LHD Experiment Research Articles

Application of container technology in LHD analysis system

The authors have developed AutoAna, an automatic registration system for physics data in LHD experiments. AutoAna has increased the number of data registrations and reduced the time required to complete them, making it possible to provide key data during three-minute intervals of short pulse experiments. The time reduction was achieved by running several analysis programs in parallel. However, problems arose due to this process, which could be solved by containerizing the execution environment of the analysis programs.

Evidence of ‘two plasmon’ decay of energetic particle induced geodesic acoustic mode

Secondary low frequency mode generation by energetic particle induced geodesic acoustic mode (EGAM) observed in LHD experiment is studied using nonlinear gyrokinetic theory. It is found that the EGAM frequency can be significantly higher than local geodesic acoustic mode (GAM) frequency in low collisionality plasmas, and it can decay into two GAMs as its frequency approaches twice GAM frequency, in a process analogous to the well-known two plasmon decay instability. The condition for this process to occur is also discussed.

Core plasma design of the compact helical reactor with a consideration of the equipartition effect

Integrated physics analysis of plasma operation scenario of the compact helical reactor FFHR-c1 has been conducted. The DPE method, which predicts radial profiles in a reactor by direct extrapolation from the reference experimental data, has been extended to implement the equipartition effect. Close investigation of the plasma operation regime has been conducted and a candidate plasma operation point of FFHR-c1 has been identified within the parameter regime that has already been confirmed in LHD experiment in view of MHD equilibrium, MHD stability and neoclassical transport.

Extended capability of the integrated transport analysis suite, TASK3D-a, for LHD experiment

The integrated transport analysis suite, TASK3D-a (Analysis), has been developed to be capable for routine whole-discharge analyses of plasmas confined in three-dimensional (3D) magnetic configurations such as the LHD. The routine dynamic energy balance analysis for NBI-heated plasmas was made possible in the first version released in September 2012. The suite has been further extended through implementing additional modules for neoclassical transport and ECH deposition for 3D configurations. A module has also been added for creating systematic data for the International Stellarator–Heliotron Confinement and Profile Database. Improvement of neutral beam injection modules for multiple-ion species plasmas and loose coupling with a large-simulation code are also highlights of recent developments.

Distorted magnetic island formation during slowing down to mode locking in helical plasmas

We report the first observation of the formation of a magnetic island before the occurrence of mode locking in helical plasma. New analysis and observation techniques applied to the ECE signal and poloidal flow in LHD experiments yield the following results. (i) A magnetic island structure is present, rotating at the end of the rotating phase. (ii) The rotation speed of the island is not uniform in space and time. The rotation of the island changes significantly at the end of the rotating phase, and the deformation increases until the mode is locked.

Development of a real-time simulation tool towards self-consistent scenario of plasma start-up and sustainment on helical fusion reactor FFHR-d1

This study closely investigates the plasma operation scenario for the LHD-type helical reactor FFHR-d1 in view of MHD equilibrium/stability, neoclassical transport, alpha energy loss and impurity effect. In 1D calculation code that reproduces the typical pellet discharges in LHD experiments, we identify a self-consistent solution of the plasma operation scenario which achieves steady-state sustainment of the burning plasma with a fusion gain of Q ~ 10 was found within the operation regime that has been already confirmed in LHD experiment. The developed calculation tool enables systematic analysis of the operation regime in real time.

