Center Stack Research Articles

NSTX tangential divertor camera

Strong magnetic field shear around the divertor x-point is numerically predicted to lead to strong spatial asymmetries in turbulence driven particle fluxes. To visualize the turbulence and associated impurity line emission near the lower x-point region, a new tangential observation port has been recently installed on NSTX. A reentrant sapphire window with a moveable in-vessel mirror images the divertor region from the center stack out to R∼80 cm and views the x-point for most plasma configurations. A coherent fiber optic bundle transmits the image through a remotely selected filter to a fast camera, for example, a 40 500 frames/s photron complementary metal-oxide-semiconductor camera. A gas puffer located in the lower inboard divertor will localize the turbulence in the region near the x-point. Edge fluid and turbulent codes UEDGE and BOUT will be used to interpret impurity and deuterium emission fluctuation measurements in the divertor.

Effect of gas fuelling location on H-mode access in NSTX

The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in NSTX. We find that gas fuelling from the centre stack midplane area produces the most reproducible H-mode access, with generally the lowest L–H threshold power in the lower single-null configuration. The edge toroidal rotation velocity of C2+ is largest (in the direction of the plasma current) just before the L–H transition with centre stack midplane fuelling and then reverses direction after the L–H transition. Simulation of these results with a guiding centre Monte Carlo neoclassical transport code (XGC) is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fuelling from either the centre stack midplane or centre stack top locations, indicating a reduced sensitivity of H-mode access to fuelling location in that shape.

Infrared camera diagnostic for heat flux measurements on the National Spherical Torus Experiment

An infrared imaging system has been installed on the National Spherical Torus Experiment at Princeton Plasma Physics Laboratory to measure the surface temperatures on the lower divertor and center stack. The imaging system is based on an Indigo Alpha 160×128 microbolometer camera with 12 bits/pixel operating in the 7–13 μm range with a 30 Hz frame rate and a dynamic temperature range of 0–700 °C. From these data and knowledge of graphite thermal properties, the heat flux is derived with a classic one-dimensional conduction model. Preliminary results of heat flux scaling are reported.

Diagnostic development for ST plasmas on NSTX**Work supported by US DOE Contract DE-AC02-76CH03073.

Spherical tori (ST) have much lower aspect ratio (a/R) and lower toroidal magnetic field than conventional tokamaks and stellarators. This paper will highlight some of the challenges and opportunities these features pose in the diagnosis of ST plasmas in the National Spherical Torus Experiment (NSTX), and discuss some of the corresponding diagnostic development that is underway. The low aspect ratio necessitates a small centre stack, with tight space constraints and large thermal excursions, complicating the design of magnetic sensors in this region. The toroidal magnetic field on the NSTX is ⩽0.6 T, making it impossible to use ECE as a good monitor of electron temperature. A promising new development for diagnosing electron temperature is electron Bernstein wave radiometry, which is currently being pursued on the NSTX. A high-resolution charge exchange recombination spectroscopy system is being installed. Since non-inductive current initiation and sustainment are top-level NSTX research goals, measurements of the current profile J(R) are essential to many planned experiments. On the NSTX several modifications are planned to adapt the motional Stark effect (MSE) technique to lower field, and two novel MSE systems are being prototyped. Several high-speed two-dimensional imaging techniques are being developed for viewing both visible and x-ray emission. The toroidal field is equivalent to the poloidal field at the outside plasma edge, producing a large field pitch (>50°) at the outer midplane. The large shear in pitch angle makes some fluctuation diagnostics like beam emission spectroscopy very difficult, while providing a means of achieving spatial localization for microwave scattering investigations of high-k turbulence, which are predicted to be virulent for NSTX plasmas. A brief description of several of these techniques will be given in the context of the current NSTX diagnostic set.This article was due to appear in issue 7 of Plasma Physics and Controlled Fusion. To access this special issue please follow this link: http://www.iop.org/EJ/toc/0741-3335/45/7.

H-mode research in NSTX

H-modes are routinely obtained in the National Spherical Torus Experiment (NSTX) and have become a standard operational scenario. L–H transitions triggered by NBI heating have been obtained over a wide parameter range in Ip, Bt, and n̄e in either lower-single-null (LSN) or double-null (DN) diverted discharges. Edge localized modes are observed in both configurations but the characteristics differ between DN and LSN, which also have different triangularities (δ). An H-mode duration of 500 ms was obtained in LSN, with a total pulse length of ∼1 s. Preliminary power threshold studies indicate that the L–H threshold is between 600 kW and 1.2 MW, depending on the target parameters. Gas injector fuelling from the centre stack (i.e. the high toroidal field side) has enabled routine H-mode access, and comparisons with low-field side (LFS) fuelled H-mode discharges show that the LFS fuelling delays the L–H transition and alters the pre-transition plasma profiles. Gas puff imaging and reflectometry show that the H-mode edge is usually more quiescent than the L-mode edge. Divertor infrared camera measurements indicate up to 70% of available power flows to the divertor targets in quiescent H-mode discharges.

