Search

Coronal mass ejections (CMEs) are some of the most energetic and violent events in our solar system. The prediction and understanding of CMEs is of particular importance due to the impact that they can have on Earth-based satellite systems, and in extreme cases, ground-based electronics. CMEs often occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal magnetohydrodynamic (MHD) instability such as the kink or torus instability. Previous experiments on the Magnetic Reconnection eXperiment (MRX) revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These "failed-tori" went through a process similar to Taylor relaxation where the toroidal current was redistributed before the eruption ultimately failed. We have investigated this behavior through additional diagnostics that measure the current distribution at the foot points and the energy distribution before and after an event. These measurements indicate that ideal MHD effects are sufficient to explain the energy distribution changes during failed torus events. This excludes Taylor relaxation as a possible mechanism of current redistribution during an event. A new model that only requires non-ideal effects in a thin layer above the electrodes is presented to explain the observed phenomena. This work broadens our understanding of the stability of MFRs and the mechanism behind the failed torus through the improved prediction of the torus instability and through new diagnostics to measure the energy inventory and current profile at the foot points., Comment: 20 pages, 18 figures. Submitted to Physics of Plasmas

We present methods for analyzing Beam Emission Spectroscopy (BES) data to obtain the plasma density evolution associated with rapid sawtooth crash events at the DIII-D tokamak. BES allows coverage over a 2D spatial plane, inherently local measurements, with fast time responses, and, therefore, provides a valuable new channel for data during sawtooth events. A method is developed to remove sawtooth-induced edge-light pulses contained in the BES data. The edge light pulses appear to be from the D α emission produced by edge recycling during sawtooth events, and are large enough that traditional spectroscopic filtering and data analysis techniques are insufficient to deduce physically meaningful quantities. A cross-calibration of 64 BES channels is performed by using a novel method to ensure accurate measurements. For the large-amplitude density oscillations observed, we discuss and use the non-linear relationship between the BES signal δI/ I0 and the plasma density variation δn e/ n e0. The 2D BES images cover an 8 × 20 cm2 region around the sawtooth inversion layer and show large-amplitude density oscillations, with additional significant spatial variations across the inversion layer that grows and peaks near the time of the temperature crash. The edge light removal technique and method of converting large-amplitude δI/ I0 to δn e/ n e0 presented here may help analyze other impulsive MHD phenomena in tokamaks.

Coronal mass ejections (CME) occur when long-lived magnetic flux ropes (MFR) anchored to the solar surface destabilize and erupt away from the Sun. This destabilization is often described in terms of an ideal magnetohydrodynamic (MHD) instability called the torus instability. It occurs when the external magnetic field decreases sufficiently fast such that its decay index, $n=-z\,\partial\,(\ln B)/\partial z$ is larger than a critical value, $n>n_{\rm cr}$, where $n_{\rm cr}=1.5$ for a full, large aspect ratio torus. However, when this is applied to solar MFRs, a range of conflicting values for $n_{\rm cr}$ is found in the literature. To investigate this discrepancy, we have conducted laboratory experiments on arched, line-tied flux ropes and have applied a theoretical model of the torus instability. Our model describes an MFR as a partial torus with footpoints anchored in a conducting surface and numerically calculates various magnetic forces on it. This calculation yields a better prediction of $n_{\rm cr}$ which takes into account the specific parameters of the MFR. We describe a systematic methodology to properly translate laboratory results to their solar counterparts, provided that the MFRs have sufficiently small edge safety factor, or equivalently, large enough twist. After this translation, our model predicts that $n_{\rm cr}$ in solar conditions often falls near $n_{\rm cr}^{\rm Sol}\sim0.9$ and within a larger range of $n_{\rm cr}^{\rm Sol}\sim(0.7,1.2)$ depending on the parameters. The methodology of translating laboratory MFRs to their solar counterparts enables quantitative investigations of the initiation of CMEs through laboratory experiments. These experiments allow for new physics insights that are required for better predictions of space weather events but are difficult to obtain otherwise., Submitted to the Astrophysical Journal

Dust vortices with a void at the centre are reported in this paper. The role of the spatial variation of the plasma potential in the rotation of dust particles is studied in a parallel plate glow discharge plasma. Probe measurements reveal the existence of a local potential minimum in the region of formation of the dust vortex. The minimum in the potential well attracts positively charged ions, while it repels the negatively charged dust particles. Dust rotation is caused by the interplay of the two oppositely directed ion drag and Coulomb forces. The balance between these two forces is found to play a major role in the radial confinement of the dust particles above the cathode surface. Evolution of the dust vortex is studied by increasing the discharge current from 15 to 20 mA. The local minimum of the potential profile is found to coincide with the location of the dust vortex for both values of discharge currents. Additionally, it is found that the size of the dust vortex as well as the void at the centre increases with the discharge current.

