out DT layers for mass production

• Simplifies target construction

– Layer is not dependent on shell thermal properties or shape

– Allows pure DT shell (eliminates plastic ablator and radiative preheat coating)

– Eliminates diffusion fill and layering time

– Little temperature control needed during fabrication

– Has increased tolerance to heating during injection

• Possible problems

– Rougher surface

– Join defect

– Temperature control (can’t use liquid or gaseous helium to

control temperature while storing)

– Cracking between cone and shell

IFSA-2001 WRM 12

Target injection/tracking

• Opportunities– If hard frozen DT can be used, heating during injection is less of

an issue– Cone-focused design could reduce effects of chamber gas on DT

fuel if injected cone end first– FI target weighs substantially more than a direct drive target, so

would be less affected by residual atmosphere and contact forces - should be somewhat easier to reproducibly deliver to target chamber center

• Issues– Tracking/timing precision for compression beams should be

similar to hot-spot ignition targets– Ignitor beams would be triggered off of compression beams with a

fixed delay– Beam pointing for ignitor beams – For cone-focus design must be

centered on tip of cone. What is allowable deviation?

IFSA-2001 WRM 13

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

Chamber pressure, torr

Ignitor beam propagation may limit chamber background gas density

Issue - if the chamber background pressure is too high, beam generated plasma creates non-linear index of refraction that could affect steering of ignitor beams.

Plasma cycles during 10 ps laser pulse at 10 cm from target. What is acceptable?

Phase retardation over propagation length, waves

IFSA-2001 WRM 14

Final optics considerations

• Compression beam requirements similar to hot spot ignition

(but may not require uniform illumination)

• Size of optics for ignitor beams may be set by laser damage or focusing requirement

• Need to understand limits for various optics options

– Gratings

– Coated focusing mirrors

IFSA-2001 WRM 15

Allowable stand-off for ignitor beams increases with optics diameter which eases optics protection

0 5 10 15 20 25 300

10

20

30

40

50

Focal length, m

Spo

t rad

ius,

mic

ro-m

Diffraction limited spot radius with final optic diameter of:

Do = 50 cmDo = 100 cm

Typical required spot size ~ 33 mSpot radius, m

Optics must be protected from (or be able to survive) x-ray, debris and neutron threats from targets.

IFSA-2001 WRM 16

Large diameter, thin transmissive gratings are being developed, will be evaluated for FI

• Fresnel lens fabricated in 80-cm diameter, 1-mm thick fused silicasubstrate using 2-mask lithography and HF etching for the Eyeglassproject. The tabs are placed in order to fold the lens.

• Monolithic 80-cm diameter, 1 mm thick fused silica Fresnel lensescan also be fabricated using the same technology.

IFSA-2001 WRM 17

50 60 70 80 90 1000

20

40

60

80

Diameter of final optics, cm

Num

ber

of ig

nito

r be

ams

Number of final optics depends on size, fluence limit and on beam coupling efficiency

Ei = 40 kJ = ignition energy

ELaser = Ei/c = laser energy

c = coupling efficiency

o = laser fluence limit, J/cm2

c = 50% , o = 1 J/cm2

c = 25% , o = 2 J/cm2

c = 25% , o = 1 J/cm2

NbeamsELaser

o

4

Do 2

Number of ignitor beams

IFSA-2001 WRM 18

3.9 mm diam.

One method to improve ignitor-beam / target coupling is the cone-focused concept

In the “cone-focused” concept for FI, the spherical capsule has a conical shell of dense material penetrating through one side to near capsule center. The implosion proceeds as usual, the cone holding open a clear path for the high intensity laser so that its energy can be deposited within ~100 µm or less of the high density core. (Ref.: Steve Hatchett, LLNL)

DT ice

Au cone

<R> = 2.0 g/cm2

Fuel capsule/ablatorTypical imploded core configuration

400 m

IFSA-2001 WRM 19

FI opens the possibility of igniting advanced fuels with reasonable driver energies

• Could tailor target to maximize charged particle output to make direct conversion more attractive– See Perkins talk

• Tritium-lean, self breeding targets opens possibilities in chamber design– See Latkowski talk

• Using advance fuels, however, gives up some of the potential advantages of FI– Requires higher driver energy– Advanced fuel targets must have a small volume which

is pure DT for ignition – increases target fabrication difficulty

IFSA-2001 WRM 20

It may be possible to efficiently burn D-D or D-3He fuels in fast-ignited targets

Fast ignition laser Slow compression driver beams

DT ignition core

DD or D3He main fuel

Cone focus hohlraum

Energy spectrum convertor

Schematic – not to scale

Advanced fuels: DD or D3He main fuel with self-breeding, fast-ignited DT ignition core

Features: – ≤ 1% T2 inventory compared to all DT targets

–  ≤ 5% of yield in fast neutrons

– > 90% charged particle output

– Advanced energy conversion

(See Perkins talk)

IFSA-2001 WRM 21

Fast ignition opens some opportunities in chamber design

• Gain with two-sided, indirect drive targets would be high enough. This would allow use of thick liquid wall chambers.

• Tritium self-sufficient, advanced fuel targets would allow use of other liquids (see Latkowski poster)HYLIFE-II

Thick-liquid wall protects structures from neutron

damage

IFSA-2001 WRM 22

Summary – the good news:FI offer many potential advantages for IFE

• High gain at low driver energy

• Lower COE and/or small size plants

• Indirect drive targets with thick liquid walls

• Reduced constraints on target fab and injection

• Possibility of using advanced fuels

IFSA-2001 WRM 23

Summary – the challenge:Significant issues musts be addressed

• Target modeling to quantify requirements for assembling main fuel

• Assessment of target fab and injection issues and development of options

• Optics development for ignitor beams

• Self-consistent design of final optics layout for ignitor beams including protection from target emissions

• Analysis of ignitor beam propagation in post-shot chamber environment and tracking/pointing/focusing to small spot size

• Chamber wall protection from target output – enhanced debris compared to hot-spot ignition and possibly asymmetric loading