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To directly detect the spatial distribution of a plasma current during the collisional merging of two field-reversed configurations (FRCs) in the FAT-CM (FRC Amplification via Translation–Collisional Merging) device, an internal current probe using Rogowski coils has been developed. An FRC is a type of magnetically confined plasma maintained by a diamagnetic plasma current flowing in the toroidal direction. Self-organized FRC formation and increased poloidal flux have been observed following the destructive perturbation during the collision in collisional merging FRC formation when two initial-plasmoids collide at a relative speed of 300–400 km/s to form one FRC. It is indicated that the toroidal plasma current is driven in those processes. In this research, an internal Rogowski probe was designed and developed to have a high-frequency response to capture a rapid change of the plasma current in a few microseconds during the collision/merging process. The FRC plasma in FAT-CM has relatively high temperature (∼100 to 200 eV) and high density (∼1020 to 1021 m−3). As a result, the probe was built to be as compact as possible to minimize disruption to the plasma. Because of its high melting point and low Z property, a machinable boron nitride ceramic was chosen to shield the Rogowski coils thermally and electrically from the plasma. All connections and seams were constructed such that the epoxy glue used for the probe assembly was not exposed to the plasma.

The radial electric field in a field-reversed configuration (FRC) plasma plays an important role in the global stability and confinement properties. Herein, we developed a new Langmuir probe array named "Skewered probe" employed in measuring the radial potential profile in the collisional merging formation of an FRC in the FAT-CM (FRC Amplification via Translation - Collisional Merging) device. Because an FRC has a strong toroidal flow, the skewered probe consists of alternately skewered ring electrodes and ceramic beads on a thin stainless-steel tube to neutralize the effect of plasma flow. The developed array has nine electrodes, one every 2 cm from r = 9-25 cm, and it measures the FRC boundary in the case when the radius of the excluded flux ranges from 10 to 20 cm. The skewered probe also has one additional electrode that measures the potential near the chamber wall as a reference for the other electrodes. The radial potential profile of the FRC formed by the collisional merging method in the FAT-CM device was measured using the probe, and the results showed that the region of negative potential gradually changed to a positive potential after merging the FRCs. It was also shown that a strong outward electric field is formed near the separatrix at n = 2 rotational instability.

In this study, a newly developed correction method with external magnetic measurements for the magnetohydrodynamics (MHD) simulation of the collisional merging formation of a field-reversed configuration (FRC) realized the estimation of the internal structure of the FRCs without invasive internal measurements. In the collisional merging formation of FRCs, an FRC is formed via merging of two initial FRC-like plasmoids at supersonic/Alfvénic velocity. An invasive diagnostic may also interfere with the collisional merging formation process. A two-dimensional resistive MHD simulation was conducted to evaluate the global behavior and internal structure of FRCs in the collisional merging formation process without invasive measurements. This code simulated the initial formation and collisional merging processes of FRCs including discharge circuits. However, the translation velocity and the pressure of initial FRCs did not simultaneously agree with the experimental values because the magnetic pressure gradient in each formation region could not be reproduced without the artificial adjustment of the initial condition. The experimentally measured current distribution was given as the initial condition of the circuit calculation in the developed correction method. The initial FRCs were successfully translated at the translation velocity and plasma pressure in the corrected simulation, both of which were equivalent to the experiments. The properties of the merged FRCs in the experiments such as volume, total temperature, and average electron density were reproduced in the corrected simulation. The detailed radial profile of the internal magnetic field of the FRC was also measured and found to agree very well with the simulation results.

An internal magnetic probe array has been developed to observe the three components of the magnetic field simultaneously in the vicinity of the collision surface of two colliding plasmoids at supersonic/Alfvénic velocity. Collisional-merging formation of a field-reversed configuration (FRC) has been conducted in the (FRC Amplification via Translation-Collisional Merging) device at Nihon University. Significant plasma heating and an increase in trapped poloidal magnetic flux have been observed during/after the collisional-merging process in the FAT-CM device. In this dynamic formation process, two FRC-like plasmoids formed by a field-reversed theta-pinch method collide in the middle of the confinement chamber at a relative speed of 200-400 km/s. Therefore, the excited shockwave is considered as one of the heating mechanisms. The developed probe array installed in the middle of the confinement chamber observes the internal structure of the magnetic field. The probe consists of 12 sets of three-axis chip inductors arranged at intervals of 40 mm. The measurement position can be varied in the radial direction. In the single translation and collisional-merging experiment, the internal magnetic probe measures the magnetic field's radial distribution with a high time resolution under noise.

The super-Alfvenic acceleration of an extremely high beta plasmoid due to a magnetic pressure gradient is evidenced in translation experiments involving the field-reversed configuration (FRC) in the FRC amplification via the translation-collisional merging (FAT-CM) device. Inside the FRC, there is a population of unmagnetized particles that forms a volume with extremely high beta. Formed using the field-reversed theta pinch method, the FRC is accelerated and injected into a quasi-static confinement magnetic field that is weaker than the formation and acceleration fields. Because the FRC is injected with nonzero axial translation speed and cannot slow down, its speed exceeds the Alfvenic and sonic speed upon entering the confinement field. This phenomenon is predicted from two-dimensional resistive magnetohydrodynamic simulations, and experimental results that are consistent with the magnetohydrodynamic approximation show that the FRC acceleration process is due mainly to magnetic pressure on the thin magnetized layer.

The accumulation and behavior of impurities is one of the most important subjects in the development of magnetically confined fusion reactors because impurities can potentially cause cooling and worsen the confinement of the hot core plasma. Tracer-encapsulated solid pellets (TESPELs) have demonstrated some results for impurity injection for fusion-reactor plasma studies [N. Tamura et al., J. Phys. Conf. Ser. 823, 012003 (2017)]. However, the TESPEL technique has several shortcomings, for example, the penetration depth and the amounts of tracer impurities. In the present study, we have developed a tracer-containing, compact-toroid (TCCT) injection system that utilizes a magnetized coaxial plasma gun (MCPG). The discharge current through the MCPG sputters and ionizes the electrode material, and the Lorenz self-force accelerates it as a plasmoid. The MCPG easily accelerates a magnetized plasmoid to speeds greater than the ion thermal velocity of several tens of kilometers per second. The accelerated and ejected plasmoid that contains the tracer ions is itself a warm, ionized plasma. Therefore, a TCCT can potentially be injected into the core region of a target plasma with less adverse effect.