Termination Scheme Research Articles

Disruption runaway electron generation and mitigation in the Spherical Tokamak for Energy Production (STEP)

Abstract Generation of Runaway Electrons (REs) during plasma disruptions is of great concern for ITER and future reactors based on the tokamak concept. The current STEP (Spherical Tokamak for Energy Production) concept design features a plasma current higher than 20 MA, thus expected to be in a regime of very high avalanche multiplication. This is confirmed by modelling runaway electron generation in unmitigated disruptions using the code DREAM, with hot-tail generation found to be the dominant primary generation mechanism and avalanche confirmed to be extremely high. Varying assumptions for the prescribed thermal quench phase (duration, final electron temperature) as well as the wall time, the plasma-wall distance, and shaping effects, all STEP unmitigated disruptions are found to generate large runaway electron beams (from 10 MA up to full conversion). The possibility of RE avoidance is first studied by idealized, i.e. radially uniform, impurity injection of a mixture of argon and deuterium, with ad-hoc particle transport arising from the stochasticity of the magnetic field during the thermal quench (TQ). Unfortunately, no such injection scenario allows runaways to be avoided while respecting the other constraints of disruption mitigation. STEP disruption mitigation system (DMS) has then been tested with DREAM, by modelling 2-stage Shattered Pellet Injections consisting of pure D2 followed by Ar+D2. Such a scheme is found to reduce the generation of REs by the hot-tail mechanism, reducing the final RE current to about 13 MA, but isn't sufficient by itself. Options for mitigating such a RE beam (benign termination scheme), as well as estimations of required RE losses during the current quench (from a potential passive RE mitigation coil) will also be briefly discussed.

Simulations of stand-off runaway electron beam termination by tungsten particulates for tokamak disruption mitigation

Stand-off runaway electron termination by injected tungsten particulates offers a plausible option in the toolbox of disruption mitigation. Tungsten is an attractive material choice for this application due to large electron stopping power and high melting point. To assess the feasibility of this scheme, we simulate runaway collisions with tungsten particulates using the MCNP program for incident runaway energies ranging from 1 to 10 MeV. We assess runaway termination from energetics and collisional kinematics perspectives. Energetically, the simulations show that 99% of runaway beam energy is removed by tungsten particulates on a timescale of 4–9 µs. Kinematically, the simulations show that 99% of runaways are terminated by absorption or backscattering on a timescale of 3–4 µs. By either metric, the runaway beam is effectively terminated before the onset of particulate melting. Furthermore, the simulations show that secondary radiation emission by tungsten particulates does not significantly impact the runaway termination efficacy of this scheme. Secondary radiation is emitted at lower particle energies than the incident runaways and with a broad angular distribution such that the majority of secondary electrons emitted will not experience efficient runaway re-acceleration. Overall, the stand-off runaway termination scheme is a promising concept as a last line of defense against runaway damage in ITER, SPARC, and other future burning-plasma tokamaks.

High-speed information reconciliation with syndrome-based early termination for continuous-variable quantum key distribution.

Information reconciliation (IR) is an indispensable component in the post-processing stage of continuous-variable quantum key distribution (CV-QKD), which adopts error-correcting codes to address the asymmetry of secret keys. Currently, low-density parity-check (LDPC) decoding in IR is a post-processing bottleneck in high-speed CV-QKD systems since the upper bound on secret key rate is higher than the information throughput delivered by decoder. In this paper, we study the relationship between the syndrome variation pattern (SVP) in iterative decoding and reconciliation frame error rate. An early termination scheme based on SVP is proposed and applied to multidimensional reconciliation, which can increase information throughput by adaptively adjusting the iteration number of iterative decoding to real-time decoding status. Furthermore, we show that only the resulting syndrome of the highest-rate code part in Raptor-like LDPC codes needs to be calculated to verify whether the reconciliation is successful by studying the convergency of resulting syndrome, which can save a large fraction of computational resources for syndrome calculation. Simulation results show that information throughput of the proposed scheme can be improved by 617.1% compared to the existing scheme when the IR efficiency reaches 97.09%. The proposed scheme points out a new direction for breaking the post-processing bottleneck in high-speed CV-QKD systems.

