SOL Width Dependence on Seperatrix Parameters in ST40 and NST

The Scrape-off Layer (SOL) power fall-off length, l_q is used to characterize the divertor heat load in tokamaks and extrapolating how it will behave under reactor relevant conditions is of utmost importance for future fusion reactors. Recent advances in understand the link between l_q and separatrix plasma quantities such as the electron density, n_{e, sep} and temperature, T_{e, sep} has led to a better understanding of the underlying, competing physical processes occurring in the edge plasma that control l_q. The SepOS framework [1] is used to analyze data from two spherical tokamaks (ST’s). ST40 is a compact, high magnetic field (B_tor ≤ 2T) ST designed, built, and operated by Tokamak Energy Ltd in the United Kingdom [2], and NSTX (B_tor ≤ 0.55T) operated by Princeton Plasma Physics Laboratory in the United States. ST40 can operate up to 0.7 MA of plasma current, I_p and ≤ 1.8 MW of neutral beam injected (NBI) power in mostly disconnected double null (DN), diverted, H and L-mode discharges. NSTX [3] operated with up to 7.5 MW of NBI power in lower single-null (LSN), diverted, H-mode discharges. Analysis of combined datasets from each ST shows a proportional relationship between l_q and the square root of the separatrix electron pressure, P_{e, sep} similar to findings from Brunner[4] when assuming Spitzer-Harm parallel thermal conductivity[5] to assign the location of the magnetic seperatrix. Further analysis is on-going.



[1] T Eich, et. al. Nucl. Fusion. 60 (2020) 056016

[2] M. Gryaznevich, O Asunta and the Tokamak Energy Ltd Team, Fus. Eng. Design. 123 (Nov 2017) 177-180

[3] M Ono, et. al. Nucl. Fusion. 41 (2001) 1435

[4] D Brunner, et. al. Nucl. Fusion. 58 (2018) 094002

[5] T Eich, et. al. Nucl. Mater. Energy. 42 (2025) 101896