Pedestal Structures Research Articles

Parameter dependence of density pedestal width and correlation with pedestal turbulence in EAST type-I ELMy H-mode plasma

The density pedestal structures in a series of type-I ELMy H-mode discharges (0.6 MA/2.5 T) with upper single null (USN) divertor configuration have been studied in EAST. To obtain the pedestal width, density profiles are fitted via the modified hyperbolic tangent (mtanh) function and the two-line method. The pedestal widths from these two methods have a similar tendency. Significant impacts of pedestal poloidal beta and pedestal collisionality on the density pedestal width have been observed for the first time in EAST. The non-linear regressions suggest the density width scaling in type-I ELMy EAST plasma are wne∝c(νped⁎)0.27−0.33⋅(βp,ped)0.5−0.58. The density fluctuations in the bottom of the pedestal from the fluctuation reflectometry imply an enhanced density fluctuation (Pfluc) and an obvious spectral broadening in the high collisionality regime of νped⁎≥3. The experimental results suggest that the pedestal turbulence would play an important role in the evolution of the pedestal structure.

Etchless pedestal chalcogenide waveguide platform for long-wave IR applications

We report the fabrication of GeAsSeTe/GeAsSe waveguides using a simple and cost-effective process. Chalcogenides are very delicate materials and can be degraded when in contact with developer solutions during photolithography and when processed using common etchants, making the use of conventional fabrication processes unattractive. In order to avoid any post-film deposition processing for the fabrication of chalcogenide waveguides, we pre-patterned pedestal structures on silicon substrates using photolithography and a simple wet-etch process followed by the deposition of chalcogenide films on the patterned structures. Using the scattered light decay fitting method, we estimated waveguide propagation losses averaging approximately 0.9 dB/cm for wavelengths between 7 and 11 µm. With these findings we show that this waveguide platform is a very attractive candidate for long-wave infrared applications.

Numerical study on predicting MHD stability of HL-2A tokamak pedestal structure

HL-2A tokamak achieved the first ELMy H-mode discharge operation in 2009 under divertor configuration, and many experimental and simulation researches have been carried out to investigate the pedestal magnetohydrodynamic (MHD) instability. However, there are still few studies on the effect of pedestal structure on MHD stability. Therefore, based on HL-2A experimental parameters, equilibria with different pedestal structures are generated by using TOQ code, and the MHD stability of the equilibria is simulated by using the BOUT++ three-field module. The linear simulations show that reducing the pedestal height, increasing the pedestal width, reducing the pedestal current density and reducing the ion density in the pedestal can improve the MHD stability of pedestal. Using the theory of dispersion relation, the simulation results are explained. Under the premise of MHD stability, the maximum pedestal heights corresponding to different pedestal widths are found, and the data are fitted to obtain an empirical formula that can predict pedestal height, and on this basis, considering the kinetic ballooning mode theory, pedestal height and width are predicted simultaneously. The effect of the pedestal structure on the MHD mode structure is investigated, it is found that the pedestal height can affect the radial width of the mode. Nonlinear simulations show that the pre-growth of instability is affected mainly by a single dominant mode, and the growth of the dominant mode to a certain size will cause the collapse of the pedestal and the eruption of the edge localized mode (ELM). The variation of ELM size after ELM eruption is synchronized with the evolution of the dominant mode. Generally, equilibria with larger linear growth rates have larger ELM sizes and a wider range of pedestal collapse in nonlinear simulations. In this work, the scanning of the pedestal data focuses mainly on the width and height of the pedestal, and other parameters such as small radius, toroidal magnetic field, plasma current, and the pedestal safety factor values will be changed in the future based on the work in this paper, with the aim of enriching the HL-2A pedestal database and predicting the pedestal structure more accurately. Finally these results will be integrated under the HL-2A integrated platform, which in turn will provide a reference for HL-2A tokamak H-mode experiments and integrated simulations.

