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Proton-boron (p11B) fusion is an attractive potential energy source but technically challenging to implement. Developing techniques to realize its potential requires first developing the experimental capability to produce p11B fusion in the magnetically-confined, thermonuclear plasma environment. Here we report clear experimental measurements supported by simulation of p11B fusion with high-energy neutral beams and boron powder injection in a high-temperature fusion plasma (the Large Helical Device) that have resulted in diagnostically significant levels of alpha particle emission. The injection of boron powder into the plasma edge results in boron accumulation in the core. Three 2 MW, 160 kV hydrogen neutral beam injectors create a large population of well-confined, high -energy protons to react with the boron plasma. The fusion products, MeV alpha particles, are measured with a custom designed particle detector which gives a fusion rate in very good relative agreement with calculations of the global rate. This is the first such realization of p11B fusion in a magnetically confined plasma., source:Magee, R.M., Ogawa, K., Tajima, T. et al. First measurements of p11B fusion in a magnetically confined plasma. Nat Commun 14, 955 (2023). https://doi.org/10.1038/s41467-023-36655-1, source:https://doi.org/10.1038/s41467-023-36655-1, identifier:0000-0001-6697-7891

This study aim is to develop further an ion cyclotron range of frequencies (ICRF) method of plasma production in stellarators based on the minority heating. The previous studies demonstrate production of low density plasma (9.5 × 1017m−3) at low power of up to 0.2 MW. The higher ICRF heating power experiments become possible after introducing a programmable ICRF power ramp up at the front of the ICRF pulse. With this trick, all the shots went with the antenna voltage within the safe range. Increase of the ICRF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved was 6 × 1018 m−3 which is 6 times higher than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the ICRF pulse. Plasma density is too low to provide good conditions for efficient plasma heating. For the reference, the ICRF heating of high density cold plasma prepared by electron cyclotron resonance heating is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna–plasma coupling was much better which result in larger heating power with the lower antenna voltage.

This report compares results ion-cyclotron range of frequencies (ICRF) plasma production at hydrogen mi-nority regime in Uragan-2M (U-2M) and Large Helical Device (LHD). The condition of the presence of the fundamental harmonic ion cyclotron resonance zone for the hydrogen inside the plasma column should be ful-filled for this method. The scenario is successful at both machines and weakly sensitive to the variation of the hydrogen concentration in the H2+He gas mixture. It should be noted that at LHD the start up is slower than at U-2M. The comparison of plasma production in ICRF with hydrogen minority at U-2M and LHD indicate that this scenario can be scaled to larger stellarator devices. The experiments made are the base for the proposal for usage this scenario for plasma production in ICRF at Wendelstein 7-X at magnetic field reduced to 1.7 T., This article is based on the presentation at the 31st International Toki Conference on Plasma and Fusion Research (ITC31).

Characteristics of the new vertical neutron camera (VNC3) installed for the study of energetic-particle transport in the relatively low neutron emission rate (Sn) in Large Helical Device (LHD) deuterium plasma is investigated. Dependence of signal of VNC3 operating with the current mode on Sn shows that accurate neutron signal is obtained using VNC3 in low Sn range with 10 ms time bin where the error of neutron counts of first vertical neutron camera (VNC1) operating with the pulse counting mode is significantly large. Time-resolved measurements of neutron emission profiles in deuterium beam heated low Sn plasmas are performed. Although the line-integrated neutron obtained by VNC3 is wider due to its larger inner diameter of the collimator compared to VNC1, the neutron profile measured by VNC3 is almost matched with the neutron profile measured by VNC1. The time-resolved neutron profile measurement in low Sn discharge with relatively short time period becomes possible using VNC3., source:https://doi.org/10.1585/pfr.16.1402039, identifier:0000-0002-0160-0468

Neutral beam (NB) fast-ion loss mechanisms in the large helical device (LHD) are investigated by the combination of the neutron measurement, the classical slowing-down simulation, and the neo-classical guiding center orbit following simulation. It is found that the neo-classical transport provides little contribution to the loss of tangentially injected NB fast-ions. For perpendicularly injected NB fast-ions, the neo-classical transport has more than 40% contribution to the NB fast ion loss. These results indicate that there are other loss mechanisms dominant in LHD plasmas. The charge exchange loss is one of the plausible candidates for the loss mechanism., source:https://doi.org/10.1585/pfr.16.2402052, identifier:0000-0003-3293-488X

