Characterization of the Fast Neutron Generators for Calibration of Fusion Neutron Diagnostics

Abstract

Modern magnetic confinement fusion devices increasingly rely on extensive neutron diagnostic configurations to measure a plethora of key plasma parameters. Measurement accuracy for these diagnostics depends heavily on in situ calibration. In order to enable said calibration, we set out to propose a reliable and powerful fast neutron source, to define a characterization plan for this source in terms of yield, flux and energy distributions, propose an optimized set of tools suitable for online monitoring of neutron source performance and its metrological characteristics. In the framework of this research activity with the ultimate aim of ITER tokamak neutron diagnostics calibration we rely on industrial-grade fast neutron generator NG-24 (D-D neutron yield ~109 n/s, D-T neutron yield ~1011 n/s) with sealed tube and stuffed, titanium target developed by FSUE VNIIA. The well-known analytical expressions for thick target NGs and our measurements using neutron spectrometers utilizing threshold reactions – diamond detector and LaCl3 scintillator for D-T and D-D neutron generators respectively – were found to be significantly coherent. These data are supported through our multi-step forward modelling including ion stopping in target, fusion reaction kinematics modelling and calculating detector response based on modelled neutron spectra. We discuss methods of uncertainty mitigation of neutron spectrometer measurements. Application of both neutron activation analysis and gas-filled neutron flux monitors during source characterization and operation allows for lowing statistical uncertainty of neutron flux measurements to 1% level over 1 minute of time resolution. Over the course of several measurement campaigns the optimal set of measurement tools have been determined including detector dimensions, required acquisition time, calibration methods.