M. S. Tillack

IFE Technology Research at UC San Diego

MAE Departmental Seminar6 October 2004

http://aries.ucsd.edu

Many people have contributed to this research

Faculty and

Staff:

C. V. Bindhu

Z. Dragojlovic

A. C. Gaeris

S. S. Harilal

F. Najmabadi

T. K. Mau

J. E. Pulsifer, MS’98

A. R. Raffray

X. R. Wang

M. R. Zaghloul

Students:

N. Basu, MS’98

D. Blair, PhD’03

L. Carlson

S. Chen

B. Christensen,

MS’04

K. Cockrell

M. Mathew

J. O’Shay

K. Sequoia

New kids

on the block:

K. Boehm

J. Hanft

J. Mar

R. Martin

E. Simpson

The inertial confinement fusion concept

The goal of “ICF” research is to ignite DT targets in order to explore high energy

density physics

Indirect DriveDirect Drive Z-pinch

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Omega

2 mm

DT ice

Au cone

Be+Cu

ρ=3. -5e gcm-3

Ignition scale“fast ignition”

1.95 mm1.69 mm1.50 mmDT Vapor0.3 mg/ccDT FuelCH Foam + DT1 mm CH+500 Å Pd

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NIF

60 beams/40 kJ

192 beams/2 MJ@3

2 MJ of x-rays

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Z

The goal of “IFE” research is to generate power economically

• HAPL: Laser driver (DPSSL or KrF) with direct drive targets and dry walls

• HI-VNL: Ion accelerator, indirect drive targets, liquid chambers

• Z-IFE: Z-pinch driver

In addition to target physics, key issues include efficient rep-rated drivers, target mass production, target injection, reliable chambers and optics

Our IFE research is focused on the key issues for IFE chambers and chamber

interfaces

Prometheus-L Reactor Building

1. Chamber walls that survive long-term exposure

2. A residual chamber medium which allows propagation of targets and beams through it

3. Final optics that survive long-term exposure

4. Cryogenic targets that survive injection and are properly illuminated

neutrons & gammas

x-rays

ions

1.Prompt transport of energy through and deposition into materials (ns-s)

2.Radiation fireball & shock propagation, mass loss from walls (1-100 s)

3.Afterglow plasma & hydrodynamics (1-100 ms)

4.Liquid wall dynamics (ms-s)

5.Long-term changes in materials

Following target explosions, several distinct stages of chamber response occur:

Wall protection and target/driver propagation depend on the details of target

emissions

Fireball forms from captured x-ray and ion energy, re-radiates on

a slower timescale

Fireball forms from captured x-ray and ion energy, re-radiates on

a slower timescale

200-400 MJ released per target

Our chamber wall research simulates thermomechanics of armor and energy

transport from ablation plumes

• High-cycle fatigue of tungsten armor

– simulations with short-pulse lasers– phenomena similar to optics

damage

• Laser plasma expansion dynamics– modeling of laser plasma– ablation plume experiments– magnetic diversion

We use laser ablation plumes to provide a surrogate plasma to study IFE target

emissions

1.5 cm

Time-resolved imaging and spectroscopy are performed with 2-ns gated camera

and PMT

An aluminum ablation plume is confined by a moderate magnetic

field5 GW/cm2, 8 ns, Al

target

0.64 T Rb

free expansion velocity v=6x106 cm/s

5 GW/cm2

The plasma beta initially is large, but

falls quickly

Similar to results without B, the initial 30-40 ns is

ballistic, followed by plume drag

The expansion is slowed after the thermal beta falls

Our IFE research is focused on the key issues for IFE chambers and chamber

interfaces

Prometheus-L Reactor Building

1. Chamber walls that survive long-term exposure

2. A residual chamber medium which allows propagation of targets and beams through it

3. Final optics that survive long-term exposure

4. Cryogenic targets that survive injection and are properly illuminated

We seek to understand the residual chamber medium and the propagation of

targets and beams through it

• Chamber dynamic response modeling and “chamber clearing”

• Target transport through the perturbed chamber

• Aerosol generation in liquid-protected walls

– explosive phase change (evaporation)– homogeneous nucleation in laser

ablation plumes (condensation)

• Laser propagation in background gas

Spartan simulation

Rapid condensation of vapor ejected from liquid-protected IFE chamber walls was

modeled numerically and experimentally

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0.15 Torr

These processes also occur in laser machining, pulsed laser deposition, and other applications

Again, lasers are used to simulate ion & x-ray deposition and response

The homogeneous nucleation rate and critical radius depend on saturation ratio &

ionization

# of atoms

• Ion jacketing (dielectric behavior of vapor) reduces the energy barrier

Without ionization With ionization

Si, n=1020 cm–3, T=2000 K

• High saturation ratios result from rapid cooling during plume expansion

• Extremely small critical radius and high nucleation rates result

Si, n=1020 cm–3, T=2000 K, Zeff=0.01

W(r)=- 34 r r3 nDn_ i +4r r2v + 1- 1/f c_ i Q2/8r f o_ i 1/r - 1/ra_ i

The condensate size distribution was measured at stagnation using atomic force

microscopy

500 mTorr He

5x108 W/cm2 5x109 W/cm2

5x109 W/cm25x108 W/cm2

5x107 W/cm2

• Correlation between laser intensity and cluster size is observed.

• Is it due to increasing saturation ratio or the presence of ions?

• Plasma temperature and density were measured spectroscopically using Stark broadening and line ratios

• Saturation ratio and ionization state were computed using these measurements and assuming local thermodynamic equilibrium

The saturation ratio is inversely proportional to laser intensity

As laser intensity increases, ionization increases but saturation ratio decreases

Maximum charge state at 50 ns, 1 mm from Al target, as derived from spectroscopy and assuming LTE.

