Fueling Location Effects on Scrape-Off Layer Response in MAST-U

Density-shoulder formation on MAST Upgrade is governed by radial particle transport,

which intensifies with higher separatrix density and collisionality, extending particle con-

finement [1]. We investigate this connection by analyzing ohmic L-mode discharges from

MAST-U, compiling a mini-database that includes short gas puffs from the low-field side

(LFS) and private-flux region (PFR) as well as no-puff baselines. Time-resolved Thomson

Scattering (TS) density profiles are combined with Langmuir probe (LP) divertor signals

to track edge evolution on millisecond timescales. LFS injections produce shoulders with a

memory effect; the density remains elevated after the valve closes, with decay times three to

eight times longer than rise times [2]. PFR puffs produce only modest, short-lived changes

with little evidence of hysteresis. Comparisons of D_α signals indicate that the PFR puffing

source is too weak relative to other divertor sources to significantly influence ne, whereas

LFS main chamber puffing shows stronger D_α responses and greater impact on density

evolution [3]. These results identify fueling location and initial edge conditions as key con-

trol parameters for shoulder dynamics and can provide new experimental constraints for

edge-transport modeling of spherical tokamaks.