D. G. Colombant

Pathway to a lower cost high repetition rate ignition facility

An approach to a high-repetition ignition facility based on direct drive with the krypton-fluoride laser is presented. The objective is development of a “Fusion Test Facility” that has sufficient fusion power to be useful as a development test bed for power plant materials and components. Calculations with modern pellet designs indicate that laser energies well below a megajoule may be sufficient. A smaller driver would result in an overall smaller, less complex and lower cost facility. While this facility might appear to have most direct utility to inertial fusion energy, the high flux of neutrons would also be able to address important issues concerning materials and components for other approaches to fusion energy. The physics and technological basis for the Fusion Test Facility are presented along with a discussion of its applications.

Test of models for electron transport in laser produced plasmas

This paper examines five different models of electron thermal transport in laser produced spherical implosions. These are classical, classical with a flux limit f, delocalization, beam deposition model, and Fokker–Planck solutions. In small targets, the results are strongly dependent on f for flux limit models, with small f’s generating very steep temperature gradients. Delocalization models are characterized by large preheat in the center of the target. The beam deposition model agrees reasonably well with the Fokker–Planck simulation results. For large, high gain fusion targets, the delocalization model shows the gain substantially reduced by the preheat. However, flux limitation models show gain largely independent of f, with the beam deposition model also showing the same high gain.

Direct-drive laser target designs for sub-megajoule energies

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30×) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S. P. Obenschain et al., Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be a...

Laser plasma instability experiments with KrF lasers

Deleterious effects of laser-plasma instability (LPI) may limit the maximum laser irradiation that can be used for inertial confinement fusion. The short wavelength (248nm), large bandwidth, and very uniform illumination available with krypton-fluoride (KrF) lasers should increase the maximum usable intensity by suppressing LPI. The concomitant increase in ablation pressure would allow implosion of low-aspect-ratio pellets to ignition with substantial gain (>20) at much reduced laser energy. The proposed KrF-laser-based Fusion Test Facility (FTF) would exploit this strategy to achieve significant fusion power (150MW) with a rep-rate system that has a per pulse laser energy well below 1 MJ. Measurements of LPI using the Nike KrF laser are presented at and above intensities needed for the FTF (I∼2×1015W∕cm2). The results to date indicate that LPI is indeed suppressed. With overlapped beam intensity above the planar, single beam intensity threshold for the two-plasmon decay instability, no evidence of instab...

Status of Light Ion Inertial Fusion Research at NRL

Abstract : High-brightness proton beams have recently been extracted from axial pinch-reflex diodes (PRD) mounted on the Gamble II generator. The source power brightness that was measured exceeded 10 TW/sq cm sq rad. Analysis of a modular Inertial confinement fusion system using such diodes shows that an operational window for transport of light-ion species exists. Multi-terawatt beams can be transported a few meters in channels a few centimeters in diameter. A proof-of-principle experiment for the required final focusing cell has been successfully carried out on Gamble II. A new barrel-shaped equatorial PRD that can be coupled to PBFA II as a single diode has also been operated on Gamble II and has demonstrated 50% ion efficiency with predominantly azimuthally-symmetric charged-particle flow. Preliminary experiments using vacuum inductive storage and plasma opening switches have demonstrated factor-of-three pulse compressions, with corresponding power and voltage multiplications for pulse durations of interest to PBFA II. In other experiments the stopping power of deuterons in hot plasmas was measured. Results show about 40% enhancement in stopping power over that in cold targets when the deuteron beam is focused on the target to about 0.25 MA/sq cm. (Author)