Development of Rogowski probe for internal plasma current measurement in the collisional merging process of field-reversed configurations.

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.

Magnetic probe array with high sensitivity for fluctuating field.

A magnetic probe array is constructed to measure precisely the spatial structure of a small fluctuating field included in a strong confinement field that varies with time. To exclude the effect of the confinement field, the magnetic probes consisting of figure-eight-wound coils are prepared. The spatial structure of the fluctuating field is obtained from a Fourier analysis of the probe signal. It is found that the probe array is more sensitive to the fluctuating field with a high mode number than that with a low mode number. An experimental demonstration of the present method is attempted using a field-reversed configuration plasma, where the fluctuating field with 0.1% of the confinement field is successfully detected.