Modelling and TOFOR measurements of scattered neutrons at JET

Abstract

In this paper, the scattered and direct neutron fluxes in the line of sight (LOS) of the TOFOR neutron spectrometer at JET are simulated and the simulations compared with measurement results. The Monte Carlo code MCNPX is used in the simulations, with a vessel material composition obtained from the JET drawing office and neutron emission profiles calculated from TRANSP simulations of beam ion density profiles. The MCNPX simulations show that the material composition of the scattering wall has a large effect on the shape of the scattered neutron spectrum. Neutron source profile shapes as well as radial and vertical source displacements in the TOFOR LOS are shown to only marginally affect the scatter, while having a larger impact on the direct neutron flux. A matrix of simulated scatter spectra for mono-energetic source neutrons is created which is folded with an approximation of the source spectrum for each JET pulse studied to obtain a scatter component for use in the data analysis. The scatter components thus obtained are shown to describe the measured data. It is also demonstrated that the scattered flux is approximately constant relative to the total neutron yield as measured with the JET fission chambers, while there is a larger spread in the direct flux, consistent with simulations. The simulated effect on the integrated scattered/direct ratio of an increase with movements outward along the radial direction and a drop at higher values of the vertical plasma

Response of LaBr 3 (Ce) scintillators to 14 MeV fusion neutrons

“…Errors are from counting statistics. Simulations of neutron transport from the plasma along the detector line of sight [25] predict the ratio of neutron fluence on the detector to the total neutron yield of JET to be (1.36 ± 0.4) · 10 -10 . This value is known with 30% uncertainty as it can vary depending on plasma shapes and neutron emission profiles.…”

Neutron spectroscopy as a fuel ion ratio diagnostic: Lessons from JET and prospects for ITER

“…We use experimental results from the time-of-flight optimized for rate ͑TOFOR͒ neutron spectrometer 15,16 at JET, first, to show that fuel ion ratio measurements with NES are possible and, second, to corroborate the modeling result of a substantial component of scattered neutrons. TOFOR views the JET plasma from a position about 20 m from the plasma center with a long, narrow collimator, similar to the ITER case.…”

Response of LaBr3(Ce) scintillators to 2.5 MeV fusion neutrons

“…Results are in good agreement. The scattering of the data points around the best fit line is mostly due to neutron transport to the detector, that depends on plasma shapes and neutron emission profiles (See details in [25]). The dashed lines in the figure correspond to the ± 30% uncertainty on the predicted slope of the LaBr 3 count rate as a function of the total JET neutron yield.…”