Development of a High Sampling Rate Data Acquisition System Working in a High Pulse Count Rate Region for Radiation Diagnostics in Nuclear Fusion Plasma Research

ORCID　0000-0003-4555-1837In this study, a high sampling rate data acquisition system with the ability to provide timestamp, pulse shape information, and waveform simultaneously under a sub megahertz pulse counting rate was developed for radiation diagnostics for magnetic confinement nuclear fusion plasma research. The testing of the data acquisition system under the high pulse counting rate condition using real signals was performed in an accelerator-based deuterium-deuterium fusion neutron source (Fast Neutron Source) at the Japan Atomic Energy Agency. We found that the pulse counts acquired by the system linearly increased up to 6 × 105 cps, and the count loss at 106 cps was estimated to be ~10%. The data acquisition system was applied to deuterium-deuterium neutron profile diagnostics in the deuterium gas operation of a helical-type magnetic confinement plasma device, called the Large Helical Device, to observe the radial profile of neutron emissivity for the first time in a three-dimensional magnetic confinement fusion device. Time-resolved measurements of the deuterium-deuterium fusion emission profile were performed. The experimentally observed radial neutron emission profile was consistent with numerical predictions based on the orbit-following models using experimental data. The data acquisition system was shown to have the desired performance.journal articl

A scintillating-fiber detector for making high-time-resolution secondary D–T neutron measurements in KSTAR

ORCID　0000-0003-4555-1837A scintillating fiber (Sci-Fi) detector for the middle neutron flux range was installed in KSTAR as part of a collaboration between the National Institute for Fusion Science and the Korea Institute of Fusion Energy. The detector could make relatively high-time-resolution measurements of secondary deuterium (D)–tritium (T) neutron fluxes to investigate the degradation of D–D-born triton confinement, which is crucial for demonstrating alpha particle confinement, particularly above 0.9 MA in KSTAR. The pulse-height spectrum of the Sci-Fi detector exhibited two peaks, the higher of which corresponded to D–T neutrons. A discrimination technique was applied to extract the D–T neutron signal, revealing the time evolution of the D–T neutron flux during relatively high plasma current discharges with a 50 ms temporal resolution. Future research will involve investigating the causes of the degradation of the triton burnup ratio above 0.9 MA in KSTAR.journal articl

The initial measurement of a compact D–T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for fusion plasma diagnostic

ORCID　0000-0002-2771-6979For developing and characterizing a novel compact D–T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for plasma diagnostics toward future D–T fusion reactors, the initial measurement was performed using the accelerator-based D–T neutron sources OKTAVIAN at Osaka University. This neutron spectrometer was designed for the detection of 3–17 MeV neutrons and operated in the proton recoil telescope configuration by installing a polyethylene converter in front of the diamond stack. The measured neutron energy spectra were obtained by summing the energy of the recoil protons deposited in the diamond stack after the coincidence of the recoil protons identified by the time coincidence analysis. The neutron energy peaks measured by the compact D–T neutron spectrometer were almost in agreement with those obtained by the Monte Carlo N-Particle transport (MCNP) simulation. The energy resolution of the compact D–T neutron spectrometer was emulated to be about 4%–5% in D–T neutron measurement. In future work, the design of the compact D–T neutron spectrometer would be optimized to measure the fusion neutron for plasma diagnostics.journal articl

Gamma ray diagnostics for high time resolution measurement in large helical device

ORCID　0000-0003-4555-1837A 1-inch LaBr3:Ce gamma ray scintillation detector, characterized by high time and energy resolution, was installed to advance energetic particle physics studies in Large Helical Device (LHD). We reduced the size of the scintillator from 3 inches to 1 inch to reinforce the radiation shielding to reduce the unwanted signal in the detector induced by fast neutrons and stray gamma rays according to the commissioning results. The radiation shielding composed of 10% borated polyethylene and lead was redesigned to suppress the gamma-ray induced signal based on the Monte Carlo three-dimensional neutron and gamma-ray transport code MCNP6. We increased the lead thickness from 50 mm to 77.5 mm to suppress the stray gamma ray effect. The gamma ray spectrum was measured in the hydrogen neutral beam heated deuterium plasma with 6LiF pellet injection. We might obtain a gamma ray peak near 0.48 MeV due to the 6Li(d,p'γ)7Li reaction.journal articl

Time-resolved deuterium–deuterium fusion born 1 MeV triton confinement study in EAST deuterium plasma

