Design of a multi-detector, single line-of-sight, time-of-flight system to measure time-resolved neutron energy spectra.

In the dynamic environment of burning, thermonuclear deuterium-tritium plasmas, diagnosing the time-resolved neutron energy spectrum is of critical importance. Strategies exist for this diagnosis in magnetic confinement fusion plasmas, which presently have a lifetime of ∼1012 longer than inertial confinement fusion (ICF) plasmas. Here, we present a novel concept for a simple, precise, and scale-able diagnostic to measure time-resolved neutron spectra in ICF plasmas. The concept leverages general tomographic reconstruction techniques adapted to time-of-flight parameter space, and then employs an updated Monte Carlo algorithm and National Ignition Facility-relevant constraints to reconstruct the time-evolving neutron energy spectrum. Reconstructed spectra of the primary 14.028 MeV nDT peak are in good agreement with the exact synthetic spectra. The technique is also used to reconstruct the time-evolving downscattered spectrum, although the present implementation shows significantly more error.

Laser diode to single-mode fiber ball lens coupling efficiency: full-wave calculation and measurements.

We show that the coupling efficiency from a laser diode (LD) to an optical fiber through a ball lens can be calculated accurately using the exact solution to Maxwell's equations for the scattering of a beam from a dielectric sphere. Our calculated results agree closely with coupling measurements from an asymmetric LD for two different ball lenses.

Ball lens reflections by direct solution of Maxwell's equations.

We calculate ball lens reflections, using the exact solution of Maxwell's equations for the scattering of a beam from a dielectric sphere. Our results are consistent to within 1 dB with measurements of backreflection to a single-mode fiber. We also calculate backreflection to an astigmatic spot laser diode.

Weighted least-squares criteria for electrical impedance tomography.

Methods are developed for the design of electrical impedance tomographic reconstruction algorithms with specified properties. Assuming a starting model with constant conductivity or some other specified background distribution, an algorithm with the following properties is found. (1) The optimum constant for the starting model is determined automatically. (2) The weighted least-squares error between the predicted and measured power dissipation data is as small as possible. (3) The variance of the reconstructed conductivity from the starting model is minimized. (4) Potential distributions with the largest volume integral of gradient squared have the least influence on the reconstructed conductivity, and therefore distributions most likely to be corrupted by contact impedance effects are deemphasized. (5) Cells that dissipate the most power during the current injection tests tend to deviate least from the background value. For a starting model with nonconstant conductivity, the reconstruction algorithm has analogous properties.

Frequency limitations of the two-point central difference differentiation algorithm.

A two-point central difference algorithm is often used to calculate the derivative of a function. This estimate is only valid over a limited frequency range. Therefore, the algorithm can be modeled as an ideal differentiator in series with a low-pass filter. The filter cutoff frequency is a function of the time between the points. We discuss the accuracy and limitations of using this algorithm on human saccadic eye movement data. To calculate the velocity of saccadic eye movements the algorithm should have a cutoff frequency of 74 Hz or above.