Density determination of the thermonuclear fuel region in inertial confinement fusion implosions

Abstract

Understanding of thermonuclear burn in an inertial confinement fusion implosion requires knowledge of the local deuterium-tritium (DT) fuel density. Neutron imaging of the core now provides this previously unavailable information. Two types of neutron images are required. The first is an image of the primary 14-MeV neutrons produced by the D+T fusion reaction. The second is an image of the 14-MeV neutrons that leave the implosion hot spot and are downscattered to lower energy by elastic and inelastic collisions in the fuel. These neutrons are measured by gating the detector to record the 6-12 MeV neutrons. Using the reconstructed primary image as a nonuniform source, a set of linear equations is derived that describes the contribution of each voxel of the DT fuel region to a pixel in the downscattered image. Using the measured intensity of the 14-MeV neutrons and downscattered images, the set of equations is solved for the density distribution in the fuel region. The method is validated against test problems and simulations of high-yield implosions. The calculated DT density distribution from one experiment is presented.

Methods for extremely sparse-angle proton tomography

“…It is in general possible to perform tomographic measurements without the restriction of all probe directions to a common plane (equivalently, without a single well-defined "axis of rotation") [28]. In that case, however, the choice of reconstruction methods is more limited.…”

“…Volegov et al have pioneered the use of tomographic techniques in three-dimensional reconstruction of plasma parameters at the National Ingition Facility [26][27][28][29]. They employ a number of approaches using different probing geometries with orthogonal basis function representations (such as spherical and cylindrical harmonic decompositions) of the function under observation.…”

Improved gamma imaging at NIF using the ceramic scintillator GYGAG

“…[1][2][3][4][5][6] Images of the burning fuel itself are routinely collected using the NIF Neutron Imaging System (NIS) [7][8][9][10][11][12] through detection of the primary 14.1 MeV fusion neutron with plastic scintillators; neutrons scattered off the dense fuel are isolated by gating the camera later in time, and provide complimentary information on the hot spot density without added complexity from the fuel temperature. [13][14][15] As they escape from the fuel region, primary 14.1 MeV neutrons may also interact with the remaining high-density carbon (HDC) shell via the 12 C(n,n ′ γ) 12 C reaction, resulting in the emission of a 4.4 MeV gamma. The gamma is also detected using scintillators and isolated by gating the camera early in time ahead of the slower neutron signals.…”

The effect of areal density asymmetries on scattered neutron spectra in ICF implosions

“…12 Scattered neutron images require sophisticated tomographic reconstruction techniques to extract the fuel density distribution. 13 Scattered neutron spectroscopy is another method commonly used to measure fuel areal densities. 14,15 It can be used at lower areal densities at which activation measurements are difficult, as is the case at OMEGA.…”