Wide dynamic range neutron flux monitor having fast time response for the Large Helical Device

Initial operation of perpendicular line-of-sight compact neutron emission spectrometer in the large helical device

The perpendicular line-of-sight compact neutron emission spectrometer (perpendicular CNES) was newly installed to understand the helically trapped fast-ion behavior through deuterium-deuterium (D-D) neutron energy spectrum measurement in the Large Helical Device (LHD). The energy calibration of the EJ-301 liquid scintillation detector system for perpendicular CNES was performed on an accelerator-based D-D neutron source. We installed two EJ-301 liquid scintillation detectors, which view the LHD plasma vertically from the lower side through the multichannel collimator. The D-D neutron energy spectrum was measured in a deuterium perpendicular-neutral-beam-heated deuterium plasma. By the derivative unfolding technique, it was found that the D-D neutron energy spectrum had a double-humped shape with peaks at ∼2.33 and ∼2.65 MeV. D-D neutron energy spectrum was calculated based on the fast ion distribution function using guiding center orbit-following models considering the detector's energy resolution. The calculated peak energies in the D-D neutron energy spectrum almost match the experiment. In addition, a feasibility study toward the measurement of the energy distribution of ion-cyclotron-range-of-frequency-wave-accelerated beam ions was performed.

Neutron-induced signal on the single crystal chemical vapor deposition diamond-based neutral particle analyzer

A diamond-based neutral particle analyzer (DNPA) array composed of single-crystal chemical vapor deposition (sCVD) diamond detectors was installed on the Large Helical Device (LHD) for measuring the helically trapped energetic particles. In high neutron flux experiments, the unwanted neutron-induced pulse counting rate should be estimated using the neutron diagnostics because a diamond detector is sensitive to neutrons as well as energetic neutral particles. In order to evaluate the quantitative neutron-induced pulse counting rate on the DNPA, the response functions of the sCVD diamond detector for mono-energetic neutrons were obtained using accelerator-based D-D and D-⁷Li neutron sources in Fast Neutron Laboratory (FNL). As a result of the neutron flux estimation by the Monte Carlo N-Particle code at the NPA position in the LHD and the response function obtained in the FNL experiment, the counting rate of the neutron-induced signal was predicted to be 1.1 kcps for the source neutron emission rate of S_n = 1 × 10¹⁵ n/s. In the LHD experiment, the neutron-induced signals were observed by closing the gate valve during the plasma discharges. It is found that the counting rates of the neutron-induced signals proportional to S_n reached 1.1 kcps at S_n = 1 × 10¹⁵ n/s. As a result of the quantitative estimation of the neutron-induced signals on the DNPA using other neutron measurements, it has become possible to accurately measure energetic neutral particles in the high neutron flux experiment.

Experimental examination of a method to estimate temporal effect by neutrons and γ-rays on scintillation light in scintillator-based soft x-ray diagnostic of experimental advanced superconducting tokamak and large helical device

Scintillators, which are more tolerant of neutrons or γ-rays than semiconductors, are a promising candidate for soft X-ray (SX) diagnostics in high neutron flux environments such as JT-60SA or ITER. Although scintillators are tolerant of radiations, neutrons and γ-rays can cause scintillation light and become noise on SX signals. Therefore, a method to estimate the temporal effect by the radiations on SX signals and an appropriate design of the radiation shield based on the estimation are required. In previous studies, it has been proposed for estimating the effect by the radiations to calculate the absorption powers due to SXs, neutrons, and γ-rays in scintillators assuming that amplitudes of scintillation light are proportional to the absorption powers. In this study, an experimental examination of this proposal is conducted in the Experimental Advanced Superconducting Tokamak (EAST). It is shown that the proposal may be valid in the examination of EAST. In addition to results in EAST, initial results of a multi-channel scintillator-based SX diagnostic in the Large Helical Device (LHD) are introduced. Although a scintillator-based SX diagnostic in LHD observes oscillations of SXs by magnetohydrodynamic (MHD) phenomena successfully, the observed temporal effect on SX signals by neutrons or γ-rays is more significant than the expected effect, which is estimated by calculating the absorption powers. One of the possible reasons for the contradiction between the results in EAST and LHD is unexpected γ-rays around the scintillators in LHD. Although the temporal effect by the radiations is significant in the current system of LHD, the degradation of amplitudes of SX signals after the deuterium plasma experiments is not observed with the current level of the fluence. The scintillator-based SX diagnostic in LHD may work as a diagnostic to research MHD instabilities in deuterium plasma experiments without additional maintenance during an experimental campaign by making the pinhole larger or setting an additional radiation shield.

