Multi-device study of characteristics of disruptive magnetohydrodynamic modes in Tokamaks

Fusion plasmas confined in tokamaks by magnetic fields often experience disruptions, i.e. sudden losses of the plasma confinement, consisting of a fast thermal quench followed by a plasma current quench, accompanied by the destabilization of the vertical position of the plasma column. Disruptions are a serious threat to the device components in terms of large heat and particle loads on the plasmafacing components and the forces applied on the various components of the device. Disruption avoidance, prediction and, in case of an unavoidable disruption, mitigation, is a prerequisite for safe, efficient operation of large tokamaks (e.g. ITER, currently under construction). Disruption onset is known to be linked with the development of magnetohydrodynamic (MHD) modes. In particular, the crossing of a critical value of the mode amplitude has been identified as a main disruption trigger in the past (de Vries, 2011). This thesis addresses multiple aspects of MHD mode development prior to a disruption. The main diagnostic instruments serving for the mode amplitude monitoring were magnetic sensors, particularly saddle and Mirnov coils. The main tool in the work is an extensive (> 1100 entries) database of discharges terminated by a disruption. The database consists of measurements from devices of various plasma size (COMPASS, ASDEX Upgrade, DIII-D, JET), but similar aspect ratios. Identical data selection criteria were applied across all devices for compiling the database. All database entries were manually classified according to the main cause leading to onset of the MHD mode. Earlier works reported that in large devices, most of the modes were static in the laboratory frame (’locked’) (de Vries, 2016). However, in general modes are often observed to rotate prior to becoming locked, as also seen in the database compiled in this work. This thesis addresses important characteristics of both rotating and locked modes in the context of the disruption initiation. The mode amplitude development is followed throughout the mode braking, the stationary phase of the mode and the disruption onset. The observations reported in this work may contribute to a set of design criteria for future disruption forecasting schemes. The work explores both existing physical models for describing mode dynamics, as well as a phenomenological description of mode behavior on the basis of the multi-machine database. A first part of the work aims to validate a model for mode locking on the basis of a reduced database of ASDEX Upgrade discharges. Not discriminating between plasma configurations, the model allows to estimate the duration of the deceleration phase, as well as the critical mode width for locking. Both quantities are important for the design of algorithms that aim to avoid disruptions by means of external actuators. The reduced ASDEX Upgrade database consists of discharges covering a broad range of plasma parameters and discharge scenarios. It was found that the model successfully describes locking of large modes in those cases where the mode deceleration started in a quasi-stationary phase of the discharge (i.e. with low variability of the global plasma angular momentum prior to mode seeding) and where deceleration took place over temporal intervals that are long in comparison with the momentum confinement time. Theoretical braking curves and locking durations predicted with the model were in good quantitative agreement with the experimental data. On the other hand, the model failed to reproduce the braking curves of modes appearing towards the end of a transient phase, e.g. during an impurity influx or when approaching the disruptive density limit. A modified mode equation of motion is proposed, which accounts for transient variation of the plasma density, e.g. during the development of a MARFE and its poloidal destabilization from its stable location at the plasma X-point. The experimental and theoretical mode braking curves were in closer accordance when the modified model was applied. The second part of the work focuses on the analysis of the duration of (quasi-) stationary modes, as well as the pre-disruptive growth of the measured mode amplitude. In addition, regression analysis is conducted of the locked phase duration and a previously derived scaling for the disruptive mode amplitude is validated (de Vries, 2016). The study was performed using the full database of disruptive discharges covering a broad range of plasma parameters, including a considerable range of plasma dimensions. The measured mode durations span several orders of magnitude in all devices, but nevertheless the median duration is seen to increase with plasma size. Several factors are discussed that could influence the locked mode duration, such as the plasma control system response to exceptional situations (e.g. switching on or off external heating modules, initiation of a fast plasma current ramp-down, plasma shape modification etc.), the specific location of the discharge within the typical device parametric space (often reflected in the disruption root cause) and the occurrence of minor disruptive events (accompanied by a partial loss of the plasma confinement). The scaling formula for the disruptive mode amplitude is reported to systematically overestimate the experimental amplitudes in case of modes detected with magnetic sensors located at the torus high-field-side. A proposal for modifying the scaling, adjusting for this case, is presented. The modified scaling equation allowed more accurate predictions of the critical mode amplitude threshold. Linear extrapolation with plasma size of both experimental and predicted times-to-disruptions and the associated fraction of disruptions suggests that, in devices with large plasma minor radius, the locked modes will grow on time scales that are long enough to allow for disruption mitigation by means of fast massive gas injection or pellet injection. A regression analysis aimed at establishing a scaling relation for the locked mode phase duration with plasma parameters. A physically plausible scaling relation could be established, which however explains only part of the variability of the data. Possible origins of the remaining scatter are discussed, such as the onset of minor disruptions in the presence of the mode, mode re-rotation under a constant external torque input etc. Application of the scaling to the ITER Baseline scenario suggests that the locked phase duration will be of the order of hundreds of milliseconds or seconds in ITER, depending on the particular disruption root cause. Such time scales are in favour of a timely disruption mitigation in ITER

