Interchange Mode Research Articles

Recent results from deuterium experiments on the large helical device and their contribution to fusion reactor development

In recent deuterium experiments on the large helical device (LHD), we succeeded in expanding the temperature domain to higher regions for both electron and ion temperatures. Suppression of the energetic particle driven resistive interchange mode (EIC) by a moderate electron temperature increase is a key technique to extend the high temperature domain of LHD plasmas. We found a clear isotope effect in the formation of the internal transport barrier in high temperature plasmas. A new technique to measure the hydrogen isotope fraction was developed in the LHD in order to investigate the behavior of the isotope mixing state. The technique revealed that the non-mixing and the mixing states of hydrogen isotopes can be realized in plasmas. In deuterium plasmas, we also succeeded in simultaneously realizing the formation of the edge transport barrier (ETB) and the divertor detachment. It is found that resonant magnetic perturbation plays an important role in the simultaneous formation of the ETB and the detachment. Contributions to fusion reactor development from the engineering point of view, i.e. negative-ion based neutral beam injector research and the mass balance study of tritium, are also discussed.

Device and network driven cellular networks architecture and mobility management technique for fog computing-based mobile communication system

In this paper, we propose a metric and performance-based mode switching procedure for 5G and beyond mobile fog networks. The network interchange mode of communication between network-centric and device-centric modes, the mode switching procedure is subject to the network’s quality of service (QoS) and quality of experience (QoE) parameters. If the quality parameters are less than the pre-set threshold value, the network selects a relay node as intermediate nodes to avail the device-centric mode of communication. During the communication, the network information’s are retrieved from the nearby fog nodes and the process by intelligent base stations. Our next contribution is a relay node assisted mobility management procedure. As the mobile nodes change the position frequently within the network service area, an efficient mobility management procedure is required to improve the network’s performance. We propose a unique mobility management procedure, it follows the movement prediction procedure and helps the mobile nodes to avail the device-centric communication in dynamic fog networks, we classify it for dynamic and static mobile users. To check the effectiveness, efficiency, and parameterized dependency of the proposed work, we formulate the process with respect to several dependent factors and comparing our results with the latest state-of-the-art networks technologies subject to communication delay and latency, energy consumption, without any compromise in node mobility and call to mobility ratio. Finally, we achieve better performance to gain the QoS and QoE.

Non-resonant global mode in LHD partial collapse with net toroidal current

A transition from an interchange mode to a non-resonant mode is found in the nonlinear magnetohydrodynamic simulation for the partial collapse in a large helical device (LHD) plasma with a net toroidal current. This transition can occur when the magnetic shear is weak and the rotational transform is close to unity in the core region. In this transition, the mode number of the dominant Fourier component is reduced. As a result of the nonlinear evolution, the (m, n) = (1, 1) component can be dominant, where m and n are the poloidal and the toroidal mode numbers, respectively. This transition is considered to be a candidate to explain the observation in the LHD experiments with the net toroidal current that show partial collapses are caused by the (1, 1) mode.

Theoretical analysis of the saturation phase of the 1/1 energetic-ion-driven resistive interchange mode

The aim of the present study is to analyze the saturation regime of the energetic-ion-driven resistive interchange mode (EIC) in the LHD plasma. A set of nonlinear simulations are performed by the FAR3d code that uses a reduced MHD model for the thermal plasma coupled with a gyrofluid model for the energetic particle (EP) species. The hellically trapped EP component is introduced through a modification of the averaged drift velocity operator to include their precessional drift. The nonlinear simulation results show similar 1/1 EIC saturation phases with respect to the experimental observations, reproducing the enhancement of the n/m = 1/1 resistive interchange modes (RIC) amplitude and width as the EP β increases, the EP β threshold for the 1/1 EIC excitation, the further destabilization of the 1/1 EIC as the population of the helically trapped EP increases and the triggering of burst events. The frequency of the 1/1 EIC calculated during the burst event is 9.4 kHz and the 2/2 and 3/3 overtones are destabilized, consistent with the frequency range and the complex mode structure measured in the experiment. In addition, the simulation shows the inward propagation of the 1/1 EIC due to the nonlinear destabilization of the 3/4 and 2/3 energetic particle modes, leading to the partial overlapping between resonances during the burst event. Finally, the analysis of the 1/1 EIC stabilization phase shows the excitation of the 1/1 RIC as soon as the flattening induced by the 1/1 EIC in the pressure profile vanishes, leading to the retrieval of the pressure gradient at the plasma periphery and the overcoming of the RIC stability limit.

