Identification and Characterization of a New Disruption Regime in ADITYA-U Tokamak

TL;DR

This paper reports the discovery of a new disruption regime in the ADITYA-U tokamak characterized by a two-phase evolution and dominated by a 2/1 drift-tearing mode, with specific empirical thresholds distinguishing it from conventional disruptions.

Contribution

It introduces a novel disruption regime with distinct dynamics and thresholds, expanding understanding of tokamak instability mechanisms.

Findings

Identified a new disruption regime with a two-phase evolution.

Disruption dominated by a 2/1 drift-tearing mode, unlike conventional modes.

Established empirical thresholds for early detection.

Abstract

Disruptions continue to pose a significant challenge to the stable operation and future design of tokamak reactors. A comprehensive statistical investigation carried out on the ADITYA-U tokamak has led to the observation and characterization of a novel disruption regime. In contrast to the conventional Locked Mode Disruption (LMD), the newly identified disruption exhibits a distinctive two-phase evolution: an initial phase characterized by a steady rise in mode frequency with a nonlinearly saturated amplitude, followed by a sudden frequency collapse accompanied by a pronounced increase in amplitude. This behaviour signifies the onset of the precursor phase on a significantly shorter timescale. Clear empirical thresholds have been identified to distinguish this disruption type from conventional LMD events, including edge safety factor, current decay coefficient, current quench (CQ) time, and CQ rate. The newly identified disruption regime is predominantly governed by the (m/n = 2/1) drift-tearing mode (DTM), which, in contrast to typical disruptions in the ADITYA-U tokamak that involve both m/n = 2/1 and 3/1 modes, consistently manifests as the sole dominant instability. Initiated by core temperature hollowing, the growth of this mode is significantly enhanced by a synergistic interplay between a strongly localized pressure gradient and the pronounced steepening of the current density profile in the vicinity of the mode rational surface.