The compact neutron emission spectrometer (CNES) having a tangential sightline was installed to observe a significant Doppler shift of the neutron energy due to the high-energy tangential neutral beam (NB) injections in the Large Helical Device (LHD) for understanding of the energy distribution of fast-ion. The CNES is based on a 1-inch diameter and 1-inch height EJ301 liquid scintillator coupled with a conventional 1-inch photomultiplier tube. The histogram of the integrated pulse signal (Qtotal) during different NBs heating phases measured by the CNES shows that the edge of Qtotal changes depending on NB directions. Using the simple derivative unfolding technique, the neutron energy spectra were unfolded from the measured Qtotal histogram. Peaks of the neutron energy shift to 2.0 MeV, 2.42 MeV, and 3.0 MeV according to the injection direction of NBs. The obtained neutron energy is almost consistent with the virgin deuterium-deuterium neutron energy evaluated by the simple two-body kinematics considering the sightline of CNES, NB injection angle, and NB injection energy., source:Sangaroon, S., Ogawa, K., Isobe, M. et al. Observation of significant Doppler shift in deuterium-deuterium neutron energy caused by neutral beam injection in the large helical device. AAPPS Bull. 32, 5 (2022). https://doi.org/10.1007/s43673-022-00036-5, source:https://doi.org/10.1007/s43673-022-00036-5, identifier:0000-0002-0160-0468

angential compact neutron emission spectrometer (CNES) based on the Cs2LiYCl6:Ce with 7Li-enrichment (CLYC7) scintillator is newly installed in the Large Helical Device (LHD). Measurement of neutron energy spectrum was performed using CNES in tangential neutral beam (NB) heated deuterium plasma discharges. The Doppler shift of neutron energy according to the direction of tangential NB injection has been obtained. When the fast ions moving away from the CNES, lower shifted neutron energy is obtained, whereas the upper shifted neutron energy is obtained when the fast ions moving toward the CNES. The obtained neutron energy is almost consistent with the virgin deuterium-deuterium neutron energy evaluated by the simple two-body kinematic calculation., source:S. Sangaroon et al 2021 JINST 16 C12025, source:https://doi.org/10.1088/1748-0221/16/12/C12025, identifier:0000-0002-0160-0468

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of plasma in the Large Helical Device (LHD), two types of ICRF antennas are used. One is the Field-Aligned-Impedance-Transforming (FAIT) antenna. It has an In-Vessel Impedance Transformer (IVIT) in the vacuum region of the antenna and shows the possibility of high-power injection despite the short antenna head. To enhance the performance more, an Ex-Vessel Impedance Transformer (EVIT) was attached outside the LHD vacuum vessel. As a result, the injectable power increased. The other is the Handshake form (HAS) antenna. Plasma can be efficiently heated by adjusting the phase difference between currents in straps. However, the injectable power from the HAS antenna was originally small. Therefore, later an EVIT was attached to it. Moreover, the transmission line in the vacuum region was remodeled to form an IVIT. By utilizing these impedance transformers, the performance of the HAS antenna was drastically improved., source:https://doi.org/10.1585/pfr.17.2405009, identifier:0000-0001-8972-6534

The characteristics of ion cyclotron range of frequencies (ICRF) minority ion heating with a hydrogen minority and deuterium majority plasma were studied by ICRF modulation injection experiments in the Large Helical Device (LHD). In recent experiments with deuterium plasma, no significant increase in the neutron emission rate due to ICRF second harmonic deuteron heating was observed. Therefore, in this study, the neutron emission rate was used to refer to the information regarding the thermal ion component. Like the results of the observations of the heating efficiencies at various minority proton ratios, the experimental results showed good agreement with the simple model simulation of ICRF wave absorption. During these experiments, the accelerated minority hydrogen ions were observed by neutral particle analyzers. The counting rates of the energetic particles were higher in the lines of sight passing through the helical ripple than across the magnetic axis, and the counting rate decreased as the minority hydrogen ion ratio increased. The dependence of the minority hydrogen ion ratio on the density of the energetic ions was consistent with the experimentally observed heating efficiencies and simulations. The heating efficiency of ICRF minority ion heating could be well explained by simple model simulation in the LHD deuterium experiment., source:Citation S. Kamio et al 2022 Nucl. Fusion 62 016004, source:https://doi.org/10.1088/1741-4326/ac359d, identifier:0000-0003-0755-2433

