Page 5 - Internal magnetic field measurements of translated and merged field-reversed configuration plasmas in the FAT-CM device

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Gota et al.
Rev. Sci. Instrum. 89, 10J114 (2018)
FIG. 6. Position-shifted Bz radial profile of collisional-merging FRC at a quiescent phase (t = 80 μs of shot 2081). The shaded areas indicate poloidal flux inside the separatrix.
flux from the internal magnetic field profile is φp ∼ 0.2 mWb at t = 46 μs, which is similar to that of the RR estimate using Eq. (2).
To study and investigate the effect of the confinement mirror field in the FRC collisional-merging process, half of the typical magnetic field strength was applied in the mirror regions (Bmirror ∼ 0.1 T, instead of ∼0.2 T); the experimental results are shown in Fig. 7. In this weak mirror-field configura- tion, the initial FRC formation and translation characteristics appear to be similar to those in the nominal mirror-field case. However, there is a clear difference and negative impact on the FRC collisional-merging process due to insufficient mag- netic field gradient at both ends of the confinement region, i.e., the magnetic mirror ratio between central and mirror regions in the confinement section was too low. As a result, the two oppositely directed colliding FRCs/plasmoids seem to bounce off each other or stay as two separate FRCs side-by-side near the mid-plane rather than fully merged in the middle of the confinement region. In this incomplete merging FRC with the weak Bmirror case, the typical field-reversed structure of the radial Bz profile cannot be observed [see Figs. 7(a) and 7(b)]. Instead, two peaks/humps of the excluded-flux radius r∆φ axial profile are clearly exhibit at around z = ±0.5 m, as seen as red circles in Fig. 7(c), which evidently indicates an unsuccessful collisional-merging process. This mirror field effect was previously studied using 2D MHD Lamy Ridge simulation,10 and the experimental results corroborate the simulation.
ACKNOWLEDGMENTS
The authors would like to acknowledge the rest of Nihon University students who contributed to the FAT-CM experi- ments and discussions. The authors also thank TAE’s share- holders for their support and trust and all fellow TAE staff for their dedication, excellent work, and extra efforts. This work was partially supported by JSPS KAKENHI Grant No. 16K06939.
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FIG. 7. (a) Contour maps of Bz and Bt radial profiles as a function of time in the weakly collided FRCs with weak mirror magnetic fields in the confinement region; (b) time slice of the radial magnetic field profiles at t = 80 μs; and (c) axial profile of the excluded-flux radius from the linear magnetic probe array at t = 80 μs (red circles) compared with a typical shot with nominal mirror fields at ∼0.2 T (blue open circles).
could be due to the FRC radial shift/motion that is not prop- erly taken into account, in particular, the case when FRC is shifted perpendicularly away from the probe array. In the case of single-sided FRC translation (Fig. 4), the estimated poloidal