Strike Position Research Articles

Plasma modelling results and shape control improvements for NSTX

New shape control implementations and dynamics studies on the National Spherical Torus eXperiment (NSTX) (Ono et al 2000 Nucl. Fusion 40 557–61) are summarized. In particular, strike point position, X-point height and squareness control, and two new system-identification methods/control-tuning algorithms were put into operation. The PID controller for the strike point was tuned by analysing the step response of the strike point position to the poloidal coil currents, employing the Ziegler–Nichols method. An offline system identification of the plasma response to the control inputs based on ARMAX (Ljung 1999 System Identification: Theory for the User (Englewood Cliffs, NJ: Prentice-Hall)) input–output models was implemented. With this tool, rough estimates of the improvements were realized and several control improvements were identified. An online automatic relay-feedback PID tuning algorithm, which has the advantage of tuning the controller in one shot, was implemented, thus optimizing the use of experimental time. Using these new capabilities, all four upper/lower/inner/outer strike points were simultaneous controlled and a combined X-point height, strike point radius control was implemented. The new and improved control with better accuracy and robustness enabled successful plasma operations with the liquid lithium divertor. Additionally this year, the first independent squareness control was developed. This will enable better optimization of the NSTX shape for stability and high performance in the future.

Training Surrogate Sensors in Musical Gesture Acquisition Systems

Capturing the gestures of music performers is a common task in interactive electroacoustic music. The captured gestures can be mapped to sounds, synthesis algorithms, visuals, etc., or used for music transcription. Two of the most common approaches for acquiring musical gestures are: 1) “hyper-instruments” which are “traditional” musical instruments enhanced with sensors for directly detecting the gestures and 2) “indirect acquisition” in which the only sensor is a microphone capturing the audio signal. Hyper-instruments require invasive modification of existing instruments which is frequently undesirable. However, they provide relatively straightforward and reliable sensor measurements. On the other hand, indirect acquisition approaches typically require sophisticated signal processing and possibly machine learning algorithms in order to extract the relevant information from the audio signal. The idea of using direct sensor(s) to train a machine learning model for indirect acquisition is proposed in this paper. The resulting trained “surrogate” sensor can then be used in place of the original direct invasive sensor(s) that were used for training. That way, the instrument can be used unmodified in performance while still providing the gesture information that a hyper-instrument would provide. In addition, using this approach, large amounts of training data can be collected with minimum effort. Experimental results supporting this idea are provided in two detection contexts: 1) strike position on a drum surface and 2) strum direction on a sitar.

Strike point control for the National Spherical Torus Experiment (NSTX)

This paper presents the first control algorithm for the inner- and outer-strike point position for a Spherical Torus (ST) fusion experiment and the performance analysis of the controller. A liquid lithium divertor (LLD) will be installed on NSTX which is believed to provide better pumping than lithium coatings on carbon PFCs. The shape of the plasma dictates the pumping rate of the lithium by channelling the plasma to LLD, where the strike point location is the most important shape parameter. Simulations show that the density reduction depends on the proximity of the strike point to LLD. Experiments were performed to study the dynamics of the strike point, design a new controller to change the location of the strike point to the desired location and stabilize it. The most effective poloidal field (PF) coils in changing inner- and outer-strike points were identified using equilibrium code. The PF coil inputs were changed in a step fashion between various set points and the step response of the strike point position was obtained. From the analysis of the step responses, proportional–integral–derivative controllers for the strike points were obtained and the controller was tuned experimentally for better performance. The strike controller was extended to include the outer-strike point on the inner plate to accommodate the desired low outer-strike points for the experiment with the aim of achieving ‘snowflake’ divertor configuration in NSTX.