The results of the plasma start-up with ICRH of U-2M RF discharges in H2+He mixture with newly implemented controlled gas H2 concentration are presented. The W7-X like ICRH antenna operated in monopole phasing with applied RF power of ∼ 100 kW. We investigated plasma start-up in the pressure range p = 6×10−4 - 9×10−2 Pa. Plasma production with an average density of up to Ne ∼ 1013 cm−3 was observed at frequencies the fundamental harmonic of the hydrogen cyclotron frequency., source:https://doi.org/10.1585/pfr.17.2402034, identifier:0000-0003-4948-0896

Tritium yields due to the deuterium-deuterium fusion reaction during the 22nd LHD experiment campaign are numerically estimated. As usual, the total tritium yields are assumed to be the same total neutron yields. In the Large Helical Device (LHD), however, it is considered that fusion reactivity of the D(d,p)T branch is lower than that of the D(d,n)3He one because the fusion reaction between a fast-deuteron and a thermal deuteron is dominant. By integrated simulation, considering the velocity distribution function of fast-deuteron, the ratio of the tritium yields to the neutron yields is estimated to be Yt/Yn ∼ 0.936. From the assumptions applied in the simulation, it is expected that this value should be still an over-estimation rather than the actual value., source:https://doi.org/10.1585/pfr.17.2402023, identifier:0000-0003-3293-488X

Understanding energetic particle transport due to magnetohydrodynamic instabilities excited by energetic particles is essential to apprehend alpha particle confinement in a fusion burning plasma. In the large helical device (LHD), beam ion and deuterium–deuterium fusion-born triton transport due to resistive interchange mode destabilized by helically-trapped energetic ions (EIC) are studied employing comprehensive neutron diagnostics, such as the neutron flux monitor and a newly developed scintillating fiber detector characterized by high detection efficiency. Beam ion transport due to EIC is studied in deuterium plasmas with full deuterium or hydrogen/deuterium beam injections. The total neutron emission rate (Sn) measurement indicates that EIC induces about a 6% loss of passing transit beam ions and a 60% loss of helically-trapped ions. The loss rate of helically-trapped ions, which drive EIC, is larger than the loss rate of passing transit beam ions. Furthermore, the drop of Sn increasing linearly with the EIC amplitude shows that barely confined beam ions existing near the confinement-loss boundary are lost due to EIC. In full deuterium conditions, a study of deuterium–deuterium fusion-born triton transport due to EIC is performed by time-resolved measurement of total secondary deuterium–tritium neutron emission rate (Sn_DT). Drop of Sn_DT increases substantially with EIC amplitude to the third power and reaches up to 30%. The relation shows that not only tritons confined in confined-loss boundary, but also tritons confined in the inner region of a plasma, are substantially transported., source:K. Ogawa et al 2021 Nucl. Fusion 61 096035, source:https://doi.org/10.1088/1741-4326/ac0d8a, identifier:0000-0003-4555-1837

The results of the first experimental series to produce a plasma using the ion cyclotron range of frequency (ICRF) in the large helical device (LHD) within the minority scenario developed at Uragan-2M (U-2M) are presented. The motivation of this study is to provide plasma creation in conditions when an electron cyclotron resonance heating start-up is not possible, and in this way widen the operational frame of helical machines. The major constraint of the experiments is the low RF power to reduce the possibility of arcing. No dangerous voltage increase at the radio-frequency (RF) system elements and no arcing has been detected. As a result, a low plasma density is obtained and the antenna-plasma coupling is not optimal. However, such plasmas are sufficient to be used as targets for further neutral beam injection (NBI) heating. This will open possibilities to explore new regimes of operation at LHD and Wendelstein 7-X (W7-X) stellarator. The successful RF plasma production in LHD in this experimental series stimulates the planning of further studies of ICRF plasma production aimed at increasing plasma density and temperature within the ICRF minority scenario as well as investigating the plasma prolongation by NBI heating., source:S. Kamio et al 2021 Nucl. Fusion 61 114004, source:https://doi.org/10.1088/1741-4326/ac277b, identifier:0000-0003-0755-2433

