G. Raupp

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Review of the ASDEX Upgrade data acquisition environment - present operation and future requirements

The data acquisition environment of the ASDEX Upgrade fusion experiment was designed in the late 1980s to handle a predicted quantity of 8 Mbytes of data per discharge. After 7 years of operation a review of the whole data acquisition and analysis environment shows what remains of the original design ideas. Comparing the original 15 diagnostics with the present set of 250 diagnostic datasets generated per shot shows how the system has grown. Although now a vast accumulation of functional parts, the system still works in a stable manner and is maintainable. The underlying concepts affirming these qualities are modularity and compatibility. Modularity ensures that most parts of the system can be modified without affecting others. Standards for data structures and interfaces between components and methods are the prerequisites which make modularity work. The experience of the last few years shows that, besides the standards achieved, new, mainly real-time, features are needed: real-time event recognition allowing reaction to complex changing conditions; real-time wavelet analysis allowing adapted sampling rates; real-time data exchange between diagnostics and control; real-time networks allowing flexible computer coupling to permit interplay between different components; object-oriented programming concepts and databases are required for readily adaptable software modules. A final assessment of our present data processing situation and future requirements shows that modern information technology methods have to be applied more intensively to provide the most flexible means to improve the interaction of all components on a large fusion device.

A "Universal Time" system for ASDEX Upgrade

For the new generation of intelligent controllers for plasma diagnostics, discharge control and long-pulse experiment control a new time system supporting steady state real-time operation has been devised. A central unit counts time at nanosecond resolution, covering more than the experiment lifetime. The broadcast time information serves local units to perform application functions such as current time readout, trigger generation and sample time measurement. Time is treated as a precisely measured quantity like other physical quantities. Tagging all detected events and sampled values with measured times as [value; time]-entities facilitates real-time data analysis, steady state protocolling and time-sorted archiving.

On-line confinement regime identification for the discharge control system at ASDEX upgrade

An algorithm is presented that identifies major confinement regimes on-line during a plasma discharge in the Tokamak ASDEX Upgrade. Apart from the ohmically heated regime with no additional heating power, low and high radiative L and H regimes are distinguished. The algorithm was successfully implemented in the ASDEX Upgrade discharge control system, allowing a dynamic control of the plasma discharge. In a first application, the control system recognized the falling back of the plasma from a high confinement regime to a low confinement regime and reacted accordingly, avoiding a disruption and enhancing the plasma performance by recovering the high confinement regime.

Plasma control in ASDEX Upgrade

Abstract In modern tokamak machines, exploration and successful development of improved plasma regimes is impossible without adequate control systems. In ASDEX Upgrade, the control tasks are performed by two systems, the continuously operating machine control and the plasma control active as long as a plasma discharge lasts. Machine control based on programmable logic controllers operates on a relatively slow timescale of τ = 100 ms to configure and monitor the machine’s technical systems. Real-time plasma controllers run on faster cycle times of a few milliseconds to feedback (FB) control plasma shape and performance quantities. During the burn of a discharge, a real-time supervisor monitors the full technical and physical system state τ = 10 ms) and applies alternate discharge program segments to optimize discharge performance or react to failures. The supervisor is fully integrated with a layered machine protection system. Plasma position and shape control in ASDEX Upgrade is particularly difficult: Since the poloidal magnetic field (PF) coils are located reactor relevant outside the toroidal magnetic field coil system and distant from the plasma, each PF coil has a global effect on all shape quantities. This makes simultaneous control of shape parameters a multivariable problem. The feedback control algorithm is based on a matrix proportional-integral-derivative method, adapted to handle saturation of coil currents, excess of coil forces, or to balance loads among coils. Control cycle time is ~3 ms. In parallel, the plasma performance control (sometimes called kinetic control) acts on particle fueling and auxiliary heating systems. It consists mainly of FB loops each controlling a single variable. These circuits can be freely combined to simultaneously control a number of different plasma quantities. A clear hierarchy in the control processes allows special real-time processes to override the programmed plasma discharge feedback action: The set of controlled quantities may be changed dynamically, depending on the plasma regime detected; stabilizing actions may be triggered when plasma instabilities grow; and discharge termination by means of impurity addition is initiated when a neural network indicates an imminent disruption. The computation of the needed plasma parameters and instability indicators requires signal inputs from many diagnostic systems during each controller cycle. Currently, a new plasma control system is being implemented as a distributed system of real-time controllers and diagnostic systems, which are connected via a deterministic communication network.