Estimation of debris relocation and structure interaction in the pedestal of Fukushima Daiichi Nuclear Power Plant Unit-3 with Moving Particle Semi-implicit (MPS) method

To provide supportive information to understand the current debris status in Fukushima Daiichi Nuclear Power Plant (NPP) Unit-3 (from hereinafter, Unit-3), the meshless, Lagrangian Moving Particle Semi-implicit (MPS) method has been developed for the evaluation of debris-structure interactions in the pedestal region of Unit-3. The developed MPS method incorporated approaches to enhance numerical accuracy and stability, such as second-order corrective matrix and particle shifting technique, and the calculation cost reduction and efficiency improvement strategies to enable plant-scale simulations. A 1/10 scale numerical model of the pedestal region and structures within of Unit-3 and sensitivity analysis conditions were established for simulation cases. The simulation results showed that the convective vapor cooling from the debris surface in the pedestal region and the debris relocation amount/the time intervals between the relocations play important roles in the structure damage due to thermal attacks from the debris.

The factors determining the evolution of edge-localized modes in plasmas driven by lower hybrid currents

Modulated lower-hybrid waves (LHWs) are injected into the Experimental Advanced Superconducting Tokamak to determine the physical principles responsible for the suppression or mitigation of edge-localized modes (ELMs). There are two cases of modulated-ELM evolution (stable and unstable cases), because of two different modulated pedestal densities. They can be attributed to additional magnetic perturbations induced by the LHWs, similarly to the effect of resonant magnetic perturbations. As regards the case of unstable modulated ELM evolution, the plasma stored energy increases as the LHWs turn on. In contrast, the central line-averaged electron density decreases, which is different from the case of ELM suppression or from the stably modulated case. The effect of LHWs or density ‘pump-out’ effect can pass across the top of the pedestal region and enter the interior of the density pedestal, causing a decrease in the electron density gradient and its value at the top of the pedestal. Simultaneously, the pressure gradient and edge bootstrap current density increase. For ELM suppression (or for the stable) case, LHWs can couple only with the plasma outside the top region of pedestal, because of the higher top value of density pedestal. Thus, LHWs can pump out the electron density significantly only in the pedestal foot region, producing a larger gradient of electron density pedestal. Statistical analysis of the data indicates that there is a threshold value of the central line-averaged electron density for each of the two modulated ELM cases. Furthermore, the ELM amplitude is modulated by LHWs with a time delay of hundreds of microseconds, which may be further evidence that LHWs have a significant impact on the evolution of ELMs and pedestal structures. All these results imply that there is a significant correlation between the ELM behavior and the electron density profiles modulated by LHWs.

Predict-first experimental analysis using automated and integrated magnetohydrodynamic modeling

An integrated-modeling workflow has been developed for the purpose of performing predict-first analysis of transient-stability experiments. Starting from an existing equilibrium reconstruction from a past experiment, the workflow couples together the EFIT Grad-Shafranov solver [L. Lao et al., Fusion Sci. Technol. 48, 968 (2005)], the EPED model for the pedestal structure [P. B. Snyder et al., Phys. Plasmas 16, 056118 (2009)], and the NEO drift-kinetic-equation solver [E. A. Belli and J. Candy, Plasma Phys. Controlled Fusion 54, 015015 (2012)] (for bootstrap current calculations) in order to generate equilibria with self-consistent pedestal structures as the plasma shape and various scalar parameters (e.g., normalized β, pedestal density, and edge safety factor [q95]) are changed. These equilibria are then analyzed using automated M3D-C1 extended-magnetohydrodynamic modeling [S. C. Jardin et al., Comput. Sci. Discovery 5, 014002 (2012)] to compute the plasma response to three-dimensional magnetic perturbations. This workflow was created in conjunction with a DIII-D experiment examining the effect of triangularity on the 3D plasma response. Several versions of the workflow were developed, and the initial ones were used to help guide experimental planning (e.g., determining the plasma current necessary to maintain the constant edge safety factor in various shapes). Subsequent validation with the experimental results was then used to revise the workflow, ultimately resulting in the complete model presented here. We show that quantitative agreement was achieved between the M3D-C1 plasma response calculated for equilibria generated by the final workflow and equilibria reconstructed from experimental data. A comparison of results from earlier workflows is used to show the importance of properly matching certain experimental parameters in the generated equilibria, including the normalized β, pedestal density, and q95. On the other hand, the details of the pedestal current did not significantly impact the plasma response in these equilibria. A comparison to the experimentally measured plasma response shows mixed agreement, indicating that while the equilibria are predicted well, additional analysis tools may be needed. Finally, we note the implications that these results have for the success of future predict-first studies, particularly the need for scans of uncertain parameters and for close collaboration between experimentalists and theorists.

