For the collisional merging field reversed configurations (FRCs), it is desired to have both FRCs to be tuned approximately same, as well as to optimize each FRC to have high temperature, and high translation speed so as to retain most of the equilibrium flux after traveling a distance to the middle plane for merging. The present study reports the experimental study of a single-translated FRC (field-reversed configuration) in the KMAX-FRC device with various diagnostics, including a triple probe, a bolometer, several magnetic probe arrays, and a novel 2D internal magnetic probe array. According to the measurements conducted in the present study, a maximum toroidal magnetic field equal to ~1/3 of the external magnetic field inside the FRC separatrix radius is observed, and the typical parameters of a single-translated FRC near the device’s mid-plane are n e ~ 2-4×1019 m-3, T e ~ 8 eV, T i ~ 5 eV , R s ~ 0.2 m , l s ~ 0.6 m, and Φ p(RR) ~ 0.2 mWb . The 2D magnetic topology measurement revealed, for the first time, the time evolution of the overall internal magnetic fields of a single-translated FRC, and an optimized operation regime is given in the paper.

A novel FRC axial compression experiment with high-speed translated θ-pinch plasma was conducted in the KMAX device. The translated north FRC was decelerated and compressed by an oncoming θ-pinch plasmastream, and the dynamic process was revealed by a 2D magnetic probe array. The FRC separatrix length is compressed to one-third of the initial value while the radius expands by ~57%, resulting in the an ~ 16% increase in the electron temperature and ~26% in the density, which matches the calculation from an adiabatic compression model. The good agreement is explained by the fast compression and particle supplementation owing to the compression with plasma. The results reported in this work may contribute to the understanding of electron heating in collision-merged FRC and provide a new compression method for the magneto-inertial fusion concept.

The collisional merging of two Alfvénic-speed translated field reversed configurations (FRCs) via magnetic reconnection is presented with a two-dimensional magnetic probe array in the KMAX-FRC experiment. The collision is accompanied by axial compression and radial expansion, resulting in the increase in the FRC’s current density and poloidal flux. A reversed-current sheet is found to form during collision, indicating the occurrence of magnetic reconnection. After merging, the ion and electron temperature are increased. By comparing three different scenarios, i.e., the single-translated FRC, the FRC colliding solely with a stream plasma, and the collisional-merging FRC, we identify that axial compression and magnetic reconnection both contribute the electron heating.

The field-reversed configuration (FRC) is a compact toroidal plasma system related to the research of alternative fusion reactors. Improving the FRC formation, which is critical to the improvement of its overall quality, is reported in this paper by using a plasma gun to assist the pre-ionization process. For the single-translated FRC, the poloidal magnetic flux increases around 30%, accompanied by an increase in the separatrix radius and plasma density, while its translation speed is found to decrease slightly. The estimated increase in particle inventory is more than 60%. The improvement of these parameters also extends the lifetime of the FRC.