A unified model of density limit in fusion plasmas

In this work we identify by analytical and numerical means the conditions for the existence of a magnetic and thermal equilibrium of a cylindrical plasma, in the presence of Ohmic and/or additional power sources, heat conduction and radiation losses by light impurities. The boundary defining the solutions’ space having realistic temperature profile with small edge value takes mathematically the form of a density limit (DL). Compared to previous similar analyses the present work benefits from dealing with a more accurate set of equations. This refinement is elementary, but decisive, since it discloses a tenuous dependence of the DL on the thermal transport for configurations with an applied electric field. Thanks to this property, the DL scaling law is recovered almost identical for two largely different devices such as the ohmic tokamak and the reversed field pinch. In particular, they have in common a Greenwald scaling, linearly depending on the plasma current, quantitatively consistent with experimental results. In the tokamak case the DL dependence on any additional heating approximately follows a 0.5 power law, which is compatible with L-mode experiments. For a purely externally heated configuration, taken as a cylindrical approximation of the stellarator, the DL dependence on transport is found stronger. By adopting suitable transport models, DL takes on a Sudo-like form, in fair agreement with LHD experiments. Overall, the model provides a good zeroth-order quantitative description of the DL, applicable to widely different configurations.

Upgrades and application of FIT3D NBI–plasma interaction code in view of LHD deuterium campaigns

This work presents an upgrade of the FIT3D neutral beam–plasma interaction code, part of TASK3D, a transport suite of codes, and its application to LHD experiments in the framework of the preparation for the first deuterium experiments in the LHD. The neutral beam injector (NBI) system will be upgraded to D injection, and efforts have been recently made to extend LHD modelling capabilities to D operations. The implemented upgrades for FIT3D to enable D NBI modelling in D plasmas are presented, with a discussion and benchmark of the models used. In particular, the beam ionization module has been modified and a routine for neutron production estimation has been implemented. The upgraded code is then used to evaluate the NBI power deposition in experiments with different plasma compositions. In the recent LHD campaign, in fact, He experiments have been run to help the prediction of main effects which may be relevant in future LHD D plasmas. Identical H/He experiments showed similar electron density and temperature profiles, while a higher ion temperature with an He majority has been observed. From first applications of the upgraded FIT3D code it turns out that, although more NB power appears to be coupled with the He plasma, the NBI power deposition is unaffected, suggesting that heat deposition does not play a key role in the increased ion temperature with He plasma.

Calibrations of the LHD Thomson scattering system.

The Thomson scattering diagnostic systems are widely used for the measurements of absolute local electron temperatures and densities of fusion plasmas. In order to obtain accurate and reliable temperature and density data, careful calibrations of the system are required. We have tried several calibration methods since the second LHD experiment campaign in 1998. We summarize the current status of the calibration methods for the electron temperature and density measurements by the LHD Thomson scattering diagnostic system. Future plans are briefly discussed.

Nonlinear excitation of subcritical fast ion-driven modes

In collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase-space. The growth of such structures is a nonlinear, kinetic mechanism, which provides a channel for free-energy extraction, different from conventional inverse Landau damping. However, such nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave–wave coupling can provide a seed, which can lead to subcritical instability by either one of two mechanisms. Both mechanisms hinge on a collaboration between fluid nonlinearity and kinetic nonlinearity. If collisional velocity diffusion is low enough, the seed provided by the supercritical mode overcomes the threshold for nonlinear growth of phase-space structure. Then, the supercritical mode triggers the conventional subcritical instability. If collisional velocity diffusion is too large, the seed is significantly below the threshold, but can still grow by a sustained collaboration between fluid and kinetic nonlinearities. Both of these subcritical instabilities can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. These results were obtained by modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave–wave coupling equations. This model is applied to bursty onset of geodesic acoustic modes in an LHD experiment. The model recovers several key features such as relative amplitude, timescales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, with sustained collaboration between fluid and kinetic nonlinearities.

Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma.

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities.

