Simulation of Hohlraum Radiation Environments and Tracer Spectra in Doped Ablator Experiments

1. Simulation of Hohlraum Radiation Environments and Tracer Spectra in Doped Ablator Experiments* J. J. MacFarlane Prism Computational Sciences, Madison, WI D. H. Cohen, K. L. Penrose, and D. Conners Swarthmore College, Swarthmore, PA D. A. Haynes University of Wisconsin, Madison, WI * In collaboration with: P. Jaanimagi, LLE, University of Rochester, Rochester, NY O. L. Landen, R. Turner, Lawrence Livermore National Laboratory, Livermore, CA J. E. Bailey, Sandia National Laboratory, Albuquerque, NM G. R. Magelssen, Los Alamos National Laboratories, Los Alamos, NM K. A. Park, A. R. Thomas-Cramer, Prism Computational Sciences, Madison, WI Presented September 10, 2001 at: Second International Conference on Inertial Fusion Sciences and Applications Kyoto, Japan.

2. Overview of OMEGA Experiments * • Indirect-drive experiments were performed at OMEGA to study the effects of mid-Z dopants (Ge-doped CH) on radiation propagation through ablator materials. • Ablator materials were attached to the ends of cylindrical halfraums (see illustrations below). In some experiments, dual ablator patches were used (one with Ge dopant, the other without). • In these experiments, Ka absorption spectroscopy of thin tracer layers (e.g., NaCl and KF) was used to track the radiation front burning into the ablator material. • In this poster, we discuss simulations of these experiments, focussing in particular on: • the halfraum radiation field, and the spectrum incident on the ablator; • the effect of non-Planckian incident spectrum on the heating of the ablator; • the sensitivity of the incident flux onto the ablator to laser pointing; • the effect of backlighter photons on the incident flux at the ablator; • beam power imbalance effects; • the uniformity of the radiation field at the ablator. *Experiments supported by the U. S. Dept. of Energy National Laser Users’ Facility Program.

3. NaCl (or KF) TR Ge-doped CH Ge-doped CH ~ 5 - 10 mm ~ 30 mm 0.5 mm Target Configuration Ablator patch, overlaid by washer (washer occults laser hot spots; reduces source size). • Parameters: • Halfraums (L = 1200 mm, R = 800 mm); LEH points toward port P7. • 15 OMEGA laser beams into the halfraum, each having 1 ns flat-topped profiles. • 3 backlighter beams. Powell scope (X-TVS view) Pb-Bi backlighter Ablator illustration:

4. VisRad Is Used to Simulate the Halfraum Radiation Field Simulatedtarget (X-TVS view) • VisRad is a 3-D view factor code used to study the radiation environment in ICF-related experiments. • Incident fluxes onto each surface element in the grid is obtain by solving the “radiosity” equation: • Bi = Ei + aiSkFik Bk, • where • Fik = configuration factor ai = albedo • Bi = sTi4 (emission flux) Ei = source term • Laser beam energy deposition is computed using a 3-D ray-trace algorithm. VisRad includes spatial beam profile models for OMEGA SG3/SG8 phase plates. • Laser beams can be pointed and target components positions and orientations can be set up utilizing the target chamber coordinate system orthe coordinate system of any other target component. • Graphical user interface and 3-D interactive graphics provide: • - convenient target grid set-up and pointing of laser beams; • - easy viewing from any diagnostic port, or any user-specified location; • - interactive graphics for display of results.

5. The Incident Flux at the Ablator Is Very Uniform View of entire halfraum 1200 mm diameter ablator region View from P7 direction • The incident flux (= s TR4) at the ablator sample is ~ 10 - 15 eV higher than throughout the rest of the halfraum. • Using beams with uniform powers, the maximum change in flux (peak-to-valley) across the central 600 mm diameter of the ablator patch is only 1.4% (0.35% in TR). • [Because of aperturing, only central ablator region is seen.]

6. Effect of Beam Pointing on Drive Temperature at Ablator • 15 “drive” beams used: • Cone 2 (5 beams at 42.0 degrees) and Cone 3 (10 beams at 58.9 degrees). • The drive temperature on the ablator can be increased by ~ 3.5 eV -- or about 8.4 % in flux -- when offsetting Cone 2 and Cone 3 by ~ 300 mm (wrt no offset case).