We have explored the effectiveness of a longitudinal gradient in Alfv\'en speed in reducing the energy of propagating Alfv\'en waves under conditions scaled to match solar coronal holes. The experiments were conducted in the Large Plasma Device at the University of California, Los Angeles. Our results show that the energy of the transmitted Alfv\'en wave decreases as the inhomogeneity parameter, $\lambda/L_{\rm A}$, increases. Here, $\lambda$ is the wavelength of the Alfv\'en wave and $L_{\rm A}$ is the scale length of Alfv\'en speed gradient. For gradients similar to those in coronal holes, the waves are observed to lose a factor of $\approx 5$ more energy than they do when propagating through a uniform plasma without a gradient. We have carried out further experiments and analyses to constrain the cause of wave energy reduction in the gradient. The loss of Alfv\'en wave energy from mode coupling is unlikely, as we have not detected any other modes. Contrary to theoretical expectations, the reduction in the energy of the transmitted wave is not accompanied by a detectable reflected wave. Nonlinear effects are ruled out as the amplitude of the initial wave is too small and the wave frequency well below the ion cyclotron frequency. Since the total energy must be conserved, it is possible that the lost wave energy is being deposited in the plasma. Further studies are needed to explore where the energy is going.

Measurements of local plasma parameters in dusty plasma are crucial for understanding the physics issues related to such systems. The Langmuir probe, a small electrode immersed in the plasma, provides such measurements. However, designing of a Langmuir probe system in a dusty plasma environment demands special consideration. First, the probe has to be miniaturized enough so that its perturbation on the ambient dust structure is minimal. At the same time, the probe dimensions must be such that a well-defined theory exists for interpretation of its characteristics. The associated instrumentation must also support the measurement of current collected by the probe with high signal to noise ratio. The most important consideration, of course, comes from the fact that the probes are prone to dust contamination, as the dust particles tend to stick to the probe surface and alter the current collecting area in unpredictable ways. This article describes the design and operation of a Langmuir probe system that resolves these challenging issues in dusty plasma. In doing so, first, different theories that are used to interpret the probe characteristics in collisionless as well as in collisional regimes are discussed, with special emphasis on application. The critical issues associated with the current–voltage characteristics of Langmuir probe obtained in different operating regimes are discussed. Then, an algorithm for processing these characteristics efficiently in presence of ion-neutral collisions in the probe sheath is presented.

In this paper, a linear trellis based precoding technique is proposed for maximizing the multiuser MIMO capacity in a multiple access channel with inter symbol interference (ISI). We use the trellis exploration algorithm to design a precoding matrix that minimizes the interference from other users by allocating power in the appropriate eigenmodes. Employing a finite number of phases, the precoder matrix for each user is first custom designed at the receiver. Then a bit sequence indicating the phase indices of the optimized precoding matrix elements for the corresponding user is fed back to the transmitter. We show that under this approach, the sum rate capacity achieved is comparable to the optimal sum-rate capacity employing the well-known water filling solution with complete channel knowledge at the transmitter for spatio-temporal vector coding (STVC). Our simulations for various multiuser MIMO cases, show that the per user precoding with limited feedback and equal power allocation strategy achieves desirable capacity gains relative to the eigenbeamforming and Grassmannian precoding.

