Performance Improvements to the Neutron Imaging System at the National Ignition Facility

A team headed by LANL and including many members from LLNL and NSTec LO and NSTec LAO fielded a neutron imaging system (NIS) at the National Ignition Facility at the start of 2011. The NIS consists of a pinhole array that is located 32.5 cm from the source and that creates an image of the source in a segmented scintillator 28 m from the source. The scintillator is viewed by two gated, optical imaging systems: one that is fiber coupled, and one that is lens coupled. While there are a number of other pieces to the system related to pinhole alignment, collimation, shielding and data acquisition, those pieces are discussed elsewhere and are not relevant here. The system is operational and has successfully obtained data on more that ten imaging shots. This remainder of this whitepaper is divided in five main sections. In Section II, we identify three critical areas of improvement that we believe should be pursued to improve the performance of the system for future experiments: spatial resolution, temporal response and signal-to-noise ratio. In Section III, we discuss technologies that could be used to improve these critical performance areas. In Section IV, we describe a path to evolve the current system to achieve improved performance with minimal impact on the ability of the system to operate on shots. In Section V, we discuss the abilities, scope and timescales of the current teams and the Commissariat energie atomique (CEA). In Section VI, we summarize and make specific recommendations for collaboration on improvements to the NIS

Experimental Evidence of a Variant Neutron Spectrum from the T(t,2n)α Reaction at Center-of-Mass Energies in the Range of 16–50 keV

Full calculations of six-nucleon reactions with a three-body final state have been elusive and a long-standing issue. We present neutron spectra from the T(t,2n)α (TT) reaction measured in inertial confinement fusion experiments at the OMEGA laser facility at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (E[subscript c.m.]) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two E[subscript c.m.], with the ⁵He ground state resonant peak at 8.6 MeV being significantly stronger at the higher than at the lower energy. The data provide the first conclusive evidence of a variant TT-neutron spectrum in this E[subscript c.m.] range. In contrast to earlier available data, this indicates a reaction mechanism that must involve resonances and/or higher angular momenta than L=0. This finding provides an important experimental constraint on theoretical efforts that explore this and complementary six-nucleon systems, such as the solar ³He(³He,2p)α reaction

Performance Improvements to the Neutron Imaging System at the National Ignition Facility

A team headed by LANL and including many members from LLNL and NSTec LO and NSTec LAO fielded a neutron imaging system (NIS) at the National Ignition Facility at the start of 2011. The NIS consists of a pinhole array that is located 32.5 cm from the source and that creates an image of the source in a segmented scintillator 28 m from the source. The scintillator is viewed by two gated, optical imaging systems: one that is fiber coupled, and one that is lens coupled. While there are a number of other pieces to the system related to pinhole alignment, collimation, shielding and data acquisition, those pieces are discussed elsewhere and are not relevant here. The system is operational and has successfully obtained data on more that ten imaging shots. This remainder of this whitepaper is divided in five main sections. In Section II, we identify three critical areas of improvement that we believe should be pursued to improve the performance of the system for future experiments: spatial resolution, temporal response and signal-to-noise ratio. In Section III, we discuss technologies that could be used to improve these critical performance areas. In Section IV, we describe a path to evolve the current system to achieve improved performance with minimal impact on the ability of the system to operate on shots. In Section V, we discuss the abilities, scope and timescales of the current teams and the Commissariat energie atomique (CEA). In Section VI, we summarize and make specific recommendations for collaboration on improvements to the NIS

Experimental Demonstration of X-Ray Drive Enhancement with Rugby-Shaped Hohlraums

Rugby-shaped hohlraums have been suggested as a way to enhance x-ray drive in the indirect drive approach to inertial confinement fusion. This Letter presents an experimental comparison of rugby-shaped and cylinder hohlraums used for D[subscript 2] and D[superscript 3]He-filled capsules implosions on the Omega laser facility, demonstrating an increase of x-ray flux by 18% in rugby-shaped hohlraums. The highest yields to date for deuterium gas implosions in indirect drive on Omega (1.5×10[superscript 10] neutrons) were obtained, allowing for the first time the measurement of a DD burn history. Proton spectra measurements provide additional validation of the higher drive in rugby-shaped hohlraums

Neutron imaging at the NIF

The National Ignition Facility neutron imaging system (NIS) is an important diagnostic for understanding ignition experiments at the NIF in late 2010. The goal of the diagnostic is to provide spatially resolved information on the production of prompt and scattered neutrons from imploded ignition targets. This information may be used to diagnose hohlraum drive symmetry and pointing conditions, or study the dynamics of DT burn within the ICF target. In this paper we will discuss NIF relevant neutron imaging issues, goals, and current requirements

Characteristics of Bremsstrahlung x-ray sources created by picosecond laser pulses and radioprotection issues for petawatt lasers

Previous experimental and numerical results have shown that in multi-MeV Bremsstrahlung x-ray sources created by picosecond laser pulses, the delivered on-axis dose increases if the laser interacts with under-critical plasmas. The effect of the laser-plasma interaction as a function of the plasma density gradient is thus first studied here, by focusing the short pulse of the Alisé laser on a solid tantalum target coated with plastic. Interaction conditions are modified by irradiating the target front side with a secondary nanosecond heating beam, prior to the main pulse. The length of the expanding plasma is modified by adjusting the delay between the interaction and heating beams. Various diagnostics give access to a whole set of consistent experimental results on the x-ray source properties which are compared successfully to self-consistent numerical simulations obtained with coupled PIC and Monte Carlo codes. All source parameters increase as the plasma length increases, in reason of the increasing number of high-energy electrons near the axis. In a second part of this talk, we use our simulation tools to predict the x-ray and neutron (produced by photonuclear interactions in the target) flash characteristics that can be expected on future PW lasers. Favourable scaling laws for the production of an intense, directional and short duration x-ray source versus intensity are observed. However, these findings also stress that radioprotection will become an important issue for future petawatt lasers operating at the kJ level