Recent Publications

Alpha particle losses due to ripple and field errors in a SPARC like tokamak

Hugo Emilio Ferrari, Agustín Mairotta, Cesar F Clauser, Ricardo Farengo, Justo Andres Gonzalez Litardo, Alexandra LeViness, Steve Scott

Universidad Nacional de Cuyo, Commonwealth Fusion Systems LLC, Centro Atomico Bariloche, Massachusetts Institute of Technology, Comisión Nacional de Energía Atómica

In this work we study the alpha particle losses due to ripple and coil misalignments in the SPARC tokamak. To calculate the effects of ripple and coil misalignment, we developed a code named B3D. The ripple field calculated by this code is combined with 2D equilibrium fields and predicted plasma profiles and used in the FOCUS code to calculate the alpha particle losses. We benchmarked our results against previous studies with the ASCOT and SPIRAL codes, obtaining excellent agreement. When the effect of coil misalignment is included, the alpha particle losses increase and localized regions with significantly larger values of the power density through the last closed flux surface appear. This appears to be due to the low order modes induced in the magnetic field by the misalignment.

Magnetic equilibrium of the CN-H1 heliac: A comparative study of design,as-built, and historically built configurations

Zhengkun Gao, Axel Koenies, Jinjia Cao, Hengqian Liu, Guodong Yu, Dong Xiang, Caoxiang Zhu, Yong-Zhi Dai, Haipeng Wu, Michael Drevlak, et al.

University of South China, Max Planck Institute of Plasma Physics Greifswald Branch, University of Science and Technology of China, Anhui Liliang Technology Development Co., Ltd, Hefei Keye Electrical Physical Equipment Manufacturing Co., Ltd

The relocation of the Australian H-1 National Facility (H-1 NF) to China and its reconstruction into the CN-H1 device necessitated a rigorous reassessment of its magnetic configuration to address systematic deviations introduced dur ing the engineering process.This paper presents a comprehensive comparative study of the magnetic equilibrium across three distinct definitions of the de vice: the idealized H-1 Design, the historically built as-built H-1 NF, and the newly reconstructed CN-H1. High-precision laser tracker metrology was em ployed to generate a high-fidelity coil model for CN-H1, incorporating the re manufactured poloidal and helical windings. Calculations of the vacuum mag netic field reveal that, compared to the H-1 NF, the CN-H1 exhibits a systematic inward radial shift of the magnetic axis, a slight increase in the rotational trans form, while maintaining a comparable level of stellarator symmetry breaking. Three-dimensional MHD equilibria were reconstructed using the VMEC code and validated against field-line tracing. Comparative analysis across represen tative configurations demonstrates that the defining characteristics of the flexi ble heliac—including high rotational transform, shear tunability, and magnetic well depth—are preserved in CN-H1. Magnetic spectra in Boozer coordinates of CN-H1 are nearly indistinguishable from the H-1 NF historically built baseline, despite coil positioning errors in the millimeter range. These results validate the engineering fidelity of the CN-H1 reconstruction and establish a precise equilib rium basis for future finite-beta plasma experiments.

Overview of the helios design: A practical planar coil stellarator fusion power plant

C.P.S. Swanson, S.T.A. Kumar, D.W. Dudt, E.R. Flom, W.B. Kalb, T.G. Kruger, M.F. Martin, J.R. Olatunji, S. Pasmann, L.Z. Tang, et al.

Quasi-single-stage optimization for advanced stellarators

Guodong Yu, Yidong Xie, Hengqian Liu, Caoxiang Zhu

Advanced stellarator design requires a balance between plasma performance and the manufacturability of three-dimensional modular coils. In conventional two-stage optimization, the coils required to realize an optimized equilibrium can be limited by engineering feasibility. Here, we develop a quasi-single-stage (QSS) framework that incorporates coil feasibility directly into plasma-boundary optimization. QSS uses the maximum normalized normal-field error, evaluated rapidly from surface currents on a uniformly offset winding surface, as a coil-feasibility surrogate. We apply this method to optimize configurations targeting quasi-axisymmetry, quasi-helical symmetry, quasi-isodynamicity, and a combination of omnigenity with piecewise omnigenity. The QSS-optimized configurations exhibit smoother plasma boundaries and winding surfaces, lower normal-field reconstruction errors, and reduced coil complexity, while preserving favourable magnetic-symmetry and transport properties. QSS provides a practical proof-of-principle strategy for co-optimizing plasma physics and coil engineering in stellarator design.

