STAR_Lite: A stellarator designed to experimentally validate non-resonant divertors

Abstract

The non-resonant divertor (NRD) offers a promising exhaust solution for stellarators, combining topological simplicity with resilience to magnetic field perturbations. To experimentally validate the robustness of non-resonant divertors in a quasi-axisymmetric (QA) configuration, we introduce STAR_Lite, a new stellarator experiment at Hampton University. This paper details the design and analysis of the first STAR_Lite coil configuration, STAR_Lite-A. The two field-period configuration manifests an NRD through X-points with zero rotational transform, at the top and bottom of the device. The divertor legs extruding from the X-points are topologically similar to the poloidal divertors of tokamaks. To expand the experimental range, STAR_Lite-A is optimized for experimental flexibility, producing a wide range of distinct QA configurations by only varying the currents in the modular coils. The NRDs not only persist across these configurations, but numerical strike-line simulations confirm that heat exhaust remains resilient to changes in coil currents, with plasma following the divertor legs and creating a toroidal, discontinuous, strike pattern. We further examine the resilience of the NRD to magnetic perturbations caused by manufacturing errors in the modular coils. We find that quasisymmetry and the existence of X-points is well-preserved under these magnetic field changes, but the rotational transform may vary substantially and displacements of the divertor X-points may lead to one X-point having a dominant effect on edge transport. Overall, our analysis indicates a compact, modular design can likely generate a resilient NRD structure while satisfying the practical constraints of a university-scale experiment.

Figures (12)

• Figure 1: (a-d) Design A viewed from different perspectives; the stable and unstable manifolds emanating from the X-point are shown in red, and the toroidal surface on which quasisymmetry is optimized is shown in purple. The four $L$-coils, and two $T$-coils in black and blue, respectively. In (e), we show students winding a full size $T$-coil at the HU Fusion Lab.
• Figure 2: (Left) Visualization of the copper cable (orange, blue, purple) wrapped around the stainless steel central spine (silver), demonstrating "spine-based" coil winding. (Right) A cross-section of the winding pack showing three layers of copper cable (orange, blue, purple) forming a hexagonal lattice around the center spine (silver).
• Figure 3: a) The quasisymmetry error as a function of the rotational transform on the optimized surface, obtained by varying the current ratio in $L$ and $T$-coils. The solid and dashed blue lines correspond to QUASR-0104183 before and after the symmetrization procedure, respectively. b) The currents in the $L$ and $T$-coils as a function of the rotational transform on the optimized surface. For each device, the field strength on the magnetic axis is $B_0$. In panels a) and b), the vertical dashed lines correspond to the rotational transforms targeted on the optimization surface. c) The $(R, Z)$ positions of the upper X-points for the three $\iota_k$-configurations as the cylindrical angle $\phi$ is varied from $0$ to $\pi$. The dot indicates cylindrical $\phi=0$, and $\phi$ increases in the clockwise direction, indicated by the arrow.
• Figure 4: Loss fraction as a function of time for two electron energies ($20\,\mathrm{eV}$ and $2.86\,\mathrm{keV}$) for the three iota configurations. The solid and dashed blue lines correspond respectively to the original QUASR-0104183 design and its symmetrized counterpart, while the solid red line corresponds to design A.
• Figure 5: (a) Rotational transform profiles for the three design A configurations all the way out to the last closed flux surface, indicated by vertical lines. The inset shows the shape of the last closed flux surface. (b) - (d) Poincaré sections and invariant manifolds associated with the X-points, for the three design A configurations (rows), at four values of $\phi$ (columns). The black curve on the Poincaré sections denotes the optimization surface on which the rotational transform is targeted.