Modeling of Injected Current Stream-Induced 3D Perturbations in Local Helicity Injection Plasmas

TL;DR

This work addresses how local helicity injection (LHI) edge current streams perturb magnetic topology in Pegasus-III by combining a Biot–Savart-like filament model of injected streams with axisymmetric equilibrium reconstructions and linear MHD response calculations using M3D-C1. It demonstrates substantial flux-surface degradation and edge-chaotic regions that begin near $Ψ_N \approx 0.37$, and shows that rotation and two-fluid effects provide significant screening of the $n=1$ perturbation, especially at the edge, while non-rotating single-fluid cases amplify resonant fields. The study further reveals that incorporating spatial spreading or oscillatory motion of the streams yields better agreement with Hall-probe magnetic power measurements than a rigid-filament model, highlighting the need for refined stream models. Overall, accurate prediction of LHI-driven topology requires realistic rotation flow constraints and more sophisticated current-stream representations to support reliable handoff to subsequent current-drive methods.

Abstract

Solenoid-free tokamak startup techniques are essential for spherical tokamaks and offer a pathway to cost reduction and design simplification in fusion energy systems. Local helicity injection (LHI) is one such approach, employing compact edge current sources to drive open field line current that initiates and sustains tokamak plasmas. The recently commissioned Pegasus-III spherical tokamak provides a platform for advancing this and other solenoid-free startup methods. This study investigates the effect of LHI on magnetic topology in Pegasus-III plasmas. A helical filament model represents the injected current, and the linear plasma response to its 3D field is calculated with M3D-C1. Poincaré mapping reveals substantial flux surface degradation in all modeled cases. The onset of overlapping magnetic structures and large-scale surface deformation begins at $Ψ_{N} \approx 0.37$, indicating a broad region of perturbed topology extending toward the edge. In rotating plasmas, both single-fluid and two-fluid models exhibit partial screening of the $n = 1$ perturbation, with two-fluid calculations showing stronger suppression near the edge. In contrast, the absence of rotation leads to strong resonant field amplification in the single-fluid case, while the two-fluid case with zero electron rotation mitigates this amplification and preserves edge screening. Magnetic probe measurements indicate that modeling the stream with spatial spreading$-$representing distributed current and/or oscillatory motion$-$better reproduces measured magnetic power profiles than a rigid filament model. The results underscore the role of rotation and two-fluid physics in screening stream perturbations and point to plasma flow measurements and refined stream models as key steps toward improving predictive fidelity.

Figures (23)

• Figure 1: Visible camera images of the startup and relaxation process during LHI on Pegasus-III: (a) injected current streams follow helical vacuum field lines; (b) streams become unstable and reconnect; (c) a tokamak-like plasma forms and is sustained by helicity input and inductive drive; (d) after injector shutoff, the perturbation dissipates and flux surfaces re-form, enabling handoff to other current drive systems.
• Figure 2: Toroidal dependence of magnetic activity, measured via the RMS of vertical magnetic field perturbation $\tilde{b}_{Z}$, normalized to the equilibrium toroidal magnetic field $B_{T}$. Data were collected from five magnetic pickup coils distributed toroidally near the outboard midplane. The red solid line corresponds to a discharge with $I{\text{inj}} = 2.6$ kA, and the black dashed line corresponds to a discharge with $I_{\text{inj}} = 6.0$ kA.
• Figure 3: Modeled system including an axisymmetric core plasma from equilibrium reconstruction (red surface), surrounded by a helical injected current stream (blue) that originates at an injector on the plasma boundary (white marker).
• Figure 4: Time evolution of the plasma current ($I_{p}$, solid red line) and injected current ($I_{\text{inj}}$, dashed black line) for the Pegasus-III plasma discharge reconstructed for this study. The $I_{inj}$ shown represents the sum over all four injectors.
• Figure 5: Overview of the reconstructed equilibrium for a Pegasus-III LHI plasma, including: (a) poloidal magnetic flux surfaces and locations of magnetic diagnostics used as constraints; and (b–d) profiles versus normalized poloidal flux $\Psi_{N}$ of the safety factor, volume-averaged toroidal current density $\langle J \rangle$, and plasma pressure $p$.