Multi-device study of characteristics of disruptive magnetohydrodynamic modes in tokamaks

Fusion plasmas confined in tokamaks by magnetic fields often experience disruptions, i.e. sudden losses of the plasma confinement, consisting of a fast thermal quench followed by a plasma current quench, accompanied by the destabilization of the vertical position of the plasma column. Disruptions are a serious threat to the device components in terms of large heat and particle loads on the plasmafacing components and the forces applied on the various components of the device. Disruption avoidance, prediction and, in case of an unavoidable disruption, mitigation, is a prerequisite for safe, efficient operation of large tokamaks (e.g. ITER, currently under construction). Disruption onset is known to be linked with the development of magnetohydrodynamic (MHD) modes. In particular, the crossing of a critical value of the mode amplitude has been identified as a main disruption trigger in the past (de Vries, 2011). This thesis addresses multiple aspects of MHD mode development prior to a disruption. The main diagnostic instruments serving for the mode amplitude monitoring were magnetic sensors, particularly saddle and Mirnov coils. The main tool in the work is an extensive (> 1100 entries) database of discharges terminated by a disruption. The database consists of measurements from devices of various plasma size (COMPASS, ASDEX Upgrade, DIII-D, JET), but similar aspect ratios. Identical data selection criteria were applied across all devices for compiling the database. All database entries were manually classified according to the main cause leading to onset of the MHD mode. Earlier works reported that in large devices, most of the modes were static in the laboratory frame (’locked’) (de Vries, 2016). However, in general modes are often observed to rotate prior to becoming locked, as also seen in the database compiled in this work. This thesis addresses important characteristics of both rotating and locked modes in the context of the disruption initiation. The mode amplitude development is followed throughout the mode braking, the stationary phase of the mode and the disruption onset. The observations reported in this work may contribute to a set of design criteria for future disruption forecasting schemes. The work explores both existing physical models for describing mode dynamics, as well as a phenomenological description of mode behavior on the basis of the multi-machine database. A first part of the work aims to validate a model for mode locking on the basis of a reduced database of ASDEX Upgrade discharges. Not discriminating between plasma configurations, the model allows to estimate the duration of the deceleration phase, as well as the critical mode width for locking. Both quantities are important for the design of algorithms that aim to avoid disruptions by means of external actuators. The reduced ASDEX Upgrade database consists of discharges covering a broad range of plasma parameters and discharge scenarios. It was found that the model successfully describes locking of large modes in those cases where the mode deceleration started in a quasi-stationary phase of the discharge (i.e. with low variability of the global plasma angular momentum prior to mode seeding) and where deceleration took place over temporal intervals that are long in comparison with the momentum confinement time. Theoretical braking curves and locking durations predicted with the model were in good quantitative agreement with the experimental data. On the other hand, the model failed to reproduce the braking curves of modes appearing towards the end of a transient phase, e.g. during an impurity influx or when approaching the disruptive density limit. A modified mode equation of motion is proposed, which accounts for transient variation of the plasma density, e.g. during the development of a MARFE and its poloidal destabilization from its stable location at the plasma X-point. The experimental and theoretical mode braking curves were in closer accordance when the modified model was applied. The second part of the work focuses on the analysis of the duration of (quasi-) stationary modes, as well as the pre-disruptive growth of the measured mode amplitude. In addition, regression analysis is conducted of the locked phase duration and a previously derived scaling for the disruptive mode amplitude is validated (de Vries, 2016). The study was performed using the full database of disruptive discharges covering a broad range of plasma parameters, including a considerable range of plasma dimensions. The measured mode durations span several orders of magnitude in all devices, but nevertheless the median duration is seen to increase with plasma size. Several factors are discussed that could influence the locked mode duration, such as the plasma control system response to exceptional situations (e.g. switching on or off external heating modules, initiation of a fast plasma current ramp-down, plasma shape modification etc.), the specific location of the discharge within the typical device parametric space (often reflected in the disruption root cause) and the occurrence of minor disruptive events (accompanied by a partial loss of the plasma confinement). The scaling formula for the disruptive mode amplitude is reported to systematically overestimate the experimental amplitudes in case of modes detected with magnetic sensors located at the torus high-field-side. A proposal for modifying the scaling, adjusting for this case, is presented. The modified scaling equation allowed more accurate predictions of the critical mode amplitude threshold. Linear extrapolation with plasma size of both experimental and predicted times-to-disruptions and the associated fraction of disruptions suggests that, in devices with large plasma minor radius, the locked modes will grow on time scales that are long enough to allow for disruption mitigation by means of fast massive gas injection or pellet injection. A regression analysis aimed at establishing a scaling relation for the locked mode phase duration with plasma parameters. A physically plausible scaling relation could be established, which however explains only part of the variability of the data. Possible origins of the remaining scatter are discussed, such as the onset of minor disruptions in the presence of the mode, mode re-rotation under a constant external torque input etc. Application of the scaling to the ITER Baseline scenario suggests that the locked phase duration will be of the order of hundreds of milliseconds or seconds in ITER, depending on the particular disruption root cause. Such time scales are in favour of a timely disruption mitigation in ITER

