Diana Grace Schroen

Dynamic hohlraum driven inertial fusion capsules

A dynamic hohlraum is formed when an imploding annular cylindrical Z-pinch driven plasma collides with an internal low density convertor. This collision generates an inward traveling shock wave that emits x rays, which are trapped by the optically thick Z-pinch plasma and can be used to drive an inertial fusion capsule embedded in the convertor. This scheme has the potential to efficiently drive high yield capsules due to the close coupling between the intense radiation generation and the capsule. In prior dynamic hohlraum experiments [J. E. Bailey et al., Phys. Rev Lett. 89, 095004 (2002)] the convertor shock wave has been imaged with gated x-ray pinhole cameras. The shock emission was observed to be very circular and to be quite narrow in the radial direction. This implies that there is minimal Rayleigh–Taylor imprinting on the shock wave. Thus, the dominant source of radiation asymmetry is not random and in principle could be significantly decreased by proper design. Due to the closed geometry of the d...

Development Path for Z-Pinch IFE

Abstract The long-range goal of the Z-Pinch IFE program is to produce an economically-attractive power plant using high-yield z-pinch-driven targets (~3GJ) with low rep-rate per chamber (~0.1 Hz). The present mainline choice for a Z-Pinch IFE power plant uses an LTD (Linear Transformer Driver) repetitive pulsed power driver, a Recyclable Transmission Line (RTL), a dynamic hohlraum z-pinch-driven target, and a thick-liquid wall chamber. The RTL connects the pulsed power driver directly to the z-pinch-driven target, and is made from frozen coolant or a material that is easily separable from the coolant (such as carbon steel). The RTL is destroyed by the fusion explosion, but the RTL materials are recycled, and a new RTL is inserted on each shot. A development path for Z-Pinch IFE has been created that complements and leverages the NNSA DP ICF program. Funding by a U.S. Congressional initiative of

Development of divinylbenzene foam shells for use as inertial fusion energy reactor targets

4M for FY04 through NNSA DP is supporting assessment and initial research on (1) RTLs, (2) repetitive pulsed power drivers, (3) shock mitigation [because of the high yield targets], (4) planning for a proof-of-principle full RTL cycle demonstration [with a 1 MA, 1 MV, 100 ns, 0.1 Hz driver], (5) IFE target studies for multi-GJ yield targets, and (6) z-pinch IFE power plant engineering and technology development. Initial results from all areas of this research are discussed.

Shock propagation, preheat, and x-ray burnthrough in indirect-drive inertial confinement fusion ablator materials

An overview of the present status of development of a hollow foam shell designed to produce high yields when used in a krypton fluoride inertial fusion energy (IFE) reactor is presented. Prototype shells have been produced from a 100 mg/cm3 density CH foam with an ~4-mm diameter and 300 μm wall thickness. A triple-orifice droplet generator was used to form the shells using solutions of an internal water phase, an oil phase (divinylbenzene monomer, dibutyl phthalate solvent, and a radical initiator), and an external water phase. The lowest percent of nonconcentricity measured for a completed shell was 3%, and the lowest average percent of nonconcentricity for a batch of shells was 7%. A technique to overcoat the shells with a 1- to 5-μm-thick full-density polymer layer using an interfacial polycondensation reaction is being developed. Methods to further optimize dimensions to produce shells that meet IFE specifications are also discussed.

Measurement of radiation symmetry in Z-pinch-driven hohlraums

The velocities and temperatures of shock waves generated by laser-driven hohlraum radiation fields have been measured in indirect-drive inertial confinement fusion (ICF) capsule ablator materials. Time-resolved measurements of the preheat temperature ahead of the shock front have been performed and included in the analysis. Measurements of the x-ray burnthrough of the ablation front and the ablator x-ray re-emission have also been made in the Cu-doped beryllium, polyimide, and Ge-doped CH ablator samples. The experiments utilize 15 beams of the University of Rochester Omega Laser [Soures et al., Phys. Plasmas 3, 2108 (1996)] to heat hohlraums to radiation temperatures of ∼120–200 eV. In the experiments, planar samples of ablator material are exposed to the hohlraum radiation field, generating shocks in the range of 10–50 Mbars. The experimental results are compared to integrated two-dimensional Lasnex [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Control. Fusion 2, 51 (1975)] calculations, in wh...

