Performance of the newly installed vertical neutron cameras for low neutron yield discharges in the Large Helical Device

Abstract

Two new vertical neutron cameras characterized by high detection efficiency were developed on the Large Helical Device in order to observe poloidal structures of helically trapped beam ions created by the perpendicularly injected positive-ion based neutral beam (P-NB) and are newly operated since 2018. In this work, the neutron fields at the vertical neutron cameras are investigated using the Monte Carlo N-particle transport code to evaluate the performance of its collimators. The results indicate that neutrons are attenuated by the heavy concrete and are well collimated through the collimator to detectors. Neutron spectra at the detector position show over 99% of uncollided 2.45 MeV neutrons. Time evolution of neutron emission profiles during the short pulse of P-NB injection is measured by the vertical neutron cameras. Peaks on the neutron emission profiles corresponding to the helically trapped beam ion are successfully obtained, as designed. The decrease in line integrated neutron flux at the peak positions after the P-NB stops is consistent with the behavior of the total neutron emission rate measured by the neutron flux monitor.

Progress on Integrated Neutron Diagnostics for Deuterium Plasma Experiments and Energetic Particle Confinement Studies in the Large Helical Device During the Campaigns from FY2017 to FY2019

“…10 (a)) [79]. In addition, the others (VNC2 and VNC3) were installed in the 1.5-L port for measuring the neutron emission profile in a relatively low-neutron emission rate range, i.e., 10 14 ∼10 15 n/s, where strong MHD instabilities can be observed [80]. VNC1 has 11 sightlines.…”

Characterization of the New Vertical Neutron Camera Designed for the Low Neutron Emission Rate Plasma in Large Helical Device

“…The VNC1 is installed at the 2.5L LHD lower port [17]. The VNC2 and VNC3 are installed at the 1.5L LHD lower port to view in different plasma cross-sections [22,23]. Note that VNC2 is beyond the scope of this paper.…”

“…The data acquired by the ADC is transferred to the data acquisition PC which is connected to the LHD experimental database. The VNC3 provides the neutron profile with the spatial resolution of ∼13 cm in FWHM at the midplane [23].…”

“…In the plasma discharges, where author's e-mail: sangaroon.siriyaporn@nifs.ac.jp the relatively strong MHD instabilities were observed, the time resolution of neutron emission profile was 10 ms with ∼10% error bar because of the relatively low neutron count due to the relatively low S n of 10 14 to 10 15 n/s [21]. To study the EP transport with the higher time resolution in relatively low S n discharges, the high detection efficiency vertical neutron cameras (the so-called VNC2 and the socalled VNC3) were developed and were used in LHD from 2018 [22,23]. This paper is devoted to characterization of the VNC3 designed for the relatively low S n discharges in LHD.…”

Characteristics of neutron emission profile from neutral beam heated plasmas of the Large Helical Device at various magnetic field strengths

“…In helical systems, experiments can be conducted in a relatively wide range of magnetic field and plasma parameter ranges because the magnetic surfaces are created primarily by external coils. Previous measurements of the neutron emission profile were mainly performed at a Bt of ~2.75 T, for which Sn is relatively high [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. In this study, the neutron emission profile was investigated for various Bt to show the ability of the neutron emission profile measurement in a fusion plasma as well as in the interest of developing a more fundamental understanding of beam ion confinement.…”