Several applications within a shipboard system involve the integration of physical systems (e.g., power systems) and cyber computing platforms such as command and control, communications, and sensing networks, and require real-time quality of service (QoS) guarantees. In this paper, the distribution of total (end-to-end) delay associated with fault diagnosis and reconfiguration of shipboard power system (SPS) is investigated from a cyber-physical systems (CPS) perspective. Specifically, a cross-layer end-to-end delay analysis framework is introduced for SPS reconfiguration. The proposed framework stochastically models the heterogeneity of actions of various subsystems involved in the reconfiguration tasks viz., generation of fault information by sensor nodes associated to the power system, processing of actions at control center to resolve fault locations and reconfiguration, and flow of information through communication network to perform necessary actions. The proposed framework then combines appropriately the output delay distributions from each subsystem to: 1) analytically predict the distribution of end-to-end delay in SPS reconfiguration after the occurrence of faults and 2) analyze and design real-time reconfiguration solutions for shipboard CPS, that meet total delay requirements. Simulations using various topological scenarios demonstrate that the proposed analytical framework closely predict the total delay associated with SPS reconfiguration.

A novel approach for the automatic reconfiguration of shipboard power systems (SPS) based on Q-learning has been investigated. Using this approach it is possible to obtain an optimal set of switches to open/close, in order to restore power to the loads, such that the weighted sum of the power delivered to the loads is maximized. This approach differs significantly from other methods previously studied for reconfiguration as it is a dynamic technique that produces not only the final reconfiguration, but also the correct order in which the switches are to be changed. Simulation results clearly demonstrate the effectiveness of this method.

A typical device for carrying out sophisticated and complex dusty plasma experiments is designed, fabricated and made operational at the Institute for Plasma Research, India. The device is named as complex plasma experimental device (CPED). The main aim of this multipurpose machine is to study the formation and behaviour of dust vortices in the absence of external magnetic field under the effect of various plasma parameters. Further, the device is equipped with advanced imaging diagnostics for studying many other interesting phenomena such as dust oscillations, three-dimensional crystalline structures, dust rotation, etc. The device is quite flexible to accommodate many innovative experiments. Detailed design of the device, its diagnostics capabilities and the advanced image analysis techniques are presented in this paper.

In power system reconfiguration, the status (ON/OFF) of switches are optimized such that maximum power is delivered to loads after the occurrence of a fault. The optimized reconfiguration is achieved by prioritizing power delivered to vital loads over semi-vital and nonvital loads. The formulation presented in this paper considers a new balanced hybrid (AC and DC) shipboard power system (SPS). Analysis of the nonconvex reconfiguration formulation is done by an appropriate nonconvex solver and by convex approximation. Unlike the nonconvex solution that is based on branch-and-bound methods, convex approximation significantly reduces complexity. It is shown that for the hybrid SPS reconfiguration problem, low complexity convex approximations are effective in finding optimal solutions. Cumulative distribution function (CDF) of the power delivered to loads is presented to showcase the system robustness against random fault scenarios. A combined objective of maximizing power delivery and minimizing the number of switching actions is included in the analysis. Tradeoff between power delivered and number of switching operations after reconfiguration has been discussed at steady state. A separate analysis is also included to observe the intermediate dynamic switch states while the reconfiguration is in progress to capture the trade-off more prominently.

Magnetic pickup loops or B-dot probes are one of the oldest known sensors of time-varying magnetic fields. The operating principle is based on Faraday's law of electromagnetic induction. However, obtaining accurate measurements of time-varying magnetic fields using these kinds of probes is a challenging task. A B-dot probe and its associated circuit are prone to electrical oscillations. As a result, the measured signal may not faithfully represent the magnetic field sampled by the B-dot probe. In this paper, we have studied the transient response of a B-dot probe and its associated circuit to a time-varying magnetic field. Methods of removing the oscillations pertaining to the detector structure are described. After removing the source of the oscillatory signal, we have shown that the time-integrated induced emf measured by the digitiser is linearly proportional to the magnetic field sampled by the B-dot probe, thus verifying the faithfulness of the measured signal.

Experimentally, the density profile in the magnetic nozzle of a helicon antenna based plasma device is seen to be modified from being centrally peaked to that of hollow nature as the external magnetic field is increased. It occurs above a characteristic field value when the ions become magnetized in the expansion chamber. The density profile in the source chamber behind the nozzle, however, remains peaked on-axis irrespective of the magnetic field. The electron temperature there is observed to be hollow and this nature is carried to the expansion chamber along the field line. In the electron energy distribution near the off axis peak location, a high energy tail exists. Rotation of these tail electrons in the azimuthal direction due to the gradient-B drift in the expansion chamber leads to an additional off-axis ionization and forms the hollow density profile. It seems that if the ions are not magnetized, then the off-axially produced additional plasma is not confined and the density profile retains the on-axis peak nature. The present experiment successfully demonstrates how the knowledge of the ion magnetization together with tail electrons significantly contributes to the design of an efficient helicon plasma based thruster.

