Experimental study of single-translated field-reversed configuration in KMAX

Abstract

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.

The dynamic axial compression of FRC with high-speed translated θ-pinch plasma

“…As expected, FRC has a hollow while θ-pinch plasma has a peaked density profile. The FRC is accelerated into the central cell by magnetic gradient, and the translation speed v z of plasma can be estimated by time-of-flight method from the APD array or the B e array in KMAX [30,33]. In this experiment, the estimated v z of north FRC is about 60 km s −1 , which is near or slightly larger than the Alfvén speed.…”

“…where r w denotes the wall radius, B 0 (B e ) represents the external background magnetic field without (with) the FRC passing through. The 2D magnetic probe array [30], which is located at the device's mid-plane, has a total of 32 measurement points (four points in the radial direction and eight points in the axial direction); each detection point has two pickup coils installed orthogonally for B z and B r measurements, and the distance between each detection point is 5 cm. The bolometer located at Z = −0.25 m consists of 16-channel AXUV array (AXUV16ELG), and the flux loop installed at Z = 0.33 m has a diameter of 0.7 m.…”

First direct experimental evidence of the merging of two colliding field reversed configurations

“…The produced main compression field has a rise-time of ∼5 µs and a peak value of ∼1400 G. Upon reaching maximum current, a crowbar switch is finally closed to slow down the decay of the coil current. When FRC is formed and ejected from the formation section, it expands and translates into the confinement chamber along the magnetic field line at a constant speed, v z = 60 km s −1 , which is near or slightly larger than the Alfvén speed [18], the typical parameters of single-translated FRC at device's mid-plane, such as electron density, electron temperature, ion temperature, separatrix radius, separatrix length and poloidal flux estimated by the rigid-rotor (RR) model [23] are n e ∼ (2-4) × 10 19 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 [18].…”

“…The spatial resolution of the probe, in either the R or Z direction, is 5 cm. Note that the 2D magnetic probe array has been carried out in the single-translated FRC experiment and has demonstrated good performance [18]. Other crucial diagnostics used in this experiment include: the B e (external magnetic field) array consists of 12 magnetic probes equally distributed along the axis from Z = −0.825 m to Z = 0.825 m and the triple probe located at Z = 0.…”

“…Here, we report on the recent CM FRC experiment on an axisymmetrical tandem mirror device named KMAX (Keda Mirror with AXisymmetricity) [14][15][16][17]. Two single-translated FRCs with similar parameters have been formed in the KMAX-FRC experiment [18]. In particular, a 2D magnetic probe array has been carried out to experimentally record the merging process.…”