Implementing a finite-state off-normal and fault response system for disruption avoidance in tokamaks

Abstract

Abstract. A finite-state off-normal and fault response (ONFR) system is presented that provides the supervisory logic for comprehensive disruption avoidance and machine protection in tokamaks. Robust event handling is critical for ITER and future large tokamaks, where plasma parameters will necessarily approach stability limits and many systems will operate near their engineering limits. Events can be classified as off-normal plasmas events, e.g. neoclassical tearing modes or vertical displacements events, or faults, e.g. coil power supply failures. The ONFR system presented provides four critical features of a robust event handling system: sequential responses to cascading events, event recovery, simultaneous handling of multiple events and actuator prioritization. The finite-state logic is implemented in Matlab R /Stateflow R to allow rapid development and testing in an easily understood graphical format before automated export to the real-time plasma control system code. Experimental demonstrations of the ONFR algorithm on the DIII-D and KSTAR tokamaks are presented. In the most complex demonstration, the ONFR algorithm asynchronously applies "catch and subdue" electron cyclotron current drive (ECCD) injection scheme to suppress a virulent 2/1 neoclassical tearing mode, subsequently shuts down ECCD for machine protection when the plasma becomes over-dense, and enables rotating 3D field entrainment of the ensuing locked mode to allow a safe rampdown, all in the same discharge without user intervention. When multiple ONFR states are active simultaneously and requesting the same actuator (e.g. neutral beam injection or gyrotrons), actuator prioritization is accomplished by sorting the pre-assigned priority values of each active ONFR state and giving complete control of the actuator to the state with highest priority. This early experience makes evident that additional research is required to develop an improved actuator sharing protocol, as well as a methodology to minimize the number and topological complexity of states as the finite-state ONFR system is scaled to a large, highly constrained device like ITER.

Progress in disruption prevention for ITER

“…In either case, with a rotating NTM controlled by ECCD, the plasma current, elongation, and EC power were then ramped down to the low plasma current limit of the control system. DIII-D has implemented an Off-Normal/Fault Response (ONFR) system to handle off-normal plasma events, and hardware faults [79]. Based on finite-state machine logic, the ONFR consists of multiple discrete operating states with rules for the transitions between states; the rules may vary with the state.…”

Integrated control of individual plasma scalars with simultaneous neoclassical tearing-mode suppression

“…In other previous work, like [12,13], integrated controllers are proposed for combined magnetic and kinetic control, although the main elements of an integrated architecture, like S & EH systems and AMs, are not present. In addition, pieces of work like [14,15] focus solely on the development of S & EH algorithms, whereas other work (e.g. [16]) centers its attention on developing strategies for AM.…”

“…In the context of developing the necessary integratedcontrol architectures for present devices and future burningplasma tokamaks, the main contribution of this work is not the synthesis of individual controllers [17][18][19] but the design and experimental testing of an integrated-control architecture powered by an optimization-based AM system for simultaneous regulation of kinetic, magnetic, and MHD-related plasma properties. The architecture integrates four types of components: (i) individual controllers for plasma kinetic variables (namely, the thermal stored-energy, W, and the volumeaverage toroidal rotation, Ω φ [17]), plasma magnetic variables (namely, the central safety factor, q 0 , and edge safety factor, q e [18]), and MHD-related variables (namely, the magnetic island width w), (ii) controllers for NTM suppression that use localized ECH & CD and are based on active-suppression techniques [20] and pre-emptive stabilization [21], (iii) the offnormal and fault response (ONFR) system [15], which offers some S & EH capabilities, and (iv) a novel AM algorithm based on nonlinear, real-time optimization. The overall control scheme is nonlinear and robust, and has been designed and tested in one-dimensional (1D) nonlinear simulations and DIII-D experiments using the ohmic coil, NBI systems, and ECH & CD launchers as actuators.…”

A machine learning approach based on generative topographic mapping for disruption prevention and avoidance at JET

“…Avoiding plasma disruptions will be one of the major concerns for the next generation of tokamaks such as ITER and DEMO. Disruptions, indeed, can cause severe damage to the structural integrity of the machines, forcing unexpected and eventually long maintenance interventions, which significantly reduce the availability of the device [1]. Avoiding disruptions or mitigating their effects requires quite different actions.…”