A database of the total neutron emission rate (Sn) in the ion cyclotron range of frequency (ICRF) heated deuterium plasma in the Large Helical Device (LHD) was developed with changing the ICRF injection power and electron density. The Sn measured in the experiment was compared with the neutron emission rate calculated by the thermal reactivity of deuterium plasma to predict Sn in long-pulse discharge and to understand the heating scheme of ICRF. The ratio of Sn evaluated by calculation and that by measurement in the electron cyclotron heating (ECH) and ICRF heating case has almost the same tendency as that of the ECH only case in which energetic deuterons are not expected. In the first campaign of deuterium ICRF plasma discharge with injection power up to 1.5 MW, neutron emission with regard to the ICRF tail deuteron was thought to be negligible. We expanded the Sn database and started building an Sn prediction code to perform a long-pulse deuterium discharge., source:https://doi.org/10.1585/pfr.15.1202088, identifier:0000-0002-5364-805X

A study of the resonant magnetic perturbation (RMP) effect on transit beam ion behavior is performed using the total neutron emission rate (Sn) measurement of the deuterium plasma in the Large Helical Device. We conducted no RMP field, one-half RMP field, and full RMP field discharges and compared Sn that reflects the global beam ion confinement information. It is determined that owing to the RMP field, Sn decreased by approximately 15 - 30%. Numerical calculations based on the classical confinement of beam ions were performed to investigate the bulk plasma parameter effect on Sn. The calculated Sn shows that the degradation of Sn by RMP is mainly caused by the degradation of the electron temperature owing to island formation which results in a shorter slowing down time of beam ions., source:https://doi.org/10.1585/pfr.14.1202159, identifier:0000-0003-4555-1837

Ion Cyclotron Range of Frequencies (ICRF) heating is one of the plasma heating methods in the Large Helical Device (LHD). The wave injected from the ICRF antenna can propagate in the plasma even if the plasma density is extremely high. It was shown that the high power absorption occurs in the plasma core with the second harmonic heating in the case of high-density deuterium plasma in LHD by the calculation with a simple model of the ICRF heating. Wave number perpendicular to the static magnetic field increases with the plasma density, and it enhances the finite Larmor radius effect in the second harmonic heating. Enhanced finite Larmor radius effect and a large amount of resonant ions enable the intense power absorption. By increasing the frequency, the third harmonic heating will also be possible. Though the intensity of the power absorption will decrease, more localized heating on the magnetic axis will be realized because the finite Larmor radius effect works better in the third harmonic heating., source:https://doi.org/10.1585/pfr.15.2402015, identifier:0000-0001-8972-6534

The compact neutral particle analyzer (CNPA) combined with the impurity pellet measurement is one of the few instruments that can directly measure the radial high energy particle distribution in the Large Helical Device (LHD). For this purpose, it is suitable to set CNPA near LHD. On the other hand, sufficient and heavy shielding against DD-neutrons, generated in deuterium experiment, is required, especially when the deuterium neutral particle beam injection heating is applied. The shield is insufficient due to the weight limitation on the stage. However, if the neutron energy is thermalized on the detector, the neutron noise can be estimated only from the total neutron yield, which is monitored by the 235U fission chamber, etc. In the experiments, the pure neutron noise on the CNPA has been measured by closing the gate-valve to avoid the charge exchange neutral particle signals. The neutron noise on the CNPA has been proportional to the total neutron yield over three-order magnitudes. This means that the neutron noise can be estimated from the total neutron yield. Therefore, the calibrated charge exchange neutral signal can be obtained simply by subtracting the estimated neutron noise from the measured signal in ordinary experiments., source:https://doi.org/10.1585/pfr.14.3402142

Plasma grid bias has been utilized for reducing an electron density in the vicinity of a plasma grid in negative ion sources for fusion, resulting in achievement of low co-extracted electron current. In this study, the effectiveness and feasibility of the plasma grid bias on beam optics optimization are demonstrated. It is shown that the beam optics strongly depends on the bias voltage and is successfully optimized at different bias voltages depending on the discharge power. Responses of the beam properties such as the perveance and the current ratio to the plasma grid bias are also shown for both hydrogen and deuterium beams., source:https://doi.org/10.1585/pfr.13.1205110

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 × 1014 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 × 106 n/cm2⋅s at the detector array for the plasma with the neutron emission rate of 1.9 × 1016 n/s., source:https://doi.org/10.1585/pfr.14.3405048