Controllable Demagnetization Strategy for Magnet Power Supply Based on Linear-Region MOSFETs

Hao Yin, Yanan Wu, Jing Lu, Jun Li, Baocan He, Liuwei Xu, Yunxiang Tian, Pengfei Wang, Nan Liu, Xiao Ma, et al.

Chinese Academy of Sciences, Institute of Energy, Hefei Comprehensive National Science Center

Multisampling Linear Active Disturbance Rejection Control with an Improved Extended-State Observer for the EAST Fast Control Power Supply

Haihong Huang, Jiacheng Wang, Haixin Wang

Hefei University of Technology

High-frequency current monitoring system for superconducting magnet insulation assessment and preliminary experimental verification

Shuliang Ma, Yezheng Xiao, Liang Guo, Jianhua Yang, Yuanyuan Ma, Xiaofeng Han, Shuqing Zhang, Tuo Wu, Yanlan Hu, Huajun Liu

Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei International Applied Superconductivity Center, University of Science and Technology of China

Optimized finite-$β$ tokamak-stellarator hybrid configurations achieved by planar dipole-field coils

Yihui Liang, Hengqian Liu, Guodong Yu, Zhenyu Zhou, Caoxiang Zhu, Yao Zhou

Tokamak--stellarator hybrids seek to combine tokamak-like compactness and confinement with stellarator-like externally generated rotational transform and steady-state operation. In this work, we build on the recent tokamak--stellarator hybrid study using planar dipole-field coils (PDCs) [Yu et al., arXiv:2605.03599], in which the fixed-position, programmable coils on an axisymmetric winding surface generate flexible three-dimensional shaping fields. Using single-stage free-boundary optimization of coil currents and plasma-equilibrium parameters, we construct vacuum and finite-$β$ configurations. The vacuum cases show controllable external transform and magnetic well. The finite-$β$ cases accommodate various density, temperature, and pressure profiles, producing quasi-axisymmetric (QA) equilibria with self-consistent bootstrap current, favorable Mercier stability, and reduced demand for external current drive. Re-optimization enables $β$ ramp-up and access to different field-period QA branches with moderate coil-current changes. At large rotational transform, a toroidally omnigenous (TO)-like configuration exhibits more favorable infinite-$n$ ideal-ballooning behavior than a QA reference with matched profiles, even though ballooning stability is not directly optimized for. These results demonstrate that PDCs provide a flexible platform for achieving optimized finite-$β$ hybrid configurations.

Integrated physics and magnet design for the ST-E1 fusion power plant

E.N.J. Maartensson, N. Welch, M. Scarpari, F. Malinowski, M. Borscz, M. Bristow, P.F. Buxton, J. Kang, M. Kruip, A.V. Langtry, et al.

