First observation of plasma healing via helical equilibrium in tokamak disruptions

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A novel detector, using stacked BGO crystals, is developed for runaway electron (REs) studies in the DIII-D tokamak. It is able to resolve fast dynamics of high-energy tail formation of REs with an ultra-high time resolution of ∼1 μs. As a cost, the detector estimates the ‘effective’ energy of a given shape of γ-ray spectra and sacrifices the energy resolution. In aid of the new measurement capability, a rapid, inhomogeneous growth of RE tail is observed in detail during a major disruption triggered by an argon pellet. It is found that both the population and energy of a well-confined RE tail significantly oscillate at the early period of the growth. The oscillation phase is locked to a slow rotating magnetohydrodynamic instability, which is briefly destabilized for only ∼1 ms at the early period of the current quench. The oscillation ceases promptly, when the mode disappears. The data suggests that the high-energy RE tail is well-confined and accelerated via a localized helical structure in the plasma core.

Section: Discussion and Summarysupporting

confidence: 79%

Experimental evidence of runaway electron tail generation via localized helical structure in pellet-triggered tokamak disruptions

Eidietis

Hollmann

et al. 2021

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“…Recently, a new tomographic inversion method [53] based on L1 regularization or lasso regularization using conventional Fourier-Bessel eigenfunctions or the Laplacian eigenfunctions has shown success in overcoming the noise problems in the tangential viewing imaging data [54]. The success stimulates the interest in the development of similar methods for the velocity-space tomography.…”

Section: Discussion and Summarymentioning

confidence: 99%

Resolving the fast ion distribution from imaging neutral particle analyzer measurements

Zeeland

Heidbrink

et al. 2020

A recently developed imaging neutral particle analyzer (INPA) on the DIII-D tokamak (Du 2018 Nucl. Fusion 58 082006) enables fast ion velocity-space tomography of high fidelity at the interrogated phase space. To accomplish this, the spatial and energy depending fast (E < 80 keV) neutral flux towards the INPA stripping foils is calculated with FIDASIM and a newly developed code INPASIM simulates the INPA instrumental response to this neutral flux. Included in INPASIM is the neutral-foil interaction, the Larmor orbit tracing between the foil and the phosphor, the phosphor response to the incident ion flux as well as camera focusing. Benefiting from heavy, localized velocity-space weights and excellent signal to noise, computed tomography using the Ridge regression method is able to successfully reconstruct fine-scale velocity-space structures produced by multiple neutral beams separated by as small as ∼3 keV in tests. Applying the inversion method to a sawtooth crash event reveals a significant profile flattening of confined passing particles across q = 1 flux surface, as well as a redistribution of fast ions into the trapped orbits at the plasma edge close to the last closed flux surface.

“…Should early prevention and asynchronous avoidance both fail to stop an oncoming disruption, mitigation and alternative softlanding methods are necessary to minimize the potential for device damage. A new soft-landing technique has been developed which greatly aids in slowing the current quench, and has routinely enabled high current (I p ∼ 1.7 MA) discharges to survive major disruption events with long ∼100 ms current quench times [18]. The technique is applied by reinforcing the destabilization of core MHD modes with large externallyapplied 3D fields after the thermal quench.…”

Section: Helical Alternative To Mitigationmentioning

confidence: 99%

“…The formation of the 3D helical state is observed and reconstructed using a combination of a dual tangentiallyviewing soft x-ray (dual-SXI) system, electron cyclotron emission, and TS measurements [18]. The successfully formation of the helical state and its later flux-surface healing is observed to depend on the plasma density after the thermal quench.…”

Section: Helical Alternative To Mitigationmentioning

confidence: 99%

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

Barr¹,

Sammuli²,

Humphreys³

et al. 2021

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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.