A tangential Fast-Ion D Alpha (FIDA) diagnostic is applied to the Large Helical Device (LHD) in order to observe energetic distribution of toroidal circulating energetic particles which are produced by tangential Negative Neutral Beams (NNB). A perpendicular Positive NB (PNB) is used as the diagnostic probe beam of the tangential-FIDA diagnostic in this observation geometry. In order to assess the appropriateness of the tangential-FIDA diagnostic, the experimental result was compared with a Silicon-diode-based Fast Neutral Analyzer (Si-FNA) which was installed on the same line of sight. As a result of the comparison, the tangential-FIDA and the Si-FNA experimental data obtained good linearity in the energy region from 60 keV to 180 keV. In addition, an enhanced FIDASIM was applied for analyzing the FIDA on the three-dimensional magnetic configuration fusion device., source:https://doi.org/10.1585/pfr.14.3402129

We compared the experimentally measured neutron emission rate with the emission rate calculated by using the simple Fokker-Planck equation with the experimental temperature and density profiles, and the deuteron density evaluated by a spectroscopy in peripheral region in LHD. The neutron emission rates evaluated by the FIT3D-DD code are approximately two times higher than experimental results in the typical magnetic configuration of LHD. In the case of low fast-ion confinement, the calculations are approximately four times higher than experimental measurements., source:https://doi.org/10.1585/pfr.14.3402126, identifier:0000-0002-5364-805X

The study of radio frequency (RF) waves was accelerated by the start of deuterium experiments in the Large Helical Device (LHD) and the introduction of wide-band and long-time data acquisition system. The mass dependency of Ion Cyclotron Emissions (ICEs) driven by perpendicular Neutral Beam Injection (NBI) was clarified by the injection of deuterium beams. Another type of ICE by the tangential beam injection was found in the LHD. Although the details of this type of ICE have not yet been elucidated, a clear linear dependency of its frequency on the magnetic field strength divided by the mass of beam species was found. Non-localized RF waves in the Lower Hybrid Wave (LHW) frequency region were also detected during the injection of a perpendicular beam. It is shown that the frequency increases with the electron density., source:https://doi.org/10.1585/pfr.13.3402043, identifier:0000-0001-8972-6534

Instabilities of high-frequency magnetosonic waves caused by energetic ions with a ring-like velocity distribution are studied with a one-dimensional electromagnetic particle code which simulates self-consistently the full ion and electron dynamics. It is shown that the magnetosonic waves with frequencies slightly smaller than the lower-hybrid resonance frequency rapidly grow to large amplitudes, in addition to electromagnetic emissions near the ion cyclotron frequency and its harmonics. The frequency of the magnetosonic wave that has the largest amplitude is in good agreement with the frequency of the most unstable mode predicted by a linear theory. The theory and simulations show that the frequency of the magnetosonic wave increases with the plasma density. The instability of the high-frequency magnetosonic waves may be an excitation mechanism for radio frequency waves in the Lower Hybrid Wave (LHW) frequency region observed in the Large Helical Device (LHD)., source:https://doi.org/10.1585/pfr.13.3403015, identifier:0000-0003-2691-5068

In order to optimize the transferred power efficiency from ICRF fast ions to bulk plasma, we have developed a code in which models of behaviors of ICRF fast ion are minimally adopted in order to save calculation time. A tendency of the transferred power efficiencies evaluated by the developed code is almost the same as that evaluated from the full analyses. In the regime with low efficiency of transferred power, the effect of the position of the resonance layer is large. The efficiency of the resonance layer through the point near the magnetic axis is found to be smaller than that of the typical ICRF resonance layer., source:https://doi.org/10.1585/pfr.10.1402077, identifier:0000-0002-5364-805X

Plasma heating experiment using the ion cyclotron range of frequencies (ICRF) heating has been carried out. Aiming at the high power and long pulse heating and application to the future fusion device, the antenna without Faraday shield was tested and newly developed antenna, called FAIT antenna, was used. Steady state experiment was progressed by using the high power ICRF heating with those antennas. Plasma discharge length about 48 minutes was achieved with the heating power of 1.2 MW and a line-averaged electron density of 1.2 × 1019 m−3. The injected heating energy reached 3.36 GJ and it is highest in the fusion plasma experiments. We will promote the high power steady state research involving the evaluation of the antennas and heating performance., source:https://doi.org/10.1585/pfr.10.3405046, source:https://doi.org/10.1585/pfr.13.1905010, identifier:0000-0003-2718-4930

