ادغام MDSplus در سیستم های زمان واقعی

RFX-mod makes extensive usage of real-time systems for feedback control and uses MDSplus to interface them to the main Data Acquisition system. For this purpose, the core of MDSplus has been ported to VxWorks, the operating system used for real-time control in RFX. Using this approach, it is possible to integrate real-time systems, but MDSplus is used only for non-real-time tasks, i.e. those tasks which are executed before and after the pulse and whose performance does not affect the system time constraints. More extensive use of MDSplus in real-time systems is foreseen, and a real-time layer for MDSplus is under development, which will provide access to memory-mapped pulse files, shared by the tasks running on the same CPU. Real-time communication will also be integrated in the MDSplus core to provide support for distributed memory-mapped pulse files.

The MDSplus data acquisition package is currently used in several nuclear fusion devices, such as RFX [1], CMOD [2], NSTX [3]. In these experiments MDSplus is used pervasively, i.e. for data acquisition, data access and presentation, graphical user interface and sequence coordination. Several other experiments employ custom data acquisition systems, but use MDSplus for data presentation, i.e. to provide a MDSplus interface to stored experiment data so that data can be accessed by data analysis programs using a standard data access library. The architecture of MDSplus is based on the concept of shot (or pulse) and therefore MDSplus handles pulsed experiments in which plasma discharge is achieved for a short period, during which the data acquisition system typically does not work, data being acquired by transient recorders which fill their local memory during the pulse. MDSplus comes in action before the shot, to download configuration parameters, and after the shot, to upload data stored in transient recorders and other devices and to store data into the pulse files. This organization fulfils the requirements for most current fusion devices, but cannot be used in the next generation of fusion devices, for which the plasma discharge is quasi-continuous. For such devices, it is not acceptable to restrict the usage of the data acquisition system to before and after the shot, and the system must carry out data acquisition and control during the pulse itself. In this case real-time constraints must be considered, while off-pulse computation is typically not subject to such constraints. Real-time constraints do not represent however the only issues to be considered for long lasting discharges, but several other features, not currently supported by MDSplus, need to be implemented in this context, such as the possibility of tagging data with time and the possibility of incremental data update. In this paper we present the ideas we developed in using MDSplus to integrate the real-time control components of RFX-mod into the MDSplus framework. It is in fact our belief that the requirements for MDSplus in real-time applications are very similar to those for long lasting discharge experiments.

The development of a distributed real-time system for the feedback control of RFX-mod and its integration in the MDSplus data acquisition system gave us the opportunity of understanding the limitations of the current version of MDSplus for real-time applications. We realized also that the requirements for real time represent most of the requirements for data acquisition and control in long-discharge experiments. Based on the lessons we learnt, we have presented some possible extensions to MDSplus which would allow a pervasive usage of the system in real-time applications. Most of the existing, powerful and tested MDSplus components can, in fact, be retained in real-time applications, in particular the expression evaluation framework. It is our belief that the new components will make MDSplus an attractive solution for control and data acquisition in future experiments.