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The contribution of the beam–beam (b–b) Coulomb collision effect on the fast ion slowing-down process is investigated. The effect is evaluated experimentally in the large helical device (LHD) from the response of the neutron emission rate to the direction of the tangential hydrogen beam, which is used with the tangential deuterium beam. In addition, to analyze the experimental results, a Fokker–Planck (F–P) code is improved. It is observed that the decay time of the neutron emission rate after the deuterium beam is turned off depends on the direction of the hydrogen beam. This trend can be explained by the b–b Coulomb collision effect. The hydrogen beam, which has the same direction as the deuterium beam, deforms the fast deuteron velocity distribution due to the b–b Coulomb collision. As a result, the neutron decay time becomes longer than that with the opposite direction hydrogen beam. Our F–P simulation shows that the b–b Coulomb collision effect contributes to the decay time of the neutron emission rate. This simulation result is qualitatively similar to the experimental result. For quantitative analysis, consideration of the fast ion spatial transport, which is neglected in the present simulation, is required., source:Citation H. Nuga et al 2019 Nucl. Fusion 59 016007, source:https://doi.org/10.1088/1741-4326/aaeaf4, identifier:0000-0003-3293-488X

For steady state operation, the feedback control of plasma density and heating power system was developed in the Large Helical Device (LHD). In order to achieve a record of the long pulse discharge, stable plasma density and heating power are needed. This system contains the radio frequency (RF) heating power control, interlocks, gas fueling, automatic RF phase control, ion cyclotron range of frequency (ICRF) antenna position control, and graphical user interface (GUI). Using the density control system, the electron density was controlled to the target density and using the RF heating power control system, the RF power injection could be stable. As a result of using this system, we achieved the long pulse up to 48 min in the electron density of more than 1 × 1019 m−3. Further, the ICRF hardware experienced no critical accidents during the 17th LHD experiment campaign in 2013., source:Shuji Kamio, Hiroshi Kasahara, Tetsuo Seki, Kenji Saito, Ryosuke Seki, Goro Nomura, Takashi Mutoh, Feedback control of plasma density and heating power for steady state operation in LHD, Fusion Engineering and Design, Volume 101, 2015, pages 226-230., source:https://doi.org/10.1016/j.fusengdes.2015.09.012, identifier:0000-0003-0755-2433

A simultaneous high ion temperature (Ti) and high electron temperature (Te) regime was successfully extended due to an optimized heating scenario in the LHD. Such high-temperature plasmas were realized by the simultaneous formation of an electron internal transport barrier (ITB) and an ion ITB by the combination of high power NBI and ECRH. Although the ion thermal confinement was degraded in the plasma core with an increase of Te/Ti by the on-axis ECRH, it was found that the ion thermal confinement was improved at the plasma edge. The normalized ion thermal diffusivity ${{\chi}_{\text{i}}}/T_{\text{i}}^{1.5}$ at the plasma edge was reduced by 70%. The improvement of the ion thermal confinement at the edge led to an increase in Ti in the entire plasma region, even though the core transport was degraded., source:H. Takahashi et al 2017 Nucl. Fusion 57 086029, source:https://doi.org/10.1088/1741-4326/aa754b, identifier:0000-0001-6984-9174

The ion cyclotron range of frequencies (ICRF) heating power injection in the hydrogen experiment in LHD was demonstrated after the upgrade of ICRF antennas. The ICRF wave couples and accelerates the energetic particles injected by perpendicular-NBIs with 40 keV. The simulation by the MORH code shows the existence of energetic particles around the ICRF third harmonic resonance layers. As the result of ICRF heating power deposition, the beta value increased by 0.2% in absolute beta mainly due to the increased energetic particle content. The increase of energetic ions particularly around 60 keV, which should be accelerated by the ICRF heating, is observed. The ICRF heating efficiency was approximately 30%–50%, estimated from the break-in-slope analysis at the turn off timing of ICRF power from the stored energy measured by diamagnetic loops. This increase of the stored energy is mostly the contribution of the increased energetic particles. The heating efficiency increases as the density increases., source:Citation S. Kamio et al 2018 Nucl. Fusion 58 126004, source:https://doi.org/10.1088/1741-4326/aadbb4, identifier:0000-0003-0755-2433