Background Intracystic bleeding is an infrequent but potentially lethal complication of pancreatic pseudocyst. Early preoperative diagnosis is an important aspect of effective management. Case outlines Three patients presented with bleeding into a pancreatic pseudocyst. One was managed by intracystic ligation and external drainage, one was managed with postoperative angiographic embolisation and the third patient underwent angiographic embolisation as definitive treatment. Discussion Massive bleeding from a pseudoaneurysm associated with pseudocyst of the pancreas carries a high mortality and morbidity rate. A high index of suspicion for patients with suggestive clinical signs and symptoms plus careful evaluation on a thin‐sliced contrast‐enhanced CT scan with early and delayed films corroborated by selective angiography leads to better management.

In a quasineutral plasma, electrons undergo collective oscillations, known as plasma oscillations, when perturbed locally. The oscillations propagate due to finite temperature effects. However, the wave can lose the phase coherence between constituting oscillators in an inhomogeneous plasma (phase mixing) because of the dependence of plasma oscillation frequency on plasma density. The longitudinal electric field associated with the wave may be used to accelerate electrons to high energies by exciting large amplitude wave. However when the maximum amplitude of the wave is reached that plasma can sustain, the wave breaks. The phenomena of wave breaking and phase mixing have applications in plasma heating and particle acceleration. For detailed experimental investigation of these phenomena a new device, inverse mirror plasma experimental device (IMPED), has been designed and fabricated. The detailed considerations taken before designing the device, so that different aspects of these phenomena can be studied in a controlled manner, are described. Specifications of different components of the IMPED machine and their flexibility aspects in upgrading, if necessary, are discussed. Initial results meeting the prerequisite condition of the plasma for such study, such as a quiescent, collisionless and uniform plasma, are presented. The machine produces δnnoise/n⩽ 1%,Luniform~ 120 cm at argon filling pressure of ~10−4mbar and axial magnetic field ofB= 1090 G.

This paper presents a design strategy for base station (BS) coordinated multi cell joint beamforming. Our objective is to maximize the average cell throughput while maintaining quality of service (QoS) for individual users. The QoS in our analysis relates to minimum signal to interference plus noise ratio (SINR) achieved by every user. Since, throughput maximization is a non convex problem, an optimal solution can not be obtained using polynomial order algorithms. However, it can be observed that existence of inter cell interference (ICI) terms are the cause of non convexity. With this knowledge, we propose two decentralized approaches based on primal and dual decomposition of the throughput maximization problem. As ICI terms are the shared variables between the base stations (BSs), these terms are exchanged between the neighboring BSs and kept fixed during an iteration. This fixes the non convex (ICI) terms, resulting in convex subproblems. We demonstrate the quality of our sub optimal decentralized solution by simulations and comparing it with optimal exhaustive search algorithms (Genie-aided algorithm). A comparison analysis is also presented for both primal and dual solutions.

Shipboard smartgrid (SSG) requires optimized power delivery under faults. This is essentially achieved by reconfiguring the switch states (ON/OFF) in a manner that ensures maximal power to be delivered to loads after the occurrence of one or more faults. This paper seeks to investigate (1) the quality of the SSG reconfiguration solution without complete knowledge of the overall system state, and (2) Communication costs that may affect the quality of the reconfiguration. To this end, a dual decomposition based distributed optimization method for shipboard smartgrid system is proposed. The shipboard system is decomposed into multiple separable subsystems with agents. The agents locally maximize the power delivery to loads for a subsystem. Specifically, each agent solves a concave dual function of the original objective subject to convex constraints. Neighboring agent subsystems interact through convex coupling constraints and fuse the local optimizations into a globally optimal state. The reconfiguration results of the proposed implementation compared to the conventional centralized methods of reconfiguration demonstrate its effectiveness. We further investigate the convergence of the proposed approach under varying network delays and quantization noise. A theoretical lower bound on convergence due to the impact of communication network is presented.

