E. L. Dewald

Fuel gain exceeding unity in an inertially confined fusion implosion

Ignition is needed to make fusion energy a viable alternative energy source, but has yet to be achieved. A key step on the way to ignition is to have the energy generated through fusion reactions in an inertially confined fusion plasma exceed the amount of energy deposited into the deuterium–tritium fusion fuel and hotspot during the implosion process, resulting in a fuel gain greater than unity. Here we report the achievement of fusion fuel gains exceeding unity on the US National Ignition Facility using a ‘high-foot’ implosion method, which is a manipulation of the laser pulse shape in a way that reduces instability in the implosion. These experiments show an order-of-magnitude improvement in yield performance over past deuterium–tritium implosion experiments. We also see a significant contribution to the yield from α-particle self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite.

Symmetric Inertial Confinement Fusion Implosions at Ultra-High Laser Energies

Ignition Set to Go One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys). Glenzer et al. (p. 1228, published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231, published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings. Laser-driven temperatures and implosion symmetry are close to the requirements for inertial-fusion ignition. Indirect-drive hohlraum experiments at the National Ignition Facility have demonstrated symmetric capsule implosions at unprecedented laser drive energies of 0.7 megajoule. One hundred and ninety-two simultaneously fired laser beams heat ignition-emulate hohlraums to radiation temperatures of 3.3 million kelvin, compressing 1.8-millimeter-diameter capsules by the soft x-rays produced by the hohlraum. Self-generated plasma optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, which produces a symmetric x-ray drive as inferred from the shape of the capsule self-emission. These experiments indicate that the conditions are suitable for compressing deuterium-tritium–filled capsules, with the goal of achieving burning fusion plasmas and energy gain in the laboratory.

Dante soft x-ray power diagnostic for National Ignition Facility

Soft x-ray power diagnostics are essential for measuring the total x-ray flux, radiation temperature, conversion efficiency, and albedo that define the energetics in indirect and direct drive, as well as other types of high temperature laser plasma experiments. A key diagnostic for absolute radiation flux and radiation temperature in hohlraum experiments is the Dante broadband soft x-ray spectrometer. For the extended range of x-ray fluxes predicted for National Ignition Facility (NIF) compared to Omega or Nova hohlraums, the Dante spectrometer for NIF will include more high energy (<2 keV) edge filter band-pass channels and access to an increased dynamic range using grids and signal division. This will allow measurements of radiation fluxes of between 0.01 to 100 TW/sr, for hohlraum radiation temperatures between 50 eV and 1 keV. The NIF Dante will include a central four-channel imaging line-of-sight to verify the source size, alignment as well as checking for any radiation contributions from unconverted...

Capsule implosion optimization during the indirect-drive National Ignition Campaign

Capsule performance optimization campaigns will be conducted at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition. The campaigns will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models using a variety of ignition capsule surrogates before proceeding to cryogenic-layered implosions and ignition experiments. The quantitative goals and technique options and down selections for the tuning campaigns are first explained. The computationally derived sensitivities to key laser and target parameters are compared to simple analytic models to gain further insight into the physics of the tuning techniques. The results of the validation of the tuning techniques at the OMEGA facility [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] under scaled hohlraum and capsule conditions relevant to the ignition design are shown ...

The high-foot implosion campaign on the National Ignition Facilitya)

The “High-Foot” platform manipulates the laser pulse-shape coming from the National Ignition Facility laser to create an indirect drive 3-shock implosion that is significantly more robust against instability growth involving the ablator and also modestly reduces implosion convergence ratio. This strategy gives up on theoretical high-gain in an inertial confinement fusion implosion in order to obtain better control of the implosion and bring experimental performance in-line with calculated performance, yet keeps the absolute capsule performance relatively high. In this paper, we will cover the various experimental and theoretical motivations for the high-foot drive as well as cover the experimental results that have come out of the high-foot experimental campaign. At the time of this writing, the high-foot implosion has demonstrated record total deuterium-tritium yields (9.3×1015) with low levels of inferred mix, excellent agreement with implosion simulations, fuel energy gains exceeding unity, and evidenc...

