Published papers

The inward pinch model for high-Z impurity ions in a toroidally rotating plasma has been improved to take account of trapped particles and the centrifugal force effect. It is found that the PHZ pinch is large for the barely trapped particles and the pinch velocity increases with the rotation velocity as in the original model.

The purpose of this study is to understand the Ar impurity transport process in the GAMMA 10/PDX. In the present study, the initial simulation using LINDA code and IMPGYRO code has been done to establish the basis of further understanding of the Ar impurity transport. The validity of the simulation model has been discussed by comparisons with those by the theoretical estimations of the friction force and the thermal force. The simulation results show that the Ar impurities with Z = 1 are transported towards the core region, since the thermal force dominates over the friction force under the present calculation conditions. The tendency does not contradict the experimental results and is reasonably explained by the theoretical prediction, i.e., the force balance between the thermal force and the friction force. The robust basis for further model validation and understanding of the Ar transport in the GAMMA 10/PDX has been established.

© 2018 Author(s). Linac4 is radio frequency (RF) driven type negative hydrogen H- ion source. In the RF system, it is required to match the impedance between the RF-system and plasma loading. In order to match the impedance, it is essential to analyze the plasma impedance. In this study, we analyze the plasma impedance by using the Electro-Magnetic Particle in Cell model. As a result, plasma impedance calculated by the Electro-Magnetic Particle in Cell model agrees qualitatively with one calculated by fluid model model in the steady state. On the other hand, plasma impedance calculated by the numerical simulation code is significantly different from one calculated by the zero dimensional equivalent circuit model during plasma ignition. The results show that the kinetic effects are necessary especially during plasma ignition. It is useful to propose more efficiency power transfer.

© 2018 Author(s). Multi cusp DC arc-discharge hydrogen negative ion (H-) source has been developed for the proton cyclotron applications. The purpose of this study is to understand the dependence of the H- production on the operation parameters and optimize the H- production in the source. In this paper, we focus on the effect of magnetic filter field on the H- volume production. The H- volume production in the source has been numerically calculated in the two case of magnetic filter field by using 3D electron transport code and 0D rate equations. The result has suggest that the effect of magnetic filter field on H- beam current could be explained by the enhanced electron density in the extraction region.

Linac4 is radio frequency (RF) driven type negative hydrogen H− ion source. In the RF system, it is required to match the impedance between the RF-system and plasma loading. In order to match the impedance, it is essential to analyze the plasma impedance. In this study, we analyze the dependence of power transfer on the input parameters (frequency, input power and H2 gas pressure) by using the plasma impedance prediction model. The results indicate that in the case of frequency = 2 MHz, which is often used in the experiment, efficient power supply is seen in the wide range of input power and H2 gas pressure. Also, if the optimum value of input power and H2 gas pressure is input, it may be possible to reduce the influence of the frequency value on the power reflection efficiency.

Molecular activated recombination (MAR) has been received attention as one of the most important processes to realize the detached state, specifically in the linear plasma device, GAMMA10/PDX. The impact of ELMs on the MAR reaction and the resultant detached state, however, have not fully been understood. We have developed a zero-dimensional model of atomic-molecular processes which takes into account MAR by separate treatment of vibrational excited states of H2 molecules, and the high energy tail of the electron energy distribution function (EEDF) during the ELM heat pulse. The results show that the rate of MAR increases during ELMs due to an increase in excited molecules.