ORCID　0000-0003-4555-1837A time-resolved deuterium–deuterium (D–D) fusion-born triton confinement study, aimed at understanding alpha particle confinement ability, was performed in Experimental Advanced Superconducting Tokamak (EAST) deuterium plasmas for the first time. A scintillating fiber detector was developed for measuring the secondary deuterium–tritium (D–T) neutrons, which provide evidence of triton slowdown, in EAST. The D–D fusion-born triton confinement experiment was performed by measuring secondary D–T neutrons in D-beam-heated D plasma with a plasma current of 400 kA. The secondary D–T neutron signal and its time evolution were obtained using pulse height discrimination analysis. The D–T neutron rate was calculated using the classical energetic ion confinement model to clarify the D–T neutron measurements. The secondary D–T neutron emission rate obtained from the numerical simulation closely agrees with the experimentally obtained results when considering the prompt loss of the tritons.journal articl

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.journal articl

Neutron Spectroscopy in Perpendicular Neutral Beam Injection Deuterium Plasmas Using Newly Developed Compact Neutron Emission Spectrometers

ORCID　0000-0002-0160-0468The presence of helically trapped fast ions in helical ripples in the large helical device (LHD), resulting from a perpendicularly injected positive-ion-source-based neutral beam (P-NB)-heated plasma and/or from ion cyclotron range of frequency (ICRF) wave-heated plasma, poses a significant concern due to the system’s lack of symmetry. In response to this challenge, compact neutron emission spectrometers (CNESs) have been strategically developed in the LHD, featuring a perpendicular line of sight relative to the plasma’s magnetic field. The first perpendicular CNES offered a vertical field of view through the deuterium plasma and incorporated a liquid (EJ-301) scintillation detector optimized for efficient operation at a high neutron emission rate. The second perpendicular CNES was operated using a Cs2LiYCl6:Ce scintillation detector enriched with 7Li (CLYC7), providing a horizontal field of view through the deuterium plasma and effectively operating in a region characterized by relatively low neutron emission rates. During plasma heating through P-NB, both perpendicular CNESs showed a neutron energy distribution, featuring a double-humped profile with two peaks at energy of approximately 2.30 and 2.74 MeV. Furthermore, the deuterium-deuterium neutron energy spectrum expected to be obtained by perpendicular CNES was calculated using the 5-D orbit following code DELTA5D and considering the Larmor motion effect. The results revealed that the experimentally obtained neutron energies were almost consistent with the neutron energies calculated based on the orbit following model. These peaks aligned with the Larmor motion of deuterons resulting from P-NB injection at the helical ripple of the LHD. In addition, we conducted neutron spectroscopy involving deuterium-deuterium interactions in a deuterium plasma simultaneously heated by both P-NB and ICRF wave, characterized by a high neutron emission rate, using perpendicular CNES based on EJ-301. The observation revealed a broadening in spectrum width, attributed to the additional ICRF wave heating.journal articl

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

Characterization of CLYC7 Scintillation Detector in Wide Neutron Energy Range for Fusion Neutron Spectroscopy

ORCID　0000-0002-0160-0468The newly developed Cs2LiYCl6:Ce crystal with7Li-enrichment (CLYC7) scintillators had been utilized as a key component in a compact neutron emission spectrometer (CNES) at the large helical device (LHD). The CNES was employed to enhance understanding the slowing-down process of neutral-beam-injected energetic beam deuterons by measuring the Doppler broadening of beam-driven deuterium–deuterium neutrons. In order to assess the detection capabilities in fusion neutron spectroscopy and to gain understanding into the energy spectrum at various neutron energies, an understanding of the response function of the detector across a wide neutron energy range is essential. Therefore, before implementing the CNES at the LHD, the detector was tested at high-performance neutron source facilities to evaluate its response to both thermal and fast neutrons. The detector exhibited excellent pulse shape discrimination (PSD) properties. However, it also exhibited complexity through the 35Cl(n, p)35S and 35Cl(n, α )32P reactions in the fast neutron measurement. To emphasize the accuracy and reliability of the detector’s response, particularly in the fast neutron energy range, Monte Carlo N-Particle (MCNP) transport simulations were employed. In order to validate the experimental results against the calculations, an energy-dependent quenching factor was determined. This analysis provided a clear understanding of the CLYC7 scintillation detector and its potential in the context of fusion neutron spectroscopy for applications in the LHD and future fusion devices.journal articl