Vertical Position Control Research Articles

(Invited) Self-Assembled Si Color Centers: Confinement to the Nanoscale Via Ultra-Low Temperature Molecular Beam Epitaxy

Single photon emission in the telecom wavelength range from isolated silicon (Si) color centers (CCs) was recently demonstrated [1,2], renewing the interest in this well-known defect class [3], originating from radiation damage in Si. It can be envisioned that single SiCCs lead to scalable deterministic group-IV-based quantum light sources [4]. Moreover, recent results for the carbon (C) based G- and T-centers outline their promising spin properties [5,6] for realizing spin-photon interfaces on a scalable Si quantum photonics platform.However, a severe obstacle remains towards their high-yield photonic integration, which is the standard SiCC fabrication process that includes single- or two-step ion implantation [1,7]. The resulting ion implantation profiles are broad and lead to a decisive lack of control over the vertical emitter position. This drawback significantly degrades the coupling efficiency to photonic structures such as resonators or waveguides. Additionally, no two SiCC emitters will be located vertically at the same position below the substrate surface, and determining the depth of isolated emitters is difficult, especially when using non-destructive methods.This contribution presents a completely different, all-epitaxial approach for fabricating variable SiCCs, departing entirely from ion implantation [8]. The presented fabrication process enables us to restrict the formation of SiCCs to a specific epilayer and control their vertical position in a structure even with sub-nm precision [8]. They self-assemble in situ during ultra-low temperature (ULT) growth [9-11] of thin Si and Si:C layers deposited at TG=200°C via molecular beam epitaxy (MBE). While the ULT growth just allows for the lattice disorder required for their formation, exceptionally pristine chamber conditions ensure a high optical quality of the SiCCs and allow fully crystalline Si overgrowth at higher TG. To verify the vertical position control, we compare the photoluminescence signal of the created CCs for a systematic variation of TG of active layer and cap vs. undoped references. The emitter density can be conveniently controlled via C doping concentration and Si:C layer thickness. Furthermore, we show the first results for electrically-pumped self-assembled SiCCs by integrating the Si:C nanolayers into ULT-grown p-i-n light-emitting diodes.

Plasma control for the step prototype power plant

In 2019 the UK launched the Spherical Tokamak for Energy Production (STEP) programme to design and build a prototype electricity producing nuclear fusion power plant, aiming to start operation around 2040. The plant should lay the foundation for the development of commercial nuclear fusion power plants. The design is based on the spherical tokamak principle, which opens a route to high pressure, steady state, operation. While facilitating steady state operation, the spherical design introduces some specific plasma control challenges: (i) All plasma current during the burn phase should to be generated through non-inductive means, dominated by bootstrap current. This leads to operation at high normalised plasma pressure βN with high plasma elongation, which in turn imposes effective active stabilisation of the vertical plasma position. (ii) The tight aspect ratio means very limited space for a central solenoid, imposing that even the current ramp up must be non-inductively generated. (iii) The compact design leads to extreme heat loads on plasma facing components. A double null design has been chosen to spread this load, putting strict demands on the control of the unstable vertical plasma position. (iv) The heat pulses associated with unmitigated ELMs are unlikely to be acceptable imposing ELM free operation or active ELM control. (v) To reduce and spread heat loads, core and divertor radiation and momentum loss has to be controlled, aiming to operate with simultaneously detached upper and lower divertors. (vi) High pressure operation is likely to require active resistive wall mode (RWM) stabilisation. (vii) The conductivity distribution in structures near the plasma must be carefully selected to reduce the growth rates for the vertical instability and the RWM without damping the penetration of the of magnetic fields from active control coils too much. This article describes the initial work carried out to develop a STEP plasma control system.