Building Preservation: The University of Illinois at Urbana-Champaign’s Stacks Assessment

This article discusses the results of two collection assessments conducted at the University of Illinois at Urbana-Champaign. The library s Preservation Committee completed the first assessment and reported its results in College & Research Libraries in 1989. The second assessment was completed in 2002 and accompanies the institution’s initiative to establish the library’s first centralized preservation and conservation program. Both assessments focused on the central stacks collection, a repository collection serving the institution’s forty-two branch and departmental libraries. Although a reanalysis of the first assessment s data was impossible, the authors attempted to draw comparisons between the two assessments’ results. After thirteen years without a preservation program and without any significant facilities improvements, the results provide insight into the results of deferred collections care and facilities maintenance and offer guidance for conducting similar studies with other research library collections.

High-resolution spectroscopic diagnostic for divertor and scrape-off layer neutral and impurity emission measurements in the National Spherical Torus Experiment

The National Spherical Torus Experiment boundary physics program presently focuses on edge power and particle flow optimization and control in β⩾25% long pulse L- and H-mode plasmas with high harmonic fast wave heating power up to 6 MW and neutral beam injection power up to 5 MW, with the emphasis on the edge physics implications resulting from the low aspect ratio geometry. To address the particle flux measurements in the divertor and plasma scrape-off layer (SOL), two spectrally filtered one-dimensional charge coupled device cameras have been fielded. The cameras utilize 2048 pixel 12-bit Dalsa CL-C6 arrays, f=85 mm lenses, and Δλ=1.5 nm bandpass interference filters. Both cameras provide mm spatial resolution, sub-ms temporal resolution and are spatially and photometrically calibrated. Midplane SOL and divertor brightness profiles of C III and deuterium species have been obtained in the L- and H-mode phases of center stack limited and diverted plasmas. The equilibria reconstructed by the EFIT code are found in agreement with the optical and infra-red measurements. In–out asymmetries in divertor recycling and carbon fluxes have been observed in L- and H-mode plasmas. The analysis of neutral recycling and impurity fluxes using the two-dimensional multifluid code UEDGE is in progress.

Three-dimensional optoelectronic stacked processor by use of free-space optical interconnection and three-dimensional VLSI chip stacks.

We present a demonstration system under the three-dimensional (3D) optoelectronic stacked processor consortium. The processor combines the advantages of optics in global, high-density, high-speed parallel interconnections with the density and computational power of 3D chip stacks. In particular, a compact and scalable optoelectronic switching system with a high bandwidth is designed. The system consists of three silicon chip stacks, each integrated with a single vertical-cavity-surface-emitting-laser-metal-semiconductor-metal detector array and an optical interconnection module. Any input signal at one end stack can be switched through the central crossbar stack to any output channel on the opposite end stack. The crossbar bandwidth is designed to be 256 Gb/s. For the free-space optical interconnection, a novel folded hybrid micro-macro optical system with a concave reflection mirror has been designed. The optics module can provide a high resolution, a large field of view, a high link efficiency, and low optical cross talk. It is also symmetric and modular. Off-the-shelf macro-optical components are used. The concave reflection mirror can significantly improve the image quality and tolerate a large misalignment of the optical components, and it can also compensate for the lateral shift of the chip stacks. Scaling of the macrolens can be used to adjust the interconnection length between the chip stacks or make the system more compact. The components are easy to align, and only passive alignment is required. Optics and electronics are separated until the final assembly step, and the optomechanic module can be removed and replaced. By use of 3D chip stacks, commercially available optical components, and simple passive packaging techniques, it is possible to achieve a high-performance optoelectronic switching system.

Atomic arrangement of dislocation defects in GaAs by HREM

Atom positions and local structure around a perfect dislocation and the central stacking fault of a Z-shape faulted dipole in deformed GaAs were evaluated numerically through high resolution electron microscopic (HREM) analysis. It was revealed around the central stacking fault, connecting the two stair-rods of the dipole, that there exists a local atomic displacement by the original faulting reaction with the motion of a Shockley dislocation. In addition, the stacking fault was found to be a unique atomic structure due to relaxation, different from that of the intrinsic stacking fault of a dissociated dislocation.

