Robust control system for plasma current and position in a tokamak with self-oscillating actuators

Plasma control at JET

We have investigated the analysis and construction of the position control system in a tokamak device. The plasma free boundary equilibrium has been numerically analyzed, especially under the influence of image flux from the ion core. Using this equilibrium analysis, we have studied the free boundary axisymmetric dynamics of plasma coupling with the external circuits. Experimentally, we have measured the step responses of the plasma position to the externally-applied error field, and sufficient agreement with the numerical analysis was obtained. The construction of the optimized control system has been studied and examined experimentally.

Real-time reflectometry – An ASDEX Upgrade DCS plugin App for plasma position and shape feedback control

マルチプロセッサシステムを用いた核融合プラズマ位置形状制御システムの開発

Nuclear fusion is, in a sense, the opposite of nuclear fission. Fission, which is a mature technology, produces energy through the splitting of heavy atoms like uranium in controlled chain reactions. Unfortunately, the by-products of fission are highly radioactive and long lasting. On the other hand, fusion is the process by which the nuclei of two light atoms such as hydrogen are fused together to form a heavier (helium) nucleus, with energy produced as a by-product. Although controlled fusion is extremely technologically challenging, a fusion-power reactor would offer significant advantages over existing energy sources. This thesis is devoted to the control of tokamaks, magnetic confinement devices constructed in the shape of a torus (or doughnut). Tokamaks are the most promising of several proposed magnetic confinement devices. The need to improve the performance of modern tokamak operations has led to a further development of the plasma shape and position control systems. In particular, extremely elongated plasmas, with high vertical-instability growth rate, are envisaged to reach the required performance for ignition. This request for better performance from the experimentalists’ side has motivated the development of the new vertical-stabilization (VS) system at the JET tokamak, which has been proposed within the Plasma Control Upgrade project. This thesis presents the activity carried out to increase the capability of the VS system and to understand the operational limits in order to assess what can be done to improve the overall performance with the existing hardware and control system so as to minimize the impact on JET operation. The first objective of this work is the analysis of the new diagnostic system and the influence of the mechanical structure on the magnetic measurements used as diagnostics by the VS controller; the main focus is on the influence on the controller performance in the presence of large perturbations. The second objective is to design a new controlled variable to increase the performance of the VS system. The third objective is to provide an equivalent model of an ELM (Edge Localized Mode), in terms of internal plasma profile parameters via best fit of the vertical velocity estimation. The last objective is to obtain a reliable and accurate model of the overall system, based on the new platform MARTe, developed at JET and useful also for other devices.

The need to improve the performance of modern tokamak operations has led to a further development of the plasma shape and position control systems. In particular, extremely elongated plasmas, with high vertical-instability growth rate, are envisaged to reach the required performance for ignition. This request for better performance from the experimentalists' side has motivated the development of the new vertical-stabilization (VS) system at the JET tokamak, which has been proposed within the Plasma Control Upgrade project. The main aim of the project is to enhance the capabilities of the plasma vertical position control system in order to operate with very highly elongated plasmas in the presence of large perturbations. This paper focuses on the new software architecture of the VS system, which relies on a highly configurable real-time framework. Due to its flexibility, the new VS system executes different control algorithms, and it schedules the one which maximizes the performance in each plasma phase.

PPCC is hybrid group of interacting controllers managed through a decision-making logic. After nine years of successfully controlling the plasma in all JET experiments, it is being overhauled to integrate the new full boundary controller Extreme Shape Controller. The difficult integration issues have spawn a re-evaluation of the overall management strategy of the many control abilities provided by PPCC. The aim of this paper is to describe the complex real-time decision logic implemented in PPCC and to explain how it has been shaped by the needs of JET plant. Furthermore, it will focus on the changes necessary to integrate the new component.

The renewed HT-7 plasma control system based on real-time Linux cluster

In order to provide efficient performance of tokamaks with vertically elongated plasma position, control systems for limited and diverted plasma configuration are required. The accuracy, stability, speed of response, and reliability of plasma position control as well as plasma shape and current control depend on the performance of the control system. Therefore, the problem of the development of such systems is an important and actual task in modern tokamaks. In this study, the measured signals from the magnetic loops and Rogowski coils are used to reconstruct the plasma equilibrium, for which linear models in small deviations are constructed. We apply methods of the H∞-optimization theory to the synthesize control system for vertical and horizontal position of plasma capable to working with structural uncertainty of the models of the plant. These systems are applied to the plasma-physical DINA code which is configured for the tokamak Globus-M plasma. The testing of the developed systems applied to the DINA code with Heaviside step functions have revealed the complex dynamics of plasma magnetic configurations. Being close to the bifurcation point in the parameter space of unstable plasma has made it possible to detect an abrupt change in the X-point position from the top to the bottom and vice versa. Development of the methods for reconstruction of plasma magnetic configurations and experience in designing plasma control systems with feedback for tokamaks provided an opportunity to synthesize new digital controllers for plasma vertical and horizontal position stabilization. It also allowed us to test the synthesized digital controllers in the closed loop of the control system with the DINA code as a nonlinear model of plasma.