Self-organized confinement in whole-device modeling of laboratory magnetospheres

Turbulent self-organization driven by global ideal interchange modes in a dipole-confined plasma is explored with self-consistent, whole-plasma simulations using a flux-tube averaged magnetohydrodynamic model in dipole magnetic geometry. We show the existence of robust particle pinch driven by ideal interchange-mode fluctuations, in which the particles are transported up the density gradient. It is found that the plasma profiles in a dipole field spontaneously relax to a marginally stable state as centrally peaked pressure and density are created by global interchange-mode transport.

A study of beam ion and deuterium–deuterium fusion-born triton transports due to energetic particle-driven magnetohydrodynamic instability in the large helical device deuterium plasmas

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 (S n) 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 S n 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 (S n_DT). Drop of S n_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.

Repeatability of Sheet Material Formation Results and Interchangeability of Processing Modes at Multi-Pass Laser Formation

Laser shaping of sheet materials is a flexible process and is carried out without force contact on the material, it allows forming, among other things, brittle, elastic and difficult-to-deformed materials. It is known that the main parameters of laser shaping are the beam power, the size of the focus zone and the speed of beam movement along the surface of the workpiece, however, the range of variation of these parameters is not unlimited, but due to the characteristics of a particular equipment. Therefore, it is necessary to develop an approach to selecting processing modes that can be selected from the range available on the equipment and at the same time obtain a predictable result. There is also a need to investigate a reproducibility of laser shaping results with a lot of pass-through processing. Actually, this study is aimed at solving these issues. In particular, the article formulates a provision on complex formation parameters that allow determining interchangeable modes of laser molding processing and varying parameters in ranges available on equipment. For this, the basic processing mode was chosen, formation was carried out with a fixed number of passes, after which, using complex parameters, alternative modes were determined and formation was carried out under these conditions with the same number of passes. The article also presents the methodology and results of experimental studies of checking the interchangeability of formation modes and the repeatability of formation results during processing along parallel and multi-directional trajectories. It was experimentally found that the deviation of the strain value obtained in alternative modes, compared to the base, and did not exceed 2.46% for a three-pass cycle and 5.8% - for a nine-pass cycle. And the repeatability of the formation results during laser shaping is quite high; the discrepancy in the deformation value did not exceed 5%, and, preferably, was lower.

Cover Feature: The PhotoFlexys: A Multi‐Purpose Reactor Platform to Simplify Process Intensification and Mechanistic Studies in Photochemistry (Chemistry ‐ Methods 6/2021)

The Cover Feature shows the “configurator” for the proposed photoreactor platform. Providing interchangeable emission wavelengths and operation modes, the reactor platform can be efficiently configured to match the diverse reaction conditions in photochemistry. Its adaptability simplifies the implementation of photochemical reactions from first batch attempts to process intensification in continuous flow. Accompanied by process analytical technologies, the reactor platform further paves the way for reliable kinetic and mechanistic studies in photochemistry. More information can be found in the Full Paper by Martin Rößler et al.

Generalization of the Heuristic Drift SOL model for finite collisionality and effect on flow shearing rate vs. interchange growth rate

We generalize the low-gas-puff Heuristic Drift (HD) model of the power scrape-off layer width to take into account both the enhanced parallel confinement time in the SOL at high collisionality, due to enhanced thermal resistivity, and the increase of the upstream temperature at very low collisionality, due to finite target temperature. We find a wide range of separatrix densities over which the original HD model is applicable. However, at the region of high separatrix density and collisionality accessible with strong gas puffs the SOL widens, in reasonable agreement with experimental data from ASDEX-Upgrade and JET. We further find that for typical low-gas-puff H-mode conditions, the projected E×B flow shearing rate in the SOL dominates over the interchange growth rate, while at the high separatrix densities at which H-Modes return to L-Mode, the interchange growth rate approximately equals the shearing rate. The result is related to that of Halpern and Ricci with respect to the steep gradient region of the SOL of inner-wall-limiter discharges, which also show HD-like scale lengths. It is also consistent with calculations of shear-flow stabilization of interchange modes by Zhang and Krasheninnikov. Taking ωs>γint as the criterion for retaining H-Mode performance, we use the generalized HD (GHD) model to predict the scaling for the H→L back transition. The power requirements to sustain H-Mode for existing machines and for ITER are in the range of a factor of 2 below the predictions for the L→H transition, consistent with the limited available studies of H-Mode hysteresis. We find reasonable agreement with the scaling of the density at the H→L back transition found by Bernert on ASDEX-Upgrade. Finally, we speculate that the shearing rate in the SOL of H-Mode plasmas contributes to the reduced core turbulence that supports the formation of the H-Mode pedestal and comment on the implications for ITER.