MHD and Heat Transfer Issues and Characteristics for Li Free Surface Flows under NSTX Conditions

In the APEX study, one of the tasks focuses on the exploration and identification of the attractive options and issues for flowing liquid lithium walls in the NSTX device. In addition to constraints imposed by the machine, the operating conditions of the flowing liquid walls along the center stack and divertor areas are guided by MHD and heat removal requirements. In this paper, we present important MHD and heat removal issues and analysis for the proposed free surface lithium flows under NSTX conditions. It is shown that of all MHD effects, the one caused by the normal magnetic field is the most important. The flow over the center stack area is not affected by MHD interaction significantly, whereas flow over the inboard divertor undergoes strong MHD drag resulting in flow thickening by several times. The flow over the outboard divertor is essentially stopped. The analysis shows that a flow with an inlet velocity of 2 m/s and film thickness of about 4 mm can be established to provide surface temperature less than 400° C for the center stack under a projected NSTX total heating power of 10 MW operation.

Engineering design of the National Spherical Torus Experiment

NSTX is a proof-of-principle experiment aimed at exploring the physics of the ‘spherical torus’ (ST) configuration, which is predicted to exhibit more efficient magnetic confinement than conventional large aspect ratio tokamaks, among other advantages. The low aspect ratio ( R/ a, typically 1.2–2 in ST designs compared to 4–5 in conventional tokamaks) decreases the available cross sectional area through the center of the torus for toroidal and poloidal field coil conductors, vacuum vessel wall, plasma facing components, etc., thus increasing the need to deploy all components within the so-called ‘center stack’ in the most efficient manner possible. Several unique design features have been developed for the NSTX center stack, and careful engineering of this region of the machine, utilizing materials up to their engineering allowables, has been key to meeting the desired objectives. The design and construction of the machine has been accomplished in a rapid and cost effective manner thanks to the availability of extensive facilities, a strong experience base from the TFTR era, and good cooperation between institutions.

Chosen solutions to the engineering challenges of the National Spherical Torus Experiment (NSTX) magnet

NSTX is one of the largest of a new class of magnetic plasma research devices known as spherical tori (ST). The plasma in a ST is characterized by its almost spherical shape with a slender open cylindrical region through its vertical axis. The so-called is located in this region. It contains magnetic windings to confine the plasma, induce the plasma current, and shape the plasma. This paper describes the engineering challenges of designing the center stack magnets to meet their operational requirements within this constrained space.

Scanning Electron Microscopy of the Interior of Cells in Hürthle Cell Tumors

Four cases of Hurthle cell tumor were examined by scanning electron microscopy after being macerated to remove all soluble components. By all morphological criteria, Hurthle cells are oncocytes with their usual augmented complement of mitochondria. The Hurthle cell mitochondria either are ovate with central stacks of cristae or elliptical or rod-like with cristae that often are finger-like. As in salivary gland oncocytes, the shelf-like cristae are anchored to the inner boundary membrane by tubular necks. In some Hurthle cells, all of the mitochondria exhibit reticulate cristae. A few mitochondria harbor a globular inclusion in their inner compartment. The Golgi apparatuses are relatively simple, consisting of imbricated saccules that are edged by small, bud-like structures. The rare lumina in the midst of clusters of Hurthle cells are lined by numerous microvilli. Thus, scanning electron microscopy of macerated Hurthle cell tumors has revealed a number of features, especially of their mitochondria, that have escaped detection by transmission electron microscopy.

From microns to nanometers: Morphology development in semicrystalline polymers by scanning force microscopy

In studies of spherulitic crystallization in polymers, many questions pertaining to the mechanism of the crystallization process have remained unanswered. A currently accepted view describes the development of spherulites from a framework of individual dominant lamellae that splay apart and branch (e.g., by a dislocation mechanism). This model, in addition, assumes that the space between the dominant lamellae is filled by subsidiary lamellae. In the center of a spherulitic entity, there is a hedritic core that occupies a relative volume that depends on the level of maturity of the spherulitic development. Typical hedritic views consist of lamellae that have a flat stack, or sheaflike, splaying appearance depending on the angle and depth of observation with respect to the central stack of lamellae. Visualization of the three-dimensional appearance of these hedrites by atomic force microscopy (AFM) and the observation of their growth in situ are the subjects of this article. First, we describe AFM studies of hedrites observed at etched surfaces of β-isotactic polypropylene (β-IPP). Evidence sup-porting splaying, branching via dislocations, and development of curved lamellae is presented. In the second part, we describe real-time hot-stage AFM in situ observations of the hedritic growth in poly(ethylene oxide) (PEO) and poly-(ε-caprolactone) (PCL).

