Tokamak-agnostic actuator management for multi-task integrated control with application to TCV and ITER

\u3cp\u3eThe plasma control system (PCS) of a long-pulse tokamak must be able to handle multiple control tasks simultaneously, and must be capable of robust event handling with a limited set of actuators. For ITER, this is particularly challenging given the large number of actuator-conflicting control requirements. To deal with these issues, this work develops a task-based approach, where a plasma supervisory controller and an actuator manager make high-level decisions on how to handle the considered control tasks, using generic actuator resources and controllers. This simplifies the interface for operators and physicists since the generic control tasks (instead of controllers) can be directly defined from the general physics goals. This approach also allows one to decompose the PCS into a tokamak-dependent layer and a tokamak-agnostic layer. The developed scheme is first implemented and tested on TCV for simultaneous β control, neoclassical tearing mode (NTM) control, central co-current drive, and H-mode control tasks. It is then applied to an ITER test scenario to prove its flexibility and applicability to systematically handle a large number of tasks and actuators.\u3c/p\u3

Real-time plasma state monitoring and supervisory control on TCV

In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation.\u3cbr/\u3e\u3cbr/\u3eIn this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state are modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule.\u3cbr/\u3e\u3cbr/\u3eThese algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements.\u3cbr/\u3e\u3cbr/\u3eIn TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECH) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation

Experiments on actuator management and integrated control at ASDEX Upgrade

\u3cp\u3eThe established field of integrated multivariable control promises improved performance and stability for strongly coupled processes, given a control-oriented model of the system. Fusion plasmas are strongly coupled, but there is currently no model which accurately reflects the nature of these complex interactions. Therefore, experiments were performed specifically to investigate coupling between controlled parameters, as a step towards designing integrated controllers in the future. The parameters chosen were core density, divertor neutral pressure and divertor temperature. For control of the plasma pressure and Neoclassical Tearing Modes, where a simple model of the coupling is known, it will be shown that linking the two controllers gives reliably good plasma performance. An additional complication with integrated control is that limited actuator resources are often oversubscribed when trying to control multiple parameters simultaneously. In order to achieve the optimum result, some form of actuator management is a pre-requisite for integrated control. An algorithm has been developed to automatically allocate Electron Cyclotron Resonant Heating gyrotrons to targets, by evaluating a cost function in real-time. Results will be shown to demonstrate the flexibility of this routine to changes in plasma state, gyrotron availability and the goals of the physics experiment.\u3c/p\u3

Real-time control of neoclassical tearing modes and its integration with multiple controllers in the TCV Tokamak

\u3cp\u3ePreliminary integrated control of NTMs, beta and model-estimated q profiles has been demonstrated experimentally in TCV for the first time. An upgrade of the supervision layer is foreseen. Dedicated NTM tests show that density affects the triggering of NTMs through global q profile modifications with central co-ECCD - too low or too high density will hinder the triggering. More detailed simulations are ongoing to further clarify these effects.\u3c/p\u3

Profile control simulations and experiments on TCV:A controller test environment and results using a model-based predictive controller

\u3cp\u3eThe successful performance of a model predictive profile controller is demonstrated in simulations and experiments on the TCV tokamak, employing a profile controller test environment. Stable high-performance tokamak operation in hybrid and advanced plasma scenarios requires control over the safety factor profile (q-profile) and kinetic plasma parameters such as the plasma beta. This demands to establish reliable profile control routines in presently operational tokamaks. We present a model predictive profile controller that controls the q-profile and plasma beta using power requests to two clusters of gyrotrons and the plasma current request. The performance of the controller is analyzed in both simulation and TCV L-mode discharges where successful tracking of the estimated inverse q-profile as well as plasma beta is demonstrated under uncertain plasma conditions and the presence of disturbances. The controller exploits the knowledge of the time-varying actuator limits in the actuator input calculation itself such that fast transitions between targets are achieved without overshoot. A software environment is employed to prepare and test this and three other profile controllers in parallel in simulations and experiments on TCV. This set of tools includes the rapid plasma transport simulator RAPTOR and various algorithms to reconstruct the plasma equilibrium and plasma profiles by merging the available measurements with model-based predictions. In this work the estimated q-profile is merely based on RAPTOR model predictions due to the absence of internal current density measurements in TCV. These results encourage to further exploit model predictive profile control in experiments on TCV and other (future) tokamaks.\u3c/p\u3

Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade

\u3cp\u3eWithin the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (n\u3csub\u3ee,sep\u3c/sub\u3e/n\u3csub\u3eG\u3c/sub\u3e ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.\u3c/p\u3