National Ignition Campaign Hohlraum energetics

The first series of experiments of the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] tested ignition Hohlraum “energetics,” a term described by four broad goals: (1) measurement of laser absorption by the Hohlraum; (2) measurement of the x-ray radiation flux (TRAD4) on the surrogate ignition capsule; (3) quantitative understanding of the laser absorption and resultant x-ray flux; and (4) determining whether initial Hohlraum performance is consistent with requirements for ignition. This paper summarizes the status of NIF Hohlraum energetics experiments. The Hohlraum targets and experimental design are described, as well as the results of the initial experiments. The data demonstrate low backscattered energy (<10%) for Hohlraums filled with helium gas. A discussion of our current understanding of NIF Hohlraum x-ray drive follows, including an overview of the computational tools, i.e., radiation-hydrodynamics codes that have been used to design the Hohlraums. The perf...

Symmetry tuning for ignition capsules via the symcap techniquea)

Symmetry of an implosion is crucial to get ignition successfully. Several methods of control and measurement of symmetry have been applied on many laser systems with mm size hohlraums and ns pulses. On the National Ignition Facility [Moses et al., Phys. Plasmas 16, 041006 (2009)] we have large hohlraums of cm scale, long drive pulses of 10 s of ns, and a large number of beams with the option to tune their wavelengths. Here we discuss how we used the x-ray self-emission from imploding surrogates to ignition capsules (symcaps) to measure the symmetry of the implosion. We show that symcaps are good surrogates for low order symmetry, though having lower sensitivity to distortions than ignition capsules. We demonstrate the ability to transfer energy between laser beams in a gas-filled hohlraum using wavelength tuning, successfully tuning the lowest order symmetry of the symcaps in different size hohlraums at different laser energies within the specification established by calculations for successful ignition.

Implosion dynamics measurements at the National Ignition Facility

Measurements have been made of the in-flight dynamics of imploding capsules indirectly driven by laser energies of 1–1.7 MJ at the National Ignition Facility [Miller et al., Nucl. Fusion 44, 228 (2004)]. These experiments were part of the National Ignition Campaign [Landen et al., Phys. Plasmas 18, 051002 (2011)] to iteratively optimize the inputs required to achieve thermonuclear ignition in the laboratory. Using gated or streaked hard x-ray radiography, a suite of ablator performance parameters, including the time-resolved radius, velocity, mass, and thickness, have been determined throughout the acceleration history of surrogate gas-filled implosions. These measurements have been used to establish a dynamically consistent model of the ablative drive history and shell compressibility throughout the implosion trajectory. First results showed that the peak velocity of the original 1.3-MJ Ge-doped polymer (CH) point design using Au hohlraums reached only 75% of the required ignition velocity. Several capsu...

Shock timing experiments on the National Ignition Facility: Initial results and comparison with simulation

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium (DT) fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the DT fuel on a low adiabat. Initial experiments to measure the strength and relative timing of these shocks have been conducted on NIF in a specially designed surrogate target platform known as the keyhole target. This target geometry and the associated diagnostics are described in detail. The initial data are presented and compared with numerical simulations. As the primary goal of these experiments is to assess and minimize the adiabat in related DT implosions, a methodology is described for quantifying the adiabat from the shock velocity measurements. Results ...

Analysis of the National Ignition Facility Ignition Hohlraum Energetics Experiments

A series of 40 experiments on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] to study energy balance and implosion symmetry in reduced- and full-scale ignition hohlraums was shot at energies up to 1.3 MJ. This paper reports the findings of the analysis of the ensemble of experimental data obtained that has produced an improved model for simulating ignition hohlraums. Last year the first observation in a NIF hohlraum of energy transfer between cones of beams as a function of wavelength shift between those cones was reported [P. Michel et al., Phys. Plasmas 17, 056305 (2010)]. Detailed analysis of hohlraum wall emission as measured through the laser entrance hole (LEH) has allowed the amount of energy transferred versus wavelength shift to be quantified. The change in outer beam brightness is found to be quantitatively consistent with LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] simulations using the predicted ener...