The large helical device vertical neutron camera operating in the MHz counting rate range

In the currently performed neutral beam (NB) -heated deuterium plasma experiments, neutrons are mainly produced by a beam-plasma reaction. Therefore, time-resolved measurement of the neutron emission profile can enhance the understanding of the classical and/or anomalous transport of beam ions. To measure radial neutron emission profiles as a function of time, the vertical neutron camera (VNC) capable of operation with a counting rate in the MHz range was newly installed on the Large Helical Device (LHD). This is the world's first neutron camera for stellarator/heliotron devices. The VNC consists of a multichannel collimator, eleven fast-neutron detectors, and the digital-signal-processing-based data acquisition system (DAQ). The multichannel collimator having little cross talk was made from hematite-doped heavy concrete, which has a high shielding performance against both neutrons and gamma-rays. A stilbene crystal coupled with a photomultiplier having high-gain-stability in the high-count rate regime was utilized as a fast-neutron scintillation detector because it has a high neutron-gamma discrimination capability at high count rates. The DAQ system equipped with a field programmable logic controller was developed to obtain the waveform acquired with a 1 GHz sampling rate and the shaping parameter of each pulse simultaneously at up to 10⁶ cps (counts per second). Neutron emission profiles were successfully obtained in the first deuterium campaign of LHD in 2017. The neutron emission profile was measured in tangentially co-injected NB-heated plasma with different magnetic axes (R _ax). The neutron counts became larger in the inward-shifted configuration, which was consistent with the total neutron rate measured by the neutron flux monitor. The radial peak position of the line-integrated neutron profile which changed according to R _ax showed that the VNC worked successfully as designed. The VNC demonstrated the expected performance conducive to extending energetic-particle physics studies in LHD.

Scintillating fiber detectors for time evolution measurement of the triton burnup on the Large Helical Device

Two scintillating fiber (Sci-Fi) detectors have been operated in the first deuterium plasma campaign of the Large Helical Device in order to investigate the time evolution of the triton burnup through secondary 14 MeV neutron measurement. Two detectors use scintillating fibers of 1 mm diameter embedded in an aluminum matrix with a length of 10 cm connected to the magnetic field resistant photomultiplier. A detector with 91 fibers was developed in the Los Alamos National Laboratory and has been employed on JT-60U. Another detector with 109 fibers has been developed in the National Institute for Fusion Science. The signals are fed into a discriminator of 300 MHz bandwidth with a pulse counter module for online measurement and a digitizer of 1 GHz sampling with 14 bits to acquire pulse shape information for offline data analysis. The triton burnup ratio has been evaluated shot-by-shot by the 14 MeV neutron measurement of Sci-Fi detectors which are calibrated by using the neutron activation system and the total neutron measurement of the neutron flux monitor using ²³⁵U fission chambers.

In situ calibration of neutron activation system on the large helical device

In situ calibration of the neutron activation system on the Large Helical Device (LHD) was performed by using an intense ²⁵²Cf neutron source. To simulate a ring-shaped neutron source, we installed a railway inside the LHD vacuum vessel and made a train loaded with the ²⁵²Cf source run along a typical magnetic axis position. Three activation capsules loaded with thirty pieces of indium foils stacked with total mass of approximately 18 g were prepared. Each capsule was irradiated over 15 h while the train was circulating. The activation response coefficient (9.4 ± 1.2) × 10^-8 of ¹¹⁵In(n, n')^115mIn reaction obtained from the experiment is in good agreement with results from three-dimensional neutron transport calculations using the Monte Carlo neutron transport simulation code 6. The activation response coefficients of 2.45 MeV birth neutron and secondary 14.1 MeV neutron from deuterium plasma were evaluated from the activation response coefficient obtained in this calibration experiment with results from three-dimensional neutron calculations using the Monte Carlo neutron transport simulation code 6.

Characterization of photo-multiplier tube as ex-vessel radiation detector in tokamak

Feasibility of using conventional photo-multiplier tubes (PMTs) without a scintillator as an ex-vessel radiation detector in a tokamak environment is studied. Basic irradiation tests using standard gamma ray sources and a d-d neutron generator showed that the PMT is responding both to gamma photons and neutrons, possibly due to the direct generation of secondary electrons inside the PMT by the impingement of high energy photons. Because of the selective sensitivity of the PMT to hard x-ray and neutrons in ohmic and neutral beam injected plasmas, respectively, it is shown that the PMT with certain configuration can be utilized either to monitor the fluctuation in the fusion neutron generation rate or to study the behavior of runaway electrons in tokamaks.