Using LQR controller for vertical position control on EAST

Vertical position control is essential for stabilizing plasma with elongated configurations. The EAST tokamak is equipped with a set of in-vessel control (IC) coils dedicated to this purpose. Currently, a PD controller with fixed parameters is used for the vertical position control of EAST plasma. However, the response of the plasma in the vertical position changes with changes in plasma configuration, which can result in different control parameter requirements. It is essential to develop a model-based fast-tuning control algorithm for ensuring stability in the vertical position under different configurations. In this study, a model-based vertical position controller tuning method based on a linear quadratic regulator algorithm (LQR) is proposed. Compared with contemporary PD controllers, the proposed model-based LQR controller can enable adjusting controller parameters based on the response of the system, achieving stable control under different vertical position responses. In the EAST experiment, the model-based LQR controller achieved stable control under a shot with a continuously increasing growth rate and reached a maximum controllable vertical displacement growth rate of 968 s−1. The robustness of the system was also demonstrated in a free drift experiment. The new vertical displacement control method can be adapted to different system states and plasma configurations and improve the controllability and safety of future devices.

Effects of walking speeds on lower extremity kinematic synergy in toe vertical position control: An experimental study.

This study aimed to investigate whether lower limb joints mutually compensate for each other, resulting in motor synergy that suppresses toe vertical position fluctuation, and whether walking speeds affect lower limb synergy. Seventeen male university students walked at slow (0.85 ± 0.04 m/s), medium (1.43 ± 0.05 m/s) and fast (1.99 ± 0.06 m/s) speeds on a 15-m walkway while lower limb kinematic data were collected. Uncontrolled manifold analysis was used to quantify the strength of synergy. Two-way (speed × phase) repeated-measures analysis of variance was used to analyze all dependent variables. A significant speed-by-phase interaction was observed in the synergy index (SI) (P < .001). At slow walking speeds, subjects had greater SI during mid-swing (P < .001), while at fast walking speeds, they had greater SI during early-swing (P < .001). During the entire swing phase, fast walking exhibited lower SI values than medium (P = .005) and slow walking (P = .027). Kinematic synergy plays a crucial role in controlling toe vertical position during the swing phase, and fast walking exhibits less synergy than medium and slow walking. These findings contribute to a better understanding of the role of kinematic synergy in gait stability and have implications for the development of interventions aimed at improving gait stability and reducing the risk of falls.

Pay load Stabilization for Offshore Cranes: A Unified Controller Based on Flatness

Crane-based handling operations represent a vital part of today's maritime sector. To guarantee both safe and efficient payload handling with ship cranes, payload oscillations induced by the sea swell have to be damped. Approaches for payload stabilization usually consider either vertical position control (Active Heave Compensation, AHC) or sway reduction (Anti Sway Control, ASC). In this paper, a unified control scheme for spatial payload stabilization is proposed, utilizing the differential flatness of the crane system. The presented framework inverts the nonlinear payload dynamics by means of the flat mapping, thus facilitating controller design. Furthermore, the approach provides a systematic way to include the sea disturbance in the controller. Redundancy of the considered knuckle boom crane is exploited to track secondary control objectives. A related cost function weighting the crane's manipulability is derived and used in an optimization-based target selector. The controller design is evaluated in simulation for varying sea disturbances. The results suggest good AHC and ASC capabilities, where the payload's position error is reduced up to 60% for light to medium sea states.

