Progress in the pulsed power Inertial Confinement Fusion program

Pulsed power accelerators are being used in Inertial Confinement Fusion (ICF) research. In order to achieve our goal of a fusion yield in the range of 200 - 1000 MJ from radiation-driven fusion capsules, it is generally believed that {approx}10 MJ of driver energy must be deposited within the ICF target in order to deposit {approx}1 MJ of radiation energy in the fusion capsule. Pulsed power represents an efficient technology for producing both these energies and these radiation environments in the required short pulses (few tens of ns). Two possible approaches are being developed to utilize pulsed power accelerators in this effort: intense beams of light ions and z- pinches. This paper describes recent progress in both approaches. Over the past several years, experiments have successfully answered many questions critical to ion target design. Increasing the ion beam power and intensity are our next objectives. Last year, the Particle Beam Fusion Accelerator H (PBFA II) was modified to generate ion beams in a geometry that will be required for high yield applications. This 2048 modification has resulted in the production of the highest power ion beam to be accelerated from an extraction ion diode. We are also evaluating fast magnetically-driven implosions (z-pinches) as platforms for ICF ablator physics and EOS experiments. Z-pinch implosions driven by the 20 TW Saturn accelerator have efficiently produced high x- ray power (> 75 TW) and energy (> 400 kJ). Containing these x-ray sources within a hohlraum produces a unique large volume (> 6000 mm{sup 3}), long lived (>20 ns) radiation environment. In addition to studying fundamental ICF capsule physics, there are several concepts for driving ICF capsules with these x-ray sources. Progress in increasing the x-ray power on the Saturn accelerator and promise of further increases on the higher power PBFA II accelerator will be described

CURRENT STATUS OF CALCULATIONS AND MEASUREMENTS OF ION STOPPING POWER IN ICF PLASMAS

En fusion inertielle conduite par les ions, on éprouve actuellement la nécessité de perfectionner les modèles de ralentissement. L'évolution des recherches montre que les lois d'échelle approchées ne sont plus suffisantes pour extrapoler les modèles actuellement utilisés. On se propose de prédire, à 10 % près, les parcours ioniques dans les cibles ICF. On expose le modèle du gaz d'électrons libres, ainsi que les profils de densité de charge atomique du type Hartree-Fock-Slater pour déterminer l'ionisation moyenne I (Z,q,E) d'une cible d'électrons. Cette méthode est systématiquement explorée afin de mettre en évidence les insuffisances de la physique sous-jacente, particulièrement pour de faibles vitesses des projectiles. Des modèles alternatifs sont également développés par d'autres auteurs à la Sandia. Des mesures expérimentales de pouvoir d'arrêt, amplifié dans les plasmas de fusion, ont été observées dans le domaine 0,3 TW/cm2 au Naval Research Laboratory. Les expérimentateurs de la Sandia étendent actuellement ces données à des états d'ionisation plus élevée et à des cibles à Z plus grand, avec l'aide de l'accélérateur PROTO-I (1,2 TW/cm2).More precise stopping power models for use in ICF target design need to be developed. The light ion beam ICF program is now moving into a phase where "ad hoc" scaling of certain key physics parameters in the stopping power models is no longer sufficient. Our goal is to predict ion ranges in ICF targets to within about 10-20%. A verified stopping power model is also essential in diagnosing target irradiation intensities ; such data can only be inferred by target response. Presently, our area of primary concern involves calculating the stopping power of the bound electrons of partially ionized atoms. One bound electron stopping power model that we are investigating uses the local oscillator model along with Hartree-Fock-Slater atomic charge density profiles to calculate I (Z,q,E), a generalized average ionization potential for the target electrons. This method is being studied systematically to look for deficiencies in the underlying physics model, especially at low projectile velocities. Another procedure uses the Generalized Oscillator Strength model to calculate the bound electron stopping. Experimental measurements of enhanced stopping power in ICF plasmas at the 0.3 TW/cm2 level have been reported by the Naval Research Laboratory. Further experiments at Sandia are aimed at extending this data base both to higher ionization states and to higher-Z targets using a 1.2 TW/cm2 proton beam on the PROTO-I accelerator

Validation of Z-pinch double-ended hohlraum energetics and capsule coupling models using Z data

The pulsed-power driven double-ended vacuum hohlraum allows highly symmetric capsule implosions, aided by the separation of the capsule from the z-pinch and its associated instabilities, spatial variations, and non-thermal spectral components. Experiments on Sandia National Laboratories' Z facility in a single-sided power feed configuration have been limited thus far to capsule drive temperatures of 75 eV or less. Nonetheless, these experiments have validated the hohlraum energetics, hohlraum coupling, and capsule coupling properties of this configuration. Implosion capsules and symmetry diagnostic capsules have provided a wealth of capsule energetics data in the form of implosion trajectories for comparison to simulation. These measurements have been compared to detailed two-dimensional Lasnex hohlraum simulations, which model the hohlraum radiation transport including the effects of z-pinch source motion as well as hohlraum absorption/re-emission and wall plasma motion

The role of strong coupling in z-pinch-driven approaches to high yield inertial confinement fusion

Peak x-ray powers as high as 280±40 TW have been generated from the implosion of tungsten wire arrays on the Z Accelerator at Sandia National Laboratories. The high x-ray powers radiated by these z-pinches provide an attractive new driver option for high yield inertial confinement fusion (ICF). The high x-ray powers appear to be a result of using a large number of wires in the array which decreases the perturbation seed to the magnetic Rayleigh-Taylor (MRT ) instability and diminishes other 3-D effects. Simulations to confirm this hypothesis require a 3-D MHD code capability, and associated databases, to follow the evolution of the wires from cold solid through melt, vaporization, ionization, and finally to dense imploded plasma. Strong coupling plays a role in this process, the importance of which depends on the wire material and the current time history of the pulsed power driver. Strong coupling regimes are involved in the plasmas in the convolute and transmission line of the powerflow system. Strong coupling can also play a role in the physics of the z-pinch-driven high yield ICF target. Finally, strong coupling can occur in certain z-pinch-driven application experiments

Simulation of heating-compressed fast-ignition cores by petawatt laser-generated electrons

In this work, unique particle-in-cell simulations to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas are reported. The simulations yield heated core parameters in good agreement with the GEKKO-PW experimental measurements, given reasonable assumptions of laser-to-electron coupling efficiency and the distribution function of laser-produced electrons. The classical range of the hot electrons exceeds the mass density-core diameter product $\rho$L by a factor of several. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-EMF impeding hot electrons on the injection side of the density maxima .This methodology is then applied to the design of experiments for the ZR machine coupled to the Z-Beamlet/PW laser.
Sandia National Laboratories is also developing a combination of experimental and theoretical capabilities useful for the study of pulsed-power-driven fast ignition physics. In preparation for these fast ignition experiments, the theory group at Sandia is modeling various aspects of fast ignition physics. Numerical simulations of laser/plasma interaction, electron transport, and ion generation are being performed using the LSP code. LASNEX simulations of the compression of deuterium/tritium fuel in various reentrant cone geometries are being performed. Analytic and numerical modeling has been performed to determine the conditions required for fast ignition breakeven scaling. These results indicate that to achieve fusion energy output equal to the deposited energy in the core will require about 5% of the laser energy needed for ignition and might be an achievable goal with an upgraded Z-beamlet laser in short pulse mode