D. Zasche

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

Edge-localized mode (ELM) triggering and pacing in an all-metal wall environment shows significant differences to a first-wall configuration containing carbon. Here we report on experiments performed at ASDEX Upgrade revisiting the issue with all plasma-facing surfaces now fully replaced by tungsten. This investigation was motivated by experimental findings indicating that ELM triggering becomes more intricate when the carbon is replaced by a metal wall. ELM pacing could no longer be achieved by magnetic triggering in ASDEX Upgrade under conditions that previously showed a positive response. Also, recent investigations at JET indicate that a lag time occurs in pellet ELM triggering when operating with the new ITER-like wall. The ASDEX Upgrade centrifuge-based launching system was revitalized and upgraded for this study, now allowing detailed analysis of the ELM trigger response. The appearance of a lag time for pellet ELM triggering in an all-metal wall environment was confirmed. While different lag time durations were found for several type-I ELMy H-mode scenarios, the magnitude of the pellet perturbation was found to cause no difference. Reducing the auxiliary heating power for ELM triggering clearly makes the pellet tool less efficient for ELM control purposes; however, this affords a major benefit when applied for fuelling. Plasma operation with benign ELM behaviour at core densities far beyond the Greenwald limit was demonstrated, this being fully reversible and not affecting the energy confinement.

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.

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.

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.

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.

Discharge Supervision Control on ASDEX Upgrade

The Axially Symmetric Divertor Experiment(ASDEX) Upgrades digital systems for plant operation and real-time discharge control are described. Experience gained during development and operation is used to derive the structure of a reference discharge control system that serves as a target system to assess the impact of enhancements in the existing system. Within this context, the extension of supervision control from technical synchronization of dedicated controllers to physics-oriented coordination is discussed, and examples oftechnical and physical applications are shown.

Impurity Accumulation in Plasma Regimes with High Energy Confinement

Investigations of impurity accumulation phenomena in ASDEX are reviewed. There are four different operating regimes where pronounced accumulation is observed and these regimes are also characterized by improved energy confinement. In particular, medium-Z metallic ions are involved in accumulation processes whereas low-Z ions appear almost unaffected. The rapid accumulation observed in the case of metallic ions may be explained by neoclassical inward drifts if we assume that the anomalous diffusion is sufficiently suppressed, some indication of this being found from laser blow-off studies. The present results, however, can only be partly explained by neoclassical theory, according to which accumulation of low-Z impurities should also occur. The temporal behaviour of accumulation and the retarding effect of proton dilution for collision dominated transport are also discussed. Finally, we conclude that the full benefits of improved energy confinement can be achieved only if the impurity influxes are kept to a sufficiently low level. Expressed in terms of concentrations under low confinement conditions we have to postulate, for ASDEX, concentrations ≲ 10−4 for metals and ≲ 2% for all light impurities.

High power ICRF heating on the divertor tokamak ASDEX

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.