The PCU JET Plasma Vertical Stabilisation Sontrol System
Modern tokamak machines are designed to run elongated X-point configurations for which the plasma is vertically unstable. To enable plasma operation beyond the vessel time the introduction of a Vertical Stabilisation (VS) System is required. Furthermore, in presence of top-bottom asymmetric passive structures, fast perturbation to the internal pressure (ELMs, H-L transition) can perturb the unstable vertical mode. This is the case for JET where H mode is restricted by the risk that an ELM could trigger a Vertical Displacement Event (VDE) leading to a disruption. The PCU enhancement project was setup with the aim of effectively increasing the JET operating space by both improving the Vertical Stabilisation System Controller and increasing the Radial Field Amplifier power. The hardware and software technological advancements that have been used allowed to implement a 200 channel, 20kHz control system with less than 2ms jitter and over 2Gops of processing capability. The software design, driven by an object oriented analysis, enabled a good compromise between real-time performances and reliability-maintainability objectives. The PCU project has adopted from its beginning a model-based approach to solve the plasma vertical stabilisation control problem. The method implies that, at all levels, the control system will be tested on a model of the machine before being validated on the real plant. The controller software is therefore to be tested against the plasma models before being used in the machine. The software was designed to allow the tests against the model, both by providing a multi-platform compatibility (although the code runs on RTAI, it can be run offline on Windows or Linux without changing the source code) and by organising a clear separation between real-time I/O, configuration and control interfaces and scientific/technical code. The first operational experience of the system will be presented, especially focussing on the first 5 months when the system has been operating in parallel to the old VS and the controller has been progressively commissioned.