Double-Ended Superposition Anti-Noise Resistance Monitoring Write Termination Scheme for Reliable Write Operation in STT-MRAM

Although resistance monitoring write termination (RM-WT) scheme for STT-MRAM can reduce the write energy, the degradation of read margin due to low tunnel magnetoresistance ratio (TMR) and intrusion of noise with process variation still seriously deteriorates the stability of the WT operation. In this paper, a double-ended superposition anti-noise write termination (DSA-WT) scheme is proposed and implemented, in which the voltage changes on both BL and SL can be superimposed to boost sensing margin (SM). Schmitt trigger (ST) is adopted to take the place of the inverter (INV) in the traditional WT scheme, which is demonstrated to be helpful for stability improvement. Based on 65-nm CMOS technology, the proposed DSA-WT scheme shows 15% ~33% sensing margin boosting under various PVT conditions and 1000 times lower read bit-error-rate (BER) compared with the other WT schemes. The write done (WD) delay and the energy-delay-product (EDP) achieve 49.6% and 47.2% improvements compared to the state-of-art self-referenced single-ended RM-WT scheme (SS-RM-WT), respectively.

Early Termination Based Training Acceleration for an Energy-Efficient SNN Processor Design.

In this paper, we present a novel early termination based training acceleration technique for temporal coding based spiking neural network (SNN) processor design. The proposed early termination scheme can efficiently identify the non-contributing training images during the training's feedforward process, and it skips the rest of the processes to save training energy and time. A metric to evaluate each input image's contribution to training has been developed, and it is compared with pre-determined threshold to decide whether to skip the rest of the training process. For the threshold selection, an adaptive threshold calculation method is presented to increase the computation skip ratio without sacrificing accuracy. Timestep splitting approach is also employed to allow more frequent early termination in split timesteps, thus leading to more computation savings. The proposed early termination and timestep splitting techniques achieve 51.21/42.31/93.53/30.36% reduction of synaptic operations and 86.06/64.63/90.82/49.14% reduction of feedforward timestep for the training process on MNIST/Fashion-MNIST/ETH-80/EMNIST-Letters dataset, respectively. The hardware implementation of the proposed SNN processor using 28 nm CMOS process shows that the SNN processor achieves the training energy saving of 61.76/31.88% and computation cycle reduction of 69.10/36.26% on MNIST/Fashion-MNIST dataset, respectively.

Sliding Window Decoding for QC-SC-LDPC Codes Under the Constraint of Implementation Complexity

Sliding window decoding has been employed as the decoding scheme of quasi-cyclic spatially-coupled low-density parity-check (QC-SC-LDPC) codes, with the decoding latency and complexity independent of the coupling length but proportional to the window size. The window size was empirically chosen at least three times the constraint length in previous works, which results in the preference of QC-SC-LDPC codes with small constraint length, to reduce the decoding latency and complexity in practical systems. However, the code design freedom and the decoder throughput are limited for these QC-SC-LDPC codes. In this paper, the optimal window size under the constraint of practical decoding complexity is analyzed employing multi-edge-type density evolution (MET-DE). It can be verified from the MET-DE analytical result that the optimal window size could achieve less than twice the constraint length for several edge spreading patterns, which largely loose this restriction in practical code design. Furthermore, it could be observed that besides the constraint length, the specific structure of the edge spreading pattern could affect the optimal window size as well, and thus the MET-DE analytical method for the optimal window size could effectively guide the design of QC-SC-LDPC codes in practical systems. Then an improved sliding window decoding scheme is proposed, including the optimal choice of window size and a parity-check based early termination scheme. Finally, the performance and robustness of the proposed sliding window decoding scheme for QC-SC-LDPC codes are demonstrated via simulation, with the decoding complexity at most twice as that of the conventional LDPC coded schemes.

