Design and initial results from the "Junior" Levitated Dipole Experiment

TL;DR

The paper presents OpenStar’s Junior, a compact levitated dipole experimental platform that integrates a high-field REBCO core magnet with an onboard HTS flux pump in a 5.2 m vacuum chamber to study magnetically confined plasmas at fusion-relevant conditions. Building on LDX and RT-1 heritage, Junior emphasizes a simplified, low-cost architecture with open diagnostic access and future full levitation capability planned for 2025. The initial results demonstrate successful magnet cooling and current ramp using the flux pump, and 17 helium-plasma shots in late 2024 reveal end-point losses and wall-pumping effects that guide future upgrades toward levitated operation and improved edge physics investigations. This work delivers a scalable, flexible platform for high-$\beta$ dipole physics and HTS-based fusion technology development, with potential impact on both basic plasma science and compact fusion concepts.

Abstract

OpenStar Technologies is a private fusion company exploring the levitated dipole concept for commercial fusion energy production. OpenStar has manufactured a new generation of levitated dipole experiment, called "Junior", leveraging recent advances made in high-temperature superconducting magnet technologies. Junior houses a ~5.6 T REBCO high-temperature superconducting magnet in a 5.2 m vacuum chamber, with plasma heating achieved via < 50 kW of electron cyclotron resonance heating power. Importantly, this experiment integrates novel high temperature superconductor power supply technology on board the dipole magnet. Recently OpenStar has completed first experimental campaigns with the Junior experiment, achieving first plasmas in late 2024. Experiments conducted with the full levitated system are planned for 2025. This article provides an overview of the main results from these experiments and details improvements planned for future campaigns.

Figures (7)

• Figure 1: Radial cross-section showing the magnetic flux of the Junior levitated dipole device. The floating core magnet is levitated by the fixed top magnet. The solid colored lines show the equilibrium flux contours for a plasma confined by the core magnet operating at $60$ % of its design current (see Sec. \ref{['sec:coremag']}). The FCFS and LCFS are represented by dashed orange lines.
• Figure 2: A photograph of the $5.2$ m wide vacuum vessel for the Junior experiment.
• Figure 3: Cross section view of the Junior core magnet showing the positions of the 14 NI HTS coils (orange boxes) and the calculated magnetic field strength, $B$.
• Figure 4: A schematic representation of a flux pump. The primary circuit (left side) is constructed from normal copper conductor and is driven with a low current oscillating waveform. The secondary circuit (right side) is made of HTS and rectifies the current through the inductive load using electromagnets to drive HTS into the normal conducting state.
• Figure 5: (a) A side view schematic of the Junior experiment showing the vacuum vessel, the core and top magnets, calculated equilibrium flux contours, and a subset of diagnostics (see main text). The first and last closed flux surfaces are shown as dashed blue lines. ECRH resonance contours at $2.45$ GHz, $6.4$ GHz, and $10.5$ GHz are shown as orange, red, and brown lines respectively. (b) A top view schematic of the Junior experiment showing the vacuum vessel, the core magnet, and a subset of diagnostics (see main text).