Femtocells are perceived as having a strong potential to provide high data-rate services with increased coverage at low cost. In this paper, we consider a WiMAX network with both macro base stations (BS) and low-power, low-cost Femto Access Points (APs). We assume that Femto-APs share the same bandwidth as macro BS and Femto-APs are connected to the macro BS by wired backhaul such as cable or DSL. One of the important technical challenges in this framework is to mitigate the interference from macro users by avoiding their spectrum. Robust and dynamic spectrum sensing for channel occupancy by macro users is therefore necessary to schedule resources for femto users. An efficient cyclostationary-based spectrum sensing method is presented to facilitate cognitive radio (CR) aided resource allocation in femtocell networks for maximizing system throughput while minimizing inter-tier interference. Contrary to the classical energy detection (ED) approach, a cyclostationary-based spectrum sensing is more effective in dealing with imperfect knowledge of in-band noise variance in practical radio signals. Also, this method is more efficient than ED in low signal-to-noise ratio which is a very important requirement in WiMAX based macrocell-femtocell overlay network.

In this paper, a linear trellis based precoder technique is proposed for maximizing multiuser MIMO capacity. We consider MIMO multiple access channel with inter symbol interference (ISI). We use the trellis exploration algorithm to form a precoding matrix that minimizes the interference from other users by allocating power in the appropriate eigen modes for each user. Employing a finite number of phases, the precoder matrix for each user is first custom designed at the receiver. Then a bit sequence indicating the phase indices of the optimized precoding matrix for the corresponding user is fed back to the transmitter. We show that under limited feedback, the sum rate capacity achieved is comparable to the optimal sum-rate capacity employing the well-known water filling solution with complete channel knowledge at the transmitter for spatio-temporal vector coding (STVC). Our simulations for various multiuser MIMO cases, show that the per user precoding strategy achieves desirable performance gain with limited feedback and equal user power allocation in terms of ergodic capacity.

In an all electric ship, the intelligent and integrated power system (IPS) that allows for reconfiguration offers better fight-through and survivability capabilities compared to manual service restoration. Automated reconfiguration involves optimizing the status of switches (ON/OFF) such that power delivered to loads is maximized after the occurrence of a fault. The priority is given to vital loads over semi-vital and non-vital loads while maximizing the power delivered. The novelty of the formulation presented in this paper is the consideration of non-radial power flow in a balanced hybrid (AC & DC) shipboard power system (SPS). Analysis of the original mixed-integer non-convex reconfiguration optimization is done by formulating appropriate low complexity convex form to reduce computation time. Comparative studies of cumulative distribution frequency (CDF) of the power delivered to loads for non-radial and radial configurations are presented to demonstrate the robustness of non-radial topology in case of multiple simultaneous faults.

Reconfiguration involves optimizing the status (ON/OFF) of switches such that power delivered to loads is maximized after the occurrence of a fault. The optimized reconfiguration prioritizes power delivered to vital loads over semi-vital and non-vital loads. The formulation presented in this paper is an extension of the work in to new balanced hybrid (ac & dc) shipboard power systems (SPS). Analysis of the non-convex reconfiguration formulation is done by appropriate non-convex solver and by convex approximation. The non-convex solution provides bounds on the optimal performance of the problem while the convex approximation reduces the complexity. Number of faults against the power delivered to loads is presented to showcase the system robustness against random fault scenarios.

In this paper we deal with shipboard power system (SPS) reconfiguration with an integrated power system (IPS) and distributed generator. The objective of reconfiguration is to determine the status of the switches such that power is delivered to the vital loads even under fault conditions. We have used a model-free reinforcement technique called Q-learning for solving the reconfiguration problem. We don't only get the final configuration but also the sequence of switches to open and close to reach the final state. To the best of the author's knowledge this is the first time reinforcement learning is being applied to shipboard power system reconfiguration.

