Operational stability and beam transport constraints in long-pulse neutral beam injection on KSTAR

Reliable, long-pulse neutral beam injection (NBI) is essential for sustaining steady-state, high-performance plasma scenarios in the Korea Superconducting Tokamak Advanced Research (KSTAR). Whereas previous developments mainly focused on enhancing beam power and ion-source stability, this paper examines the technological and transport constraints encountered during KSTAR discharges lasting >100 s. During the stable reference discharge (Shot #34705), the monitored G1-grid cooling-water temperature rise approached a quasi-steady level within approximately 40 s under the present operating and cooling conditions, indicating a stable thermal response in the monitored channel. In a degraded operational scenario, however, an ion-source gas-supply fault altered the source pressure, inducing a beam perveance drift from 1.6 to 1.5 microperveance, where 1 microperveance corresponds to 10 − 6 A V − 3 / 2 . The estimated effective neutral beam power delivered to the plasma was lower at 90 s than at 20 s, whereas the extracted ion beam power changed only modestly. Although the analysis is based on two representative discharges and does not provide statistical verification of a unique causal mechanism, the combined power and heat-load observations are qualitatively consistent with increased post-extraction transport losses associated with perveance drift. Analyses of beam-duct pressure, local temperature, and particle trajectories further suggest that pressure-dependent re-ionization and the subsequent deflection of re-ionized particles by vertical stray magnetic fields may contribute to localized downstream beamline heating. This interpretation is consistent with previous Monte Carlo simulations predicting increased transmission losses under elevated-pressure and stray-field conditions. Altogether, these findings indicate that reliable long-pulse NBI operation requires integrated management of beam optics, chamber pressure, and the local magnetic environment to mitigate transport-related constraints.