Use of virtual actuators in ASDEX Upgrade control

Actuator management is an essential part of a modern tokamak plasma control system. It has to deal with a large number of control task simultaneously, needs to be able to operate close to stability limits and to avoid disruptions. In the ASDEX Upgrade tokamak experiment, the process of actuator management development is ongoing. As a first step, we have removed direct assignement of physical actuators to controllers responsible for the control task execution. Instead, the controller is communicating to the so-called virtual actuator, which groups an arbitrary number of actuators of the same type (so far ECRH only) and distributes the controller command between them. The virtual actuator has been implemented and used on ASDEX Upgrade in the 2019 experimental campaign. It gave valuable contributions to several physics experiments. First, the implementation of the virtual actuator enabled beta(p) control by ECRH power, which is of interest especially for I-mode and ELM-free H-mode discharges. Second, it was used in the electron temperature control experiments, which is a useful control tool for transport studies. Third, the concept was used for the density limit disruption avoidance experiments where central ECRH power is applied to recover the plasma from a disruptive zone. The current paper describes the use of the virtual actuator in these cases and based on the gained experience, it suggests future developments and improvements of the virtual actuator on ASDEX Upgrade discharge control system.