Disruption prediction at JET through deep convolutional neural networks using spatiotemporal information from plasma profiles

In view of the future high power nuclear fusion experiments, the early identification of disruptions is a mandatory requirement, and presently the main goal is moving from the disruption mitigation to disruption avoidance and control. In this work, a deep-convolutional neural network (CNN) is proposed to provide early detection of disruptive events at JET. The CNN ability to learn relevant features, avoiding hand-engineered feature extraction, has been exploited to extract the spatiotemporal information from 1D plasma profiles. The model is trained with regularly terminated discharges and automatically selected disruptive phase of disruptions, coming from the recent ITER-like-wall experiments. The prediction performance is evaluated using a set of discharges representative of different operating scenarios, and an in-depth analysis is made to evaluate the performance evolution with respect to the considered experimental conditions. Finally, as real-time triggers and termination schemes are being developed at JET, the proposed model has been tested on a set of recent experiments dedicated to plasma termination for disruption avoidance and mitigation. The CNN model demonstrates very high performance, and the exploitation of 1D plasma profiles as model input allows us to understand the underlying physical phenomena behind the predictor decision.

Efficient SOVA decoding and enhanced early termination mechanism based on new attenuation factors

This paper proposes two attenuation factors (AF’s) to improve the performance of the soft-output Viterbi algorithm (SOVA) as well as to enhance the early termination mechanism in turbo decoding. The mean square difference between the systematic bipolar coded symbols and the a-posteriori information is used to estimate the first AF. The second AF is computed online for each iteration based on the correlation coefficient between the extrinsic and a-priori information instead of intrinsic information as customary to calculate in literature. The second factor is used in the early termination (ET) scheme which is practically useful to terminate iterations when there is no significant improvement is achieved. In addition, a method for offline computing the AF’s that cover a specific range of signal-to-noise power ratio is provided which results in a reduction of utilization and latency estimates with a shallow degradation in performance. The results show that the proposed scheme outperforms the previous related works by about 0.2 dB at bit error rate (BER) of 10 -5 using interleaver depth of 512 and reducing the average number of iterations (ANI) by about 3 iterations.

Third-Order Many-Body Expansion of OSV-MP2 Wave Function for Low-Order Scaling Analytical Gradient Computation.

We present a many-body expansion (MBE) formulation and implementation for efficient computation of analytical energy gradients from the orbital-specific-virtual second-order Møllet-Plesset perturbation theory (OSV-MP2) based on our earlier work (Zhou et al. J. Chem. Theory Comput. 2020, 16, 196-210). The third-order MBE(3) expansion of OSV-MP2 amplitudes and density matrices was developed to adopt the orbital-specific clustering and long-range termination schemes, which avoids term-by-term differentiations of the MBE energy bodies. We achieve better efficiency by exploiting the algorithmic sparsity that allows us to prune out insignificant fitting integrals and OSV relaxations. With these approximations, the present implementation is benchmarked on a range of molecules that show an economic scaling in the linear and quadratic regimes for computing MBE(3)-OSV-MP2 amplitude and gradient equations, respectively, and yields normal accuracy comparable to the original OSV-MP2 results. The MPI-3-based parallelism through shared memory one-sided communication is further developed for improving parallel scalability and memory accessibility by sorting the MBE(3) orbital clusters into independent tasks that are distributed on multiple processes across many nodes, supporting both global and local data locations in which selected MBE(3)-OSV-MP2 intermediates of different sizes are distinguished and accordingly placed. The accuracy and efficiency level of our MBE(3)-OSV-MP2 analytical gradient implementation is finally illustrated in two applications: we show that the subtle coordination structure differences of mechanically interlocked Cu-catenane complexes can be distinguished when tuning ligand lengths; and the porphycene molecular dynamics reveals the emergence of the vibrational signature arising from softened N-H stretching associated with hydrogen transfer, using an MP2 level of electron correlation and classical nuclei for the first time.