Finite electron temperature gradient effects on blob formation in the scrape-off layer of a tokamak plasma

Nonlinear coherent dense plasma structures—‘blobs’—that form in the turbulent edge plasma of a tokamak play an important role in the anomalous nature of the plasma transport in that region. A plasma blob is normally formed when another kind of coherent structure—a radially elongated streamer structure—breaks due to differential stretching in the radial and poloidal directions. We investigate the nature of such a blob formation in the scrape-off layer region by taking into account electron temperature effects. It is found that the shear related to the poloidal gradient of the poloidal electric field plays a major role. A blob is predicted to form when this shear exceeds the interchange mode growth rate within the radially elongated region. Our theoretical estimate of this extended criterion for blob formation is validated from three-dimensional numerical simulation results using the BOUT++ framework and could be useful in the interpretation of blob formation in the presence of finite electron temperature gradient that is applicable in the H and L mode discharges.

Linear MHD analyses of locked-mode-like instabilities in LHD

To investigate the driving mechanism of the locked-mode-like instability observed in the large helical device, we reconstruct the magnetohydrodynamic (MHD) equilibria consistent with the measurement and identify a dominant MHD instability in the precursor phase based on linear MHD analyses. From the dependence of the linear growth rate on the magnetic Reynolds number, the radial mode structure of the electrostatic potential fluctuation and other indices, the ideal interchange mode is found to be dominant. Moreover, it is found that the Mercier parameter, D I, becomes much larger than 0.3 during the phase, while the precursor has constant frequency and fluctuation amplitude. Therefore, D I ≫ 0.3 is a good index of the on-set condition of the minor collapse itself. It is also found that the achievement of D I ≫ 0.3 is due to the movement of the resonant surface to the inner plasma region, which corresponds to the larger pressure gradient region.

Dissipative Polarization Domain Walls in a Passive Coherently Driven Kerr Resonator.

Using a passive, coherently driven nonlinear optical fiber ring resonator, we report the experimental realization of dissipative polarization domain walls. The domain walls arise through a symmetry breaking bifurcation and consist of temporally localized structures where the amplitudes of the two polarization modes of the resonator interchange, segregating domains of orthogonal polarization states. We show that dissipative polarization domain walls can persist in the resonator without changing shape. We also demonstrate on-demand excitation, as well as pinning of domain walls at specific positions for arbitrary long times. Our results could prove useful for the analog simulation of ubiquitous domain-wall related phenomena, and pave the way to an all-optical buffer adapted to the transmission of topological bits.

Three-dimensional characteristics of the quasi-single helical state in the KTX

The characteristics of various quasi-single helicities (QSHs) in the Keda Torus eXperiment (KTX) are investigated in ideal magnetohydrodynamics (MHD) simulations with self-organized helical equilibria. It is found that in the core plasma region the negative magnetic shear imposes a substantial influence on the stabilization of interchange modes, which can enhance the magnetic fluctuations of the dominant single mode. The prominent reversal shear plays a critical role on the transition to the QSH phase. This paper shows that the QSH state with a toroidal field periodicity N fp = 6 is expected to stably achieve by the negative magnetic shear in the future KTX experiments. In addition, the plasma confinement effected by subdominant modes is estimated using particle drift computations in the KTX. As the amplitude of residual subdominant modes increases the radial drift is significantly enhanced, which indicates a drastic loss of ions arises from the subdominant modes with sufficient amplitudes. The result appears to agree with experimental observations in the Madison Symmetric Torus (MST) (Bonofiglo 2019 Phys. Rev. Lett. 123 055001). This work may shed a light on the transition mechanism between the multiple helicities (MH) phase and the QSH phase in reversed-field pinch (RFP) facilities. Moreover, to improve the confinement of RFP in the QSH scenario, the critical importance of subdominant mode amplitudes on the ion confinement should be greatly considered.

Construction of metal transfer modes maps for an ER4130 filler metal in GMAW process

Metal transfer modes (MTMs) maps were constructed for GMAW process using ER4130 and 98%Ar-2%O2 shielding gas. There is no available MTMs maps for this filler metal which is used to obtain matching strength in welds of AISI 4130/4140 steels. These maps serve as tools to establish the MTM given a welding current and voltage, which is useful when an engineer is trying to qualify welding procedures according to construction codes. The maps were built analyzing current and voltage signals recorded at 5000 samples/second during bead-on-plate welds. The main advantage of this methodology is its simplicity of instrumentation without expensive cameras, but has low resolution and it is difficult to identify finer characteristics of MTMs, such as subgroups (repelled globular, streaming, rotational spray), drop diameter, explosive transfer, etc. Several MTMs were identified in the signal analysis and grouped into natural MTMs (short circuit, globular and spray) and interchangeable modes (short-circuit-globular, globular-spray and short-circuit-globular-spray).