Unicompartmental Knee Resurfacing: Enlarged Tibio-Femoral Contact Area and Reduced Contact Stress Using Novel Patient-Derived Geometries

Advances in imaging technology and computer-assisted design (CAD) have recently enabled the introduction of patient-specific knee implant designs that hold the potential to improve functional performance on the basis of patient-specific geometries, namely a patient-specific sagittal and coronal curvature, as well as enhanced bone preservation. The objective of this study was to investigate the use of a novel implant design utilizing a patient specific sagittal J-curve on the femoral component combined with a novel constant, patient-derived femoral coronal curvature and to assess tibio-femoral contact area and contact stress on a femur matched curved tibial polyethylene insert. Mean contact area and standard deviations were 81±5, 96±5 and 74±4 mm2 for the heel strike, toe off and mid-stance positions, respectively. Mean contact stress and standard deviations were 23.83±1.39, 23.27±1.14 and 20.78±0.54 MPa for the heel strike, toe off and mid-stance positions, respectively. Standard deviations of the measurements were small, not exceeding 6-7% confirming the consistency of loading conditions across different flexion angles. The results were comparable to those reported for standard, off-the-shelf fixed-bearing implants with paired femoral and tibial geometries. These data show that a constant coronal curvature can be applied to a patient-specific implant by measuring coronal curvatures across the femoral condyle in each patient and by deriving an average curvature. This novel approach combines unique benefits of patient-specific geometry with proven design concepts for minimizing polyethylene wear.

Effect of Post-traumatic Tibiotalar Osteoarthritis on Kinematics of the Ankle Joint Complex

Knowledge of joint kinematics in the healthy and diseased joint may be useful if surgical techniques and joint replacement designs are to be improved. To date, little is known about the kinematics of the arthritic tibiotalar joint and its effect on the kinematics of the subtalar joint. Kinematics of the ankle joint complex (AJC) were measured in six patients with unilateral post-traumatic tibiotalar osteoarthritis in simulated heel strike, midstance and toe off weight bearing positions using magnetic resonance and dual fluoroscopic imaging techniques. The kinematic data obtained was compared to a normal cohort from a previous study. From heel strike to midstance, the arthritic tibiotalar joint demonstrated 2.2 +/- 5.0 degrees of dorsiflexion while in the healthy controls the tibiotalar joint plantarflexed 9.1 +/- 5.3 degrees (p < 0.01). From midstance to toe off, the subtalar joint in the arthritic group dorsiflexed 3.3 +/- 4.1 degrees whereas in the control group the subtalar joint plantarflexed 8.5 +/- 2.9 degrees (p < 0.01). The subtalar joint in the arthritic group rotated externally 1.2 +/- 1.0 degrees and everted 3.3 +/- 6.1 degrees from midstance to toe off while in the control group 12.3 +/- 8.3 degrees of internal rotation and 10.7 +/- 3.8 degrees eversion (p < 0.01 and p < 0.01, respectively) was measured. The current study suggests that during the stance phase of gait, subtalar joint motion in the sagittal, coronal, and transverse rotational planes tends to occur in an opposite direction in subjects with tibiotalar osteoarthritis when compared to normal ankle controls. This effectively represents a breakdown in the normal motion coupling seen in healthy ankle joints. Knowledge of ankle kinematics of arthritic joints may be helpful when designing prostheses or in assessing the results of treatment interventions.

Erosion and deposition behaviour of a-C:H layers in the private flux region of the JET MKII-HD divertor

Material deposition was measured in the private flux region of JET by means of a quartz microbalance (QMB) for ∼11 900 s of successive plasma discharges with moderate additional heating (2–3 MW). The QMB was located under the load bearing septum replacement plate with a view towards the inner divertor. In total a ∼270 nm thick hydrogenated amorphous carbon layer was measured assuming a density of 1 g/cm 3 correlated with a fluence of 2.36 × 10 23 D + ions into a toroidal section of 1 cm length of the inner divertor. The area of the QMB in the private flux region was deposition dominated when the inner strike point position was on the vertical tiles, line-of-sight with the quartz crystal, and turned into erosion solely by moving the strike point from vertical tile 3 to horizontal tile 4. The most likely reason is a change of the C/D flux ratio of the particles impinging on QMB turning this area from a deposition to an erosion dominated region.