Optimal topology design of internal stiffeners for machine pedestal structures using biological branching phenomena

Naturally evolved biological structures exhibit the optimal characteristics of light weight, high stiffness, and high strength. Based on the growth mechanism of biological branch systems in nature, an optimization method for internal stiffener plate distribution in box structures is suggested. Under the given load and support conditions, the internal stiffener plates of machine pedestal structures grow, bifurcate, and degenerate towards the direction of maximum overall structural stiffness in accordance with the adaptive growth law. The optimal and distinct distribution of internal stiffener plates with the most effective load path is thus obtained. Based on this, a size optimization for lightweight design is conducted, in which the self-weight of the structures is taken as the design objective, and the natural vibration frequency and static stiffness in the direction that is sensitive to machining accuracy are set as constraints. Finally, an optimized structure is obtained. The effectiveness of the proposed method is verified by using a precision grinder bed as an example. The results of numerical simulation and 3D–printed model experiment indicate that both the dynamic and the static performance of the optimized structure are improved, while the structural weight is reduced by compared with the initial structure. The suggested design method provides a new solution approach for the design optimization of machine pedestal structures.

CFETR equilibrium with self-consistent pedestal structure

The Chinese Fusion Engineering Test Reactor (CFETR), which aims to fill the gaps between the International Thermonuclear Experimental Reactor (ITER) and DEMO, is in the design stages. Plasma equilibrium is an important basis for tokamak plasma research. We present a method for constructing CFETR equilibria with a self-consistent pedestal structure, using an EPED-like model and the Sauter bootstrap current model. With this method, the pedestal structures of the snowflake (SF) divertor configuration for CFETR are calculated as well. It is shown that the pedestal heights and widths of SF+ and SF- are slightly larger than those in the standard divertor configuration.

Control of Sidewall Profile in Dry Plasma Etching of Polyimide

Spin-on polyimide is an organic thin film often used as a sacrificial layer for surface micromachining due to its high thermal stability, ease of removal, and compatibility with many materials and processes used in the realization of microelectromechanical systems (MEMS). The incorporation of sloped sidewalls in polyimide for fabricating pedestal structures is crucial in order to provide strong anchors in free-standing MEMS devices, especially in cases having a high aspect ratio and/or where structural materials have limited deposition conformality. This paper demonstrates a reliable reactive ion etching (RIE) methodology for tuning the polyimide sidewall angle, ranging from a vertical sidewall up to an angle of about 25° from the vertical. The key modifications to the process parameter space include changes to the process temperature and chamber pressure. This paper also presents a novel lift-off process, which is based on the use of an interfacial polymer layer to facilitate removal of an overlying silicon oxide hard mask. This procedure allows polyimide sacrificial layers employing a silicon oxide hard mask to be used on samples that have exposed silicon oxide layers elsewhere on the chip that are required to remain intact during hard mask removal. Therefore, this lift-off process is applicable in situations where the silicon oxide hard mask removal cannot be accomplished by wet etching in hydrofluoric acid solutions.

