Taylor limit studies for local helicity injection plasma startup

Weberski, Justin D.; Aslin, Mary W.; Bongard, Michael W.; Goetz, John A.; Nornberg, Mark D.; Peery, Jilliann K.; Redd, Sophie; Reusch, Joshua A.; Rodriguez Sanchez, Cuauhtemoc; Schaefer, Carolyn E.; Sontag, Aaron C.; Tierney, Timothy N.; Diem, Stephanie J.

doi:10.1063/5.0261523

Taylor limit studies for local helicity injection plasma startup

Journal Article · Tue Jun 24 00:00:00 EDT 2025 · Physics of Plasmas

DOI:https://doi.org/10.1063/5.0261523· OSTI ID:2589123

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Univ. of Wisconsin, Madison, WI (United States)

Solenoid-free tokamak startup techniques can simplify the design and reduce the cost of tokamak-based fusion energy systems. The newly commissioned Pegasus-III spherical tokamak provides a dedicated platform for developing a scalable solenoid-free startup approach. Local helicity injection (LHI) is one promising technique being developed on Pegasus-III, which uses compact edge current sources to drive open field line current I_inj that is redistributed via helicity-conserving instabilities to initiate tokamak plasmas. These instabilities relax the system toward a minimum energy state described by Taylor relaxation theory. This relaxation process imposes a global current limit for LHI plasmas, referred to as the Taylor limit I_TL ⁠, which can dictate the plasma current I_p evolution. Experiments on Pegasus and Pegasus-III show that increasing this limit leads to more effective utilization of the available helicity input and higher I_p plasmas. Initial Pegasus-III experiments have verified the expected I_TL $$\varpropto$$ (I_injB_T)^1/2 scaling holds for B_T and I_inj up to 0.3 T on axis and 12 kA ⁠, respectively. Additionally, data from different injector arrays show that the width of the injector aperture directly alters I_TL and can be leveraged for future injector design optimization. Throughout these scaling studies, discharges were overdriven at I_p = I_TL by providing excess helicity input. Intermittent bursts of n = 1 magnetic activity consistent with large-scale reconnection events occurred more often as the system was increasingly overdriven leading to a flattening of the λ = μ₀J_∥ / |B| profile, where J_∥ is the parallel current density.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of Wisconsin, Madison, WI (United States). Pegasus Toroidal Experiment

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

Grant/Contract Number:: SC0019008

OSTI ID:: 2589123

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 6 Vol. 32; ISSN 1070-664X; ISSN 1089-7674

Publisher:: AIP PublishingCopyright Statement

Country of Publication:: United States

Language:: English

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Taylor limit studies for local helicity injection plasma startup

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