ELMs and strike point movements

A detailed study of position changes of plasma strike points before and after edge localized modes (ELMs) in JET was carried out. A hypothesis being tested is that in an ELM previously closed edge field lines would open up, releasing plasma current and leading to the formation of a new, smaller separatrix. It was observed that after each ELM strike points have shifted a few centimetres towards the plasma centre (up in JET). In some cases a transient (<100 µs), upwards large (>10 cm) jump of strike positions was observed first. It was followed by an equally fast jump down to the shifted strike positions. Such behaviour has not been described in previous computational models of the ELM. Therefore two novel instability mechanisms are presented, which contribute to explain the changes in strike point position: an X-point instability, due to positive toroidal current density at the X-point, and a diamagnetic instability, due to negative inboard toroidal current density.

Tungsten erosion in the outer divertor of JET

Erosion of tungsten in the outer JET divertor was determined with a set of tungsten coated divertor tiles during the 2001–2004 discharge campaign. The tungsten marker was strongly eroded, with the largest erosion at the outer strike point position, where more than 75% of the initial W disappeared. Strong erosion is also observed at the outer baffle and horizontal apron of tile 8, where about half of the tungsten has been removed. These numbers are lower boundaries, because the W was locally completely eroded. The tungsten erosion is inhomogeneous on a microscopic level and depends on the micro-topography of the rough surface: large erosion with complete disappearance of the W layer is observed on plasma-facing areas of microscopic ridges, while a smaller erosion and sometimes even deposition of carbon is found on the far side of ridges and in pores.

Surface studies of tungsten erosion and deposition in JT-60U

In order to study tungsten erosion and migration in JT-60U, 13 W tiles have been installed in the outer divertor region and tungsten deposition on graphite tiles was measured. Dense local tungsten deposition was observed on a CFC tile toroidally adjacent to the W tiles, which resulted from prompt ionization and short range migration of tungsten along field lines. Tungsten deposition with relatively high surface density was found on an inner divertor tile around standard inner strike positions and on an outer wing tile of a dome. On the outer wing tile, tungsten deposition was relatively high compared with carbon deposition. In addition, roughly uniform tungsten depth distribution near the upper edge of the inner divertor tile was observed. This could be due to lift-up of strike point positions in selected 25 shots and tungsten flow in the SOL plasma.

SOLPS modelling of ASDEX upgrade H-mode plasma

A low density H-mode plasma has been selected for detailed inter-ELM modelling by the SOLPS code package, with the coupled treatment of its plasma (fluid code B2) and neutral (Monte-Carlo code Eirene) parts. Good quality measured midplane density and temperature profiles, covering the pedestal region and stretching far into the SOL, as well as several other parameters and profiles measured in the divertor, have enabled testing the consistency of code solutions with experiment. Once the upstream, midplane profiles have been fitted, and the global parameters (e.g. input power into the computational grid, radiated power) matched, the code reproduces experimental profiles and control parameters in the divertor with an accuracy within a factor of 2. Deviations of modelled parameters from the experiment were found around the strike point position where most of the power was deposited on the target. The deviations are consistent among themselves and all point to one common problem with the modelling: the predicted divertor electron temperature is too low and the density too high, compared with the experiment. The largest inconsistency between the code and experiment was in the magnitude of the peak Hα radiation in the outer divertor, which was larger by a factor of 2 in the code simulations. In addition, the code predicts a somewhat higher sub-divertor neutral flux but lower carbon impurity content in the edge plasma than in the experiment, as well as lower CIII emission. The discrepancy between Hα profiles can to a large degree be attributed to profile effects: the simulated Hα emission profiles are narrower than in the experiment, reflecting the tendency of the neutral–plasma mix to congregate excessively around the strike point in the modelling. At the same time, the integrated Hα emission matches very well with the experiment.Extensive sensitivity studies of the influence of variations in input parameters and assumptions of the code on the modelled divertor conditions have been conducted. They have not resulted in an identification of any SOLPS input/control parameters capable of removing the main disagreement between the code output and experiment. A possibility of parallel transport effects related to low collisionality to increase the effective plasma temperature near the strike point position or of increased perpendicular transport by neutrals (due to some missing reactions in Eirene) to widen the target profiles, will be explored in the future.

