Description
Recent experiments in the TJ-II stellarator using multi-pellet injection into Neutral Beam Injection (NBI)-heated plasmas have established a new enhanced confinement regime [1,2]. These discharges achieved record energy confinement times exceeding ISS04 scaling law predictions, alongside record densities and the highest ion temperatures ($T_i$) measured in the device. These conditions provide a unique opportunity to test neoclassical transport theory under high-density, high-ion-temperature conditions. This work presents a comparison between experimentally measured radial electric field ($E_r$) profiles and neoclassical simulations, highlighting the role of ambipolarity in the transition to enhanced confinement.
Simulations were performed using the monoenergetic approach via the MOCA and DKES codes to resolve the ambipolar radial electric field. The modeling reveals that the increased $T_i$ and density profiles following pellet injection drive the ambipolar $E_r$ toward more negative values, particularly in the density gradient region. These results show a consistent dependence on pellet fueling and align with experimental measurements obtained through Heavy Ion Beam Probe (HIBP) and Charge-Exchange Recombination Spectroscopy (CXRS)..
The results demonstrate that neoclassical predictions are quantitatively consistent with the experimental $E_r$ measurements obtained in these high-performance discharges. Such alignment provides a robust validation of neoclassical theory as a predictive tool for $E_r$ behavior in this TJ-II enhanced stellarator regime.
[1] K.J. McCarthy et al. Nucl. Fusion 64 066019 (2024). DOI:10.1088/1741-4326/aa9f8a
[2] I. García-Cortés et al. Phys. Plasmas 30 072506 (2023). DOI: 10.1063/5.0151395