K. Behler

Design of a digital multiradian phase detector and its application in fusion plasma interferometry

We discuss the circuit design of a digital multiradian phase detector that measures the phase difference between two 10 kHz square wave TTL signals and provides the result as a binary number. The phase resolution of the circuit is 1/64 period and its dynamic range is 256 periods. This circuit has been developed for fusion plasma interferometry with submillimeter waves on the ASDEX Upgrade tokamak. The results from interferometric density measurement are discussed and compared to those obtained with the previously used phase detectors, especially with respect to the occurrence of phase jumps. It is illustrated that the new phase measurement provides a powerful tool for automatic real-time validation of the measured density, which is important for feedback algorithms that are sensitive to spurious density signals.

Real-time feedback control of the plasma density profile on ASDEX Upgrade

The spatial distribution of density in a fusion experiment is of significant importance as it enters in numerous analyses and contributes to the fusion performance. The reconstruction of the density profile is therefore commonly done in offline data analysis. In this paper, we present an algorithm which allows for density profile reconstruction from the data of the submillimetre interferometer and the magnetic equilibrium in real-time. We compare the obtained results to the profiles yielded by a numerically more complex offline algorithm. Furthermore, we present recent ASDEX Upgrade experiments in which we used the real-time density profile for active feedback control of the shape of the density profile.

ASDEX Upgrade MHD Equilibria Reconstruction on Distributed Workstations

The identification of MHD equilibrium states on the ASDEX Upgrade tokamak is a prerequisite for interpreting measurements from a wide range of diagnostics which are correlated with the shape of the plasma. The availability in realtime of plasma parameters related to the MHD state is crucial for controlling the experiment. Function Parameterization is used as a standard tool to determine the position, shape, and other global parameters of the plasma as well as the MHD equilibrium flux surfaces. The recently developed interpretive equilibrium code CLISTE now enables the calculation of MHD equilibria on an intershot timescale. These calculations are parallelized by the use of a Message Passing Interface (MPI).

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.

Real-Time Diagnostics and their Applications at ASDEX Upgrade

Abstract For applications of advanced plasma control schemes, many computers that execute complex algorithms need to communicate with low latency so that result data are promptly available for operating adequate actuators that can directly influence the plasma behavior. ASDEX Upgrade has completed the commissioning phase of its real-time diagnostic framework serving that purpose. Several applications were successfully tested, and progress toward a full feedback neoclassical tearing mode stabilization loop is evident. The new real-time diagnostics comprise several new diagnostics capable of acquiring raw data (up to 1 MHz, up to 60 channels), processing the raw data (calibrate, transform, evaluate, etc.) and transmitting the results over suitable networks to other computers, all in real time. Projects for machine safety (divertor cooling and hot spot detection), physics studies [regulation of density peaking by application of electron cyclotron resonance heating (ECRH)], and real-time state monitors (ECRH deposition calculation) have demonstrated the capabilities of the new diagnostics and the control framework. The control system can now operate its actuators in line with decisions based on algorithms with rather high complexity. Adding new control algorithms has become a distributed effort with manageable overhead.

Recent results from divertor operation in ASDEX upgrade

The results of divertor studies on ASDEX Upgrade, at currents of up to 1.2 MA and heating powers up to 10 MW are described, with emphasis on the ELMy H-mode. The spatial and temporal characteristics of their heat load, and the simulation of ELMs by a time-dependent scrape-off layer code are described. High gas puff rates were found to lead to a large increase in divertor neutral pressure, at modest changes in ne, and to a strong reduction in time-averaged power flow and complete detachment from both target plates in between ELMs. Using pre-programmed puffs of neon and argon, the radiative power losses could be raised to 75% of the heating power, in H-regime discharges, and the regime of enhanced divertor neutral pressure was found also to lead to an improved pumping of recycling impurities.

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.

Next generation discharge control system for ASDEX upgrade

Based on present achievements in tokamak control and considering requirements for reactor-oriented research, a next generation discharge control system is presented. Decomposition of control functions mapped onto dedicated controllers and diagnostics gives a distributed system integrated with a common real-time network to publish process and reference values. Basic mechanisms for cycle administration, data exchange and process management of the prototype system currently being implemented are shown.

Fast Sampling Upgrade and Real-Time NTM Control Application of the ECE Radiometer on ASDEX Upgrade

Abstract The ASDEX Upgrade tokamak employs a 60-channel electron cyclotron emission (ECE) radiometer diagnostic for the measurement of radial electron temperature profiles of the plasma. The data acquisition (DAQ) portion of the system has now been upgraded to sample at 1 to 2 MHz, and accordingly, electron temperature fluctuations from 500 kHz to 1 MHz may be measured. The high spatial resolution of [approximately]1 cm and flexible magnetic field coverage from 1.5 to 3.0 T remain unchanged. The system can now provide observations of plasma phenomena on the magnetohydrodynamic timescale, such as neoclassical tearing modes (NTMs) and toroidal Alfvén eigenmodes (TAEs). The upgraded and existing DAQ systems may be run in parallel for comparison, and some of the first plasma measurements using the two systems together are presented, along with an example of localization of [approximately]120-kHz TAEs in the fast ECE data. A principal planned application of the upgraded radiometer is integration into a real-time NTM stabilization loop using targeted deposition of electron cyclotron resonance heating (ECRH) or electron cyclotron current drive. For this loop, it is necessary to determine the locations of the NTM and ECRH deposition using ECE measurements. The NTM location is determined via correlation between ECE and Mirnov coil measurements, and results of this technique for (2,1) and (3,2) NTMs are presented. ECRH deposition is located by observing the modulation signature of the injected ECRH power in ECE measurements. Several additional applications enabled by the upgraded radiometer are also discussed.