X-Ray Spectral Measurements of Cryogenic Capsules Imploded by OMEGA

Transcription

1 X-Ray Spectral Measurements of Cryogenic Capsules Imploded by OMEGA F. J. Marshall University of Rochester Laboratory for Laser Energetics kt =.65 kev (v ice = 1.5 nm) (v ice = 4.4 nm) 4 kt =.94 kev th Annual Meeting of the American Physical Society Division of Plasma Physics Orlando, FL November 27

2 Summary X-ray spectral measurements diagnose conditions in the compressed plasma Time-integrated, space-resolved core x-ray spectra are obtained with both pinhole cameras and Kirkpatrick Baez microscopes dispersed by transmission gratings. The shapes of the spectra allow inference of the core electron temperature (kt e ) from the slope of the spectrum at high energies. In selected cases, the surrounding main fuel layer areal density (tr fuel ) is inferred from absorption at low energies. These results provide important benchmarks for the predictions of 1-D and 2-D hydrocode simulations. E16279

3 Collaborators J. P. Knauer, T. C. Sangster, J. A. Delettrez, P. W. McKenty, R. Epstein, V. N. Goncharov, and B. Yaakobi Laboratory for Laser Energetics University of Rochester Related talk to follow by R. Epstein (JO3.5)

4 Grating-Dispersed Imaging X-ray emission from the core of an imploded OMEGA target is space and spectrally resolved, enabling the estimation of kt hot and tr cold KB microscope or pinhole camera Grating Diffracted x-rays m ZntR Z I = I e colde / hot E kt hot Film Target 1 18 m E1628 Hot core Transmitted hard x rays Cold imploded shell I (kev/kev) Energy (kev) F. J. Marshall et al., Phys. Rev. E 49, 4381 (1994). F. J. Marshall et al., Phys. Plasmas 7, 16 (2).

5 Cryogenic targets should exhibit an exponential tail with low energy absorption by the cold fuel Thermal bremsstrahlung from the hot core* Z38 2 Z12 / Zho/ kt ff e i f ] og = 6.8 # 1 Z N NT e g ^o, Th erg s 1 cm 3 f ff ] og\ e Zho/ kt hot Absorption by the main fuel layer (free-free absorption)* I(o) = I hot e ntr fuel n(o) = o 3 Z 2 N e N i T 1/2 t 1 (1 e hv/kt ) g ff (o,t) cm 2 g 1 n(o)? t E 3 T 1/2 (optical depth? t 2 R) where g ff (o,t) is the velocity-averaged Gaunt factor. ff *G. B. Rybicki and A. P. Lightman, Radiative Processes in Astrophysics (Wiley and Sons, New York, 1979). See also R. Epstein (JO3.5) talk to follow E16281

6 Typical x-ray spectra of OMEGA cryogenic targets are exponential with little sign of absorption OMEGA shot nm D 2 layer, 1.5-nm CD shell GtRH p = 174 mg/cm 2 v ice = 3.2 nm 2 4 kt = 1.1 kev 6 8 Core images (2 to 7 kev) 2-nm pinhole in KB1 4-nm FWHM KB3 opposite side (5-nm res.) 2-nm regions E16282

7 The low-energy portion of the spectrum has been measured with multiple methods to verify the shape OMEGA DT cryo shot 4715 (nom. a = 1.3) 91-nm DT layer (v ice = 1.6 nm), 3.6-nm CD shell kt =.88 kev 2 4 Crystal spec Grating spec LILAC kt = 1.63 kev 6 8 E16284

8 DRACO simulations explain the lack of absorption at low x-ray energies as being due to evolved perturbations OMEGA shot 4424 (nom. a = 2) 91-nm DT layer (v ice =.8 nm), 3.6-nm CD shell 2 4 KB1 KB3 DRACO 2-D (i = 3 to +3) LILAC *Unstable implosion due to a high in-flight-aspect-ratio E16283 Core image (2 to 7 kev) 5-nm FWHM 2-nm regions DRACO density contours at stagnation*

9 A significantly different core size and x-ray spectra are seen when higher tr is achieved OMEGA cryogenic-d 2 target, grating-dispersed x-ray images 1-nm-thick CD shell GtRH p = 22 mg/cm 2 5-nm-thick CD shell GtRH p = 15 mg/cm 2 1-mm regions nm regions (2 to 7 kev) E nm FWHM 51-nm FWHM

10 A significant difference is seen in both measured tr and x-ray spectrum when thicker CD shells are used E OMEGA cryo shots with 95-nm-thick D 2 layers Shot nm CD shell kt = 1.5 kev GtRH p = 22 mg/cm 2 v ice = 2.4 nm 2 4 Shot nm CD shell kt =.8 kev GtRH p = 15 mg/cm 2 v ice = 3.3 nm nm resolution nm FWHM nm FWHM 2-nm regions (2 to 7 kev)

11 DRACO simulations are in excellent agreement* with the observed x-ray spectrum for high tr and moderate ice roughness E DRACO with view angle variations OMEGA shot Grating spec kt = 1.5 kev LILAC kt = 1.52 kev 6 8 * with the exception of the very lowest energies where absorption by the cold fuel should occur Z (nm) 95-nm D 2 layer 1.2-nm CD shell GtRH p = 22 mg/cm 2 v ice = 2.4 nm R (nm) 25 DRACO density contours at stagnation t(g/cm 3 )

12 A large increase in observed absorption may be correlated with improved ice-layer uniformity E OMEGA cryo shots (nom. a = 2.5) 95-nm D 2 layers, 1-nm CD shells average GtRH p = 183 mg/cm 2 Shot v ice = 1.5 nm kt =.94 kev GtRH x *. 14±2 mg/cm Shot v ice = 4.4 nm kt =.65 kev GtRH x *. 34±17 mg/cm 2 Fits to measurements 6 8 * dependent on assumed t, T, and Gaunt factor see R. Epstein (J3.5 to follow) GMXI CID x-ray images Shot nm FWHM Shot nm FWHM 2-nm regions (2 to 7 kev)

13 Summary/Conclusions X-ray spectral measurements diagnose conditions in the compressed plasma Time-integrated, space-resolved core x-ray spectra are obtained with both pinhole cameras and Kirkpatrick Baez microscopes dispersed by transmission gratings. The shapes of the spectra allow inference of the core electron temperature (kt e ) from the slope of the spectrum at high energies. In selected cases, the surrounding main fuel layer areal density (tr fuel ) is inferred from absorption at low energies. These results provide important benchmarks for the predictions of 1-D and 2-D hydrocode simulations. E16279