Effects of Injected Current Streams on MHD Equilibrium Reconstruction of Local Helicity Injection Plasmas in a Spherical Tokamak

Weberski, Justin D.; Bongard, Michael W.; Diem, Stephanie J.; Fonck, Raymond J.; Goetz, John A.; Nornberg, Mark D.; Reusch, Joshua A.; Schaefer, Carolyn E.; Sontag, Aaron C.

doi:10.1007/s10894-024-00460-5

Effects of Injected Current Streams on MHD Equilibrium Reconstruction of Local Helicity Injection Plasmas in a Spherical Tokamak

Journal Article · Sat Oct 12 00:00:00 EDT 2024 · Journal of Fusion Energy (Online)

DOI:https://doi.org/10.1007/s10894-024-00460-5· OSTI ID:2466166

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University of Wisconsin-Madison, WI (United States); University of Wisconsin-Madison
University of Wisconsin-Madison, WI (United States)

Open field line currents are intrinsic to DC helicity injection plasma startup and pose a challenge for inferring the plasma equilibrium with standard reconstruction analysis. Local helicity injection (LHI) is a type of DC helicity injection which uses small, modular current sources to drive force-free current along helical field lines to produce tokamak plasmas. MHD modeling and magnetic measurements during LHI indicate the injected current streams remain coherent as helical structures on the outboard edge of a core toroidal plasma that is tokamak-like in a toroidally averaged sense. To extract core plasma equilibrium properties, external magnetic diagnostics corrected for contributions from the injected current streams are fitted by a standard Grad-Shafranov equilibrium code. An iterative approach for estimating and subtracting the stream contributions from the diagnostic signals is described and applied to a model equilibrium database to reduce systematic errors introduced by the streams. Convergence is usually attained with 2 to 4 iterations, with derived equilibrium parameters matching the prescribed axisymmetric core values to within estimated experimental uncertainties. Accurate recovery of core parameters occurs when the ratio of the net toroidal windup current from the streams to the core plasma current is less than 0.2, which is typically satisfied in most experiments.

View Accepted Manuscript (DOE)

Research Organization:: University of Wisconsin-Madison, WI (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: SC0019008

OSTI ID:: 2466166

Journal Information:: Journal of Fusion Energy (Online), Journal Name: Journal of Fusion Energy (Online) Journal Issue: 2 Vol. 43; ISSN 1572-9591

Publisher:: SpringerCopyright Statement

Country of Publication:: United States

Language:: English

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