Diagnostics for initial plasma operations on the National Spherical Torus Experiment

The goals of the diagnostic set for initial plasma operations on the National Spherical Torus Experiment (NSTX) are to provide information on the fundamental discharge parameters necessary to characterize NSTX plasmas, and guide NSTX operations for optimized performance. While most of these diagnostics involve relatively straightforward techniques requiring little or no research and development, and rely heavily on reusing existing equipment, a few pose considerable challenges due to the NSTX geometry. Magnetic diagnostics and visible discharge imaging will be the primary systems for plasma control and equilibrium. Confinement and transport studies will be supported by electron temperature, density, and impurity profile diagnostics. Scrapeoff layer and divertor physics will be investigated with spectroscopic and bolometric diagnostics, thermocouples, and edge Langmuir probes. X-ray fluctuation diagnostics and magnetic pickup coils will be provided for MHD stability analysis. The latter impose special difficulties for NSTX, because of the high temperature environment and the very limited space on the center stack. This article provides an overview of the diagnostics planned for the first phase of NSTX, and highlights the design features needed to satisfy the difficult constraints the device imposes on magnetic diagnostics.

Structure and Climb of Faulted Dipoles in GaAs

Z-shape faulted dipoles in deformed GaAs were investigated by using high resolution electron microscopy. There is no difference in the core structure of the α and β dislocations of stair-rod and 90° Shockley partials, nor in the dissociated stacking fault width. The central stacking fault is found to generate large local atomic displacements and exhibits a different structure from that of the intrinsic stacking fault of a dissociated dislocation considered so far. 90° Shockley partials of Z-shape faulted dipole climbed through absorption of interstitials during electron irradiation.

Atomic Arrangement of a Z-Shape Faulted Dipole within Deformed GaAs

Atom positions and local structure around the central stacking fault of a Z-shape faulted dipole within deformed GaAs have been evaluated numerically, for the first time, through high resolution electron microscopic analysis. From comparison with the numerical image computation, the stacking fault is found to generate large local atomic displacements and exhibits a different structure from that of the intrinsic stacking fault of a dissociated dislocation considered so far. The atomic arrangement on and around the stacking fault is discussed.

Short-term, post-burial change in a humic rendzina soil, Overton Down Experimental Earthwork, Wiltshire, England

The 1992 excavation of the Overton Down Experimental Earthwork provided a unique opportunity to study pedogenic change in a humic rendzina soil 32 years after burial beneath a chalk bank containing a central stack of turves. Investigations focused on soil structure, porosity, organic matter, plant remains, pH, carbonate, iron, and magnetic susceptibility. Standard methods of chemical analysis and micromorphological description were complemented by quantitative analysis of soil thin sections. The results demonstrate both the speed of change (e.g., compaction and organic decomposition have reduced the thickness of the Ah horizon by >50% in places), and the way the microenvironment of burial can significantly affect the nature and direction of change (e.g., differences in pH, carbonate, soil structure, and earthworm activity between soils buried beneath the chalk rubble and the central turf stack). These have important implications for the preservation of biological and cultural remains, and for the maintenance of their stratigraphic integrity. © 1996 John Wiley & Sons, Inc.

Measurement and separation of very fine liquid and solidsparticles in the chemical industry

In the Chemical industry, the amounts of waste gases to be purified are relatively low, compared with for example power stations. However, the complexity of the different gases and aerosols is much greater. The Chemical industry therefore can not only rely on general concepts of purification, but must come up with a number of individual solutions. This means, that waste gas streams are not merely purified at a central waste gas stack, but also mostly at their very source, the individual production plant. This paper deals with some examples.

Computer-generated three-dimensional representation of dictyosomes reconstructed from electron micrographs of serial ultrathin sections

ABSTRACT Computer-generated three-dimensional reconstructions of higher plant dictyosomes from electron micrographs are presented from different viewing angles in the form of shaded perspective displays. The image processing has revealed complicated structures indicating a central cisternal stack surrounded by a cloud of vesicles and vesicular and tubular protrusions. The entire dictyosome is suggested as being an approximately spherical body that is marginally penetrated by plates and tubules of the endoplasmic reticulum.