Permeation measurements for investigating atomic hydrogen flux and wall pumping/fuelling dynamics in QUEST

► We characterized a permeation probe to investigate atomic hydrogen flux in QUEST. ► Atomic hydrogen fluxes during several operation parameters are estimated. ► Positive correlation between permeation flux and H α intensity is observed. ► Fuel particle balances are performed to investigate progress in wall conditioning. ► Correlation between the atomic flux and wall pumping behavior is observed. In order to investigate the overall atomic hydrogen background and the dynamic characteristics of wall pumping/fuelling phenomenon, a permeation probe system has been developed and applied in the spherical tokamak QUEST. Reliability of measurements, within ±3% accuracy and a positive correlation with the hydrogen line emission over three orders of magnitude have been demonstrated for more than 3000 various plasma discharges. By comparison of the experimental permeation (flux) curves with the numerically simulated curves, the net incident atomic hydrogen flux is evaluated in the range of 1 × 10 19 H m −2 s −1 to 4 × 10 20 H m −2 s −1. The atomic flux has been investigated as a function of various plasma operation parameters like RF power, gas pressure and magnetic configuration. Using the static particle balance and permeation measurements, the progress in wall conditioning has been investigated. An inverse correlation between the atomic hydrogen flux and improvement in wall pumping has been observed over the two campaigns.