Space programs summary no. 37-46, volume IV FOR the period June 1 to July 31, 1967. Supporting research and advanced development

Spacecraft guidance and control, propulsion, telecommunications, and systems analysis, and space science researc

Multi-device study of temporal characteristics of magnetohydrodynamic modes initiating disruptions

Disruptions in tokamaks are often preceded by magnetohydrodynamic (MHD) instabilities that can rotate or become locked to the wall. Measurements from magnetic diagnostics in the presence of MHD mode precursors to disruptions can yield potentially valuable input to the plasma control system, with a view to disruption avoidance, prediction and mitigation. This paper presents an exploratory analysis of the growth of MHD modes and corresponding time scales on the basis of magnetic measurements in multiple tokamaks. To this end, a database was compiled using disruptive discharges from COMPASS, ASDEX Upgrade, DIII-D and JET, manually classified according to disruption root cause, and characterized by a great diversity of operational conditions and mode dynamics. The typical time during which a mode can be detected using saddle coils and the duration of the locked mode phase in the database both extend over several orders of magnitude, but generally the time scales increase with plasma size. Several additional factors are discussed that can influence these durations, including the disruption root cause. A scaling law for the locked phase duration was estimated, yielding predictions toward ITER of the order of hundreds of milliseconds or even seconds. In addition, a scaling law for the mode amplitude at the disruption onset, proposed earlier by de Vries et al. (2016), is applied to the database, and its predictive capabilities are assessed. Despite significant uncertainty on the predictions from both scaling laws, encouraging trends are observed of the fraction of disruptions that may be detected with sufficient warning time to allow mitigation or even avoidance, based solely on observations of MHD mode dynamics. When combined with similar analysis of measurements from diagnostics that are sensitive to other disruption precursors, our analysis methods and results may contribute to the reliability, robustness and generalization of disruption warning schemes for ITER

An active feedback recovery technique from disruption events induced by m=2 n=1 tearing modes in ohmically heated tokamak plasmas

