A control oriented strategy of disruption prediction to avoid the configuration collapse of tokamak reactors

Abstract

The objective of thermonuclear fusion consists of producing electricity from the coalescence of light nuclei in high temperature plasmas. The most promising route to fusion envisages the confinement of such plasmas with magnetic fields, whose most studied configuration is the tokamak. Disruptions are catastrophic collapses affecting all tokamak devices and one of the main potential showstoppers on the route to a commercial reactor. In this work we report how, deploying innovative analysis methods on thousands of JET experiments covering the isotopic compositions from hydrogen to full tritium and including the major D-T campaign, the nature of the various forms of collapse is investigated in all phases of the discharges. An original approach to proximity detection has been developed, which allows determining both the probability of and the time interval remaining before an incoming disruption, with adaptive, from scratch, real time compatible techniques. The results indicate that physics based prediction and control tools can be developed, to deploy realistic strategies of disruption avoidance and prevention, meeting the requirements of the next generation of devices.

Fig. 1. Magnetic topology and disruption precursors.

a Topology of the magnetic fields in a tokamak. b Time evolution of typical disruption precursors during normal operation and in the phase leading to a disruption. In agreement with the literature, the beginning of the current quench is considered the disruption time. Avoidance consists of the remedial actions, allowing to recover a healthy plasma state to continue the experiments. Prevention measures are meant to terminate the discharge quickly, before the actual occurrence of the disruption. When a disruption is unavoidable, because there is no time for avoidance or prevention, mitigating its consequences with appropriate tools, such as shattered pellets or massive gas injection, is the only remaining option.

Fig. 2. Main radiation patterns.

The main radiation patterns leading to a radiation collapse of the plasma in JET with a metallic wall, as revealed by the maximum likelihood tomography and visible imaging. a Anomalous high radiation in the core due to accumulation of high Z impurities, mainly W. b Anomalous low field radiation. The crescent shape excessive emission of is typically an effect of heavy impurities in configurations, in which, due to the combination of neoclassical and anomalous transport, they accumulate in the outer equatorial plane. c Radiation instability of the MARFE (Multifaceted Asymmetric Radiation from the Edge) type^,, typically consequence of impurity seeding or excessive high density in the attempt to reduce the power load on the divertor. d MARFE radiation observed by the visible camera (colour proportional to pixel intensity).

Fig. 3. Fast time resolution tomography regions.

The four macro regions, whose emitted power is determined with the fast time resolution tomography described in the section ‘Methods’.

Fig. 4. Alarm statistics.

a Distribution of the first alarms in disruptive pulses, particularised for the current flat top and ramp down. b Distribution of the first alarms in safe pulses, also particularised for the current flat top and ramp down. The locked mode does not trigger any false alarms, therefore not causing any unnecessary mitigation. The majority of the first alarms in safe discharges are due to the appearance of blobs of radiation in the core. The presence of such anomalies has been checked with the ML tomography; consequently these alarms would not result in negative interventions by the control system, because the proposed remedial action of increasing the power deposition in the core would only improve the plasma performances.

Fig. 5. Control logic.

Block diagram of the detection and control logic proposed in the present work.