Neutron Diagnostics in the Large Helical Device

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To obtain rapid change of the neutron emission rate caused by magnetohydrodynamic (MHD) instabilities, measurements of the neutron flux with the high temporal resolution are made on the Large Helical Device (LHD) with a fast-neutron scintillation detector called neutron fluctuation (NF) operated in current modes. The time response of this detector is up to 5 microseconds. We checked the unwanted signal level caused by neutrons/gamma-rays inside the PMT. The intensity of the unwanted signal was negligibly small compared with the intensity of the scintillation signal as expected. The signal of NF has linear dependence with total neutron emission rate measured by neutron flux monitor in MHD quiet discharges. Rapid decreases of the neutron flux associated with bursting MHD modes excited in a neutral-beam-heated LHD plasma are observed with NF. We found that the neutron flux rapidly decreases within 2 ms.

Section: Methodsmentioning

confidence: 99%

Neutron Flux Measurement Using a Fast-Neutron Scintillation Detector with High Temporal Resolution on the Large Helical Device

Ogawa

Isobe

Kawase

et al. 2018

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“…The horizontal and vertical axes denote the simulated and measured neutron decay times, τ sim n and τ exp n . These neutron decay times, τ exp n and τ sim n , are estimated by the weighted least-square fitting in S peak n exp(−t/τ n ), which is weighted by the variance of the measured neutron emission rate [5], where S peak n is the peak value of the neutron emission rate. The y error-bars displayed in Fig.…”

Section: Benchmark Of the Neutron Estimationmentioning

confidence: 99%

“…In general, deuterium fusion plasmas yield fast neutrons due to the deuterium-deuterium (DD) fusion reaction. Because the neutron measurement plays an important role in the studies of fast ion confinement, LHD is equipped with the neutron diagnostics systems [5]. In the present fusion devices, such as LHD, the fusion reaction between thermal ion and accelerated fast ion is dominant instead of the reaction between thermal ions.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Energetic Particle Confinement in LHD Using Neutron Measurement and Simulation Codes

Nuga

Seki

Ogawa

et al. 2019

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The fast ion confinement time in the Large Helical Device (LHD) is investigated by using the neutron measurement and simulations. To estimate the fast ion confinement time, a series of short pulse neutral beam (NB) injection experiments have been performed. Additionally, the NB heating simulation code, CONV_FIT3D, has been extended to estimate the neutron emission rate in LHD. We estimate the fast ion confinement time from the differences between the measured and the simulated neutron decay times after the NB is turned off. It is found that the fast ion confinement time of three tangential NBs have similar values τ c ∼ 0.5 sec and the confinement time of the perpendicular NB is approximately τ c ∼ 0.06 sec. Additionally, by using the confinement time and the ion ratio data estimated from the simulation result, we can obtain a simulation result similar to the measured data.

“…The source neutron energy is assumed to be 99.5% of 2.45 MeV and 0.5% of 14.1 MeV. The 14.1 MeV neutron is generated by the triton burnup process [10]. Figure 4 shows neutron and gamma-ray spectra on the center line of the O-port with several radial distances from the center of the torus for the maximum neutron emission rate of 1.9 × 10 16 n/s.…”

Section: Radiation Environment Around Lhdmentioning

confidence: 99%

Neutronics Analyses for Shield Upgrading of the Compact Neutral Particle Analyzer for LHD Deuterium Plasma Experiments

Nishitani

Ozaki

Saito

et al. 2019

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The Compact Neutral Particle Analyzer (CNPA) is an E//B type neutral particle energy analyzer using permanent magnets. CNPA is installed on the diagnostic platform and connected to the outside horizontal port. Based on the preliminary shielding design, 15 cm thick borated polyethylene shields were installed before the fast deuterium plasma campaign. However, neutron induced noises could not be ignored in plasmas with the neutron emission rate higher than ∼1 × 10 14 n/s. We carried out the detailed neutronics analyses for the original neutron shield by using the MCNP6 code, which takes into account the detailed structure. The neutron streaming from the beam inlet and the penetration of the bulk shield are approximately 60% and 40% contribution to the neutron flux at the detector array, respectively. Consequently, 5 cm borated polyethylene plates are added to the bulk shield and an extension shield of borated polyethylene cylinder shield with 20 cm in diameter and 35 cm in length is added around the beam inlet, where the neutron flux is expected to be 1.9 × 10 6 n/cm 2 · s at the detector array for the plasma with the neutron emission rate of 1.9 × 10 16 n/s.