Methodology of Plasma Shape Reachability Area Estimation in D-Shaped Tokamaks

This paper suggests and develops a new methodology of estimation for a multivariable reachability region of a plasma separatrix shape on the divertor phase of a plasma discharge in D-shaped tokamaks. The methodology is applied to a spherical Globus-M/M2 tokamak, including the estimation of a controllability region of a vertical unstable plasma position on the basis of the experimental data. An assessment of the controllability region and the reachability region of the plasma is important for the design of tokamak poloidal field coils and the synthesis of a plasma magnetic control system. When designing a D-shaped tokamak, it is necessary to avoid the small controllability region of the vertically unstable plasma, because such cases occur in practice at a restricted voltage on a horizon field coil. To make the estimations mentioned above robust, PID-controllers for vertical and horizontal plasma position control were designed using the Quantitative Feedback Theory approach, which stabilizes the system and provides satisfactory control indexes (stability margins, setting time, overshoot) during plasma discharges. The controllers were tested on a series of plasma models and nonlinear models of current inverters in auto-oscillation mode as actuators for plasma position control. The estimations were made on these models, taking into account limitations on control actions, i.e., voltages on poloidal field coils. This research is the first step in the design of the plasma shape feedback control system for the operation of the Globus-M2 spherical tokamak. The developed methodology may be used in the design of poloidal field coil systems in tokamak projects in order to avoid weak achievability and controllability regions in magnetic plasma control. It was found that there is a strong cross-influence from the PF-coils currents and the CC current on the plasma shape; hence, these coils should be used to control the plasma shape simultaneously.

Non-inductive plasma vertical position measurement for the 1056s discharge on EAST.

Vertical position stability plays a crucial role in maintaining safe and reliable plasma operation for long-pulse fusion devices. In general, the vertical position is measured by using inductive magnetic coils installed inside the vacuum vessel; however, the integration drift effects are inherent for steady-state or long-pulse plasma operation. Developing a non-magnetic approach provides a fusion reactor-relevant steady-state solution that avoids the negative impact of integration drift. In this paper, we compare the non-inductively determined vertical position achieved by line-integrated interferometer and polarimeter measurements to that employing an inductive flux loop for a 1056s discharge recently achieved on EAST (Experimental Advanced Superconducting Tokamak). Experimental results show that the non-inductive measurement is more robust than flux loops after 300s if the integrator is not reset to suppress integrator drift. Real-time vertical position control using the non-inductive system is proposed for the next EAST experimental campaign.

Improved Plasma Vertical Position Control on TCV Using Model-Based Optimized Controller Synthesis

Elongated plasmas lead to improved performance in tokamaks but make the plasma prone to vertical instability, which requires active feedback control, a critical issue for future fusion reactors. Vertical control was optimized for the TCV tokamak by applying modern control theory to electromagnetic models for the plasma-vessel-coils dynamics. Two different optimal combinations of poloidal field coils for vertical control actuation are derived from linear plasma response models and used on different timescales for controlling the plasma vertical position. On fast timescales, the priority is input minimization, while on long timescales position control is designed to be compatible with shape control. A structured H-infinity design extending classical H-infinity to fixed-structure control systems was subsequently applied to obtain an optimized controller using all available coils for position control. Closed-loop performance improvement was demonstrated in dedicated TCV experiments, showing a reduction of input requirement for stabilizing the same plasma, thus reducing the risk of power supply saturation and consequent loss of vertical control. This novel algorithm is adaptable to different plasma equilibria as it is designed for model-based automated coil selection and controller tuning, thus avoiding extensive experimental gain scans when performing plasma discharges in TCV. The presented technique is general and can be applied to any present tokamak with independent coils or for the design of future tokamak magnetic control systems.

Full conversion from ohmic to runaway electron driven current via massive gas injection in the TCV tokamak

Full conversion from ohmic to runaway electron (RE) driven current was observed in the tokamak à configuration variable (TCV) following massive injection of neon through a disruption mitigation valve into a low-density limited circular plasma. Following a partial disruption, a stable 200 kA RE beam is maintained for more than 1 s. Controlled ramp-down of the RE beam with adjustable decay rate was demonstrated. Control of the beam vertical position was achieved down to a RE current of 20 kA. RE beam formation is observed in elongated plasma configurations up to κ = 1.5. A reproducible scenario for RE beam generation without loss of circulating current is of particular interest for disruption modelling applications.

Design considerations of the European DEMO’s IR-interferometer/polarimeter based on TRAVIS simulations

Interferometry is the primary density control diagnostic for large-scale fusion devices, including ITER and DEMO. In this paper we present a ray tracing simulation based on TRAVIS accounting for relativistic effects. The study shows that measurements will over-estimate the plasma density by as much as 20°. In addition, we present a measurement geometry, which will enable vertical position control during the plasma’s ramp-up phase when gap-reflectometers and neutron cameras are still blind.