Bitwise Early Termination of Multiuser Detection for IDMA Systems

This letter presents an early termination scheme for iterative multiuser detection in interleave division multiple access (IDMA) systems. Instead of handling an entire frame as a whole, each bit in the frame is separately considered. If further iterations are found to be dispensable in determining a bit, the estimate of the bit is immediately finalized, and all the calculations associated with the bit are bypassed afterward, diminishing the computational complexity. To confirm the dispensability, all log-likelihood ratios (LLRs) associated with a bit are examined. Once their magnitudes exceed a threshold, their statistical measures are unlikely to change even after a few more iterations, implying that further iterations are unnecessary. As a countermeasure for infrequent behaviors deviating from such an expectation, the signs of the LLRs in two consecutive iterations are compared. If they all match, the bitwise procedure is terminated. For the remaining bits not fulfilling the criterion, the procedure continues as usual. As a result, in the realm of practical signal-to-noise ratios, a significant number of iterations and operations can be discarded while sustaining the near-optimal error rate.

Design of PAMBE-based selective-area growth compliant ultra-low leakage GaN mixed-conduction vertical diodes for high-power applications * *This work was supported by the Office of Naval Research (ONR) under the ONR Award N00014-17-1-2681 (Program Manager: Lynn J Petersen).

Modern high-speed power applications require diodes with high-current capacity, larger reverse blocking voltage, lower reverse recovery transients, and lower leakage. GaN mixed-conduction vertical diodes are an attractive option for these operations. Experimentally reported GaN diodes have two major shortcomings, namely, higher leakage due to non-optimum design and conventional processing methods (e.g. ion implantation and dry etching), introducing huge leakage components. In this work, we design ultra-low leakage GaN buried p-base merged p–i–n Schottky and buried p-base merged p–i–n junction barrier controlled Schottky diodes, capable of reducing leakage current by more than five orders of magnitude by virtue of strong reduced surface field effect. Moreover, these designs are fully compliant with our earlier reported silicon nitride shadowed selective-area growth (SNS-SAG) methodology, capable of reducing the leakage by at least four orders of magnitude. In combination with SNS-SAG and highly efficient dielectric vertical sidewall edge termination scheme, these designs provide attractive options for higher performance.

Self-Referenced Single-Ended Resistance Monitoring Write Termination Scheme for STT-RAM Write Energy Reduction

Essential design requirements for a sense amplifier (SA) used in the resistance monitoring write termination (RM-WT) scheme are suggested to reduce the write energy of spin-transfer-torque random access memory (STT-RAM) while achieving a write pass yield comparable to that of a conventional write operation. In addition, a self-referenced single-ended RM-WT (SS-RM-WT) scheme is proposed. To reduce the offset voltage, a single-ended sensing circuit (SE-SC) is used in the SA. A data-aware input voltage-transfer method is also adopted in the SE-SC to maximize the input voltage difference. By adopting a capacitor between the output of the SE-SC and the input of an inverter generating a logical output used for the write termination, the conflict between maintaining and changing the output of the SE-SC is resolved. The simulation results using the industry-compatible 65-nm technology HSPICE model parameters show that the proposed SS-RM-WT scheme achieves a 44% write energy saving on average without increasing the write error rate. Area overhead is only 11.8% for a 256-kb STT-RAM array, whereas that of the previous self-referenced RM-WT schemes is up to 42.5%.

How long is forever in the laboratory? Three implementations of an infinite-horizon monetary economy

We compare three implementation schemes of an infinite-horizon monetary economy with discounting. Under the standard random termination scheme and its block variation, the economy lasts for an indefinite number of periods and the discounting factor is captured by the probability the economy continues to the next period. These schemes rely on the belief the experimenter can credibly implement a game that lasts an arbitrarily long time. We also propose a new method that does not rely on such belief. Under this scheme, subjects participate in an experiment for a fixed number of periods where the discount factor is captured by a weighting factor that shrinks payoffs over time. Dynamic incentives are preserved by paying subjects their continuation value based on past market prices. Results show dynamic incentives are preserved and behavior is similar in all three implementations. Researchers may decide among these approaches depending on the research question of interest and more practical concerns, such as the ease of implementation and the need to collect data for multiple supergames when the discount factor is high.

