Neutron wall emission in tokamaks and plasma diagnostics

The wall emission (WE) background accompanying observations of fusion plasma neutrons is a limiting factor in high accuracy neutron diagnostics. It is an important issue to solve for envisaged burning plasma experiments such as with ITER. This work represents the first study of the spectral shape and magnitude of the WE spectrum. The salient features are defined and analyzed with respect to their parametric dependencies based on model descriptions of the plasma neutron source and calculations of the neutron transport. Simple analytical expressions for neutron scattering are used to display the dependence on reaction kinematics, nuclear structure and plasma conditions. Monte Carlo calculations are used for comprehensive descriptions of the WE spectrum distinguishing between single and different multiple scattering contributions. Special consideration is paid to the errors in predicting WE spectra and to the practicality for adopting measurement based descriptions. Illustrations are given describing the impact of the WE background on essential neutron diagnostic measurements proposed for ITER and the means for controlling and correcting for such interference. Specific improvements in the neutron scattering data base relevant in describing WE spectrum are identified.