Current losses induced by edge localized modes in JET tokamak plasmas

We have investigated the effect of edge localized modes (ELMs) on the total plasma current in JET tokamak plasmas using the earth leakage current detection circuit connected to the in-vessel saddle coils. These measurements, and the geometry of the saddle coil circuit, indicate that, in typical H-mode plasmas, up to ∼300 A (corresponding to a maximum of ∼150 A per MA of plasma current) can be deposited at each ELM in regions rather far away (∼50 cm) from the strike point position and divertor target plates. These current losses decrease with increasing ELM frequency (fELM), being practically below or around the detection limit for fELM > 60 Hz, and show some saturation for increasing diamagnetic energy losses for ‘slow’ Type-I ELMs at fELM < 10 Hz. The measurements reported here are consistent with the ELM phenomenon being associated to a peeling of the edge magnetic flux surfaces.

Effect of plasma configuration on carbon migration measured in the inner divertor of JET using quartz microbalance

The deposition of carbon in the louvre area of the inner divertor of JET was measured by means of a Quartz Microbalance diagnostic (QMB) during 806 exposures (total of about 6479s) in various divertor conditions. Exposure time was controlled by means of a shutter. The overall integrated frequency shift of the deposition crystal from the start of the measurements to the end was 23640Hz, corresponding to an average carbon deposition flux of 5.5×10−8g/cm2s or 2.8×1015C/cm2s. Extrapolating this to the total time of 26.4 hours of divertor plasmas with the MKII GB SRP divertor (gas box divertor with septum replacement plate) yields a total amount of 35.4g of carbon layers deposited on the louvre region. It was found that the deposition increases significantly with decreasing distance of the strike point position to the louvre entrance. Elmy-H-mode discharges with the strike point on the horizontal target dominate the carbon layer formation on the QMB.

Query

Although I've never participated in a strike, I've seen enough of them to grasp their eternal dichotomy: one believes either that unions manoeuvre themselves into a strike position by virtue of employer weakness, or that they are forced there by employer intransigence. And between those two poles?

Neutral pressure dynamics in the upper plenums in the DIII-D tokamak

We present the dependence of the neutral pressure rise on strike point geometry in the inner and outer plenums of the upper divertor in the DIII-D tokamak. The upper outer plenum pressure increases in the proximity of the plenum opening in a manner similar to previous reports of the lower outer plenum pressure. The inner plenum pressure measurement is modulated up to a factor of two by edge-localized modes (ELMs) particle flux, due to the small plenum volume and resulting short equilibration time constant. Reasonable agreement is obtained between the measured dependence of the outer plenum pressure on strike point position and simulations with an analytic neutral transport model, using the time-averaged (i.e. long compared with the ELM period) divertor profiles as inputs. The modelling of inner plenum pressure data with time-averaged profiles traces through the middle of the data range. In addition, modelling with two other sets of profiles, one averaged in the vicinity of ELMs and the other between ELMs, covers the upper and lower envelopes of the data. These new data sets represent an independent test and the agreement confirms that the basic physics in the model is responsible for the dependence of pressure rise on strike point geometry in the long mean free path limit.

Carbon Deposition in the Inner JET Divertor Measured by Means of Quartz Microbalance

A Quartz Microbalance (QMB) system was implemented in the inner divertor region of JET in order to measure in situ and time resolved (minimum exposure time ≥ 0.1 s) material fluxes (mainly carbon) and layer deposition. The system has been developed to operate at temperatures up to 200°C. The aim is to investigate carbon transport to the remote areas, and hence the tritium retention in dependence on plasma conditions. This question is still a major concern for the ITER operation. The mass sensitivity of the system is Sm = 1.5 × 10-8 [g/Hz cm2]. First reliable measurements were made during the C5 campaign (March–May 2002; ≈ 1000 plasma discharges). The results presented are based on 74 selected exposures (694 s) under various conditions (strike point position, input power, neutral pressure, ELM frequency). Most influencing on the carbon deposition in the remote area seems to be the geometry i.e. the strike point position on the divertor tiles. In average 1.9 × 10-4 C-atom are deposited per deuterium ion flowing into the inner divertor.