7. Effect of Backlighter on Incident Spectrum at Ablator The B/L beams’ focus points occur near the plane of the LEH. Spot size on backlighter is ~ 300 mm (diameter). Figure below shows radiation drive temperature on ablator with (solid curve) and without (dotted curve) backlighter beams. The backlighter beams increase TR by ~ 3.5 eV at early times and ~ 1.4 eV at late times. Incident spectrum on ablator (left) shows spectrum is non-Planckian due to hot spots, but backlighter contributes little to total spectrum.

8. 69 50 25 45 Effect of Beam Imbalance on Wall Temperature Seen by Dante Beams 25, 50, 45, & 69 are in the Dante field of view. Approximately 50% of the flux seen by Dante is due to laser hot spots. View from Dante (H16) Figure (right) shows Shot 19526 beams energies were relatively low for beams 25 and 69. When taking into account actual beams powers: -- the flux at Dante decreases by 8.7% -- inferred temperature decreases by ~ 3 - 4 eV.

9. Spectra Were Obtained Using the LXS Spectrometer • From our April 2000 campaign, we obtained time-dependent LXS spectra from germanium-doped plastic and undoped ablators. • The LXS was in TIM 4 (P6), and had an angle offset (downward) of 9 degrees. View from LXS (~ P6) • 6 backlighter beams were used (Cone 1: beams 10, 28, 31; Cone 2: beams 17, 20, 33). • Delays of 300 ps and 600 ps both extended length of backlighter time and provided timing information.

10. Experimental Spectra Undoped plastic 1.5% germanium dopant • The above figures show LXS spectra from a Ge-doped plastic (right) and an undoped ablator (left). • Spectra show hints of Cl Ka absorption, and suggest radiation wave propagates more slowly in the Ge-doped ablator case. • Absorption of Ka spectral lines appears weak. Reasons for this are not fully understood at this time. Possibilities: • Salts used for tracers not uniformly distributed. • Contamination of spectra by other hot plasma sources.

11. Calculation of Drive Temperatures at Ablator April 2000 Halfraum Shots • To determine drive temperature at the ablator, VisRad simulations were performed using: • Time-dependent albedos based on 1-D rad-hydro simulations (red curve). • Time-dependent x-ray conversion efficiencies (hx) based on: • Contraints by Dante data (dotted green curve) • Simple model [assumed] (solid green curve) • Measures beam laser powers <PL(t)> (blue curve) • To match the Dante data, (anomalously?) low values for hx are required. • Such low values were not needed in analyzing previous experiments (see upper right figure).

12. Calculated Drive Temperatures at Ablator • View factor calculations were performed to compute the flux incident on the ablator and the wall temperature seen by Dante. • The drive temperature at the ablator is slightly higher (up to ~ 5%) than the wall temperature seen by Dante. • Dante sees laser hot spots due to several beams. Only ~ 50% of the flux seen by Dante is due to wall reemission.

13. Calculated Temperature Distributions in the Ablator Sample • Using the time-dependent incident spectra computed from VisRad, the ablator plasma conditions were computed from 1-D radiation-hydrodynamics simulations. • Comparisons between simulations using Planckian and non-Planckian spectra are shown at right. • Results using non-Planckian incident spectra are shown as solid curves. • Results using Planckian incident spectra are shown as dotted curves. • Summary of radiation-hydrodynamics results (shown for undoped ablator case): • Temperatures in the thin NaCl tracer initially rise more rapidly due to its higher opacity. • Results for non-Planckian and Planckian incident spectra are roughly similar, with differences being more pronounced at early times (~ a factor of 2 in the tracer at early times).

14. SPECT3D Simulations Were Performed to Predict Ka Absorption Spectra • SPECT3D* is a collisional-radiative code used to post-process 1-D and multi-D radiation-hydrodynamics datasets. Given the spatial distribution of T and r in a plasma, it computes high-resolution spectra, fluxes, and images (e.g., framing camera, Schleirren, radiographs) for the plasma. • Material radiative properties can be modeled at several levels of detail: • DCA1: • -- LTE2 or non-LTE atomic level populations computed explicitly; • -- b-b, b-f, and f-f opacities / emissivities computed on-line; • -- used to obtain spectra of low- to mid-Z elements. • non-DCA: • -- atomic level populations not computed on-line; • -- opacities from LTE or non-LTE multi-group opacities; -- generally used for higher-Z materials. • Here, our 1-D rad-hydro calculations are post-processed to compute synthetic Ka absorption spectra due to the NaCl tracer. * MacFarlane, Thomas-Cramer, Park, and Zeng, “Spect3D Imaging and Spectral Analysis Suite,” Prism Computational Sciences Report No. PCS-R-027, July 2001. 1 DCA => detailed configuration accounting. 2 LTE => local thermodynamic equilibrium.

