outline spectroscopic investigations of implosions on the ... · the national ignition facility...

LLNL-PRES-668439 This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC

Spectroscopic Investigations of Implosions on the National Ignition Facility

ICTP-IAEA Workshop on Modern Methods in Plasma Spectroscopy

March 23-27, 2015 Trieste, Italy

Lawrence Livermore National Laboratory LLNL-PRES-668439 2

Outline   Inertial Confinement Fusion (ICF)

•  the National Ignition Facility (NIF)

•  ICF basics

  Implosions and instability growth

  Spectroscopy with radiation transport

  Spectroscopy / radiography


The National Ignition Facility (NIF) is a 192 beam laser system in operation at LLNL since 2009


National Ignition Facility (NIF) overview


NIF target chamber + positioner


Outline   The list of bullets will start from here

diagnostic port

beam port


Indirect-drive Inertial Confinement Fusion (ICF) at the National Ignition Facility (NIF)

NIF also provides a platform for High Energy Density (HED) experiments

  NIF uses 192 laser beams to deliver 1-2 MJ of energy over several ns

  ~75% of the energy is converted to X-rays in the hohlraum

  Radiation field inside the hohlraum is ~thermal at Tr ~ 300 eV


Ignition will require a convergence ratio of ~35

This requires extreme precision and control to maintain symmetry

ρ

50 million degrees 100 g/cc

Hot spot

alph

60 μm

~2 mm diameter


NIF uses a graded-doped CH capsule with a layer of frozen DT fuel inside a high-Z hohlraum

Hohlraum Wall: – Au

Laser Beams (24 quads through each LEH arranged to illuminate two rings on the hohlraum wall)

Laser Entrance Hole (LEH) with window

Hohlraum Fill – He at 0.96 mg/cm3

10.01 mm

Graded-doped CH Capsule

Solid DT fuel layer

Cryo-cooling Ring

Aluminum assembly sleeve

° ° ° °

Capsule Fill Tube: – SiO2 , 10 µm OD


Recent implosions have achieved significant α-heating

0

5

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1305

0113

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1013

0802

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0927

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2014

0225

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0414

0311

1405

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0520

1407

0714

0819

Yie

ld (k

J)

Yield from fuel compression Yield from self heating

start of experiments

2012

mid-2014

Ignition (G>1)

Alpha-heating

g ( )

p g

ignition parameter

O. Hurricane, et al, Nature 506, 343 (2014)


Significant improvements have come from better stability

High foot shots trade convergence for stability

"low-foot"

Radiation Temperature Drive

"high-foot"

300

0

T rad

(eV)

t (ns) 0.0 10. 20.

100

200

5.0 15.


Material interfaces are Rayleigh-Taylor unstable during compression (either acceleration or deceleration)

Perturbations which grow too large can inject material into the hot spot, resulting in cooling

Fill tube - 10 µm SiO2

DT ice

DT gas

undoped ablator

doped ablator (varying Z and %)

rnggr

Dopant shields fuel against preheat from Au M-band emission

Ice

Ablator

Hotspot

Growth of “seeds” results in perturbations of shell areal density (ρR)


The amount of material mixed into the hot spot correlates with the implosion yield

Mix mass inferred from X-ray emission relative to neutron yield - T. Ma et al, PRL 111 (2013)

P. Patel

mix “cliff”


•  CH mix of ~100 – 1000 ng is need to explain low-foot results •  Known capsule perturbation seeds

•  Capsule roughness •  Fill tube •  Tent •  Individual defects

•  Simulations do not predict this much CH will enter the hot spot

Diagnosing / verifying hydrodynamic growth of perturbations is critical to understanding implosions

We are developing methods to measure growth and mix in the late stages of the implosion

-800

-600

-400

-200

0

200

400

600

800

0 20 40

Def

ect H

eigh

t (nm

)

Defect Width (μm)Either seeds are larger than we think – or –

Hydrodynamic growth is larger than we simulate ⇒


Perturbation growth brings Ge dopant into the hot spot

temperature (6 keV)

electron density (2.5 x 1026 cm-3)

material density (1000 g/cm3)

gas

CH

ice

50 μm

•  2D high-resolution HYDRA simulation •  Perturbation due to fill tube •  ~4 ps before peak compression