Long-pulse Plasma Discharges by Upgraded ECH System in the LHD

Until 2009, three high-power, over 1MW each, 77GHz gyrotrons have been installed and applied to the LHD experiment. In addition, a 154GHz gyrotron of 1MW was installed in 2012. The 77GHz gyrotrons suffer gradual increases of internal pressure during long-pulse operation delivering power to LHD. To mitigate the problem, quasi-steady operation by combination of on-off operations of the 77GHz gyrotrons was performed. Applying two 77GHz gyrotrons alternately with intervals of two minutes and an 84GHz gyrotron continuously, a 30 min. long-pulse discharge with the line average electron density n e_ave of 0.7×1019 m−3 and the central electron temperature T e0 of 1.7keV was achieved by the time average injection power P inj of 260kW in 2012, showing significant progress in sustained density from the former 65 min. discharge with n e_ave of 0.15×1019 m−3 and T e0 of 1.7keV by P inj of 110kW of 84GHz wave. In 2013, one of the 77GHz gyrotron was improved to furnish a sub-window to remove stray radiation inside the tube. And the new 154GHz gyrotron was applied to the long-pulse discharge experiment. Using three gyrotrons: 154, the improved and an existing 77GHz with P inj of 340kW in total, higher temperature plasma having steep temperature gradient typical for internal transport barrier, with n e_ave of 1.1×1019 m−3 and T e0 of 3.5keV was quite stably sustained for 325s.

Study of high-performance plasmas in the Large Helical Device

Recent results from LHD experiments are presented together with the progress of the related fusion technology supporting the experiment. An ultra-long pulse discharge with central electron and ion temperatures of 2 keV and electron density of 1.2 x 1019m−3 was successfully maintained for 48 minutes. Such a high performance plasma was heated and sustained by electron cyclotron resonance heating (ECH) with newly developed gyrotrons and ion cyclotron range of frequency (ICRF). Due to the continuous heating with the relatively high power of 1.2 MW, the total heating energy during the long pulse discharge reached 3.36 GJ, which is the world's record. The highest central ion temperature of 8.1 keV was also recorded in the last experimental campaign. In these experiments, it is essential to reduce the edge electron density, i.e., edge neutrals. Dedicated wall conditioning was performed to evacuate neutrals in the vessel wall before the main discharge with the high heating power. This process realizes the quite low recycling for particles, which results in the reduction of the edge plasma density.

Plasma-induced magnetic responses during nonlinear dynamics of magnetic islands due to resonant magnetic perturbations

Resonant magnetic perturbations (RMPs) produce magnetic islands in toroidal plasmas. Self-healing (annihilation) of RMP-induced magnetic islands has been observed in helical systems, where a possible mechanism of the self-healing is shielding of RMP penetration by plasma flows, which is well known in tokamaks. Thus, fundamental physics of RMP shielding is commonly investigated in both tokamaks and helical systems. In order to check this mechanism, detailed informations of magnetic island phases are necessary. In experiments, measurement of radial magnetic responses is relatively easy. In this study, based on a theoretical model of rotating magnetic islands, behavior of radial magnetic fields during the self-healing is investigated. It is confirmed that flips of radial magnetic fields are typically observed during the self-healing. Such behavior of radial magnetic responses is also observed in LHD experiments.

Integration of Large-Scale Simulations and Numerical Modelling Tools in Close Link with the LHD Experiment

An integrated transport analysis suite, TASK3D-a, has been developed with the emphasis on establishing close-link with the LHD experiment database. The suite makes possible energy transport analyses for huge cases, from which the systematic understandings can be elucidated. A statistical approach for implementing largescale simulation results into the integrated modelling has been tested. The importance of strengthening trilateral links among experiments, large-scale simulations, and integrated numerical modelling tools such as TASK3D, is crucial for promoting systematic understandings, performing validations, and then increasing the predictive capabilities.

Development of Transport Model in Reactor Plasmas based on LHD Experiment Scaling

We propose a transport model based on a systematic analysis of local transport for a large number of Large Helical Device discharges. This model allows us to more accurately predict the plasma performance compared with early researches in the helical plasmas. The proposed model has been applied to the integrated modeling code, TASK3D. We have developed a code to predict the confinement performance in the reactor-relevant highbeta regime. In the predictions of high-beta discharges maintained by NBIs, when field strength increases from 0.43 T to 0.9 T, the achievable beta value decreases by about 1%, but the magnetic Reynolds number increases about 10 times larger. In the prediction to add the perpendicularly NBs with the “re-entering fast ion” effect, the achievable beta value increase by about 0.3%. On the other hand, the magnetic Reynolds number barely changes.