ASDEX-Upgrade Discharge Control and Shot Management

ASDEX Upgrades fully digital control system is described. Discharge control consists of 6 real time computers for discharge and system monitoring, position and shape control and extended plasma control, all synchronized by a supervisor for discharge phase switching. The timing system is integrated giving absolute time for discharge control and diagnostics. Event-dependent operation is supported. Hierarchically organized protection systems are closely coupled with the discharge and machine control systems. All systems run under a software platform for automated experiment operation.

Protection Strategy in the ASDEX Upgrade Control System

The global protection strategy of ASDEX Upgrade has three goals: protection of personnel, protection of the machine and termination of potentially dangerous discharges The new discharge termination system reacts to deviations from the discharge schedule before machine limits are violated. Its integration into the discharge control system allows for smooth termination under central control.

Plasma Shape Control Design in ASDEX Upgrade

At ASDEX Upgrade shape control has been implemented in addition to the existing position and current control loops to complete the digital feedback control with poloidal field coils. The primary goal was the feedback control of the divertor strike points, though other parameters like gaps and geometrical moments are configurable as well. The approach comprises a multivariable control concept whose core is a matrix PI-controller designed for dynamically decoupled and stationary accurate adjustment and robust operation. The controller gains are computed with a transfer function collocation method. The algorithm has been embedded in ASDEX Upgrades digital discharge control and has successfully been used in experiment.

Novel aspects of plasma control in ITER

ITER plasma control design solutions and performance requirements are strongly driven by its nuclear mission, aggressive commissioning constraints, and limited number of operational discharges. In addition, high plasma energy content, heat fluxes, neutron fluxes, and very long pulse operation place novel demands on control performance in many areas ranging from plasma boundary and divertor regulation to plasma kinetics and stability control. Both commissioning and experimental operations schedules provide limited time for tuning of control algorithms relative to operating devices. Although many aspects of the control solutions required by ITER have been well-demonstrated in present devices and even designed satisfactorily for ITER application, many elements unique to ITER including various crucial integration issues are presently under development. We describe selected novel aspects of plasma control in ITER, identifying unique parts of the control problem and highlighting some key areas of research remaining. Novel control areas described include control physics understanding (e.g., current profile regulation, tearing mode (TM) suppression), control mathematics (e.g., algorithmic and simulation approaches to high confidence robust performance), and integration solutions (e.g., methods for management of highly subscribed control resources). We identify unique aspects of the ITER TM suppression scheme, which will pulse gyrotrons to drive current within a magnetic island, and turn the drive off following suppression in order to minimize use of auxiliary power and maximize fusion gain. The potential role of active current profile control and approaches to design in ITER are discussed. Issues and approaches to fault handling algorithms are described, along with novel aspects of actuator sharing in ITER.

The new ASDEX Upgrade real-time control and data acquisition system

Abstract ASDEX upgrade investigates the integration of confinement, stability and exhaust issues into an operating scenario for ITER and a future fusion reactor. Since commissioned in 1990 the systems used to feedback control plasma position and shape as well as performance have continuously been enhanced. To overcome performance limitations and improve connectivity and steady state capability, a new plasma control system is being implemented. For the new system, adequate and reliable communication mechanisms are essential to integrate the realtime discharge control and data acquisition. We present communication methods and the process organisation of the new system and show that the new concept allows easy performance scaling. We demonstrate how existing periphery and new realtime diagnostics interface to control applications. This facilitates the realisation of novel and sophisticated control tasks combining multiple diagnostics and actuators for common physical goals.