Page 2 - Plasma and Fusion Research,ISSN 1880-6821

Plasma and Fusion Research: Regular Articles
Volume 13, 3402098 (2018)
Confinement Region V-Formation Antenna
Mirror coil
Theta-Pinch Coil
Quasi-steady state confinement coil
-3.0 -2.0
Fig. 1 Schematic view of the FAT-CM device.
cess at Alfvénic velocity. Quasi-static confinement coils are placed along the confinement region.
Initial FRCs are formed by the FRTP method in two formation sections. In a typical FAT-CM operation, the theta-pinch circuits generate the negative bias field of ∼ 0.038 T, followed by the main reversal/compression field of ∼ 0.40 T with the rise time of ∼ 4 μs. The formation pro- cess also employs a pre-ionization method by ringing theta discharge. The quasi-steady state confinement coils cre- ate a forward field of 0.03∼0.07 T to confine plasmas. A working gas of D2 is puffed into the both formation sections at 6 ms before the start of the bias field. Initial FRC plasmas, ∼ 0.07 m in radius and ∼ 1.0 m in length, are formed at ∼ 6 μs after the initiation of the compression field in the both formation sections, and are ejected from the re- spective formation regions at ∼ 30 μs. The translated FRCs collide each other in the middle of the confinement cham- ber, the merged FRC plasma state is obtained at ∼ 60 μs, at which the plasma radius is ∼ 0.22 m and the length is ∼2m.
3. Initial Results of Collisional Merging Experiment
Figure 2 shows the comparison of single FRC transla- tion (from each formation) and collisional merging one in the typical FAT-CM experiment. The data are measured by magnetic pick-up coil near the mid-plane of the confine- ment chamber (z = +8 cm). The excluded-flux radius is estimated as
R-Formation
Theta-Pinch Coil
􏰀 B0 rΔφ=rw 1−B,
e
the magnetic mirrors. In the case of collisional merging, radial expansion of the plasma is clearly observed and the plasma size/flux remains large compared with the single translation case.
Ion Doppler spectroscopy is employed to estimate the ion temperature of FRC plasmas. The estimated temper- ature of the merged FRC is 50∼120eV for an emission spectrum of C-III. The emission intensity in the single translated FRC case appears to be insufficient to estimate temperature. The electron density, measured by laser inter- ferometry, is 0.5 ∼ 1 × 1020 m−3 in both cases. The relative speed of the colliding two FRCs is ∼ 300 km/s that is faster than Alfvénic speed of ∼ 200 km/s calculated by the elec- tron density and external magnetic field. Therefore, shock heating may occur in the collisional merging FRC.
-1.0 0 1.0 2.0 3.0 Z(m)
where, rw is the radius of the metal confinement chamber, B0 is the magnetic field without plasma, respectively. The estimated excluded-flux radius is known to be compara- ble to the radius of flux null surface for equilibrium. As shown in Fig. 2, global behavior of each FRC in the R- and V-formation are comparable. In the case of single FRC for- mation/translation, FRC is ejected at a speed of ∼ 70 km/s and accelerated by the external magnetic field gradient. When the FRC enters into the confinement region, it im- mediately expands radially from ∼ 6 cm to ∼ 12 cm in ra- dius, as shown in Fig. 2 (b). After passing through the mid- plane, the FRC with the accelerated speed of ∼ 150 km/s is then decelerated and bounced off back-and-forth between
assuming rigid-rotor (RR) profile that is consistent with the internal field measurements for translated FRC [8]. Here, rs is the separatrix radius. The apparent increase in the excluded-flux radius and the estimated poloidal flux, seen in Figs. 2 (b) and 2 (c), indicates that the collisional merg- ing process has thermalized the initial kinetic energy of the two translating FRC’s.
4. Simulation Results
Figure 3 shows 2D MHD simulation results computed for the same discharge conditions as experiments, shown in Fig. 2. The simulation results approximately agree with
(1)
The poloidal flux is estimated as 3
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Fig. 2
Comparison of the single translated and merged FRCs; showing time evolutions of (a) external field Be, (b) ex- cluded flux radius rΔφ, and (c) estimated poloidal flux φp near the midplane (z = +8 cm).
φp = 0.31πBers /rw, (2)