Behavior of Plasma Response Field in Detached Plasma

Sustaining a detached plasma is important for the reduction of the heat load on a divertor. The externally controlled methods such as gas puffing in the divertor region, etc. have been studied for maintaining detached plasmas. In the recent LHD experiments, a resonant magnetic perturbation (RMP) is utilized to establish the detached plasma, which is one of the candidates to reduce the divertor heat load. The finite plasma response field has been detected when the RMP is imposed, and its behavior is investigated from the viewpoint of the magnetic island dynamics. A parameter of the phase difference, Δθ (defined as the phase difference between the plasma response field and the RMP), changes from antiphase to in-phase while the plasma transits from the attached to the detached states. Finally, when the Δθ reaches at certain critical value, Δθcrit, the plasma enters the detached state. Since this critical value does not have a dependency on plasma beta and collisionality, Δθcrit seems to be a threshold to establish the detached plasma. From the viewpoint of magnetic island dynamics, the behavior of Δθ approaching in-phase implies the “trend” of island growth. The detached plasma might be established in the case of the tendency for the plasma response field to reinforce the island width.

Development of Integrated Transport Code, TASK3D, and Its Applications to LHD Experiment

The integrated transport code for helical plasmas, TASK3D, has been developed both by modifying modules in TASK to be applicable to three-dimensional magnetic configurations, and by adding new modules for stellarator-heliotron specific physics and incorporating three-dimensional equilibria. In this paper, these module developments so far are collectively introduced, and recent progress on the applications of TASK3D to heat transport analyses of LHD plasmas is introduced.

Experimental study of the poloidal flow effect on magnetic island dynamics in LHD and TJ-II

The dynamics of a magnetic island is studied by focusing on the poloidal flows in the helical devices LHD and TJ-II. An experimental result implying the temporal increment of the E × B poloidal flow prior to the magnetic island transition from growth to healing is observed. The direction of the poloidal flow is in the electron-diamagnetic direction in LHD and in the ion-diamagnetic direction in TJ-II. From the magnetic diagnostics, it is observed that a current structure flowing in the plasma moves ∼π rad poloidally in the electron-diamagnetic direction during the transition in LHD experiments. These experimental observations from LHD and TJ-II show that the temporal increment of the poloidal flow is followed by the transition (growth to healing) of the magnetic island regardless of the flow direction and suggests the fact that a significant poloidal flow affects the magnetic island dynamics.

Development of Portal Web Pages for the LHD Experiment

Because the LHD project has been operating with the cooperation of many institutes in Japan, the remote participation facilities play an important role. Therefore, NIFS has been introducing these facilities to its remote participants. Because the authors regard Web services as essential tools for the current Internet communication, Web services for remote participation have been developed. However, because these services are dispersed among several servers in NIFS, users cannot find the required services easily. Therefore, the authors developed a portal Web server to list the existing and new Web services for the LHD experiment. The server provides services such as summary graph, plasma movie of the last plasma discharge, daily experiment logs, and daily experimental schedules. One of the most important information from these services is the summary graph. Usually, the plasma discharges of the LHD experiment are executed every three minutes. Between the discharges, the summary graph of the last plasma discharge is displayed on the front screen in the control room soon after the discharge is complete. The graph is useful in evaluating the last discharge, which is important information for determining the subsequent experiment schedule. Therefore, it is required to display the summary graph, which plots more than 10 data diagnostics, as soon as possible. On the other hand, the data-appearance time varies from one diagnostic to another. To display the graph faster, the new system retrieves the data asynchronously; several data retrieval processes work simultaneously, and the system plots the data all at once.