Design and optimization of an advanced time-of-flight neutron spectrometer for deuterium plasmas of the large helical device

A time-of-flight neutron spectrometer based on the Time-Of-Flight Enhanced Diagnostic (TOFED) concept has been designed and is under development for the Large Helical Device (LHD). It will be the first advanced neutron spectrometer to measure the 2.45 MeV D–D neutrons (DDNs) from helical/stellarator plasmas. The main mission of the new TOFED is to study the supra-thermal deuterons generated from the auxiliary heating systems in helical plasmas by measuring the time-of-flight spectra of DDN. It will also measure the triton burnup neutrons (TBNs) from the d+t reactions, unlike the original TOFED in the EAST tokamak. Its capability of diagnosing the TBN ratios is evaluated in this work. This new TOFED is expected to be installed in the basement under the LHD hall and shares the collimator with one channel of the vertical neutron camera to define its line of sight. The distance from its primary scintillators to the equatorial plane of LHD plasmas is about 15.5 m. Based on Monte Carlo simulation by a GEANT4 model, the resolution of the DDN energy spectra is 6.6%. When projected onto the neutron rates that are typically obtained in LHD deuterium plasmas (an order of 1015 n/s with neutral beam injection), we expect to obtain the DDN and TBN counting rates of about 2.5 · 105 counts/s and 250 counts/s, respectively. This will allow us to analyze the DDN time-of-flight spectra on time scales of 0.1 s and diagnose the TBN emission rates in several seconds with one instrument, for the first time in helical/stellarator plasmas

Fusion product diagnostics based on commercially available chemical vapor deposition diamond detector in large helical device

Fusion product diagnostics based on four commercially available single-crystal chemical vapor deposition (s-CVD) diamond detectors are installed in the Large Helical Device (LHD) in order to understand energetic ion confinement. Characteristics of s-CVD diamonds were surveyed using alpha and D-T neutron sources. It is found that the energy resolutions of s-CVD diamonds for ∼ 5 MeV alpha particles and 14 MeV neutrons are 1%–3% and ∼ 1.7%, respectively. Moreover, the response of four s-CVD diamond detectors to alpha particles and D-T neutrons is almost identical. The installation positions of the diamond detectors in the vacuum vessel are searched for, based on the loss points of charged fusion products reckoned by Lorentz orbit calculations. Energy- and time-resolved measurement of fusion product flux will progress in further understanding of energetic ion confinement in LHD

Characterization of Liquid Scintillator-Based CNES for Deuterium–Deuterium Neutron Emission Spectroscopy in the LHD

ORCID　0000-0002-0160-0468The compact neutron emission spectrometers (CNESs) based on conventional liquid (EJ-301) scintillation detectors were characterized in this work. The CNESs were employed for deuterium–deuterium neutron emission spectroscopy in the Large Helical Device (LHD). Prior to the installation, the EJ-301 scintillation detectors were characterized at the Fast Neutron Laboratory (FNL) of Tohoku University. In order to discriminate the neutron and γ -ray signals, the charge comparison method was used. The neutron energy spectrum was successfully unfolded from the measured recoil proton energy spectrum of the EJ-301 scintillation detector using the derivative unfolding technique. The detector’s energy resolution was examined. In the LHD, EJ-301-based CNESs with both tangential and perpendicular line-of-sight were employed for characterization. Furthermore, the characterization of the CNES was performed in the deuterium–deuterium experiment campaign. The operational capabilities of CNESs in the LHD were examined. Deuterium–deuterium neutron emission spectroscopy in various approaches of neutral beam (NB)-heated plasmas was conducted using CNES. Because of the high energy of the injected fast ions from tangential NB injection, an investigation of the Doppler effect on deuterium–deuterium neutron energy was conducted. The upper and lower shifted deuterium–deuterium neutron energies were observed when the fast ions moved toward and away from the tangential CNES, respectively. As expected, no notable energy shift was observed from fast ions injected by the perpendicular NB injection. Additionally, the 5-D orbit following code DELTA5D, which takes into account Larmor motion effects and the detector’s energy resolution, was utilized to compute the expected deuterium–deuterium neutron energy spectrum that would be measured by the CNES. There was a concurrence between the calculations and experimental results