Tomohiko Asai

We report experimental evidence for the self‐organized formation of a field-reversed configuration (FRC)-like high‐ β plasmoid inside a magnetic mirror field. The plasmoid forms when two plasmoids counter‐injected along an axis perpendicular to the mirror field axis collide at high speed and then merge. This ‘orthogonal merging’ geometry, which differs from conventional axisymmetric FRC merging along the device axis, can (in principle) preserve both axial ends of the mirror for reactor-relevant components such as divertors and direct energy-conversion stages. In the FRC Amplification via Translation–Collisional Merging device, two deuterium plasmoids with a line-integrated density of (1–1.5) × 10 ²⁰ m ⁻² were accelerated to ∼150 km s ⁻¹ and merged at a relative speed of ∼300 km s ⁻¹ , producing a localized high-density region within the mirror. Excluded-flux probes, internal magnetic probes, interferometry, and ion Doppler spectroscopy revealed (i) direct reversed-field signatures during the early post-collision and reformation phases, (ii) a transition from paramagnetic to diamagnetic rotation, and (iii) subsequent spin-up. Representative mid-plane interferometry measurements show that the bulk plasma density persists on a timescale comparable to the decay of the excluded-flux signal, while the longest-lived non-invasive excluded-flux traces indicate a persistence time of approximately 130 μ s, substantially longer than the estimated classical diffusion time in a simple mirror (μ s with a mirror ratio of R ∼ 2.9). Although the full magnetic topology was not reconstructed non-invasively, the combined observations are consistent with the formation of an FRC-like high- β state containing a closed-field component that reduces parallel losses relative to a simple mirror. These results indicate a feasible pathway toward mirror–FRC hybrid operation. They also broaden the options of FRC formation and high-speed fueling in linear systems while maintaining the availability of the axial ends for reactor-relevant hardware.

The damping process of magnetic turbulence induced by the collisional merging of field-reversed configurations (FRCs) was investigated using the FAT-CM device at Nihon University. Two FRC-like plasmoids collided at super-Alfvénic and supersonic speeds. During the collision phase, intense magnetic turbulence developed and temporarily disrupted the FRC-like reversed magnetic field structure. Nevertheless, the plasmoids relaxed into an FRC-like state within several tens of microseconds. High-frequency turbulence components were rapidly damped, particularly under kinetic conditions, suggesting the possible involvement of kinetic effects in the damping process. Additionally, energy conversion from kinetic to magnetic and subsequently to thermal energy was observed. These results suggest that kinetic effects may play an important role in both the rapid damping of magnetic turbulence and the associated plasma heating during the collisional merging process.

We experimentally investigate the generation and temporal evolution of toroidal ion flow in field-reversed configurations (FRCs) produced by the field-reversed theta-pinch method. Immediately after formation, the plasma exhibits rotation in the paramagnetic direction, which subsequently reverses to diamagnetic rotation within about 10 μs, reaching velocities up to 20 km/s. Complementary ion Doppler spectroscopy of the core region and Mach probe measurements in the scrape-off layer (SOL) reveal near rigid-body rotation in the core and microsecond-scale viscous transfer of angular momentum toward the SOL. The observed dynamics cannot be fully accounted for by conventional spin-up mechanisms such as particle loss, end-shorting, or flux decay. A comparison with collisional-merging FRCs demonstrates that rapid axial compression and the accompanying magnetic field reconfiguration drive an initial paramagnetic spin-up, after which the plasma self-organizes into stable diamagnetic rotation.

In this paper, we use hybrid simulation to elucidate the plasma heating mechanism due to waves excited in Field-Reversed Configuration (FRC) plasma. The plasma parameters are a separatrix radius of 0.16 m and a separatrix length of 1.16 m (x-point position is z = ± 0.57 m). The wave excitation antenna consists of two loop antennas with a radius of 0.3 m and is placed at a position of z = ± 0.5 m. The current waveform of the antenna is a sine wave with a maximum current value of 30 kA and a frequency of 160 kHz. The simulation results showed that the excited waves caused compression/expansion of the plasma, and at the same time, the temperature of the plasma increased or decreased at the compressed/expanded position. When waves are applied, a 23% increase in the volume-averaged ion temperature in the separatrix is observed compared to the case without waves applied. On the other hand, no increase in electron temperature is observed. For the electron fluid, the adiabatic condition is well established, and temperature changes are observed as the plasma compressed and expanded. On the other hand, for ions, kinetic energy perpendicular to the magnetic field lines increases during compression, and part of this energy is transferred to the energy of the parallel component by collisionless pitch angle scattering, resulting in heating due to the so-called magnetic pumping.