Tokamak Energy Ltd

ST-E1 is Tokamak Energy’s commercially competitive fusion power plant design featuring a lifetime high-temperature superconducting (HTS) magnet cage, which has completed its pre-concept design stage. A central challenge at the pre-concept stage is the need to iteratively and consistently integrate the development of the magnet cage, core plasma physics, and power-exhaust systems, while avoiding serial design loops and late discovery of infeasible operating scenarios. Accurate representation of inductive current drive is particularly critical, as it tightly couples magnetic equilibria, flux swing, and HTS operational limits across these systems. To address this challenge, a novel integrated workflow has been developed to enable rapid, early-stage iteration during the pre-concept design phase. A key novelty is the deliberate de-coupling of magnetic equilibria generation from coil optimisation and detailed engineering, allowing a broad range of time-dependent operating scenarios to be explored without repeated high-fidelity coil redesign. The workflow combines Metis, a fast 1.5D fixed-boundary modelling tool, with FreeGS, a free-boundary equilibrium solver, to generate flat-top and time-dependent ramp-up scenarios together with the associated flux-swing traces and magnetic equilibria. These outputs are passed to SCOPE, an in-house tool that optimises poloidal field and central solenoid coils based on physics requirements while respecting HTS operational limits. The resulting coil designs are then refined within a dedicated coil-engineering workflow performing higher-fidelity structural, electromagnetic, and HTS analyses. The integrated workflow has been applied to evaluate a wide range of ramp-up, ramp-down, and flat-top scenarios, enabling consistent down-selection of technically achievable operating points across all phases of ST-E1operation. The resulting set of coils and operating scenarios collectively satisfy the full system requirements of the ST-E1 pre-concept design.

Nuclear technology considerations and neutronics for the ST-E1 fusion power plant

Kamya Chandrasekhar, Christopher Lister Wilson, Samara Michelle Levine, Sandeep Irukuvarghula, Vandeep Bharatbhai Godhani, Jonathan David Naish, Ariel Aníbal Márquez, Michael J Loughlin

Tokamak Energy Ltd, Oak Ridge National Laboratory

This paper presents Tokamak Energy’s pre-concept design of ST-E1, a low aspect ratio tokamak fusion reactor. Efficacy of radiation shielding in protecting the high temperature superconducting magnets systems is a major driver dictating tokamak size, plant lifetime, and balance of plant. It is crucial that neutronics and material considerations are incorporated into plant design from the earliest design stages and comprehensively taken into account across all areas through the systems engineering framework. At Tokamak Energy, a dedicated workflow for plant-scale neutronics simulations has been developed using G4Tokamak, an in-house application based on the Geant4 toolkit. This workflow enabled faster and more efficient design iterations, and its results were verified through benchmarking against MCNP. A four-part shielding architecture, comprising of the centre column, vacuum vessel, outboard horizontal and vertical elements, was designed to balance neutronics performance, thermal management, structural demands, and cost. The shielding system, at this pre-concept stage, demonstrates the feasibility of limiting the neutron flux to the magnet systems to support a full power life of 30 years, and limiting the nuclear heating of the cold mass within cryogenic cooling capacity. Activation studies highlight maintenance and long-term waste management challenges, with non-reduced activation ferritic martensitic steels and plasma facing components showing the highest activation. Collectively, these results establish a comprehensive neutronics and materials framework, baseline core shielding design for ST-E1, and demonstrate its readiness for progression to detailed concept development.

Proof-of-concept of heater-assisted variable dump resistor for fusion-magnet fast discharge system

Seokho Nam, Soo Hwan Park, Jinsub Kim, Kwang Pyo Kim, Young Min Park, Seunghyun Song, Yeon Suk Choi, Yojong Choi

Korea Institute of Fusion Energy (KFE), Korea Basic Science Institute (KBSI)

Towards joint optimization of stellarator coils and support structures

Lanke Fu, Alan A. Kaptanoglu

The support structure is an integral part of the design of nuclear fusion reactors, especially 3D stellarator devices. A practical reactor's coils and support structures must have three competing qualities: an accurate magnetic field for good confinement, sufficient rigidity to protect the brittle high-temperature superconductor (HTS) from damage, and a simple geometry for low-cost construction. In existing devices, the coil geometry is often optimized without knowledge of the support structures' design and the coils' true stress and deformation. The support structures are then placed by hand through repeated finite element analyses (FEA) until engineering requirements are met. This makes the structural design of stellarator coil systems lengthy and labor-intensive. Using new developments in differentiable structural mechanics, we present coil-fem, an open-source software tool that integrates support differentiable FEA into the stellarator coil optimization loop. It enables the integrated optimization of coil geometry and support clamp locations to simultaneously reduce magnetic field errors and stresses in the coil body. We also present the first combined coil-support optimization in the stellarator literature. Using a penalty term based on coil-fem, we produced a coil set with 2.4x lower RMS von Mises stress and similar field error compared to an unoptimized baseline.