Full conversion from Ohmic to runaway electron driven current via massive gas injection in the TCV tokamak

Abstract Full conversion from Ohmic to runaway electron (RE) driven current was observed in the TCV tokamak following massive injection of neon through a disruption mitigation valve into a low-density limited circular plasma. Following a partial disruption, a stable 200 kA RE beam is maintained for more than one second. Controlled ramp-down of the RE beam with adjustable decay rate was demonstrated. Control of the beam vertical position was achieved down to a RE current of 20 kA. RE beam formation is observed in elongated plasma configurations up to κ=1.5. A reproducible scenario for RE beam generation without loss of circulating current is of particular interest for disruption modelling applications.

Stabilized Controller for Jet Actuated Cantilevered Pipe Using Damping Effect of an Internal Flowing Fluid

Fluid jet actuation is a potential actuation technique for continuum robots. It can generate and rapidly control a relatively large force using a small and lightweight structure because a significant amount of energy can be transported through its internal channels. Recently, jet-actuated flying continuum robots have been developed using this advantageous characteristic. However, a challenging issue in controlling the robot is the fluid structure interaction between the flexible body and the internal flowing fluid. This interaction often causes instability in the pipe conveying fluid. In this study, as a first step to address this issue, we propose a stabilized controller (vertical position control) for a jet-actuated two-dimensional cantilevered pipe with a nozzle unit at the tip using the damping effect of the internal flowing fluid and verify the controller with a real robot. Specifically, a model is constructed with the net force of the jets as the control input. A simple controller that can constantly decrease the energy function is proposed by utilizing the damping effect of the flowing fluid. Numerical simulations verify the stability of the system regardless of the flow velocity. In particular, fluid damping mainly suppresses the higher-order mode oscillations. Moreover, the stability of the system can be improved by adjusting the controller gains. We also conduct experiments using an actual robot to verify the simulation results. The vibrations can be damped by the fluid effect, and the stability can be improved using the proposed controller.

New horizontal and vertical field coils with optimised location for robust decentralized plasma position control in the IGNITOR tokamak

The paper deals with the improvement of the poloidal system of the IGNITOR tokamak, and the design and simulation of the robust decentralised control systems of the plasma’s vertical and horizontal position. The results were obtained on a linear model that is constructed from the circuit equations for the coils, plasma, and vacuum vessel (VV) currents, and from the force balance equations for the plasma. Two new coils that are located close to the VV have been introduced in the IGNITOR poloidal system; specifically, a Horizontal Field Coil (HFC) that is designated for the plasma vertical position control and a Vertical Field Coil (VFC) that is used for the horizontal position control. For the first time in the design of D-shaped tokamaks poloidal systems, the location of the HFC was optimised to achieve the maximum size of the plasma controllability region in the vertical direction and the location of the VFC was determined from the optimisation of the plasma position response to the voltage step function. Although the ideal position of the VFC with fast response time is on the equatorial plane, its location usually is occupied by the other devices such as ports, diagnostic system, and heating systems, so the separated two-section VFC is used. Careful calculation shows the performance is degraded by only 20 % in this layout. For the improved IGNITOR poloidal system, the new robust decentralised plasma position control systems were designed by means of two robust approaches: H∞-optimisation and Linear Matrix Inequalities (LMI). The results of the simulation of the control systems with two types of actuator models as the multiphase thyristor rectifier and the voltage inverter in Pulse- Width Modulation (PWM) mode for the new IGNITOR poloidal system are presented.

