Exploring the impact of pedestal collisionality on NSTX H-mode confinement*

In the National Spherical Torus Experiment (NSTX), H-mode energy confinement and transport in the core are strongly influenced by collisionality, B_tτ_E,th ∝ν^*_e ^-0.67 [1]. We extend this study to explore the impact of collisionality on the H-mode pedestal using the same NSTX data set. In these NSTX discharges, a lower collisionality regime was made accessible by employing lithium evaporation for wall-conditioning between plasma shots. The NSTX data shows improved confinement in the core and a significant reduction in electron transport accompanied by a broadening in the electron temperature profiles at lower (electron) collisionality, leading to narrower pedestal widths. The pedestal width-height scaling is determined using the PyPed and equilibrium reconstruction tools [2] on electron temperature and density profiles obtained from the Thomson scattering diagnostic. The peeling-ballooning mode stability boundaries for the NSTX pedestal at different collisionalities will be evaluated and compared with existing stability studies on other devices.

 

[1] Kaye et al., Nucl. Fusion 53 (2013) 063005.

[2] Osborne et al., Nucl. Fusion 55 (2015) 063018.