Design of selective-area growth compatible fully-vertical GaN p-i-n diodes with dielectric vertical sidewall appended edge termination schemes

While a slew of edge termination schemes for gallium nitride (GaN) power devices have been proposed and experimentally demonstrated to date, all of them suffer from the inability to achieve breakdown voltage close to ideal parallel-plane breakdown voltage. Further, they are exclusively processed using implantation or dry etching based methods, both of which are known to introduce additional defects and lattice damage leading to large leakage components. In this work, we develop and design novel dielectric vertical sidewall appended edge termination (DiVSET) schemes that are surface-charge resilient and capable of achieving ideal parallel-plane breakdown voltage. These edge termination schemes are compatible with plasma-assisted molecular-beam epitaxy facilitated silicon nitride shadowed selective-area growth (SNS-SAG) processing protocol, recently developed by us. The SNS-SAG protocol is uniquely capable of processing smooth, lattice damage-free GaN interfaces and vertical sidewalls that can reduce the leakage current by several orders of magnitude compared to conventional implant and dry etching based GaN processing. Together with the SNS-SAG processing, the DiVSET schemes offer an enabling technology for high-performance ultra-low leakage GaN power devices.

Towards the Prediction of SpaceWire Radiated Emissions Employing an LVDS Signal Emulator

The majority of the past and current ESA’s, NASA’s, and JAXA’s science missions utilize broadly SpaceWire (SpW) Links, with Low Voltage Differential Signaling (LVDS) for the physical layer, to transfer mission data. Electromagnetic Compatibility (EMC) and Electromagnetic Interference (EMI) problems caused by harnesses’ radiated emissions remain a constant open issue in every mission due to the scientific payload sensitivity. A common cause of radiated emissions from shielded cable bundles can be traced to the currents flowing on the shield of the harness. This work aims to present a complete methodology for predicting the Electric field emissions in the case of an operating SpW link, while it can be expanded to any shielded cable which utilizes LVDS. The physical and protocol layer of the link are implemented via a developed LVDS Signal emulator, handling both the characteristics of the signals at the outputs of the LVDS line drivers and the effects of their transmission across the link along with the termination scheme, in order to calculate the common mode current flowing on the shield. The radiated emissions can then be estimated considering the ground material and a detailed harness topology, employing traveling wave antenna theory. The validity of this approach is assessed by comparing the predictions with actual link radiated emissions measurements. Simulation results are in good agreement with the measurements, rendering this methodology a fast tactic for the pre-compliance procedure for the early design stage of the science space mission.

GaN Integrated Bridge Circuits on Bulk Silicon Substrate: Issues and Proposed Solution

A discrete GaN power transistor’s substrate is typically connected to its source electrode. However, on the GaN-on-Si power IC platform, the high-side transistor (HS-transistor) and low-side transistor (LS-transistor) share a common substrate that cannot be simultaneously connected to both source electrodes of the two transistors. Thus, the termination of the common substrate remains an undecided issue. In this work, comprehensive TCAD simulations are exploited to reveal the influences of various substrate termination schemes. It is found the common substrate inevitably leads to severe degradation in the dynamic ${R} _{\mathrm{ ON}}$ due to back-gating effects. The mechanisms for the degradations vary with the substrate termination scheme, and will be discussed in detail. To address these issues, we propose a new GaN power IC platform on an engineered bulk silicon substrate, and study the new platform with TCAD simulations. The proposed platform provides a local electrical substrate (a p+ island) for each GaN power transistor. The source electrode of each GaN transistor is connected to its local electrical substrate, while all devices share a common mechanical substrate. The junctions between the local substrates and the underlying n-layer provide an effective isolation between GaN transistors. The back-gating effects are completely suppressed for the GaN integrated bridge circuit.

