Stephen P. Obenschain

Direct-Drive Laser Fusion; Status and Prospects

Techniques have been developed to improve the uniformity of the laser focal profile, to reduce the ablative Rayleigh–Taylor instability, and to suppress the various laser–plasma instabilities. There are now three direct-drive ignition target designs that utilize these techniques. An evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor.

Laser‐plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

Evidence in the second‐harmonic emission for self‐focusing of a laser pulse in a plasma

Short‐pulse (300 psec), high‐intensity (1014−1015 W/cm2) Nd‐laser light was propagated into variable scale length plasmas (Ln≡n/∇n=200–400 μm at 0.1 critical density) preformed by long‐pulse (4 nsec), low‐intensity (≂6×1012 W/cm2) irradiation of planar targets. For high short‐pulse intensities (≥5×1014 W/cm2), time‐integrated images show filament‐shaped regions of second‐harmonic (2ω0) emission from the low density (0.01≤ne/nc≤0.2) region of the ablation plasma. Two‐dimensional computer calculations of the hyrodynamics and laser beam propagation indicate that these filaments are consistent with ponderomotive self‐focusing of the short pulse. A theoretical model that explains the 2ω0 generation mechanism within low‐density filaments is also presented.

Ablative acceleration of planar targets to high velocities

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

Computational modeling of direct-drive fusion pellets and KrF-driven foil experiments

FAST is a radiation transport hydrodynamics code that simulates laser matter interactions of relevance to direct-drive laser fusion target design. FAST solves the Euler equations of compressible flow using the Flux-Corrected Transport finite volume method. The advection algorithm provides accurate computation of flows from nearly incompressible vortical flows to those that are highly compressible and dominated by strong pressure and density gradients. In this paper we describe the numerical techniques and physics packages. FAST has also been benchmarked with Nike laser facility experiments in which linearly perturbed, low adiabat planar plastic targets are ablatively accelerated to velocities approaching 107 cm/s. Over a range of perturbation wavelengths, the code results agree with the measured Rayleigh–Taylor growth from the linear through the deeply nonlinear regimes. FAST has been applied to the two-dimensional spherical simulation design to provide surface finish and laser bandwidth tolerances for a ...

Production of high energy, uniform focal profiles with the Nike laser

Abstract Nike, a KrF laser facility at the Naval Research Laboratory, is designed to produce high intensity, ultra-uniform focal profiles for experiments relating to direct drive inertial confinement fusion. We present measurements of focal profiles through the next-to-last amplifier, a 20 × 20 cm 2 aperture electron beam pumped amplifier capable of producing more than 120 J of output in a 120 ns pulse. Using echelon free induced spatial incoherence beam smoothing this system has produced focal profiles with less than 2% tilt and curvature and less than 2% rms variation from a flat top distribution.

Stability of large-area electron-beam diodes

In this letter, we report on experimental measurements of an instability in the large-area electron-beam diodes used to pump krypton–fluoride (KrF) lasers. The instability is identified as the transit-time instability and it is shown that it modulates the electron beam (spatially and temporally), producing a wide spread in the energy and momentum distributions of electrons emerging from the diode. These effects can enhance the energy deposited in the foils and adversely affect the energy-transfer efficiency to the KrF gas. Analysis and simulations of the instability suggest that resistively loaded slots in the cathode should eliminate the instability.

Laser interaction in long-scale-length plasmas

Absorption of a short‐pulse, high‐intensity Nd‐laser beam (vacuum irradiance of 1014 to 1015 W/cm2) by preformed plasmas of different density scale lengths is investigated. Increased effects of plasma instabilities are found at longer scale lengths. The amount of backscattered light increases with plasma scale length and limits the absorption fraction at the longest scale length. The onset of suprathermal electron production, deduced from observations of energetic (20 to 50 keV) x rays, occurs at lower laser irradiance for longer‐scale‐length plasmas. A correlation between energetic x rays and 3ω0/2 emission suggests that the suprathermal electrons are produced by a plasma instability at quarter‐critical density. At higher intensities there is evidence for severe perturbations of the preformed plasma and for self‐focusing of the incident beam.

Conceptual design of a 2-MJ KrF laser fusion facility

A detailed KrF amplifier model is first benchmarked against new data and then used to design higher-energy modules with segmented pumping. It is found that segmentation with unpumped regions does not carry with it any penalty in efficiency because the distributed geometry has reduced losses from amplified spontaneous emission (ASE) that counteract the fluorine absorption of unpumped regions. A 68-kJ module is designed, incorporating a new water line geometry and a combined switch/bushing. The electrical parameters of the module are calculated in detail. The effect of multiplexed beamline energy on facility size is discussed, and an energy of 100 to 200 kJ is found to be optimal. Two 68-kJ modules are combined in a 136-kJ multiplexed beam line, incorporating incoherent spatial imaging, that fits within a compact beam tunnel. A total of 16 such beam lines are arranged on four floors to deliver 64 beams to a target; the net energy is 2.0 MJ. Detailed calculations of prepulse ASE energy are given, and the levels are designed to be low enough not to initiate a prepulse plasma. The basic geometrical uniformity of target illumination is shown to be better than 0.3% for a 64-beam illumination geometry that has a high degree of symmetry. A test of the 68-kJ module would be necessary to verify the projected specific laser energy and facilitate more detailed design of this fusion laser.

Flash x radiography of laser‐accelerated targets

Flash x radiography of ablatively accelerated planar foils has provided quantitative measurements and qualitative observations regarding several parameters of critical interest to direct illumination laser fusion. A 1.05‐μ, 3.3‐ns driver beam was focused onto carbon foils in a large (0.7–1‐mm diameter) spot to reduce edge effects. From images produced by a backlighting x‐ray flash, we have measured overall coupling efficiency, smoothing of laser nonuniformities, target velocity, and ablation pressure. The high velocity targets maintain a localized, high density (≳3% of solid). In contrast to other workers’ recent measurement of pressure from x‐ray imaging, our x‐radiographic results, including pressure, are in general agreement with earlier NRL studies. Our results have also provided further insights into double foil interactions and planar target preheat measurements.