We present experimental results of magnetic feedback control on the m=2, n=1 tearing mode in RFX-mod operated as a circular ohmically heated tokamak. The feedback suppression of the non-resonant m=2, n=1 Resistive Wall Mode (RWM) in q(a)<2 plasmas is a well-established result of RFX-mod. The control of the tearing counterpart, which develops in q(a)>2 equilibrium, is instead a more difficult issue. In fact, the disruption induced by a growing amplitude m=2, n=1 tearing mode can be prevented by feedback only when the resonant surface q=2 is close to the plasma edge, namely 2<q(a)<2.5, and the electron density does not exceed approximately half of the Greenwald limit. A combined technique of tearing mode and q(a) control has been therefore developed to recover the discharge from the most critical conditions: the potentially disruptive tearing mode is converted into the relatively benign RWM by suddenly decreasing q(a) below 2. The experiments demonstrate the concept with 100% of successful cases. The q(a) control has been performed through the plasma current, given the capability of the toroidal loop-voltage power supply of RFX-mod. We also propose a path for controlling q(a) by acting on the plasma shape, which could be applied to medium size elongated tokamaks

First observations of Rydberg blockade in a frozen gas of divalent atoms

This thesis details the first measurements of Rydberg dipole blockade in a cold ensemble of divalent atoms. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Owing to the divalent nature of strontium, one electron can be excited to the Rydberg state, whilst the other lower-lying electron is available to undergo resonant optical excitation to autoionising states, which ionise in sub-nanosecond timescales. The remaining ions that are recorded on a micro-channel plate are proportional to the number of Rydberg atoms. The development of a narrow linewidth laser system necessary for an additional stage of cooling is explained and characterised. Two frequency stabilisation schemes are discussed: one to address the short-term laser frequency instabilities based on the Pound-Drever-Hall technique; the other to address the long-term laser frequency instabilities based on Lamb-dip spectroscopy in an atomic beam. The cooling dynamics on the narrow cooling transition is studied experimentally and modelled via theoretical simulations

Molecular Dynamics Simulations of the Bacterial Outer Membrane Channels TolC and OprM & dxTuber, a Biomolecular Cavity Detection Tool based on Protein and Solvent Dynamics

