Streaked X-Ray Imager for Observation of Oscillationsof Perturbed Ablation Fronts in Planar ICF Targets

During Shock Transit

O. Gotchev, V. N. Goncharov,P. A. Jaanimagi, J. P. Knauer,F. J. Marshall, and D. D. MeyerhoferUniversity of RochesterLaboratory for Laser Energetics

Backlighter laser beams(driver beams not shown)U

back-lighter CH target

Beshield

Al debrisfilter

KB mirrorassembly

Be substrate

Slit (0.09 mm x6 mm)

Photo-cathodeYCsI

τsubτBe

τAl

τCH40

p q

RIr2

43rd Annual Meeting of theAmerican Physical SocietyDivision of Plasma Physics

Long Beach, CA29 October–2 November 2001

E11171

• An experiment is proposed to confirm the oscillationsof ablation- front perturbations, caused by the dynamicoverpressure of the blowoff plasma.

• A new x-ray diagnostic system is being developedfor this experiment.

• The experimental requirements, setup, and diagnosticare described here.

*Also known as rocket effect [V. N. Goncharov, Phys. Rev. Lett. 82, 2091 (1999)].

Summary

Experimental observation of oscillations at the ablationfront will confirm the dynamic overpressure effect

E11172

Outline

• Motivations for experimental observation

• Experimental requirements

• Experimental setup

– diagnostic components

– characterization

• Conclusions

E11174

Dynamic overpressure is the main physical mechanismstabilizing ablative Richtmyer–Meshkov (RM) growth

• A thermally induced increase in theexhaust velocity at the peak createsdynamic overpressure δPd.

• Classical RM:

ξ ~ kUsξ0t

• With ablation:

Va ~ ∇T ~ I1/3

δ(∇T) → δVa → δVbl → δPd

ξ ~ ξ0cosωt, ω = k√VbtVa

Laserbeam

ρbl

Ablatedplasma

Shock front

Us

δPd < 0

Coldtarget

Vbl

Heat flux

δPd > 0 Ablation front

dt

Vbl

Heat flux

Lateralflow

E11177

Oscillations induced by dynamic overpressureare easier to detect for short-wavelength modes

Single modesurface perturbations

on a 40-µm CH foil

I = 2 × 1014 W/cm21Goncharov, Phys. Rev. Lett. 82, 2091 (1999).2Aglitsky et al., 31st Annual Anomalous Absorption Conference, June, 2001.

6

4

2

0

–2

0.2 0.4 0.6 0.80.0

Time (ns)

Theoretical results

20 µm

15 µm

δ(ρR

)(ρ

R) 0

• The oscillations areobservable only beforethe onset of RT growth.

• 40- or 60-µm-thick planarCH foils are used to providesufficient shock transit time.

• The period1 of oscillationsTCH 3/[Va(µm/ns)√k(µm–1)] ns.

• Phase variation <π/4 observedin reference 2.

E11178

Grazing-incidence x-ray optics with a high collecting anglewill be coupled to a high-dynamic-range streak camera

• Current x-ray pinhole framing cameras do not provide optimalthroughput to diagnose mass perturbations on > 40-µm-thick CH foils.

• A streaked device will allow for continuous record revealing detailsin the evolution of ablation-front pertubations.

• A Kirkpatrick–Baez microscope with Ir-coated mirrors is the front endof the apparatus while a high-current streak tube (PJx) coupled directlyto a high-resolution CCD is the detector.

• The KB resolution is ~ 3 µm on axis (6× magnification).

• The PJx has a PSF with a FWHM of 18 µm. A magnified image willbe registered at the PJx photocathode.

E10562

An optimal magnification that accommodates the resolutionrequirements and the size of the CCD detector is providedby the chosen geometry

• 6× magnification at current configuration

• Projected resolution of better than 5 µm over a 200-µm field of view.

• The radius of curvature for each concave mirror is = 4250 mm.

• f = 77.5 mm, p = 90 mm, q = 560 mm

Backlighter laser beams(driver beams not shown)U

back-lighter CH target

Beshield

Al debrisfilter

KB mirrorassembly

Be substrate

Slit (0.09 mm x6 mm)

Photo-cathodeYCsI

τsubτBe

τAl

τCH40

p q

RIr2

E11279

Optimal throughput depends on the grazingangle and the coating material

• Optimal:

– Material is iridium.

– Angle of incidence i = 2.1°.

10

8

6

4

2

00 1 2 3 4 5

ΩR

2 xx (

sr)(

× 10

–7)

Angle of incidence (deg)

Al

Ir

Zerodur

Effective throughputof a KB mirror pair

Rxx = materialreflectivityat 1.4 keV

Ω = solidangle (sr)

Pt

E11280

The choice of mirror material was experimentally verified

• Zerodur glass substrates with surface roughness < 2 Å rmsare coated with ~500 Å Ir on top of ~100 Å Cr for improved adhesion.

Ref

lect

ivit

y1 2 3 4 5 6

Energy (keV)

Smootheddata

Theoretical,ideal surface

Theoretical,measured

surface rms

Ir coating reflectivity at 1.2°

1.0

0.8

0.6

0.4

0.2

0.01.5 2.0 2.5 3.0 3.5 4.0

Ref

lect

ivit

y

Superpolished Zerodursubstrate at 1.5°

Theoretical

Measured

Energy (keV)

1.0

0.8

0.6

0.4

0.2

0.0

E10570

The point-spread function (PSF) for an objecton axis shows better-than-5-µm spatial resolution

• The image spot size at the 10% level is less than 15 µm(box size is 30 µm). A division by the magnification M = 6yields 2.5 µm resolution at target center.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Raytrace calculation

E11171a

Experimental observation of oscillations at the ablationfront will confirm the dynamic overpressure effect

• Dynamic overpressure* stabilizes (RM) pertubation growthat the ablationt fron during shock transit and thus reducesthe seeds of the subsequent RT growth.

• An experiment is proposed to confirm the oscillations of ablation-front perturbations, caused by the dynamic overpressureof the blowoff plasma.

• A new x-ray diagnostic system is being developed for this experiment.

• The experimental requirements, setup, and diagnostic aredescribed here.

*Also known as rocket effect [V. N. Goncharov, Phys. Rev. Lett. 82, 2091 (1999)].

Summary/Conclusions