The termination process of long pulse plasma discharges in the Large Helical Device (LHD) have been observed with fast framing cameras, which shows that the reason for the termination of the discharged has been changed with increased plasma heating power, improvements of plasma heating systems and change of the divertor configuration, etc. For long pulse discharges in FYs2010-2012, the main reason triggering the plasma termination was reduction of ICRF heating power with rise of iron ion emission due to electric breakdown in an ICRF antenna. In the experimental campaign in FY2013, the duration time of ICRF heated long pulse plasma discharges has been extended to about 48 minutes with a plasma heating power of ∼1.2 MW and a line-averaged electron density of ∼ 1.2 × 1019 m−3. The termination of the discharges was triggered by release of large amounts of carbon dusts from closed divertor regions, indicating that the control of dust formation in the divertor regions is indispensable for extending the duration time of long pulse discharges., source:https://doi.org/10.1585/pfr.10.3402040, identifier:0000-0003-0655-7347

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., source:https://doi.org/10.1585/pfr.9.3402038, identifier:0000-0003-0755-2433

Studies of energetic particle transport due to energetic-particle-driven Alfvénic instability have progressed using neutron and energetic particle diagnostics in Large Helical Device deuterium plasmas. Alfvénic instability excited by injecting an intensive neutral beam was observed by a magnetic probe and a far-infrared laser interferometer. The interferometer showed Alfvénic instability composed of three modes that existed from the core to the edge of the plasma. A comparison between the observed frequency and shear Alfvén spectra suggested that the mode activity was most likely classified as an Alfvénic avalanche. A neutron fluctuation detector and a fast ion loss detector indicated that Alfvénic instability induced transport and loss of co-going transit energetic ions. The dependence of the drop rate of the neutron signal on the Alfvénic instability amplitude showed that significant transport occurred. Significant transport might be induced by the large amplitude and radially extended multiple modes, as well as a large deviation of the energetic ion orbit from the flux surface., source:K. Ogawa et al 2022 Nucl. Fusion 62 112001, source:https://doi.org/10.1088/1741-4326/ac6f66, identifier:0000-0003-4555-1837

Three-dimensional trajectories of incandescent dust particles in plasmas were observed with stereoscopic fast framing cameras in a large helical device. It proved that the dust is located in the peripheral plasma and most of the dust moves along the magnetic field lines with acceleration in the direction that corresponds to the plasma flow. ICRF heated long pulse plasma discharges were terminated with the release of large amounts of dust from a closed divertor region. After the experimental campaign, the traces of exfoliation of carbon rich mixed-material deposition layers were found in the divertor region. Transport of carbon dust is investigated using a modified dust transport simulation code, which can explain the observed dust trajectories. It also shows that controlling the radius of the dust particles to less than 1 mm is necessary to prevent the plasma termination by penetration of dust for the long pulse discharges. Dust transport simulation including heavy metal dust particles demonstrates that high heating power operation is effective for shielding the main plasma from dust penetration by an enhanced plasma flow effect and a high heat load onto the dust particles in the peripheral plasma. It shows a more powerful penetration characteristic of tungsten dust particles compared to that of carbon and iron dust particles.

Understanding energetic particle (EP) confinement is one of the critical issues in realizing fusion reactors. In stellarator/helical devices, the research on EP confinement is one of the key topics to obtain better confinement by utilizing the flexibility of a 3D magnetic field. A study of EP transport in the Large Helical Device (LHD) has been performed by means of escaping EP diagnostics in hydrogen plasma operation. By starting deuterium operation of the LHD, the confinement study of EPs has progressed remarkably using newly developed comprehensive neutron diagnostics providing information for EPs confined in the core region. The total neutron emission rate (Sn) increases due to the relatively low deviation of the beam ion orbit from the flux surface with the inward shift of the magnetic axis. The Sn has a peak around the electron density of 2 × 1019 m−3 to 3 × 1019 m−3, as predicted. It is found that the fraction of beam–beam components in Sn is evaluated to be approximately 20% by the Fokker–Planck models TASK/FP in the plasma with both co- and counter-neutral beam injections. The equivalent fusion gain in DT plasma achieved 0.11 in a negative-ion-based neutral beam heated plasma. Time evolution of Sn following the short pulse neutral beam injection into the electron–cyclotron-heated low-beta plasma is reproduced by drift kinetic simulation, indicating that transport of a beam ion injected by a short pulse neutral beam can be described with neoclassical models in magnetohydrodynamic quiescent low-beta plasmas. The vertical neutron camera works successfully, demonstrating that in the co-neutral beam-injected plasma, the neutron emission profile shifts according to the magnetic axis position. The shift of the neutron emission profile is reproduced by orbit-following models. The triton burnup study is performed for the first time in a stellarator/heliotron to understand the alpha particle confinement. It is found that the triton burnup ratio, which largely inc, source:https://doi.org/10.1088/1741-4326/ab14bc, identifier:0000-0003-4555-1837