SOL effects on the pedestal structure in DIII-D dischargesaaThis manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form

Analysis with the SOLPS code suite (Schneider R. et al 1992 J. Nucl. Mater. 196 810; Wiesen S. et al 2015 J. Nucl. Mater. 463 480–4; Bonnin X. et al 2016 Plasma Fusion Res. 11 1403102) explains the differences in pedestal structure associated with different ion drift directions in DIII-D. Core transport models predict that fusion power scales roughly as the square of the pressure at the top of the pedestal, so understanding the effects that determine pedestal structure in steady-state operational scenarios is important to help project scenarios developed in DIII-D to ITER and other devices. Both experiments and modeling indicate that scrape off layer (SOL) conditions are important in optimizing the pedestal structure for high-beta steady-state scenarios. The SOLPS code is used to provide interpretive analysis of the pedestal, and SOL conditions are used to examine the nature of flows and fueling on the pedestal structure; including the effect of drifts in the fluid model. This analysis shows that flows driven by the ion drift outwards when the drift flows toward the x-point, in a single-null divertor configuration (favorable direction for reduced H-mode power threshold), and they drift inwards when the drift flow is away from the x-point (unfavorable direction). It is hypothesized that these flows decrease the density gradient of the pedestal in the favorable direction, thereby stabilizing the kinetic ballooning mode (KBM) and increasing the pedestal width. Comparisons of the pedestal structures in similarly shaped DIII-D steady-state plasmas confirm this change, showing increased density pedestal width, lower peak density and lower separatrix density with the favorable drift direction. The pedestal temperature is higher at a lower density, resulting in an increased pedestal pressure, which indicates that the increased particle flux does not significantly degrade energy confinement. Modeling cases with constant drift direction, but with a change between the more open lower divertor and more closed upper divertor, show that there is increased fueling inside a pedestal with a more open geometry. The pedestal fueling rate for both attached and detached cases is always lower when there is a more closed divertor geometry than in cases when there is a more open divertor geometry.

Inter-ELM evolution of the pedestal structures in type-I ELMy H-mode plasmas with LHW and NBI heating on EAST

The evolution characteristics of type-I ELMy high-confinement mode pedestal are examined in EAST based on the recently developed Thomson scattering system. The influence of the plasma current on pedestal evolvement has been confirmed experimentally. In the higher Ip case (500 kA) the pedestal height shows an increase trend until the onset of next ELM and in the lower Ip cases (300 and 400 kA), however, this buildup saturates at the first ∼30% of the ELM cycle. In contrast, the width increases only during the first ∼70% of the ELM cycle and then keeps almost stable in three Ip cases, but resulting in different widening size of ∼1.5, 1 and 0.5 cm for 300, 400 and 500 kA respectively. Experimental results show that the pedestal pressure width has good correlation with poloidal beta as where the fitting coefficient 0.16 is not changed with different plasma currents but a little larger than that of other machines. For each current level, the pedestal density increases while the pedestal temperature decreases. But with increasing platforms, the pedestal height prior to the ELM onset shows a near quadratic (within error bars) increase. Experimental measurements demonstrate that the decrease of with increasing comes mostly from the reduction of the plasma temperature drop, while the pedestal density height keeps relatively stable. Additional injection of LHW has been proved to modify the pedestal structure which should be responsible for the remaining scatter of the experimental data.

Fabrication and optical characterization of pedestal micro-structures on DUV210 polymer: waveguides structures towards micro-resonators

The paper presents the global design, fabrication and optical characterizations of pedestal waveguides and 2D microresonators made of DUV210 polymer. These particular geometries are achieved thanks to specific deep-UV lithography procedures allowing sub-lambda development coupled to chemical etching able to shape such pedestal configuration on various optical microstructures. Scanning electron microscopy images have confirmed their features. Moreover, such families of pedestal structures were characterized experimentally, including the optical losses measurements for the waveguides and the optical resonance responses for two kinds of microresonators. Optical studies of single mode propagation losses measurements have been performed by a cut back method leading to values close to 20 dB/cm at a 635 nm wavelength. Additionally, resonant spectral analysis has been performed into pedestal rings and racetracks microresonators with a broadband laser source centered at 795 nm, demonstrating the presence of expected theoretical resonances. Free spectral range values of 2.86 nm and 2.51 nm have been measured for these new designed pedestal resonators on DUV210 related to quality factor values superior to 520 and 610 respectively.