Parametric system-level models for position-control of novel electromagnetic free flight microactuator

This work investigates a new type of microactuator for optical applications and a new type of assembly that incorporates free flight phases to overcome the lack of ball bearings in microsystems. Presented design employs electromagnets to manipulate the flying object, conceived as a freely moving micromirror without any limitation of the maximum deflection angle. A fast and precise system-level model enables the micromirror's vertical position-control, thus achieving the desired accuracy for optical applications. This paper evaluates the performance of three system-level models based on magnetostatic finite element simulations. These are a lookup table, a semi-analytical compact model and a parametric reduced order model constructed by matrix interpolation method. All system-level models compete in computational time and accuracy, achieving an excellent match to the reference solution. Finally, an application within a control loop demonstrates their feasibility.

О создании системы управления вертикальным положением плазмы в Токамаке КТМ

The Tokamak KTM plant with the aspect ratio of A = 2 and a plasma current of around 750 kA, which is being put into operation in Kurchatov (Kazakhstan), will be able to produce heat loads on the tested samples of materials that are characteristic of the ITER plant, and owing to additional HF heating the plasma pulse duration will reach about 4 s. One of the stages of this work involves studies aimed at obtaining an efficient plasma vertical position control system. The system includes special fast control windings located between the toroidal winding and the inner chamber, and also a fast-acting power supply with feedback. Currently, the tokamak inner chamber with all main systems and windings has been assembled, and a plasma discharge with duration of about 100 ms has been obtained in the limiter configuration. To support operation of the plasma vertical position control system, an own power supply has been designed and assembled, which consists of a thyristor rectifier connected in series with a controlled voltage inverter on IGBT transistors, which has a significant margin in power and response speed. Preliminary computation experiments were carried out in the MATLAB environment, which have confirmed the adequacy of the selected power supply and the system for discharging the energy stored in the windings to the water-cooled ballast resistance. With the remaining tokamak systems having sequentially been put into operation and the first experimental data on plasma motion characteristics in the divertor configuration having become available, it is planned to carry out a series of program experiments aimed at developing and optimizing the feedback system controller algorithm in the power supply of the KTM tokamak fast control windings.

Plasma flux expansion control on the DIII-D tokamak

A new controller has been developed with help of the flexible divertor poloidal-field coil set of the DIII-D tokamak, to aid in the precise control of the flux expansion in the scrape-off layer. The single-input multiple-output architecture ensures flexibility through a complementary set of orthogonal actuator direction to guarantee minimum effect on existing controlled variables, e.g. radial and vertical position control of the X-point. A non-linear free-boundary simulation code (GSevolve) is used for simulating the closed-loop response and for verifying the implementation of the control algorithm on the DIII-D plasma control system. First results of the experimental commissioning of the new controller during 2020 DIII-D campaign are also presented.

A robust controller for multi rotor UAVs

Unmanned aerial vehicles (UAVs) are safety-critical systems that often need to fly near buildings and over people under adverse wind conditions and hence require high manoeuvrability, accuracy, fast response abilities to ensure safety. Under extreme conditions, the dynamics of these systems are strongly nonlinear and are exposed to disturbances, which need a robust controller to keep the UAV and its environment safe. In this paper a novel robust nonlinear multi-rotor controller is introduced based on essential modifications of standard dynamic inversion control, which makes it insensitive to payload changes and also to large wind gusts. First a robust attitude controller is established, followed by lateral and vertical position control in a customary outer loop. The controllers take into account thrust limitations of the aircraft and theoretical proof is provided for robust performance. The control scheme is illustrated in simulation with a realistic nonlinear dynamical model of an aircraft that includes rotor dynamics and their speed limitations to show robustness. Lyapunov stability methods are used to prove the stability of the robust control system.