Control of divertor heat load by Ar injection with keeping high performance in ELMy H-mode plasmas on JT-60U

In ELMy H-mode plasmas of Japan Atomic Energy Research Institute Tokamak-60 Upgrade, extension of a high confinement regime toward high density and high radiation-loss-power fraction has been investigated by Ar injection. With injecting Ar and putting the outer strike point on the top of divertor dome, confinement improvement, electron density, and radiation-loss-power fraction reached the ITER relevant regime; the thermal energy confinement time was as high as that the International Tokamak Experimental Reactor ELMy H-mode scaling IPB98( y,2), the electron density is ∼80% of the Greenwald density, and the radiation-loss-power fraction is ∼0.8. Although electron density profile is slightly peaked with Ar injection, the confinement improvement is mainly attributed to the improvement of ion transport. Moreover, the maximum divertor heat load due to ELMs has been reduced by a factor of 3–5 with keeping high confinement. ELM activity can be controlled by injecting Ar and changing the strike point position.

Analyses of erosion and re-deposition layers on graphite tiles used in the W-shaped divertor region of JT-60U

Erosion and re-deposition profiles were studied on graphite tiles used in the W-shaped divertor of JT-60U in June 1997–October 1998 periods, operated with all-carbon walls with boronizations and inner-private flux pumping. Continuous re-deposition layers were found neither on the dome top nor on the outer wing, while re-deposition layers of around 20 μm thickness were found on the inner wing, in the region close to the dome top. On the outer divertor target, erosion was found to be dominant: maximum erosion depth of around 20 μm was measured, while on the inner target, re-deposition was dominant: columnar structure layers of maximum thickness at around 30 μm on the inner zone while laminar/columnar-layered structures of maximum thickness around 60 μm were found on the outer zone. Poloidal distributions of the erosion depth/re-deposition layer thickness were well correlated with the frequency histograms of strike point position, which were weighted with total power of neutral beam injection, on both the outer and inner targets. Through X-ray photoelectron spectroscopy, composition of the re-deposition layers at a mid zone on the inner target were 3–4 at.% B and <0.6 at.% O, Fe, Cr, and Ni with remaining C. Boron atoms are mostly bound to C atoms but some may precipitated as boron.

A new plasma condensation phenomenon in the W7-AS island divertor

In the W7-AS stellerator with island divertor very high densities can be achieved in the high-density H-mode. As a consequence, the plasma in the divertor detaches as previously shown by the reduction of the particle flux at some strike point positions and an upward movement of the ionisation front. Our spectroscopic measurements prove the simultaneous formation of a high-density (8×10 20 m −3), low temperature (0.2 eV) region at the detachment transition in the divertor on top. This plasma is strongly recombining, as a result a strong up/down pressure asymmetry develops. Since flux amplification at the strike points was not found in the island divertor, we suppose that a nonlinear thermal condensation instability leads to the observed high-density region.

Evaluation of currents and charges in low-voltage surge arresters due to lightning strikes

This paper presents an evaluation of the values of currents and charges absorbed by surge protective devices (SPDs) connected in low-voltage open-wire overhead distribution networks in the case of direct lightning strikes to primary lines. Also, some information about overvoltages magnitude is included. The calculations have been performed using the ATP (Alternative Transients Program). The modeling of the system components includes the insulation characteristics (voltage versus time to breakdown) of the primary and secondary insulators and a distribution transformer model for high frequencies which takes into account the load conditions. Some parameters of interest are taken into consideration in the analysis, such as ground resistances of poles and consumers, lightning strike position and crest value of the stroke current.

Evaluation of lightning location data employing measurements of direct strikes to a radio tower

A lightning location systems (LLS) provides large scale information for lightning strikes to ground. In addition to the event time and strike point position, the LLS gives estimates for the lightning peak current. For the end user of LLS data it is important to know the technical limits of the applied network in terms of detection efficiency, accuracy of the stroke location and peak current estimate. To establish a ground truth reference for comparison of natural lightning current parameters and the data of ALDIS (Austrian Lightning Detection & Information System), a telecommunication tower on Gaisberg near the city of Salzburg (Austria) was instrumented for direct lightning current measurement. Since the start of the tower experiment in 1998 we have recorded more than 200 lightning flashes to the tower. Based on the analysis of the current waveforms nearly all of them are so called upward initiated discharges. GPS time synchronization of both data sets allows a precise correlation of individual events measured at the tower and reported by the lightning location network.