Energetic particle transport and loss induced by helically-trapped energetic-ion-driven resistive interchange modes in the Large Helical Device

Abstract

In this work, energetic-ion confinement and loss due to energetic-ion driven magnetohydrodynamic modes are studied using comprehensive neutron diagnostics and orbit-following numerical simulations for the Large Helical Device (LHD). The neutron flux monitor is employed in order to obtain global confinement of energetic ions and two installed vertical neutron cameras (VNCs) viewing different poloidal cross-sections are utilized in order to measure the radial profile of energetic ions. A strong helically-trapped energetic-ion-driven resistive interchange mode (EIC) excited in relatively low-density plasma terminated high-temperature state in LHD. Changes in the 2 neutron emission profile due to the EIC excitation are clearly visualized by the VNCs. The reduction in the neutron signal for the helical ripple valley increases with EIC amplitude, which reaches approximately 50%. In addition to the EIC experiment, orbit-following simulations using the DELTA5D code with EIC fluctuations were performed to assess the energetic-ion transport and loss. Two-dimensional temporal evolution results show that the neutron emissivity at the helical ripple decreases significantly due to the EIC. The rapid reduction in neutron emissivity shows that the helically-trapped beam ions immediately escape from the plasma. The reduction in the VNC signals for the helical ripple valley and the total neutron emission rate increase with increasing EIC amplitude, as observed in the experiment. Calculated line-integrated neutron emission results show that the profile measured by VNC1 has one peak, whereas the profile measured by VNC2 has two peaks, as observed in the experiment. Although the neutron emission profile for VNC2 has a relatively wide peak compared with the experimental results, the significant decrease in neutron signal corresponding to the helical ripple valley was successfully reproduced.

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.…”

“…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.…”

Observation of significant Doppler shift in deuterium-deuterium neutron energy caused by neutral beam injection in the large helical device

“…By starting the deuterium experiment in LHD in March 2017, global information of confined fast-ions can be obtained using a comprehensive neutron diagnostic [7][8][9][10] because neutrons are mainly created by the fusion reaction between fast-ion and bulk plasma [11]. Time evolution of neutron emission profile obtained in energetic-ion-driven resistive interchange mode discharge in the experiment and numerical simulation revealed the substantial radial transport of beam ions [12,13]. For a deeper understanding of the excitation of fast-ion-driven MHD instability and resonance between fast-ion and fastion-driven MHD instability, the energy distribution of fast-ion is necessary.…”

Time-resolved secondary triton burnup 14 MeV neutron measurement by a new scintillating fiber detector in middle total neutron emission ranges in deuterium large helical device plasma experiments

“…In the deuterium experiment performed in the large helical device (LHD) [14][15][16], energetic particle confinement studies are largely advanced using a variety of neutron diagnostics [17][18][19][20][21][22][23]. Sci-Fi detectors play a part in comprehensive neutron diagnostics.…”

“…In the LHD, plasma discharge with MHD instabilities possessing relatively large amplitudes is excited in the S n range of 10 13 n/s to 10 15 n/s [29,30]. Although beam ion transport due to MHD instabilities has been intensively studied by vertical neutron cameras [18][19][20], the effect of deuterium-deuterium (DD) fusionborn triton transport is unclear because there is almost no DT neutron count in vertical neutron cameras [31]. To understand the DD fusion-born triton transport due to the MHD instabilities, time-resolved measurement of secondary DT neutrons with a 1 ms time bin over this S n range is required.…”