© 2019 Elsevier B.V. In future fusion reactors, tungsten (W) is currently regarded as one of the most feasible candidates for the plasma-facing components (PFCs). Once the W impurities sputtered from the PFCs penetrate into the core plasma, the large radiation cooling and fuel dilution, which leads to the deterioration of the core plasma performance, take place. To understand and to control W impurity transport are indispensable for future fusion reactors. In order to understand W transport, we are continuing to develop the kinetic impurity transport code IMPGYRO for SOL/divertor in fusion reactors. The IMPGYRO has obtained following unique features compared to other existing kinetic impurity transport codes through the 15-years development activity; (i) the exact Larmor motion of impurity ions are computed so that the effects of drifts are automatically taken into account, (ii) the Coulomb collision between impurities and background plasma ions are modelled by the Binary Collision Method which kinetically calculates more precise friction and thermal forces, and (iii) the background plasma transport and impurity transport are computed self-consistently by the coupled calculation with SOL/divertor plasma transport code such as SOLPS-ITER. Although the IMPGYRO transport model has significantly improved from the initial development phase, the modelling improvement of the IMPGYRO has never been described in detail. In this paper, (a) the current W generation/transport model, and (b) the coupling strategy between the SOLPS-ITER and the IMPGYRO are summarized. The new results (c) the check of the prompt re-deposition model of the IMPGYRO, and (d) the further extension of the Coulomb collision modelling of the IMPGYRO towards the neoclassical (NC) transport handling capability has been performed. The effects of the NC transport processes on a simple circular tokamak geometry are compared with the anomalous diffusion by means of the extended IMPGYRO to obtain better understanding of cross-field transport process of W. The results show that the NC transport process may become non-negligible compared to the anomalous diffusion at least in this configuration. Based on these recent improvements, the present version of the IMPGYRO is very useful tool for understanding and prediction of W transport in SOL/divertor regions of fusion devices.

The purpose of this study is to make clear the effects of the plasma blob on the impurity transport in the Scrape-Off Layer (SOL) regions. By using the two-dimensional (2D) interchange turbulence model with plasma transport along magnetic field line, we have simulated the plasma blob formation and transport in the SOL regions. We have studied the effects of the plasma blob on the impurity transport by using impurity equations of motion in the trace impurity limit. The results show that the plasma blob transport direction is not only the radial direction but also poloidal direction due to the plasma transport along the magnetic field line. Also, the impurity ions are transported radially outward with the plasma blob transport and the velocity of the impurity ions transport is the same order to the plasma blob velocity radially outward. These results implied that the impurity transport with the plasma blob may possibly be not only diffusion but also convection. Moreover, this motion of the impurity ions seems like eddy motion. As a next step, we study relation between this eddy motion of impurity ions and the plasma blob motion. That is our future work.

© 2019 Author(s). Negative hydrogen (H-) ion sources have a wide range of applications. The general requirement for these H-ion sources is to produce intense H-ion beams with good beam optics. The purpose of this paper is to clarify the effects of the beam extraction voltage on the beam divergence angle by three-dimensional Particle-in-Cell (PIC) modeling. Perveance matching has been studied for a wide range of the extraction voltage with the model geometry of a Linac4 H-ion source. The extracted H-beam divergence angle is evaluated for extraction voltages ranging from 7 to 14 kV by using the Keio-BFX PIC code. The results show divergence minima in the range of 9.2-11.5 kV for the case without surface H-production, which correspond to experimental results. The dependence of divergence on the extraction voltage is explained by the change of the shape of the meniscus. In particular, a flat meniscus corresponds to low beam divergence, and particles extracted through the center of the meniscus have a lower divergence than particles extracted near the Plasma Electrode.

© 2019 IOP Publishing Ltd. This research numerically investigates the plasma behavior during neutral gases puffing into the west end-cell of the linear plasma confinement device GAMMA 10/PDX. A multi-fluid code named 'LINDA' is developed and applied at the west end-cell of the device to explore the processes of plasma detachment and energy loss mechanism during plasma-neutral interactions. The ion heat flux limiter is newly introduced in the LINDA code. In the paper, we investigate the influence of the ion heat flux limiter in our divertor plasma models to understand how the ion heat flux limiter affects the computed plasma parameters. The upstream plasma parameters are ni = 1 × 1019 m-3,Te = 30 eV and Ti = 100 eV. The effect of ion heat flux limiting reduces the ion temperature and heat flux in the direction of the target plate. The ion temperature profile is shown to be similar when the heat flux limiting factor (α i) is varied from 0.2 to 2.0. The effect of Hydrogen (H) and Argon (Ar) gas puffing has been investigated under the condition of ion heat flux limiting factors α i = 1.0. The ion temperature is shown to be similar between Ar and without Ar puffing. On the other hand, T e reduces remarkably for Ar puffing by enhancing the radiation cooling of Ar. The ion and electron loss are greatly enhanced for combined puffing of Ar and H. The ionization loss is found to be slightly higher and the charge-exchange loss is found to be slightly lower during the ion heat flux limiting case. These outcomes may contribute to understanding the plasma detachment processes in fusion devices.