Situating High-Speed Railway Stations within Local Urban Contexts: Passenger Satisfaction with Intermodal Integration at the Hong Kong HSR Station

With more high-speed railway (HSR) being built worldwide, its be er integration into the 'urban fabric' is imperative. Focusing on the travel experience, this paper presents an analysis of the intermodal integration of HSR with the local public transport system at the newly opened HSR station in Hong Kong. A participatory survey was conducted with arrival passengers making intermodal transfers to local destinations using the metro, bus, and taxis. Participants were asked to recall and rate their travel experience from the exit of HSR station to the public transport stops. This is the first study that integrates both objective and subjective measurements of regional–local intermodal integration holistically from a passenger perspective. In the survey, respondents generally reported a positive experience. However, there were signi ficant diff erences for diff erent interchange modes. At a more theoretical level, ten service a ributes including both instrumental and aff ective dimensions were found to have signi ficant in fluence on the intermodal interchange experience. Furthermore, the importance-satisfaction analysis suggests that three instrumental factors (ticket purchase, time coordination, interchange signage) are in most urgent need of improvement, while good service quality of four aff ective factors (walking environment, cleanliness, congestion-free waiting area, and luggage delivery) should be maintained and strengthened.

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

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.

Ultrafast reprogrammable multifunctional vanadium-dioxide-assisted metasurface for dynamic THz wavefront engineering

In this paper, for the first time, a new generation of ultrafast reprogrammable multi-mission bias encoded metasurface is proposed for dynamic terahertz wavefront engineering by employing VO2 reversible and fast monoclinic to tetragonal phase transition. The multi-functionality of our designed VO2 based coding metasurface (VBCM) was guaranteed by elaborately designed meta-atom comprising three-patterned VO2 thin films whose operational statuses can be dynamically tuned among four states of “00”-“11” by merely changing the biasing voltage controlled by an external Field-programmable gate array platform. Capitalizing on such meta-atom design and by driving VBCM with different spiral-like and spiral-parabola-like coding sequences, single vortex beam and focused vortex beam with interchangeable orbital angular momentum modes were satisfactorily generated respectively. Additionally, by adopting superposition theorem and convolution operation, symmetric/asymmetric multiple beams and arbitrarily-oriented multiple vortex beams in pre-demined directions with different topological charges are realized. Several illustrative examples successfully have clarified that the proposed VBCM is a promising candidate for solving crucial terahertz challenges such as high data rate wireless communication where ultrafast switching between several missions is required.

Ion kinetic effects on linear pressure driven magnetohydrodynamic instabilities in helical plasmas

The linear MHD (magnetohydrodynamic) stability for high beta plasmas in the inward shifted Large Helical Device (LHD) configurations has been investigated for a wide range of magnetic Reynolds numbers $S$ using numerical simulations based on the kinetic MHD model with kinetic thermal ions where the beta is the ratio of the plasma pressure to the magnetic pressure. It is found that the dependence of the linear growth rate of the resistive ballooning modes on the $S$ number changes from $\unicode[STIX]{x1D6FE}\propto S^{-1/3}$ to $\unicode[STIX]{x1D6FE}\propto S^{-1}$ by the kinetic thermal ion effects so that the resistive ballooning modes are significantly suppressed as the $S$ number increases. For a high $S$ number comparable to experimental values, the most unstable modes are interchange modes. The kinetic thermal ion effects change the most unstable interchange mode from the ideal mode to the resistive mode. This transition of the interchange modes by kinetic thermal ion effects is consistent with the shift of the marginal stability boundary for the ideal interchange modes observed in the LHD experiments.

Linear gyrokinetics of electron–positron plasmas in closed field-line systems

Linear gyrokinetic simulations of magnetically confined electron–positron plasmas are performed for the first time in the geometry and parameter regimes likely to be relevant for upcoming laboratory experiments. In such plasmas, the density will be sufficiently small as to render the plasma effectively collisionless. The magnetic field will be very large, meaning that the Debye length will exceed the gyroradius by a few orders of magnitude. We show the results of linear simulations in flux tubes close to the current carrying ring and also in the bulk of the plasma, demonstrating the existence of entropy modes and interchange modes in pair plasmas. We study linear stability and show that in the relevant configurations, almost complete linear stability is attainable in large swathes of parameter space.

A new explanation of the sawtooth phenomena in tokamaks

The ubiquitous sawtooth phenomena in tokamaks are so named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974, it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new theory for the sawtooth phenomena in auxiliary heated tokamaks, which is motivated by our recent understanding of “magnetic flux pumping.” In this theory, the role of the (m,n)=(1,1) mode is to generate a dynamo voltage, which keeps the central safety factor, q0, just above 1.0 with low central magnetic shear. When central heating is present, the temperature on axis will increase until at some point, and the configuration abruptly becomes unstable to ideal MHD interchange modes with equal poloidal and toroidal mode numbers, m=n>1. It is these higher order modes and the localized magnetic stochasticity they produce that cause the sudden crash of the temperature profile, not magnetic reconnection. Long time 3D MHD simulations demonstrate these phenomena, which appear to be consistent with many experimental observations.