The onset distribution of rotating $m,n=2,1$ tearing modes and its consequences on the stability of high-confinement-mode plasmas in DIII-D

Journal Article · 31 October 2023 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/ad0488 · OSTI ID:2204433

Bardóczi, L. ; Richner, N. J. ; Logan, N. C.

Abstract Analysis of a multi-scenario database of over 13 000 DIII-D H-mode discharges shows that the $m,n=2,1$ magnetic islands are dominantly pressure gradient driven, stochastically triggered non-linear instabilities at all edge safety factor ( $q_{\text{95}}$ ) values. The instability onset time closely follows the exponential distribution in intermediate and high $q_{\text{95}}$ scenarios and is characterized by near constant onset rate ( λ ), in accordance with Poisson-point processes. This implies that the plasmas are operated in marginally stable conditions, characterized by a small threshold for instability growth and variations in the trigger amplitude and/or the stabilizing mechanisms with temporally uniform random distribution in this database. While the majority of the tearing modes occur in the first current-profile relaxation time of the ${\beta }_{\text{N}}$ flattop, constant λ throughout the ${\beta }_{\text{N}}$ flattop shows that the tearing onset is insensitive to the evolution of the equilibrium current profile. In low $q_{\text{95}}$ scenarios, where a large fraction of the plasmas are operated at low torque, λ increases over the course of the ${\beta }_{\text{N}}$ flattop, showing that these plasmas evolve toward more unstable conditions. The onset rate rapidly increases with ${\beta }_{\text{N}}$ , while it does not show a clear dependence on the current gradient at the mode rational surface. Overall, these observations support that the majority of the analyzed 2,1 tearing modes are non-linear, neoclassically driven instabilities and classical stability does not play a dominant role in their onset.

• Research Organization: General Atomics, San Diego, CA (United States)
• Sponsoring Organization: USDOE; USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: AC52-07NA27344; FC02-04ER54698; SC0014264
• OSTI ID: 2204433
• Report Number(s): DOE-GA--54698
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 12 Vol. 63; ISSN 0029-5515
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: IAEA
• Language: English