DDR4 Data Channel Failure Due to DC Offset Caused by Intermittent Solder Ball Fracture in FBGA Package

This paper shows that an intermittent AC coupling defect occurring in a DDR4 data channel will cause more intermittent errors in DDR4, compared to such defect in DDR3. The intermittent AC coupling defect occurs due to intermittent fracture in DDR4 package solder ball. The defect causes DC offset in DDR4, which shifts the data signal or data eye and results in DDR4 data channel failure. The DC offset occurs due to the asymmetric nature of pseudo open drain termination scheme. DDR4 data channel response is compared with DDR3 channel. It is shown that pseudo random binary sequence (PRBS) pattern will always cause failure for DDR4, but PRBS will only cause failure in DDR3 if the sequence of consecutive 0’s or 1’s in PRBS pattern is long enough to cause threshold violation. As a result there will be more intermittent errors in DDR4 compared to DDR3. The defect due to fracture in solder ball is modelled by an AC coupling capacitor. A 1nF AC coupling capacitor corresponding to a solder ball fracture of height about 1nm is used to show the difference between DDR4 and DDR3 response.

Energy-Efficient Optimal Admission Control for Body Area Networks

This paper proposes an optimal Admission Control (AC) scheme for Body Area Networks (BANs). The scheme captures BAN characteristics, such as limited buffer size and power consumption of sensor nodes, and QoS-sensitive heterogeneous traffic types. Unable to admit a new flow, the Hub initially considers a novel mechanism of adjusting some of the already admitted (or new) flows' connection parameters to free up enough bandwidth. Adjustment does not violate a flow's QoS requirements but comes at the cost of extra energy consumption. An efficient scheme is proposed that traverses viable adjustment options until the least costly is found. When adjustment does not suffice, a combination of adjusting a set of flows and terminating some existing ones is considered. For the flow termination scheme, a bandwidth-efficient algorithm is proposed that terminates the least number of flows that have lower priory than the one requesting to join. The optimality of the proposed AC scheme for BANs is also studied. Compared to conventional schemes, our results show on average an increase of up to 33% in the number -and considerable improvement in terms of the priority- of admitted flows, at a diminutive cost of less than 1.5% on the nodes' energy consumption.

A 96-MB 3D-Stacked SRAM Using Inductive Coupling With 0.4-V Transmitter, Termination Scheme and 12:1 SerDes in 40-nm CMOS

A 28.8-GB/s 96-MB 3D-stacked SRAM is presented. A total of eight SRAM dies, designed in a 40-nm CMOS process, are vertically stacked and connected using an inductive coupling wireless link with a low-voltage NMOS push-pull transmitter that reduces the power of the link by 35% with a 0.4-V power supply. The SRAM utilizes an inverted bit insertion scheme that compensates for the degradation of the first transmitted bit, a coil termination scheme that aims to eliminate the ringing of 3D inductive coupling bus, and a 12:1 SerDes that minimizes power consumption and area overhead in inductive coupling channels. Low-power, large-capacity, 3-cycle latency 3D-stacked SRAM for a DNN accelerator is achieved with the combination of these techniques to serve as a replacement of 3D-stacked DRAM. The performance of the proposed 3D-SRAM is compared with HBM DRAM and achieves more than 50% lower energy consumption. The scaling scenario of the SRAM module is discussed in light of the scaling of the inductive coupling technology and logic process.

The Trigger and Termination Scheme for the Event Mode of the Lyman-alpha Solar Telescope (LST) onboard the ASO-S Mission

The Advanced Space-based Solar Observatory (ASO-S) is the China's first comprehensive solar dedicated satellite, scheduled to be launched around 2022. The Lyman-alpha (Lyα) Solar Telescope (LST) is one of the payloads of the ASO-S, consisting of three scientific instruments and two Guide Telescopes (GTs). The scientific instruments include a Solar Disk Imager (SDI), a White-light Solar Telescope (WST), and a Solar Corona Imager (SCI), which are aimed to observe the whole evolving process of two types of violent eruptions on the Sun, i.e., solar flares and coronal mass ejections, in multiwavelengths with high temporal and spatial resolutions. To achieve this goal, all LST instruments have included an observation mode (event mode) dedicated to event observations. In this mode, all instruments take images at a higher cadence (the SCI takes full-frame coronal images while SDI and WST take only partial frame images around the eruption region). The time and location information of the eruption events can be effectively obtained by monitoring the local brightness enhancement of the onboard images processed with median filtering and pixel rebinning. The obtained information will provide an important electronic input for the automatic switching between different observation modes of LST.