The case for toroidally symmetric limiters within DIII-D

Toroidally symmetric limiters within DIII-D would symmetrize the SOL near the wall, enabling measurements taken at/near the wall to be representative of plasma conditions at every toroidal angle. This would create a significantly more diagnosable and interpretable far-SOL, analogous to how poloidal divertors create a toroidally symmetric near-SOL. This would enable DIII-D to make reliable measurements at/near the wall and to take a leading role in investigating the issue of unacceptably high levels of plasma-wall interactions in reactor-scale devices. The current DIII-D wall is protected by three discrete midplane "bumper limiters" at different toroidal angles. This creates a complicated 3D SOL wherein each measurement at the wall depends on which toroidal angle it was taken at. Field line tracing calculations show that the toroidal variation in the connection length can vary by a factor of ~3. Toroidal limiters are shown to eliminate this variation. Additionally, a 3D far-SOL does not allow 2D codes, such as SOLPS-ITER and OEDGE, to reliably simulate conditions near the wall. A 2D far-SOL would enable reliable studies of the filamentary nature of the far-SOL upon the main-wall, such as the relationship between density shoulder formation and connection length.