Development and experimental qualification of novel disruption prevention techniques on DIII-D

Abstract

Novel disruption prevention solutions spanning a range of control regimes are being developed and tested on DIII-D to enable ITER success. First, a new real-time control algorithm has been developed and tested for regulating nearness to stability limits and maintaining safety-margins. Its first application has been for reliable prevention of vertical displacement events (VDEs) by adjusting plasma elongation (κ) and the inner-gap between the plasma and inner-wall in response to real-time open-loop VDE growth rate (γ) estimators. VDEs were robustly prevented up to average open-loop growth rates of 800 rad s−1 with initial tunings, with only applying shape modification when near safety limits. Second, the disruption risk during fast, emergency shutdown after large tearing and locked modes can be significantly improved by transitioning to a limited topology during shutdown. More than 50% of emergency limited shutdowns after locked modes reach a final normalized current I N < 0.3 before terminating, scaling to the 3 MA ITER requirement. This is in contrast to diverted shutdowns, the majority of which disrupt at I N > 0.8. Despite improvements, these results highlight the critical importance of early prevention. Third, a novel emergency shut down method has been developed which excites instabilities to form a warm, helical core post-thermal quench. The current quench extends to ∼100 ms and avoids VDEs and runaway electron generation. Novel real-time machine learning disruption prediction has been integrated with the DIII-D proximity controller, and a real-time compatible multi-mode MHD spectroscopy technique has been developed. Results presented here were enabled by a focused effort, the disruption free protocol, in DIII-D’s 2019–20 campaign to complement disruption prevention experiments with a large piggy-back program. In addition to testing novel techniques, it is estimated to have helped avoid 32 potential disruptions in piggyback operations with rapid, early shutdowns after large rotating n = 1 or locked modes.

Avoiding tokamak tearing instability with artificial intelligence

“…At each time point, the AI controller observes the plasma profiles and determines control commands for beam power and triangularity. The PCS algorithm receives these high-level commands and derives low-level actuations, such as magnetic coil currents and the individual powers of the eight beams [35][36][37]. The coil currents and resulting plasma shape at each phase are shown in Fig.…”

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…The effectiveness of fast emergency shutdown for disruption prevention during plasma current ramp down after locked mode detection in single-null plasmas at ITER-relevant normalized-currents shows that with optimization at least 50% of ramp down disruptions were delayed until after the plasma current I p was reduced to ITER-safe normalized-current levels (figure 3(a)) [6]. Key to the shutdown result is for the I p ramp down phase to transition to an inner wall limited (IWL) shape after the locked mode precursor to disruption is detected and emergency ramp down initiated.…”

“…A recently developed algorithm for real-time regulation of proximity-to-instability boundaries has been applied for robust vertical displacement event (VDE) prevention in DIII-D experiments [6]. The algorithm uses either a physics-based or neural-net-based VDE growth-rate estimation to monitor stability, and modifies plasma shaping in real-time to prevent the growth-rate from reaching uncontrollable limits.…”

Database-wide hazard modelling of the onset of DIII-D tearing modes with field features

“…This type of reasoning is sometimes known as ‘proximity control’ and is an active research topic at DIII-D (Barr et al. 2021). A reactor grade tokamak cannot maintain operation in a plasma state where a truthful hazard function returns a value comparable to the inverse discharge time.…”