TSC simulation of vertical position control by the Bang-bang method on EAST

Vertical instability is a potentially serious hazard for elongated plasma. Many methods have been proposed to control the plasma vertical position. The Bang-bang method, which is designed for the inside coil (IC) voltage mode based on optimal control theory and the RZIp rigid plasma response model, has been successfully used for plasma vertical position control in the EAST 2014 experimental campaign (Wang et al 2016 Fusion Eng. Des. 112 692–8). The Tokamak Simulation Code (TSC) is a numerical model of tokamak plasma and the associated control systems. It studies the evolution of the magnetic field in a rectangular computational domain using the Maxwell magnetohydrodynamic equations for plasma coupled with the boundary conditions to the circuit equations for the poloidal field coils. In this paper, the numerical model of the Bang-bang method has been implemented in the TSC code, which is used to simulate shot 52444. Simulated trajectories of the vertical position, IC voltage, and current fit the experimental data well. On the basis of benchmarked calculation, a numerical investigation of the Bang-bang controller has been performed using the TSC. This controller was able to work with 8 mm random noise and dz = 41.8 mm displacement. As a predictive tool, the TSC code is used to investigate the the maximum controllable vertical displacement value for different maximum current, and evaluate the ability to control the vertical position by the Bang-bang method.

Overview of disruptions with JET-ILW

The paper presents an analysis of disruptions occurring during JET-ILW plasma operations covering the period from the start of ILW (ITER-like wall) operation up to completion of JET operation in 2016. The total number of disruptions was 1951 including 466 with deliberately induced disruptions. The average rate of unintended disruptions was 16.1 %, which is significantly above the ITER target at 15 MA. The pre-disruptive plasma parameters are: plasma current Ip = (0.82–3.38) MA, toroidal field BT = (0.98–3.4) T, safety factor q95 = (1.52–9.05), plasma internal inductance li = (0.58–1.86), Greenwald density limit fraction FGWL = (0.04–1.61), with 720 X-point plasma pulses from a subset of 1420 unintended disruption shots. Massive gas injection (MGI) has been routinely used in protection mode both to terminate pulses when the plasma is at risk of disruption and to mitigate against disruption effects. The MGI was mainly triggered by the n = 1 locked mode (LM) amplitude exceeding a threshold or by the disruption itself, namely, either dIp/dt (specifically, a fast drop in Ip) or the toroidal loop voltage exceeding threshold values. For mitigation purposes, only the LM was used as a physics precursor and threshold on the LM signal was used to trigger the MGI prior to disruption. Long lasting LM (≥ 100 ms) do exist prior to disruption in 75% of cases. However, 10% of non-disruptive pulses have a LM which eventually vanished without disruption. The plasma current quench (CQ) may result in 3D configurations, termed as asymmetrical disruptions, which are accompanied by sideways forces. Unmitigated vertical displacement events (VDEs) generally have significant plasma current toroidal asymmetries. Unmitigated non-VDE disruptions also have large plasma current asymmetries presumably because there is no plasma vertical position control during the CQ and so they too are subject to large vertical displacements. MGI is a reliable tool to mitigate 3D effects and correspondingly sideways forces during the CQ. The vessel structure loads depend on the force impulse and force time behaviour, including their rotation. The toroidal rotation of 3D configuration may cause resonance with the natural frequencies of the vessel components in large tokamaks such as ITER. The JET-ILW amplitude-frequency interdependence of toroidal rotation of 3D configurations is presented.

Accuracy Constraints Improve Symmetric Bimanual Coordination for Children with and without Unilateral Cerebral Palsy

ABSTRACTPurpose: To evaluate the influence of accuracy constraints on functional symmetric bimanual coordination for children with Unilateral Spastic Cerebral Palsy (USCP). Methods: Ten children with USCP (average age: 9.6; MACS levels: I–II), ten typically developing children, and ten adults lifted a tray with a water bottle on top. Two accuracy constraints of handle size and cap condition were manipulated. Results: Children with USCP exhibited greater bilateral asymmetry in hand vertical position, timing, upper arm, and elbow control than other groups. Smaller handle decreased bilateral timing differences at lift onset and offset, and decreased bilateral elbow asymmetry at reach and lift offset. Without a cap (accuracy constraint), they showed greater trunk involvement, and less bilateral vertical position and lift offset timing differences (all p < .05). Conclusions: Children with USCP showed impaired symmetric bimanual coordination. Higher accuracy constraints improved some bimanual spatial and temporal control. Therefore, task accuracy constraints should be manipulated carefully for training.