© Gerardo Giruzzi. JT-60SA, the largest tokamak that will operate before ITER, has been designed and built jointly by Japan and Europe, and is due to start operation in 2020. Its main missions are to support ITER exploitation and to contribute to the demonstration fusion reactor machine and scenario design. Peculiar properties of JT-60SA are its capability to produce long-pulse, high-β, and highly shaped plasmas. The preparation of the JT-60SA Research Plan, plasma scenarios, and exploitation are producing physics results that are not only relevant to future JT-60SA experiments, but often constitute original contributions to plasma physics and fusion research. Results of this kind are presented in this paper, in particular in the areas of fast ion physics, high-beta plasma properties and control, and non-linear edge localised mode stability studies.

A multi-cusp DC arc-discharge hydrogen negative ion (H−) source has been developed for proton cyclotron, which is used for Boron Neutron Capture Therapy (BNCT). The purpose of this study is to understand the dependence of H− production on the operation parameters and optimize H− production in the source. In this paper, we focus on the effect of magnetic filter field on H− volume production. The H− density in the extraction region has been numerically estimated in the three patterns of magnetic filter field by using 3D electron transport code and 0D rate equations. The result suggests that the magnetic filter field which is localized in the vicinity of the extraction region is suitable for the efficient H− volume production.

© 2019 IAEA, Vienna. A global kinetic simulation model of collisional impurity transport is developed for evaluating the radial particle flux of tungsten impurity in the edge region of a tokamak plasma. Here, the plasma including the impurity and the background ion is presupposed to be quasi-steady. The simulation model is based on the drift-kinetic equation of the impurity affected by the friction force and the thermal force, which were formulated in the previous study (Homma et al 2016 Nucl. Fusion 56 036009). The model is implemented in a drift-kinetic δf simulation code. We find that the magnetic drift term in the drift-kinetic equation causes the 'global effect' on the impurity transport. Here, the global effect means that the solution δfz of the drift-kinetic equation (and also the radial particle flux) on a magnetic flux surface is influenced by the values of the solution δfz all over the edge region.

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim In order to study the potential impurity seeding operation regime of the future fusion devices, the first application of the integrated divertor code SONIC to the Ar + Ne mixed-impurity seeding operation of JT-60SA steady-state high-β plasma has been carried out. In the case, Ne is added to Ar-only seeding, the separatrix electron density has fell into the desired low separatrix electron density of the scenario. This is mainly because the D+ flow velocity towards the inner divertor has been increased by the Ne seeding. The resultant friction force transports Ar impurities towards the inner divertor region, while impurities are stagnated in the top of scrape-off layer (SOL) in the Ar-only seeding case. The higher impurity radiation power in the divertor regions and lower one in the SOL region above the X point have been obtained in mixed-impurity seeding cases, which show similar tendency as the Ar + Ne mixed-impurity seeding experiment in JT-60 U. At the core edge, Zeff has been slightly increased and the radiation power has been decreased as the Ne seeding rate increases. The core plasma/impurity transport has been also evaluated by the TOPICS code using the impurity density at the core edge computed by the SONIC as a boundary parameter. The results show lower Zeff and radiation power, and higher electron temperature in the core in the mixed-impurity seeding cases. Above possible contributors to the better energy confinement indicate that the mixed-impurity seeding operation might be more effective than Ar-only seeding operation.