A reduced-complexity decision-directed timing recovery method for continuous phase modulation (CPM) is presented. Using the well-known and popular pulse amplitude modulation (PAM) representation-or Laurent representation- of CPM, we develop two different formulations of a PAM-based timing error detector (TED). We consider the general M-ary multi-h CPM model in our development and use the family of aeronautical telemetry CPMs for our numerical examples. We show by analysis that the two TED formulations have identical ldquoS-curverdquo characteristics, and are free of false lock points, but have slightly different performance when applied to specific CPM examples, particularly in the multi-h case. There is no unanimous winner between the two formulations in all cases; however, for both formulations, we show the TEDs are able to perform close to the theoretical limit given by the modified Cramer-Rao bound. As such, the proposed TEDs provide an important synchronization component for reduced-complexity PAM-based CPM receivers that has heretofore been missing.

Observation of two well-separated dust vortices in an unmagnetized parallel plate DC glow discharge plasma is reported in this paper. A non-monotonic radial density profile, achieved by an especially designed cathode structure using a concentric metallic disk and ring of different radii, is observed to produce double dust tori between cathode and anode. PIV analysis of the still images of the double tori shows oppositely rotating dust structures between the central disk and the ring. Langmuir probe measurements of background plasma shows a non-uniform plasma density profile between the disk and the ring. Location and sense of rotation of the dust vortices coincides with the location and direction of the radial gradient in the ion drag force caused by the radial density gradient. The experimentally observed dust vorticity matches well with the calculated one using hydrodynamic formulations with shear in ion drag dominating over the dust charge gradient. These results corroborate that a radial gradient in the ion drag force directed towards cathode is the principal cause of dust rotation.

Inverse mirror plasma experimental device has been designed and fabricated for detailed experimental investigation of phase mixing and wave breaking of plasma oscillation/wave. The device produces quiescent magnetized plasma over a wide operating range using multifilamentary source with low filament spacing in cusp geometry along with a flexible transition magnetic field region between the plasma source chamber and the main chamber. Argon plasma has been produced in the device over a wide pressure range from 1.7 × 10(-5) mbar to 9 × 10(-4) mbar, achieving plasma densities in the range of ∼10(9) cm(-3)-10(12) cm(-3) and temperatures in the range of ∼1.7 eV-5 eV. To fulfill a desired prerequisite of having quiescent plasma (δn/n ≤ 1%) for realizing phase mixing of nonlinear plasma oscillation and other wave experiments, a quiescent magnetized plasma is obtained: typical quiescence, δn/n ∼ 0.5% at 10(-4) mbar and B(main) ∼ 1 kG. The potential of the multifilamentary plasma source has been experimentally explored using a flexible transition magnetic field and the usual control features of a filament discharge. Probe measurements reveal that the plasma to be axially and radially uniform, an excellent scenario for wave launching and studying its propagating and phase mixing characteristics.

Observation of dust cloud rotation in parallel-plate DC glow discharge plasma is reported here. The experiments are carried out at high pressures (∼130 Pa) with a metallic ring placed on the lower electrode (cathode). The dust cloud rotates poloidally in the vertical plane near the cathode surface. This structure is continuous toroidally. Absence of magnetic field rules out the possibility of E × B induced ion flow as the cause of dust rotation. The dust rotational structures exist even with water cooled cathode. Therefore, temperature gradient driven mechanisms, such as thermophoretic force, thermal creep flow, and free convection cannot be causing the observed dust rotation. Langmuir probe measurement reveals the existence of a sharp density gradient near the location of the rotating dust cloud. The gradient in the density, giving rise to a gradient in the ion drag force, has been identified as the principal cause behind the rotation of dust particles.

This chip decodes MPEG-1 audio and video in real-time when connected to a single 80 ns 256 k/spl times/16 DRAM. An MPEG-1 system stream, optionally embedded in a CD data stream, is sent to the chip on either an 8 b host bus or a serial bus. The host interface contains a code FIFO that buffers input bit streams before they are written to the audio, video or overlay bitstream buffers in DRAM. The MPEG system stream is processed by interrupting the on-chip CPU after a packet of compressed data has been written to DRAM. The CPU reads the system stream headers out of the code FIFO and initiates a block transfer of the next packet of compressed data to DRAM. The chip uses less than 5% of the clock cycles for system stream processing. The chip alternates between audio decoding and video decoding, with the audio portion using 15% of the clock cycles and video using 80%.