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 Sn = 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 Sn reached 1.1 kcps at Sn = 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., source:Review of Scientific Instruments 91, 113304 (2020); https://doi.org/10.1063/5.0020460, source:https://doi.org/10.1063/5.0020460, identifier:0000-0003-0755-2433

In order to study fast ion confinement and transport mechanisms both in quiescent plasmas and in the presence of magneto-hydrodynamic (MHD) activity, an electric field parallel to the magnetic field type neutral particle analyzer (E|B-NPA) was installed using Micro-Channel Plates (MCPs) in the Large Helical Device (LHD). The E|B-NPA is an established diagnostic used to measure the energy distribution of charge exchange neutral particles escaping from plasmas corresponding to hydrogen, deuterium, and tritium, respectively. The LHD deuterium experimental campaign started in 2017. To obtain more details of MHD behavior, we improved on time resolution performance of the E|B-NPA using a high-speed Pre-Amplifier and Discriminator (PAD) and Latching Scaler Module (LSM). For the last LHD experimental campaign, the E|B-NPA was installed on nearly the equatorial plane of the vacuum chamber to measure the passing energetic particles generated by tangentially injected neutral beams. As the results of the E|B-NPA commissioning, it is confirmed that the E|B-NPA successfully detected the energetic particles and obtained the energy distributions corresponding to hydrogen. Also, the effect of the neutron to E|B-NPA is estimated using the other neutron diagnostics during the experiments., source:Y. Fujiwara et al 2020 JINST 15 C02021, source:https://doi.org/10.1088/1748-0221/15/02/C02021

For understanding the physics of energetic particles, the deuterium experimental campaignsstarted in Large Helical Device (LHD) from March 2017. To investigate the behavior ofenergetic particles, a Fast-ion D Alpha (FIDA) diagnostic was installed on the LHD. In theFIDA diagnostic, the Doppler-shifted D alpha light from fast-neutrals are utilized as signals ofenergetic particles, where these fast-neutrals are produced by the charge exchange processbetween fast-ions in the plasma and actively injected neutrals by neutral beam (NB). Theadvantages of the FIDA diagnostic are the velocity and the spatially resolved measurement offast-ions at the crossing point between its line of sight (LOS) and the incident line of NB. Themost recent FIDASIM is enhanced to simulate signals produced in three-dimensional magneticconfigurations. The new version of FIDASIM uses the fast-ion distribution function producedby GNET as input to simulate FIDA signals at LHD. In order to validate the new version of thecode, measurements of radial profiles of fast-ions using the FIDA diagnostic are performed inmagnetohydrodynamic (MHD)-quiescent plasmas. The measured spectra are in good agreementwith the theoretical prediction by 3D-supporting FIDASIM at the center of the plasma(R = 3.5 m~3.7 m, reff/a99 = −0.28~0.05) on the LHD when the line averaged electron densityis ne_avg < 1.23 × 1019 m−3. On the other hand, the measured spectra are in disagreement withthe theoretical prediction by 3D-supporting FIDASIM at even the center of the plasma when theline averaged electron density is ne_avg ≥ 1.23 × 1019 m−3., source:Citation Y. Fujiwara et al 2020 Nucl. Fusion 60 112014, source:https://doi.org/10.1088/1741-4326/abae84, identifier:0000-0003-2985-060X

0000-0003-0755-2433, One of the purposes of the UTST device is to demonstrate of magnetic reconnection heating using external coils to conduct field line merging. In order to measure the ion heating, a Doppler spectroscopy system was developed. By increasing the reconnection magnetic field to 17 mT from 4 mT after the UTST upgrade, the ion heating was observed for the first time in the UTST reconnection experiments. The ion temperature increased to 50 eV from 15 eV due to reconnection during the plasma merging.