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Velocity correction for neutron activation diagnostics at the NIF
Hans G Rinderknecht1, R Bionta1, G Grim1
1Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
The Review of Scientific Instruments
|November 8, 2018
Summary
Hotspot velocity in inertial confinement fusion affects neutron diagnostics. Accounting for these velocity-induced spectral changes is crucial for accurate areal density asymmetry measurements.
Area of Science:
- Nuclear Fusion Science
- Plasma Physics
- Diagnostic Techniques
Background:
- The velocity distribution of fusion hotspots influences neutron energy spectra and viewing angle.
- Neutron activation diagnostics (NADs) are sensitive to these spectral changes, impacting areal density (ρR) asymmetry measurements.
- Accurate interpretation of NAD data requires accounting for velocity-induced effects.
Purpose of the Study:
- To investigate how hotspot velocity affects NAD responses.
- To quantify the impact of velocity-induced spectral changes on NAD activation.
- To develop a method for correcting NAD data and assessing uncertainties.
Main Methods:
- Analysis of three mechanisms: Doppler shift in activation cross section, kinematic focusing of neutron fluence, and Doppler shift in scattering cross section.
- Inference of hotspot velocity using neutron time-of-flight measurements (D-T and D-D fusion neutrons).
- Evaluation of uncertainties in spherical-harmonic fits considering velocity correction and detector variations.
Main Results:
- Hotspot velocity was inferred from neutron time-of-flight data.
- The inferred hotspot velocity successfully explained observed NAD activation asymmetry in a calibration shot.
- A method for evaluating uncertainties in NAD data analysis was developed.
Conclusions:
- Hotspot velocity is a significant factor influencing NAD measurements.
- Velocity corrections are essential for accurate determination of areal density asymmetry.
- The proposed method enhances the reliability of NAD data interpretation in fusion experiments.
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