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The first EMC3-EIRENE modelling for the JT-60SA edge plasmas with/without the resonant magnetic perturbation field (RMP) was conducted. Both core degradation in the vacuum approximation and strike-point splitting were observed due to the application of the RMP, as in previous studies in other devices. Toroidally averaged divertor heat fluxes with and without the RMP were fitted by using a one-dimensional diffusive model function, showing similar profiles. To extract the modulation components of the divertor heat flux profile, the proper orthogonal decomposition (POD) method was applied. By using the POD outputs, a modulation ratio with respect to the toroidally averaged profile was quantitatively calculated.

© 2020 IAEA, Vienna. A preceding study (Homma et al 2018 Contrib. Plasma Phys. 58 629-37) presented an advanced model for thermal force whose applicable condition is extended from collisional to relatively low collisional plasma. According to this extended model, the thermal force reduces when plasma collisionality decreases. In the present study, the extended thermal force model has been implemented into a SOL-divertor integrated simulation code SONIC, in order to study the impact on impurity transport in DEMO-relevant scrape-off layer (SOL) plasma, due to thermal force reduction. A set of test simulations has been carried out supposing a reference steady-state operation scenario of the Japanese DEMO fusion reactor concept (JA DEMO). The thermal force has reduced by as much as 20%-70%, reflecting relatively lower collisionality in DEMO SOL plasma. The simulation results have demonstrated that the introduction of the collisionality dependence of thermal force leads to as much as 20%-80% of effective decrease in impurity density and its content ratio widely over the SOL upstream area under the DEMO relevant condition. Compared to the case with the conventional thermal force model, relative change rate of impurity content at representative poloidal positions are as follows: low-field side (LFS) X-point-59%, poloidal top area-22%, HFS upstream area up to-80%, and no major impact around LFS upstream.

We are developing a Lagrange (LG)-Monte-Carlo (MC) scheme for three-dimensional (3D) SOL/Divertor plasma fluid modeling. By using test particles, the scheme is suitable for handling 3D complex geometries. The semi-implicit treatment of the pressure gradient term enables us to improve the robustness of the coupling of the continuity and the momentum equations. Detailed numerical checks of the integrated scheme of LG-MC have been done for a simple 1D geometry. Benchmark tests between the new LG-MC and a conventional Finite-Volume scheme were carried out and good agreement was obtained. A first test calculation for a 3D cylindrical geometry has been also successfully done.

© 2020 IOP Publishing Ltd. Aimed at obtaining key physics that determine the controllability of impurity transport in the scrape-off layer (SOL)/divertor regions, the integrated divertor code SONIC has been further extended to handle three or more impurity species kinetically. The extended SONIC code has been applied to the steady-state high-beta scenario-like plasma of JT-60SA as a testbed. We first performed a Ne transport simulation on the fixed Ar-seeded background plasma. Different radiation power distribution along the magnetic field line was obtained between Ar and Ne. The Ar radiation is strong around the top region of the SOL, which is mainly due to the line radiation of highly charged Ar ions trapped by the thermal force. In contrast, the Ne radiation is strong around the high-field side near the X point, mainly due to the line radiation of Ne7+ trapped by the balance between the thermal force and the frictional force with D+ parallel flow. We performed a parametric survey of Ne seeding rate as a second step. The effects of Ne transport on the plasma are self-consistently computed. The Ne impurities are injected into the plasma with a fixed puff rate of Ar. Even a small Ne seeding rate of 0.02 Pa m3 s-1 results in lower Ar radiation power in the SOL and core edge than in the Ar-only case. This is mainly due to the high D+ parallel flow velocity towards the inner divertor in the Ar + Ne seeding case. The resultant frictional force transports the Ar impurities towards the inner divertor region. When the line radiation of Ne7+ is switched off in the simulation, such high D+ parallel flow cannot be seen. These results suggest that the line radiation of Ne7+ has a key role for the high D+ parallel flow. The results show the possibility of impurity transport control in the SOL by mixed-impurity seeding.