B. A. Hammel, et al, Phys. Plasmas 18, 056310 (2011)

CH+Ge


0.001

0.01

0.1

1

flux

(erg

s/s/

Hz/s

ter)

140001200010000800060004000frequency (eV)

The spectrum reflects emission over a wide range of conditions and the effects of radiation transport

  K-shell emission from hot Ge (>2 keV)   1s 2p absorption from warm Ge (300 eV)   2p 1s fluorescence from cold Ge (200 eV)

“K-shell” features provide information on mix

1.2

1.0

0.8

0.6

0.4

0.2

0.0

flux

(erg

s/s/

Hz/s

ter)

11000105001000095009000frequency (eV)

“He-αα” + “Ly-α”

Ge K-edge fluorescence

1s 2p

CH attenuation

He-β

2p 1s

(e(eflu

x (e

flux

(eeflu

x (e

flux

(eflu

x (e

flux

(eflu

x (e(((e

flux

(ex

(ex

(eflu

x (e(

ux (e(

flux

(fl

//

rgs/

s/rg

s/s/

rgs/

s/rg

s/s/

rgs/

s/rg

s/s/

rgs/

s/rg

s/s/

rgs/

s/rg

s/s/

rgs/

s//rg

s/s/

rgs/

s/rg

s/s///

rgs/

sgs

/srg

s/s

rgs//gssss

Hz/s

teHz

/ste

Hz/s

teeeeHz

/steete

Hz/s

teteste

Hz/s

teHz

/st

Hz/s

Hz//

HHzHHHr)r)r)))r)))r)r)r)rrr


Monochromatic imaging accentuates features

Fluorescence is locally weak but extends over a large area

position (μm)

posi

tion

(μm

)

position (μm)

10.22 keV (He-α) 9.85 keV


Experimental spectra show optically thick features similar to simulations

S. P. Regan, et al, Phys. Rev. Let. 111, 045001 (2013)

Co

nti

nu

um

-su

btr

acte

din

ten

sity

(J/

keV

/ste

r)

9.8 10.0 10.2Photon energy (keV)

10.4 10.6 10.8

2.5

2.0

1.5

1.0

0.5

0.0B-like Ge

Be-like Ge

Li-like GeHe-like Ge

Ge Hea + satelliteemission

Ge Ka emissionFluorescentinner-shell transitionsof Ne-like to N-like Ge

Ge Lya + satellite emission

2|2 fits

Best fits

This 0D analysis only addresses the “He-α” emission

•  Shot N120219

•  Capsule doped with Si, Ge, and Cu

•  Best fit uniform conditions: Te ≈ 3 keV ne ≈ 1025 cm-3 (ρR)Ge ≈ 0.3 mg/cm2

τ He-α ≈ 10


•  Count photons emitted at a given frequency Nν •  Infer total atom population Ni from

•  Complications: •  Emission rate per atom, , depends strongly on Te , ne

estimate Te , ne from spectral features

•  Optical depth effects (lines + shell)

approximate with escape factors, shell ρr fitting parameter

•  Non-uniformity in space and/or time

•  Non-emitting mass is invisible (but likely unimportant)

The basic 0D analysis method counts photons

The resulting mass estimates have large uncertainties

Nν =Niην (ne,Te )Δt

ην


1D modeling provides additional information

Radiation transport connects the spectral features

Hot spot:DT gas + mix

“Ice”:DT + mix

unablated shell:cold, dense CH + Ge

3-shell model (O. Ciricosta, Oxford U.)