Currently, the fast evolution of cameras in recent years has made them promising tools for diagnostics of Tokamak. The solution presented in this paper consists of a prototype of high-speed visible image acquisition and processing system (HVIAPs) dedicated for experimental advanced superconducting tokamak shape and position control. Graphics processing unit (GPU) and field-programmable gate array (FPGA) are typically used as accelerators or co-processors in addition to a CPU. Such a heterogeneous computing system can combine the advantages of its individual components. The main imaging equipment components used for this system are four high-resolution fast cameras equipped with fiber interface. Image data from the cameras are received by the frame grabber card based on the FPGA and transmitted to the GPU via the peripheral component interconnect express interface. The software support for the system includes low-level drivers and application programming interface libraries for all components and the algorithm developed for image processing. The offline image process results are compared to equilibrium fitting code, which is a commonly used reconstruction method, with an average error of 1.5 cm. The total processing time for one frame is less than 0.3 ms.

This paper deals with the problem of designing a plasma current, position, and shape controller for the ITER-FEAT tokamak. Exploiting the features of the plant, we show that it is possible to design separately two control loops: a first loop which stabilizes the vertical position by means of a simple derivative action, and a second loop which drives the plasma current and the geometrical shape descriptors as close as possible to the reference values. The second loop is designed by assigning the dominant closed loop poles.

A simple model and its application in designing the plasma position control system in the Saskatoon Torus Medium (STOR-M ) Tokamak are described. Estimating the model parameters, the design of a controller, plasma position measurement, the design of a power driver, and the implementation and testing of the complete system are included. The following assumptions are made to simplify the plasma position model: the plasma is treated as a moving axisymmetric, current-carrying filament; the iron-core of the transformer is approximated by an infinitely long cylinder; and the STOR-M vacuum vessel is constructed of type-304L stainless steel, is circular in cross-section, and is 4 mm in thickness. The model was very useful in the early stages of the design, but due to practical constraints the parameters of the model could not be determined very accurately. A least-squares-error algorithm was used offline to determine revised estimates of the model's parameters. Based on these values, the PID (proportional-integral-derivative) controller was tuned. A 30 kA plasma has been successfully contained for a time of 50 ms, which is the maximum time required for this machine.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Robust Control of Muti Input Multi Output(MIMO) systems are important class of problems and their design is challenging one. This paper proposes an efficient algorithm for automatic synthesis of Quantitative Feedback Theory(QFT) based robust controllers. The proposed method uses Interval Analysis, hybrid optimization and consistency techniques. Hull consistency is used to prune the initial input domain by removing the inconsistent values which are not part of the solution. Hybrid optimization part combines interval global optimization and nonlinear local optimization methods. The proposed overall algorithm is demonstrated to design a robust MIMO QFT controller for uncertain Magnetic levitation System (ECP 750).

It is pointed out that plasma position and shape control calls for fast algorithms for plasma boundary identification, i.e. methods capable of determining the shape in a time interval not longer than the sampling time of the controller. The authors present an online identification algorithm which realizes a trade-off between accurate plasma shape reconstruction and computational burden. The proposed identification has been tested with reference to some types of ITER (International Thermonuclear Experimental Reactor) plasmas.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper describes a multi-winding transformer as a plasma initiator in tokamak devices and the application to the design of poloidal field coil power supply system. The scale of tokamak device has been considerably increased based on some experimentally obtained scalings of a energy confinement, because the plasma performance has been improved with the current and size parameters. Since large tokamaks require the hybrid functional poloidal field coil system, the each coil power supplies must prepare the high voltage generation circuit for the plasma breakdown, and it causes increase in the number of current interrupters, disconnectors and the resisters. Therefore, the plasma position and shape control tend to be difficult especially in the start-up phase of the discharge. The plasma initiator proposed in this paper makes the plasma initiation circuit to be simple by unification of the high voltage generation circuit. It has similar ampere-turn and permeance distribution with the tokamak device and produces the magnetic field necessary for plasma breakdown. An application to FER tokamak proposed by JAERI is also presented.