Comparison of a copper foil to a copper wire-array Z pinch at 18 MA

The Z-pinch-driven hohlraum (ZPDH) [J. H. Hammer et al., Phys. Plasmas 6, 2129 (1999)] is a promising approach to high yield inertial confinement fusion currently being characterized in experiments on the Sandia Z accelerator [M. E. Cuneo et al., Phys. Plasmas 8, 2257 (2001)]. Simulations show that capsule radiation symmetry, a critical issue in ZPDH design, is governed primarily by hohlraum geometry, dual-pinch power balance, and pinch timing. In initial symmetry studies on Z without the benefit of a laser backlighter, highly-asymmetric pole-hot and equator-hot single Z-pinch hohlraum geometries were diagnosed using solid low density foam burnthrough spheres. These experiments demonstrated effective geometric control and prediction of polar flux symmetry at the level where details of the Z-pinch implosion and other higher order effects are not critical. Radiation flux symmetry achieved in Z double-pinch hohlraum configurations exceeds the measurement sensitivity of this self-backlit foam ball symmetry di...

Mesoscale simulation of shocked poly-(4-methyl-1-pentene) (PMP) foams.

Results from the first solid foil implosion on the 18-MA Z accelerator are reported. The foil implosion is compared to a 300-wire-array implosion with the same material and the same diameter, height, and total mass. Though both the foil and the array produced comparable x-ray yields, the array’s radiation burst was twice as powerful and half as long as the foil’s. These data along with x-ray backlighting images and inductance measurements suggest that the foil implosion was more unstable than the wire-array implosion.

THE CHALLENGE OF AN IFE FOAM CAPSULE OVERCOAT

Hydrocarbon foams are commonly used in high energy-density physics (HEDP) applications, for example as tamper and ablation materials for dynamic materials or inertial confinement fusion (ICF) experiments, and as such are subject to shock compression from tens to hundreds of GPa. Modeling of macro-molecular materials like hydrocarbon foams is challenging due to the heterogeneous character of the polymers and the complexity of voids and large-scale structure. Under shock conditions, these factors contribute to a relatively larger uncertainty of the post-shock state compared to that encountered for homogenous materials; therefore a quantitative understanding of foams under strong dynamic compression is sought. We use Sandias ALEGRA-MHD code to simulate 3D mesoscale models of poly-(4-methyl-1-pentene) (PMP) foams. We devise models of the initial polymer-void structure of the foam and analyze the statistical properties of the initial and shocked states. We compare the simulations to multi-Mbar shock experimen...

Significant reduction of instability growth in magnetically driven liner implosions.

Abstract The baseline design for the laser-driven Inertial Fusion Energy (IFE) target is a 4.6 mm foam capsule with a polymer overcoat of 1 to 5 microns. The specifications for this overcoat include surface finish, permeation properties, uniform wall thickness and conformal coating of the foam shell. Many of these specifications are not unlike the full density polymer National Ignition Facility targets, but the foam shell adds to the fabrication difficulty. Since the foam surface is composed of open cells, creating the overcoat by typical vacuum deposition processes would start by replicating the foam surface making it very difficult to achieve the required surface specification. Instead an overcoat is made using interfacial polymerization at the edge of the foam surface. This is done by filling the foam shell with an organic solvent containing one reactant, then placing the shell into water containing another reactant. The reaction occurs only at the interface of the two solutions. This technique was pioneered at the Institute of Laser Engineering (Osaka University) with 0.8 mm diameter methacrylate shells. The process was later extended to 0.9 mm diameter resorcinol-formaldehyde and divinyl benzene (DVB) shells. For the High Average Power Laser Program target we need to extend the process to 4.6 mm diameter DVB foam shells. The properties of the DVB foam and the larger diameter of the shell make it more difficult to produce a gas tight shell. This report will explain how we are adapting the process and the results to date.