InAsSb/InAsPSb multiple quantum well disk cavities with pedestal structures on a GaSb substrate for mid-infrared whispering-gallery-mode emission beyond 4 μm

The mid-infrared whispering-gallery-mode disk cavities with InAs0.85Sb0.15/InAs0.53P0.23Sb0.24 multiple quantum wells active medium on a GaSb substrate were fabricated. For this material system in the mid-infrared range, fabrication techniques were developed to form the disk cavity structure. The smooth sidewalls of the disk cavities were achieved by appropriate gas mixture flow ratio of BCl3/Ar in the inductively coupled plasma-reactive ion etching. In addition, selective wet etching technique was used to form the pedestal of the disk cavity using dilute hydrofluoric acid with good selectivity. For efficient confinement of the whispering gallery modes along the radial direction, the extent of the lateral etching was carefully controlled. The processed 30-μm-diameter disk cavities were optically pumped, and the whispering gallery modes with wavelengths around 4.1 μm can be observed up to 90 K.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 μm and λ=3.7 μm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 μm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

Transfer of GaN LEDs From Sapphire to Flexible Substrates by Laser Lift-Off and Contact Printing

We fabricate flexible GaN-based light-emitting diode (LED) systems by laser lift-off (LLO) and transfer printing methods. LLO enables transferring a whole GaN LED layer from sapphire onto a silicon handling wafer to provide a stable platform for any shape of LED. Polymer pedestal structures underneath the LEDs support efficient transfer printing of the patterned LED array from the silicon handling wafer to a flexible substrate. We demonstrate the efficacy of the technique by presenting 9 × 9 LED arrays on polyethylene terephthalate and heart-shaped LED pixels on a piece of paper with transfer yields of 92% and 79%, as well as their successful illumination.

Modifications to the edge radial electric field by angular momentum injection in JT-60U and their implication for pedestal transport

The first detailed measurements of ion-impurity dynamics for NBI-heated ELMy H-modes at the edge of the JT-60U tokamak are reported. We investigated the ability of external momentum/power input to modify and control the radial electric field, Er, and pedestal structures. The relationship between Er and pedestal structures of ion-impurity density, ni, and temperature, Ti, during the ELMing H-mode phase for various momentum input directions (i.e. co-, balanced- and counter-NBI) and input powers from perpendicular NBI are compared with the ELM-free phase. The observed trend is that the edge Er-well width increases in the co-NBI discharge, while the Er value at the base of the Er-well becomes more negative in the counter-NBI discharge. The scale length for both ni and Ti in the pedestal is ∼2 cm and values are ∼1 for both ELM-free and ELMing phases with different magnitudes of Er (and/or Er shear). Characteristics of the turbulent density fluctuation, in addition to a uniform toroidal MHD oscillation (i.e. n = 0), during both ELM-free and ELMing phases are also reported.

Enterohaemorrhagic Escherichia coli requires the spectrin cytoskeleton for efficient attachment and pedestal formation on host cells

Recent work has demonstrated that the spectrin cytoskeleton is a host cell target, exploited during intestinal bacterial disease. Here we show that the highly virulent intestinal pathogen enterohaemorrhagic Escherichia coli (EHEC) is also reliant upon the spectrin cytoskeleton during key pathogenic events. Immunofluorescent microscopy demonstrated that the core components of the spectrin cytoskeleton (spectrin, adducin, and protein 4.1 [p4.1]) are recruited to sites of EHEC attachment and localized at pedestal structures along with the EHEC pedestal specific proteins IRSp53 and IRTKS. Further studies involving siRNA-mediated knockdowns of spectrin, adducin, or p4.1 revealed that those proteins are needed for efficient docking of EHEC to host cells, are involved in recruiting IRSp53 to the pedestal and are necessary for pedestal formation. These findings identify the spectrin cytoskeleton as a major host cell cytoskeletal network involved in critical EHEC pathogenic events.