An FPGA-based digital control platform with explicit stability guarantee for the CAT-1 levitated dipole magnet

Yunhui Liu, Yaoyu Xie, Guoshu Zhang

East China University of Technology

Planar coil design for the Helios stellarator fusion power plant

T.G. Kruger, T. Elder, D.A. Gates, R.H. Wu, C.P.S. Swanson, S.T.A. Kumar, D.W. Dudt, J.R. Olatunji, E.R. Flom, J. von der Linden

Thea Energy, Inc.

Performance of the W7-X ECRH gyrotron diamond output window at 2 MW operation

G. Aiello, B. Gorr, J. Jelonnek, H.P. Laqua, F. Legrand, A. Meier, T. Scherer, S. Schreck, D. Strauss, M. Thumm

Karlsruhe Institute of Technology (KIT), Max Planck Institute for Plasma Physics (IPP), Thales

Equilibrium optimization of the Helios planar coil stellarator power plant

D.W. Dudt, M. Avida, E. Flom, T.G. Kruger, S.T.A. Kumar, M.F. Martin, C.P.S. Swanson, J. von der Linden, R.H. Wu, D.A. Gates

Thea Energy, Inc.

Numerical evaluation of fatigue and fracture behavior of the CFETR CSMC helium inlet/outlet structure under cyclic electromagnetic load

Xianwei Wang, Peng Han, Wenlong Xu, Haikuo Zhao, Jie Zhou, Aihua Xu, Houxiang Han, Xiulian Li

Jiangsu University of Technology, Institute of Plasma physics, Chinese Academy of Sciences, Changzhou Vocational Institute of Mechatronic Technology, Hangzhou Chicheng Digital Technology Co., Ltd.

Fault diagnosis of fusion magnet power supply commutation failure based on Glow-PCNN

Zhenyu Xue, Liansheng Huang, Xiaojiao Chen, Xiuqing Zhang, Shiying He, Ying Zuo

Institute of Plasma Physics, Chinese Academy of Sciences, Anhui University of Science and Technology

Estimating coil features from an equilibrium

E. Rodríguez, W. Sengupta

Max Planck Institute for Plasma Physics, Princeton University

We present an explicit theoretical framework for constructing artificial modular coils for vacuum stellarator fields based solely on equilibrium properties, achieved through the formulation of a current potential defined on flux surfaces. Contours of constant Boozer toroidal angle can be directly interpreted as proxy coils, and so we demonstrate that key measures of coil complexity—particularly coil non-planarity—are strongly governed by local magnetic-field properties. This approach shows promise as predictor for more realistic coil configurations, providing both a pathway toward deeper understanding of equilibrium–coil relationships and a potential practical proxy for coil design.

Permanent magnet optimization of stellarators with coupling from finite permeability and demagnetization effects

Armin Ulrich, Mason Haberle, Alan A. Kaptanoglu

New York University

Permanent magnets provide an attractive path for shaping university-scale stellarator magnetic fields. Previous work has shown that greedy permanent magnet optimization (GPMO) can produce sparse, grid-aligned arrays that match target surfaces with high accuracy under an ideal rigid-remanence model. Here, we extend this approach to a greedy permanent magnet optimization with macromagnetic refinement (GPMOmr) by accounting for magnet–magnet and magnet–coil coupling from finite permeability and demagnetizing interactions, and apply it to the published magnet grid from the MUSE stellarator design. Finite-permeability effects produce degree-scale tilts and few-percent magnitude changes in individual magnets. When the same model is embedded in the greedy loop, GPMOmr achieves final errors within a few percent of the uncoupled GPMO code, while producing more nonuniform magnetization patterns. Our formulation provides a fast and practical tool for quantifying and incorporating finite-permeability and other coupling effects in permanent-magnet stellarator designs, and offers a framework for extending permanent-magnet optimization to higher field strengths and to materials with stronger coupling.