Charge Separation Caused by Fast Ion Trajectories and Losses

Neutral beam injection (NBI) is ionized inside the plasma, forming fast ions and electrons. While electrons remain near the ionization flux surface, fast ions move across flux surfaces, causing charge separation and generating a radial electric field Er [1].

imilarly, charge separation arises from various fast ion losses. Electrons associated with fast ions lost to the chamber walls (wall losses) remain confined in the plasma. Charge-exchange (CX) losses, enhanced near the plasma edge by elevated neutral density, generate thermal ions on outer flux surfaces. The resulting thermal ions and the associated electrons from the original fast ion ionization are also shifted.

We evaluate the contributions of different charge separation mechanisms for various NBI directions. FIDASIM [2] simulates NBI attenuation, defining electron density, EBdyna [3] provides fast ion density and losses, DEGAS2 [4] computes 2D background neutrals for CX. Confined fast ions and CX losses generate opposing charge separation for co- and counter-NBI, with different spatial profiles. Wall losses shift positive charge outward in all cases. Charge separation from CX and wall losses peaks in the pedestal and edge plasma regions. The resulting radial electric field can contribute to the overall E×B shear in the pedestal. Since transition to QHmode is limited by E×B shear threshold [5], we evaluate the NBI induced charge separation in a DIII-D QH-mode discharge.