The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current () reaches 46.2 A from two beam sources with the averaged current density being 190 A m−2 for 2 s, and the extracted electron to accelerated ion current ratio () increases to 0.39 using 5.6 V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIFS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, increases to 55.4 A with the averaged current density being 233 A m−2 for 1.5 s using the shot extraction gap length. The low of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below., source:Ikeda K, Tsumori K, Nakano H, Kisaki M, Nagaoka K, Kamio S, Fujiwara Y, Haba Y and Osakabe M 2019 Exploring deuterium beam operation and the behavior of the co-extracted electron current in a negative-ion-based neutral beam injector Nuclear Fusion 59 076009., source:https://doi.org/10.1088/1741-4326/ab0fca, identifier:0000-0001-9781-231X

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 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., source:https://doi.org/10.1088/1741-4326/ab6da0, identifier:0000-0003-4555-1837

The neutron emission profile of deuterium plasma in the Large Helical Device was measured with a multi-sightline vertical neutron camera under various magnetic field strength conditions. It was found that the line-integrated neutron emission profile shifts outward in the co-neutral beam (NB) case and inward in the counter NB case. Here, co- and counter directions correspond to enhance and reduce the poloidal magnetic field directions, respectively. The shift becomes more significant when the magnetic field decreased in strength. The experimentally obtained neutron emission profile was compared with the orbit-following models simulated through the DELTA5D code. The calculated neutron emission profiles vary according to the magnetic field strength because of the change of beam ion orbit and the slowing down due to the plasma parameter changes. Although a relatively narrow profile was obtained in the calculations at the inboard side for the co-NB case in the relatively low field condition, the profiles obtained through calculation and experiment were almost qualitatively aligned., source:Ogawa K, Isobe M, Sugiyama S, Spong D A, Sangaroon A, Seki R, Nuga H, Yamaguchi H, Kamio S, Fujiwara Y, Kobayashi M I, Jo J and Osakabe M 2021 Characteristics of neutron emission profile from neutral beam heated plasmas of the Large Helical Device at various magnetic field strengths Plasma Physics and Controlled Fusion 63 065010., source:https://doi.org/10.1088/1361-6587/abf575, identifier:0000-0003-4555-1837

The isotope effect on impurities and bulk ion particle transport is investigated by using the deuterium, hydrogen, and isotope mixture plasma in the Large Helical Device (LHD). A clear isotope effect is observed in the impurity transport but not the bulk ion transport. The isotope effects on impurity transport and ion heat transport are observed as a primary and a secondary effect, respectively, in the plasma with an internal transport barrier (ITB). In the LHD, an ion ITB is always transient because the impurity hole triggered by the increase of ion temperature gradient causes the enhancement of ion heat transport and gradually terminates the ion ITB. The formation of an impurity hole becomes slower in the deuterium (D) plasma than the hydrogen (H) plasma. This primary isotope effect on impurity transport contributes the longer sustainment of the ion ITB state because the low ion thermal diffusivity can be sustained as long as the normalized carbon impurity gradient R/Ln,c, where , is above the critical value (~−5). Therefore, the longer sustainment of the ITB state in the deuterium plasma is considered to be a secondary isotope effect due to the mitigation of the impurity hole. The radial profile of H and D ion density is measured using bulk charge exchange spectroscopy inside the isotope mixture plasma. The decay time of H ion density after the H-pellet injection and the decay time of D ion density after D-pellet injection are almost identical, which demonstrates that there is no significant isotope effect on ion particle transport., source:K. Ida et al 2019 Nucl. Fusion 59 056029, source:https://doi.org/10.1088/1741-4326/ab0e41, identifier:0000-0002-0585-4561

Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to Ie/Iacc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m2. The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign., source:https://doi.org/10.1063/1.5128529, identifier:0000-0001-9781-231X

In the Large Helical Device (LHD), the development of high-power and long-pulse Ion Cyclotron Range of Frequencies (ICRF) heating system is ongoing. The developed Field-Aligned-Impedance-Transforming (FAIT) antenna has the potential for high-power injection of more than 1.8 MW. Here, to achieve this injection power, a power combination system was developed. An optimized power combiner was designed by repeated simulations, and then was fabricated and installed in the ICRF transmission system. Control of the power and the phase of incident waves into the input ports of the power combiner is important for the power combination. Therefore, a real-time control system was developed, and prompt reduction of power loss was demonstrated. As a result, combined powers of more than 2 MW for 6 s and 1 MW for 10 min were successfully achieved., source:https://doi.org/10.1016/j.fusengdes.2018.12.039, identifier:0000-0001-8972-6534

The transmission line is one of the most important components of ion cyclotron range of frequencies (ICRF) heating devices. In the case of unexpected trouble on the line, such as a breakdown, immediate power-off is necessary in order to avoid severe damage on the line. Breakdowns are difficult to detect with a reflection monitor, since the reflection may originate from a change in the antenna-plasma coupling. In the Large Helical Device (LHD), a Fault Detection System (FDS) for the transmission line was developed, which detects the breakdown utilizing the unbalance of three signals from the both ends of the line. For the precise balancing in the normal condition, the calibration is iteratively conducted. FDS is insensitive to the change of the antenna impedance, therefore, FDS can detect breakdown clearly. Frequency shift is also detectable with the FDS applied to a long transmission line. Therefore, the self-oscillation accompanying frequency shift could be detected in addition to breakdown., source:https://doi.org/10.1016/j.fusengdes.2017.03.004, identifier:0000-0001-8972-6534

The behavior of carbon impurities in deuterium plasmas and its impact on thermal confinement were investigated in comparison with hydrogen plasmas in the Large Helical Device (LHD). Deuterium plasma experiments have been started in the LHD and high-ion-temperature plasmas with central ion temperature (T i) of 10 keV were successfully obtained. The thermal confinement improvement could be sustained for a longer time compared with hydrogen plasmas. An isotope effect was observed in the time evolution of the carbon density profiles. A transiently peaked profile was observed in the deuterium plasmas due to the smaller carbon convection velocity and diffusivity in the deuterium plasmas compared with the hydrogen plasmas. The peaked carbon density profile was strongly correlated to the ion thermal confinement improvement. The peaking of the carbon density profile will be one of the clues to clarify the unexplained mechanisms for the formations of ion internal transport barrier and impurity hole on LHD. These results could also lead to a better understanding of the isotope effect in the thermal confinement in torus plasma., source:Citation K Mukai et al 2018 Plasma Phys. Control. Fusion 60 074005, source:https://doi.org/10.1088/1361-6587/aac06c, identifier:0000-0003-1586-1084

The neutral beam (NB) fast ion confinement in the Large Helical Device (LHD) is studied for several full field (????????∼2.75 T) magnetic configurations by a combination of neutron measurement and simulations. To investigate the NB fast ion confinement, we have performed a series of short-pulse NB injection experiments. The experiment results are analysed by the integrated code TASK3D-a. From this investigation, the effective particle diffusion coefficients of the tangential and perpendicular NBs are approximately ????eff∼0.1 m2 s−1 and ????eff∼1 m2 s−1 in the standard configuration. It is clarified that the NB fast ion confinement improves when the plasmas are shifted inward. Moreover, it is also found that the simulation, which considers the deuteron dilution effect due to the presence of impurity ions, can describe a neutron emission rate consistent with the measurement., source:Journal of Plasma Physics , Volume 86 , Issue 3 , June 2020 , 815860306 DOI: https://doi.org/10.1017/S0022377820000525[Opens in a new window] NASA ADS Abstract Service., source:https://doi.org/10.1017/S0022377820000525, identifier:0000-0003-3293-488X

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., source:Review of Scientific Instruments 91, 083505 (2020); https://doi.org/10.1063/5.0010302, source:https://doi.org/10.1063/5.0010302, identifier:0000-0002-0160-0468