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

Masline, R.; Bykov, I.; Moyer, R. A.; Wingen, A.; Guterl, J.; Rudakov, D.; Wampler, W. R.; Wang, H. Q.; Watkins, J. G.; Orlov, D. M.

doi:10.1088/1741-4326/ac9cf1

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

Journal Article · Tue Dec 06 00:00:00 EST 2022 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/ac9cf1· OSTI ID:1902578

; Bykov, I.; Moyer, R. A.;

;

; Watkins, J. G.;

We assess the toroidal magnetic field B_t asymmetry in DIII-D due to a misalignment of the toroidal field coils with respect to the poloidal magnetic field coils and vacuum vessel. The peak-to-peak variation of the divertor strike point (SP) radius is measured to be 1 cm, with an n=1 toroidal pattern. We use the center of a narrow carbon deposition band on tungsten-coated divertor tiles just inside the outer strike point (OSP) as a proxy for the divertor strike point location. The band occurred in a series of reverse B_t discharges with the OSP positioned on the divertor inserts due to strong E×B 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) is a 5mm shift towards 310° toroidal direction. These measurements agree well with previous measurements of the 3D magnetic field distribution, simulations performed by the MAFOT field line integration code, and recent Langmuir probe measurements in the Small-Angle-Slot (SAS) divertor. Comparison these measurements in the SAS divertor also indicate that there is a tilt (in conjunction with the shift) of the B_t coil field of 0.04° toward the toroidal angle of 215°. Previous measurements suggested a field misalignment of 4.6 mm in the 270° toroidal direction, and a tilt of 0.06° towards the 114° toroidal direction, which is similar to the results reported here, but also suggests that the B_t field misalignment has evolved over time with more than 100,000 coil pulses over the intervening 20 years. Furthermore, these studies will be important for better understanding the radial variation of the toroidal strike line in DIII-D, designing the new generation of the SAS divertor, and developing an understanding of the impact of error fields on tokamaks with tightly baffled slot divertors.

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Research Organization:: General Atomics, San Diego, CA (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)

Grant/Contract Number:: FG02-07ER54917; AC05-00OR22725; FC02-04ER54698; FG02-05ER54809; NA0003525; SC0018030

OSTI ID:: 1902578

Alternate ID(s):: OSTI ID: 1894539; OSTI ID: 1895329; OSTI ID: 2282931

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Vol. 63 Journal Issue: 1; ISSN 0029-5515

Publisher:: IOP PublishingCopyright Statement

Country of Publication:: IAEA

Language:: English

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