Saturation ratio at 1 mm, derived from spectroscopy and assuming LTE.

Our IFE research is focused on the key issues for IFE chambers and chamber

interfaces

Prometheus-L Reactor Building

1. Chamber walls that survive long-term exposure

2. A residual chamber medium which allows propagation of targets and beams through it

3. Final optics that survive long-term exposure

4. Cryogenic targets that survive injection and are properly illuminated

The final optic in a laser-IFE plant sees line-of-sight exposure to target emissions

• Laser-induced damage

• x-rays

• ions

• neutrons and -rays

• contaminants

Damage threats:

• 5 J/cm2

• 2 yrs, 3x108 shots

• 1% spatial nonuniformity

• 20 m aiming

• 1% beam balance

Mirror requirements:

We are developing damage-resistant final optics based on grazing-incidence metal

mirrors

The reference mirror concept consists of a stiff, light-weight, radiation-resistant substrate with a thin metallic coating optimized for high reflectivity (Al for UV, S-polarized, shallow )

Al reflectivity at 248 nm

~50 cm85˚

Laser damage is thermomechanical in nature: high-cycle fatigue of Al bonded to

a substrate

300 nm Coating

300

305

310

315

320

325

330

335

340

0.E+00 1.E-08 2.E-08 3.E-08 4.E-08 5.E-08 6.E-08Time, s

Temperature, K

SurfaceInterfaceSiC (0.5 um)SiC (1 um)SiC (2.5 um)SiC (5.0 um)

q”=10 mJ/cm2Al: 20-500 nmSiC: 10 m

S-N curve for Al alloy

Basic stability

High cycle fatigue

Differential thermal stress

Testing is performed at the UCSD laser plasma and laser-matter interactions laboratory

cubedumpcube1/2 waveplatebeam diagnosticsdumpviewing portspecimenmount

400 mJ, 25 ns, 248 nm

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Pure Al can have large grains, resulting in slip plane transport and grain boundary

separation (data at 5 J/cm2, 50 shots)

)(in

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C

Several fabrication techniques have been explored to enhance damage

resistance

• Monolithic Al (>99.999% purity)

• Thin film deposition on polished substrates

– sputter coating, e-beam evaporation

– Al, SiC, C-SiC and Si-coated substrates

• Electroplating

• Surface finishing

– polishing, diamond-turning

– magnetorheological finishing

– friction stir processing

• Advanced Al alloys

– solid solution hardening

– nanoprecipitation hardening

Finer-grained electroplated Al withstands higher fluence, but eventually goes unstable

At 18.3 J/cm2 laser fluence: Grain boundaries still separate Damage is “gradual” at 18.3 J/cm2

Mirror survived 105 shots

At 33 J/cm2 laser fluence: Rapid onset (2 shots) Severe damage (melting) probably starts with grains

High shot count data extrapolates to acceptable LIDT; end-of-life exposures are

still needed

In addition, we are continuing to develop improve- ments such as “Al-on-Al”, hardened alloys, etc.

Our IFE research is focused on the key issues for IFE chambers and chamber

interfaces

Prometheus-L Reactor Building

1. Chamber walls that survive long-term exposure

2. A residual chamber medium which allows propagation of targets and beams through it

3. Final optics that survive long-term exposure

4. Cryogenic targets that survive injection and are properly illuminated

Targets play a central role in many of the critical issues for

IFE

MIRROR R 50 m

TRACKING10 m STAND-OFF

5 m

CHAMBERR 5 mT ~1500 C

ACCELERATOR8 m1000 gCapsule velocity out 400 m/sec

INJECTORACCURACY

TRACKINGACCURACY

GIMM R 30 m

R 6.5mT ~ 1000C

1.Mass production 500,000/day, $0.25/target, sub-m uniformity

2.Injection 400 m/s, 1-5 mm accuracy

3.Tracking/steering 20/200 m accuracy, ~64 beams

4.Survival 18˚K target in a 1000˚C turbulent chamber

We collaborate with General Atomics on several target-related

tasks

1.Target fabrication• indirect drive target layering

via external thermal control

2.Target injection• sabot transport• capsule steering

3.Target tracking/beam steering• interface with beam steering

system

4.Target survival

Target steering is possible in the chamber using a short-pulse guide laser

• Use shortest pulse possible for minimum ablation depth: 15 fs

• Use highest pulse energy to achieve maximum impulse (subject to total power and rep rate constraint)

• assume 100 kW for 15 ms, 100 kHz – 1 J pulses• assuming 1 mm2 contact, 1016 W/cm2 if the full beam hits• 1014 W/cm2 is a more likely value

• Instead of steering 64 heavy mirrors, why not steer one 4-mg target?

• Can be accomplished using an annular guide-laser beam in the chamber

• Biggest concern is amount of ablation needed and degradation of target surface due to that ablation

Analysis of ablation depth and impulse was performed using the 1D Hyades rad-hydro

code

• 850-nm laser pulse, 15 fs FWHM, 1014 W/cm2

• 500-nm Au coating, 100 m CH substrate

• run code until plume heating of surface is negligible and acceleration phase is complete (1 ns)

DT CH

Au

feathered grid

3 nm

16 nodes

Ablation depth and expansion velocity of Au

d ~2.5 nm total ablationm ~2.5 g/cm2 accelerated to

<v> ~ 2x105 cm/smv ~0.5 (g-cm/s)/cm2

assuming 1 mm2 contact and 5 mg target, each “kick” results in 1 cm/s correction of the target transverse velocity

For more information on our research and student

opportunities,visit our web sitehttp://

aries.ucsd.edu