The field-reversed configuration (FRC) amplification via a translation–collisional merging device was used to conduct experiments where two initial FRC-like plasmoids produced by the field-reversed theta pinch method were translated and accelerated to supersonic/Alfvénic velocities before being subjected to collisional merging. The temporal evolution of the toroidal flow was measured throughout each experimental phase. The plasmoids initially rotated in the ion paramagnetic direction relative to the open-field region and subsequently spun up in the diamagnetic direction. The toroidal angular momentum of the plasmoids was conserved during the translation and acceleration phases but not during the collisional merging process. As the reversed magnetic field reformed, a toroidal flow formed in the paramagnetic direction and then accelerated in the diamagnetic direction. This behavior appears to be a unique feature of collisional merging and cannot be solely explained by the coherent magnetic field connection between the two initial plasmoids. This paper discusses the temporal evolution of the toroidal flow and the implications for the collisional merging process.

TAE Technologies’ fifth-generation fusion device, C-2W (also called ‘Norman’), is the world’s largest compact-toroid device and has made significant progress in field-reversed configuration (FRC) plasma performance. C-2W produces record breaking, macroscopically stable, high-temperature advanced beam-driven FRC plasmas, dominated by injected fast particles and sustained in steady state, which is primarily limited by neutral-beam (NB) pulse duration. The NB power supply system has recently been upgraded to extend the pulse length from 30 ms to 40 ms, which allows for a longer plasma lifetime and thus better characterization and further enhancement of FRC performance. An active plasma control system is routinely used in C-2W to produce consistent FRC performance as well as for reliable machine operations using magnet coils, edge-biasing electrodes, gas injection and tunable-energy NBs. Google’s machine learning framework for experimental optimization has also been routinely used to enhance plasma performance. Dedicated plasma optimization experimental campaigns, particularly focused on the external magnetic field profile and NB injection (NBI) optimizations, have produced a superior FRC plasma performance; for instance, achieving a total plasma energy of ∼13 kJ, a trapped poloidal magnetic flux of ∼16 mWb (based on the rigid-rotor model) and plasma sustainment in steady state up to ∼40 ms. Furthermore, under some operating conditions, the electron temperature of FRC plasmas at a quiescent phase has successfully reached up to ∼1 keV at the peak inside the FRC separatrix for the first time. The overall FRC performance is well correlated with the NB and edge-biasing systems, where higher total plasma energy is obtained with higher NBI power and applied voltage on biasing electrodes. C-2W operations have now reached a mature level where the machine can produce hot, stable, long-lived, and repeatable plasmas in a well-controlled manner.

This study successfully developed a refueling technique for a field-reversed configuration (FRC) via axial plasmoid injection and demonstrated it on the FAT-CM device at Nihon University. The target FRC is generated using the collisional-merging formation technique combined with conical theta-pinch formation. Plasmoids with an FRC-like configuration are coaxially injected from both ends of the FAT-CM device toward the preexisting target FRC. Postinjection, the system achieves equilibrium, resulting in increases by factors of 1.8 and 2.4 in the total inventory and plasma energy, respectively, compared to cases without injection. This method effectively accomplishes FRC refueling while preserving the intrinsic characteristics of a simply connected, axisymmetric configuration and a high beta value approaching unity. Therefore, this approach offers potential for repetitive refueling in the reactor stage having a FRC plasma core. Experimental outcomes are compared with magnetohydrodynamic simulation results. In the collisional merging process, the characteristics of the pre-collision plasmoids, such as the strong toroidal rotation and coherent FRC-like magnetic field structures of the FRC, are not preserved. Experimental environments have been constructed to investigate such unique properties of the resulting FRCs.

The radial motion of a field-reversed configuration (FRC) during translation at supersonic/Alfvénic speed was effectively suppressed by the global gyro-effect induced by toroidal flow. In magnetohydrodynamic approximation, an FRC is inherently unstable against radial motions such as tilt and wobble. Surprisingly, the magnetic structure of the FRC remained intact even during translation at speeds exceeding the ion sonic and Alfvén speeds, despite the presence of radial motions. Optical observations were conducted on the radial motion and toroidal flow of translated FRCs under various electric boundary conditions at the device end. The implementation of end-shorting led to an increased toroidal flow rate, consequently reducing the radial displacement of the FRC during translation. This control of toroidal flow from the device’s end not only suppressed radial motion but also facilitated head-on collisions in the formation of FRCs by the collisional merging technique. The findings emphasize the significance of toroidal flow control in preserving the integrity of FRC’s simply connected geometry and enhancing its formation performance.

Rapid magnetic reconnection was experimentally observed to occur within a microsecond timescale, leading to the field-reversed configuration (FRC) comprising a single closed magnetic field structure. The compressibility of the plasma within the FRC played a crucial role in accelerating the coalescence process, making it considerably faster than the resistive process. A strong similarity between the observed experimental results and those of the coalescence of solar flare loops indicates that astrophysical phenomena and laboratory plasmas are governed by a common physics.