Conditions in the hot spot   Ge emission features   continuum radiation

Hot spot radiation + cold shell absorption   K-shell edge   K-α fluorescence

Photon energy (eV)

Tim

e-in

tegr

ated

si

gnal

(J/k

eV/s

ter)

4-shell model allows separate mixed and unmixed regions (in pressure balance) in the hot spot

4-shell fit hot spot parameters - mix : 40 ng (34 – Regan PRL) mass: 6.8 μg (6.5 – neutron yield)


•  “He-α” emission – hot spot Te, ne •  He-α red wing – “ice” Te, ne •  K-shell jump – shell ρR

•  K-shell position, slope – shell Te, ne + continuum lowering model

•  K-α fluorescence – shell Te

Features are sensitive to physical parameters and models

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0.3

0.2

0.1

0.0

signa

l (J/

keV/

ster

)

1110photon energy (keV)

shell Te 100 eV 150 eV 200 eV 250 eV 300 eV

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

sign

al (

J/ke

V/s

ter)

1110photon energy (keV)

shell Te = 100 eV mod. Ecker-Kroll non-degenerate Stewart-Pyatt

ne = 1025

cm-3

Tf = 350 eV


Argon in the gas provides backlighting to analyze the growth of perturbations

Machined perturbations (mode ~60) are easily visible

105

106

107

108

109

powe

r (W

/sr)

17.817.617.417.217.016.8time (ns)

no Ar 1% Ar

above 8 keV above 10 keV

peak compression comp

shock bounce ock bo

simulated X-ray power Low-foot High-foot

mode 60 mode 90 mode 60 mode 90

These images are from an earlier time and use an external Vanadium backlighter


Simulated diagnostics diagnose the shell ρR

70% He 30% D 1% Ar

•  Innermost layer doped with Cu •  Images above / below K-edge

obtained with Ross filter pairs


Intensity jump across K-edge provides information on ρR

Analysis of a synthetic spectrum reproduces the ρR variations of a rippled shell

Iν = Bν (T )e−τν , τν = ρR( )κν

materials∑

hot spot Te

t

•  Analysis requires Te of hot spot - provided by spectroscopy

•  Integrate over filter response function •  ρR of ablated material estimated from simulation

lnI↓B↑

I↑B↓

⎛

⎝⎜⎞

⎠⎟=

ρR( )shκ sh

↓ + ρR( )abκ ab

↓

ρR( )shκ sh

↓ + ρR( )abκ ab

↓⎡⎣

⎤⎦ ν↓ ν↑( )3 + fdop ρR( )sh Δκ dop

ablated material

shell


Initial experiments look promising •  Target with pre-imposed ripples •  Inhomogeneity along ripples due to machining imperfections •  Spectra show emission from shock bounce and peak compression •  Spacing, ρR vs time give capsule velocity, growth factors

An improved spectrometer (coming soon) will enable better quantitative analysis of ρR

μ

μ

17.33ns, R~150μm

μ

μ17.46ns, R~130μm

L. Pickworth


Recent work has focused on the effect of the “tent” – a thin plastic membrane which supports the capsule

Rayleigh-Taylor growth is seeded where the tent

leaves the capsule

~ 50-100 nm thick plastic (Formvar)

Tent

The tent can induce large (~40%) defects in the shell ρR


In a DT target, a jet of CH penetrates the shell and cold DT enters the hot spot

Yield is reduced by ~50%

50 nm tent: Tangential +14 deg

CH does not quite reach the hot-spot


An appropriate dopant could provide a spectroscopic signature of the jet / bubble resulting from the tent scar

These simulations are of a Symcap, in which DT ice is replaced by an equivalent mass of CH


An appropriate dopant could provide a spectroscopic signature of the jet / bubble resulting from the tent scar

These simulations are of a Symcap, in which DT ice is replaced by an equivalent mass of CH


12x10-3

10

8

6

4

2

0

radia

tive

powe

r (er

g/s/

Hz/s

ter)

20x10315105frequency (eV)

9º 18º 27º 36º 45º 54º 63º 72º 81º 90º

The tent scar allows radiation to escape the hot spot

Radiative energy loss may also be significant

90°

0°

temperature CH density t t CH d it100 μm 100 μm

100 μm

Escaping radiation spectrum at multiple viewing angles


  Spectral features •  Ly- and He- emission, K-α fluorescence

•  K-edge absorption

•  Continuum shape

•  Time-dependence

  Diagnostics for •  Hot spot temperature

•  Shell optical depth

•  Shell velocity, growth factors

Spectroscopy is providing information on NIF implosions

Spectroscopic simulations are an important part of the experimental design process

outline spectroscopic investigations of implosions on the ... · the national ignition facility...

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