The multidrug resistance of bacteria is a serious phenomenon in current medical treatment. Beginning with the introduction of antibiotics more and more bacterial strains achieved resistance against these chemical compounds and over the years a competition between antibiotic drug discovery and bacterial drug resistance arose. The well studied Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa serve in this work as a model organisms for bacterial resistance against antibiotics. Both bacteria evolved multidrug resistant strains through several strategies, including the expelling of harming compounds through efflux systems. The over expression of these efflux systems in the bacterial membranes are responsible for resistance against many antibiotic compounds. The AcrA/B-TolC efflux system induces resistance of E.coli against a broad range of antibiotics. Ranging from the inner membrane towards the outer membrane, the efflux system spans the entire periplasmic space. The system consists of the inner membrane transporter AcrB, the membrane fusion protein AcrA and the outer membrane channel TolC. TolC itself cooperates with several inner membrane transporters and facilitates the export of harming compounds across the outer membrane. Due to this versatility TolC could become a target of drug treatment. A disabled or blocked TolC could prevent drug extrusion via systems that use TolC as an exit gate. At the time of writing the gating functionality of TolC is not known in detail. To gain insights into TolC functionality two series of unbiased molecular dynamics (MD) simulations were performed. Whereas the first series was carried out in absence of AcrB the second one was executed in presence of the AcrB docking domain (AcrB-DD). For the first series unbiased MD simulations between 150-300 ns in a Palmitoyloleoylphosphatidylethanolamine (POPE) / NaCl / water environment were calculated. In most of these simulations TolC opens and closes freely on extracellular side hinting at the absence of a gating functionality on this side. On periplasmic side a double aspartate ring restricts substrate passage in all simulations and grasping-like motions were noticed for the tip loops of helix 7 & 8. A consecutive binding of two sodium ions inside the lower periplasmic part of TolC occured in one simulation, which induced a stabilized closed state on periplasmic side. TolC remained closed on periplasmic side unless all ions were removed from the simulation box indicating a sodium dependent lock on this side. For the second series of MD simulations we added the AcrB-DD to the previously described system setup based on orientations of a previously published data driven modeled structure. Four unbiased 150 ns MD simulations were calculated and in one of these simulations the docking domain spontaneously docks onto TolC. The latter simulation was extended to a simulation time of 1.05 μs resulting in a tighter binding between AcrB and TolC with regards to the modeled structure. A preferred open conformation on extracellular hints analogue to TolC only simulations at the absence of a lock on extracellular side. On the AcrB-facing side TolC's tip loops located at helix 7 & 8 opened up and were stabilized by the AcrB docking domain. However, the double aspartate ring remained closed until the end of the simulation, meaning that either the simulation time is too short to observe an opening of TolC or that another part of the AcrA/B-TolC efflux system is missing to open TolC. In Pseudomonas aeruginosa OprM had been identified as a TolC homologue protein. OprM is part of the multidrug efflux system MexA/B-OprM and acts as an exit duct for several inner membrane transporters. Also for OprM the gating mechanisms are not known in detail at time of writing. To explore OprM's gating mechanisms it has been simulated in a POPE / NaCl / water environment. During all five 200 ns long MD simulations OprM opens and closes freely on extracellular side suggesting also for OprM the absence of a gating mechanism on extracellular side. The tip loops of helix 7 & 8 on periplasmic side open up in a way comparable to TolC simulations and in contrast to TolC no closing motions were noticed for these helices for OprM. In OprM a single aspartate ring limits substrate passage on the inner membrane facing side of OprM. In contrast to TolC simulations a slight opening of this aspartate ring was measured in all five simulations. The absence of heightened sodium densities near the periplasmic entrance regions could mean that either longer simulation time is needed to observe a sodium induced closure of OprM or that the periplasmic access is regulated only by the aspartate ring. Despite the absence of heightened sodium densities in the aspartate ring region, clear peaks of high sodium densities identified sodium pockets between the equatorial region and the aspartate ring region formed by Asp171 and Asp230. Voids inside of proteins can indicate substrate binding sites, ion pockets, pathways through channel proteins, their open and closed states and active sites. Over the years numerous cavity detection tools have been introduced to identify and highlight these voids. All available cavity detection tools were based on static structures and present cavities for single protein conformations only. With dxTuber we developed and introduced a novel cavity detection tool based on an ensemble of protein conformations. It uses averaged protein and solvent density maps, which are derived from MD trajectories, as input. With this technique protein dynamics are taken into account and cavities are detected through the separation of protein external solvent from protein internal solvent. Protein internal solvent can be grouped into cavities and stored in the commonly used PDB file format. Individual cavities can be separated via the atom name field of the PDB file format. dxTuber itself can calculate cavity volume and the cross-sectional area of a single cavity along a principle axis. For convenience a graphical user interface (GUI) and a command line interface (CLI) of dxTuber are released under the GPL v2

Parental Perspectives on Twenty-First Century Learning Environments in Private Middle Schools: A Phenomenological Study

Over the last decade instructional technology has experienced tremendous growth in adoption and implementation throughout K-12 schools; pedagogy has shifted to keep pace. Within this growth of technological and pedagogical adoption and implementation a lag has emerged. While teachers and administrators have worked hard to maintain the pace with regard to changes, a major stakeholder (i.e. the parents), have struggled to keep up. The purpose of this transcendental phenomenological study was to better understand the experiences of parents with middle school students enrolled in private, twenty-first century learning model/technology-rich ACSI schools in South Florida. The theory guiding this study was Schlossberg’s transition theory as it addresses the progression of parents from elementary through middle school and on to high school. Participants in this study included parents of middle school students enrolled in technology-rich ACSI schools in South Florida. Phenomenological analysis identified common four themes across four schools, Socio-economic levels, and degree attainment levels. These were: Technology change & strategic consideration, parental control, parental isolation, and parent pacing. Implications for the research suggested that improved communication and more granular approach by schools in reaching out to parents could have a significant positive impact parents experiences. Recommendations for future research are provided