15. Synthetic Chlorine Ka Spectra Calculated streaked spectra. Spectral lineouts (calculated). • Calculated spectra show the onset of Ka absorption at ~ 200 psec. • This is due to vacancies appearing in the L-shell => F-like Cl. • The tracer is “pre-heated” by high energy photons that penetrate the ablator material. • The appearance of He- and Li-like Cl is not predicted until after 600 psec. This appears to be at odds with the measured LXS spectra.

16. Summary • View factor simulations of our OMEGA experiments show: • the incident flux onto the ablator is very uniform; • the incident spectrum is non-Planckian due to laser hot spots; • the backlighter photons contribute little (~ a couple %) to the radiation field at the ablator; • beam power imbalance can affect hohlraum temperature inferred from Dante data because Dante directly sees several laser hot spots; •  offsetting Cone 2 laser beams wrt Cone 3 can raise the drive temperature at the ablator sample by 3 - 4 eV; • the drive temperatures at the ablator are higher (~ 5%) than the wall temperatures inferred by Dante. • Rad-hydro & Ka spectral simulations predict that: • the NaCl thin tracer layer is pre-heated early due to high-energy photons; • the onset of F-like chlorine Ka absorption is predicted to occur by ~ 200 psec; • the onset of He- and Li-like chlorine Ka absorption is predicted to occur after 600 psec, which appears to be at odds with experimental spectra. •  LXS experimental spectra show hints of Cl Ka absorption, but data may suffer from: • -- target fab issues (non-uniform layering of NaCl => salt crystals) • -- contamination by stray hot plasma sources (?).

17. Additional VisRad Capabilities • Supports target grid generation for a variety of 2-D and 3-D primitives (including disks, cylinders, spheres, rectangles, and pre-configured cylindrical and tetrahedral hohlraums). • Provides capability for surface removal (e.g., drilling holes in cylinders). • Multiple coordinate systems are supported. User can readily view angles to laser ports and diagnostic ports in the coordinate system of any target component. • Currently supports OMEGA laser system. Others (e.g., NIF) can be added in the future. • Provides 3-D interactive graphics to view: • Target grid setup • Laser beams • Temperature and flux distributions • Properties of individual (“picked”) surface elements, including incident spectrum. • Surface occultation effects

18. Selected Examples of Target Grids That Can be Generated by VisRad • Geometric Primitives: • Spheres • Cylinders • Disks • Rectangles • Boxes • Cylindrical Hohlraums • Cylindrical Halfraums • Tetrahedral Hohlraums • Surface Removal Shapes: • Cylinders • Boxes Intersecting cylinders (two views) Cylindrical halfraum with backlighter and opposing sample Z-pinch hohlraum Cylindrical hohlraum with side package Tetrahedral hohlraum

19. Target Components List Laser Beams List Viewing Controls Color Bar VisRad User Interface Provides for Convenient Setup of Targets and Viewing Results • Developed using Qt* UI software, VisRad is supported on Windows and UNIX platforms. Mac support is expected in the future. • VisRad is supported by on-line documentation. • VisRad main window (above) * From Trolltech AS, Oslo, Norway

20. PRISM Computational Sciences: Who We Are • Prism CS is a small business that focuses on: • basic plasma physics research (fusion, astrophysics); • software development (plasma physics applications; e.g., VisRad, Spect3D). • While being a privately-owned small business, we have close ties with several educational institutions: • University of Wisconsin (Engr. Physics and Astronomy departments) • Swarthmore College (Dept. of Physics & Astronomy) • University of Rochester (LLE: Spect3D work) • Recent collaborations also include: University of Toledo (astrophysics), Harvard University (atomic data for astrophysics applications). • Employees backgrounds: • Ph. D. physicists (3 full-time; 2 part-time) • M.S. computer scientist (1 full-time) • Students: U. Wisc. (1) and Swarthmore (3) • Our goals are: • Perform basic research • Develop software tools that assist other scientists in performing research.

22. Synthetic Cl Ka Spectra: Model vs. Dante-Inferred XRC • If we use x-ray conversion efficiencies

23. Calculated Drive Temperatures at Ablator

24. Target Configuration

25. Sensitivity to Non-Planckian Inc. Spectrum