Plasma regimes and research goals of JT-60SA towards ITER and DEMO

The JT-60SA device has been designed as a highly shaped large superconducting tokamak with a variety of plasma actuators (heating, current drive, momentum input, stability control coils, resonant magnetic perturbation coils, W-shaped divertor, fuelling, pumping, etc) in order to satisfy the central research needs for ITER and DEMO. In the ITER- and DEMO-relevant plasma parameter regimes and with DEMO-equivalent plasma shapes, JT-60SA quantifies the operation limits, plasma responses and operational margins in terms of MHD stability, plasma transport and confinement, high-energy particle behaviour, pedestal structures, scrape-off layer and divertor characteristics. By integrating advanced studies in these research fields, the project proceeds ‘simultaneous and steady-state sustainment of the key performances required for DEMO’ with integrated control scenario development applicable to the highly self-regulating burning high-β high bootstrap current fraction plasmas.

Dynamin2 GTPase and Cortactin Remodel Actin Filaments

The large GTPase dynamin, best known for its activities that remodel membranes during endocytosis, also regulates F-actin-rich structures, including podosomes, phagocytic cups, actin comet tails, subcortical ruffles, and stress fibers. The mechanisms by which dynamin regulates actin filaments are not known, but an emerging view is that dynamin influences F-actin via its interactions with proteins that interact directly or indirectly with actin filaments. We show here that dynamin2 GTPase activity remodels actin filaments in vitro via a mechanism that depends on the binding partner and F-actin-binding protein, cortactin. Tightly associated actin filaments cross-linked by dynamin2 and cortactin became loosely associated after GTP addition when viewed by transmission electron microscopy. Actin filaments were dynamically unraveled and fragmented after GTP addition when viewed in real time using total internal reflection fluorescence microscopy. Cortactin stimulated the intrinsic GTPase activity of dynamin2 and maintained a stable link between actin filaments and dynamin2, even in the presence of GTP. Filaments remodeled by dynamin2 GTPase in vitro exhibit enhanced sensitivity to severing by the actin depolymerizing factor, cofilin, suggesting that GTPase-dependent remodeling influences the interactions of actin regulatory proteins and F-actin. The global organization of the actomyosin cytoskeleton was perturbed in U2-OS cells depleted of dynamin2, implicating dynamin2 in remodeling actin filaments that comprise supramolecular F-actin arrays in vivo. We conclude that dynamin2 GTPase remodels actin filaments and plays a role in orchestrating the global actomyosin cytoskeleton.

Marcatili’s extended approach: comparison to semi-vectorial methods applied to pedestal waveguide design

This paper deals with a theoretical study of pedestal waveguides. An extension of the Marcatili method has been developed in order to adapt this analytical method to pedestal structures. Simulations are performed for two different T-pedestal waveguide (T-PW) configurations corresponding respectively to a high and a lower core to pedestal widths ratio (T-PW I and T-PW II). Each configuration is simulated considering two core widths (2 and 4 µm) and a core height ranging from 1 to 2 µm at a 670 nm wavelength. Then, this extended Marcatili method has been compared with a semi-vectorial finite difference method (SVFD) and a spectral method developed by Galerkin, both based on a numerical approach. The simulation of the T-PW structure with these three methods shows a good congruence since the relative differences between Marcatili's method and the numerical methods remain below 6%. Then, the three approaches are applied to study the modal birefringence minimization in the case of pedestal structures. Simulations are typically performed for waveguide height and width values ranging, respectively, around (1.6–2.6) µm and (1.8–6) µm, with pedestal widths ranging around (0.4–0.8) µm, at a 670 nm wavelength. The authors stress a specific property of pedestal configurations: by judiciously adjusting the dimensional parameters (core and pedestal width and core height), the birefringence can be completely screened out.