Abstract
The compact neutron spectrometer used at the ASDEX Upgrade tokamak is characterised to obtain its response matrix. This paper describes the characterisation procedure and the derived response matrix, based on a campaign at the PTB ion accelerator facility (PIAF) and on the subsequent time-of-flight (TOF) analysis of neutrons from a field with a broad energy distribution. The response of mono-energetic neutrons generated at the PIAF is used as reference for the TOF analysis. The detector's response functions for spectrum deconvolution are obtained by Gaussian broadening of the simulated responses to fit the experimental ones, using a maximum-entropy ansatz. In this way, the response functions are smooth enough to ensure a reliable unfolding of pulse height spectra into neutron emission spectra, which provide information on the fast ion velocity distribution in neutral beam heated tokamak plasmas.
Highlights
In tokamak research, measurements of the fast ion distribution are gaining attention as the confinement of suprathermal ions plays a key role in terms of fusion performance, potential damage of the plasma facing components, and interplay with magneto-hydrodynamic (MHD) instabilities such as Alfven eigenmodes,1,2 neoclassical tearing modes,3 and sawteeth,4 all representing serious concerns for next-step tokamaks like ITER.Neutron emission spectra (NES) provide energy-resolved information on the fast ion distribution, whenever beamtarget reactions dominate the neutron production, which is the case in Neutral Beam Injection (NBI) discharges, in the ASDEX Upgrade tokamak5 as well as in most NBI heated devices
This paper describes the characterisation procedure and the derived response matrix, based on a campaign at the PTB ion accelerator facility (PIAF) and on the subsequent time-of-flight (TOF) analysis of neutrons from a field with a broad energy distribution
Since pulses are digitalised via a digital acquisition system, the pulse shape is defined as the ratio between the pulse integral in a short and a long time interval after the pulse peak,7 whereas the Pulse Height (PH) is the total pulse integral
Summary
Measurements of the fast ion distribution are gaining attention as the confinement of suprathermal ions plays a key role in terms of fusion performance, potential damage of the plasma facing components, and interplay with magneto-hydrodynamic (MHD) instabilities such as Alfven eigenmodes, neoclassical tearing modes, and sawteeth, all representing serious concerns for next-step tokamaks like ITER. Diagnostic systems to measure energy resolved NES are usually expensive and require a large dedicated volume.. Diagnostic systems to measure energy resolved NES are usually expensive and require a large dedicated volume.6 These limitations are overcome by Compact Neutron Spectrometers (CNSs). CNSs cannot determine the neutron energy for each event occurring in the detector, but it rather allows to measure the deposited energy from recoil protons, for all possible scattering angles. The determination of the energy distribution of the incoming neutrons requires a deconvolution of the measured PHS, using the detector’s characteristic response functions. The detector’s response matrix has to be determined with high accuracy and it requires a sufficient degree of smoothness. The details of the characterisation procedure and the determination of the detector response matrix are the subject of this paper.
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