Misalignment of magnetic field in DIII-D assessed by post mortem analysis of divertor targets

We assess the magnetic field toroidal asymmetry in DIII-D present due to a misalignment of the toroidal field coils with respect to the vessel structure. The peak-to-peak variation of the radial strike point (SP) location is measured to be 1.0 cm, with n=1 toroidal pattern. We use the center of a narrow C deposition band present on tungsten-coated divertor tiles just inside the outer SP as a proxy for the magnetic strike point location. The band occurred in a series of rev. B_t discharges during the Metal Rings Campaign due to strong ExB drift transport of C from the inner to the outer SP through the private flux region. The variation in band radius (and hence the magnetic SP) will be compared to measurements of the 3D magnetic field distribution [1], simulations performed by the TRIP3D field line tracing code, and recent Langmuir probe measurements in the Small-Angle-Slot (SAS) divertor [2]. These studies will be important for better understanding the radial variation of the toroidal strike line in DIII-D and for designing the new generation of the SAS divertor.

[1] M. Schaffer, et al, Bull. Am. Phys. Soc. 52, 164 (2007)

[2] J.G. Watkins et al., PSI-23