Page 5 - Collisional merging formation of a field-reversed configuration in the FAT-CM device

Nucl. Fusion 59 (2019) 056024
T. Asai et al
Figure 6. 2D MHD simulation, indicating collisional merging process of FRCs in the typical condition of FAT-CM experiments.
φp_RR and φp are approximately equal (~0.2 mWb) because of thinner SOL and small shift, and is less than the trapped flux of initially formed FRC in the formation region.
4. MHD simulations
Figure 6 shows contour maps of plasma pressure computed by the 2D resistive MHD code for the typical conditions of the FAT-CM experiments. The shape of the simulated FRC approximately agrees with the experimental results: separa- trix radius ~0.23 m, and translated velocity in the confine- ment region ~200 km s−1 (relative velocity ~400 km s−1). Collided FRCs bounce off each other once at the midplane and two separate FRCs reflect at the magnetic mirrors. Finally, merged FRC is formed through a second collision process of FRCs. While the 2D MHD simulation does not reproduce kinked magnetic field structure—appearance of toroidal field—and recovery of the FRC structure, global behavior and its time scale is consistent with experimental results. Collisional-merging of FRCs have an advantage in point of suppressing the bouncing and reduces axial asym- metry with reduced particle/energy loss especially in the reflection process at the downstream mirror compared to single-sided translation. According to the simulation, almost all of the kinetic energy is converted back into the internal energy of the merged FRC plasma. Reduced expansion at the mirror enables us to install in vessel antennas for wave excitation [18].
5. Discussion
The collisional merging formation of a FRC at super Alfvénic velocity has been successfully demonstrated in the FAT-CM device. The averaged electron density of the merged FRC is ~2.5 × 1020 m−3, which is ten times higher than the previous experiments performed in a C-2U device at TAE. Initial mag- netized plasmoids are formed by the FRTP method in two for- mation sections within the tapered theta-pinch coil. The initial plasmoids are accelerated by the gradient of the external guide magnetic field and then injected into the confinement chamber. The translated FRCs collide in the middle of the confinement chamber at the relative velocity in the range of 300–400 km s−1 which is approximately two times faster than the Alfvén velocity estimated by the plasma parameters at vicinity of separatrix of the translated FRC.
Although during collisional merging, the plasmoid expandes radially by more than a factor of two and internal magnetic structure is severely perturbed and then tidy reversed magnetic field profile is re-organized in the quasi-equilibrium phase as shown in figure 4. The re-organized FRC has larger amount of magnetic flux compared with the initial FRC in the formation region even taking the most underestimated value. This exper- imental result evidences that the magnetic flux is not conserved during the process in which the two plasmoids collide with each other at the relative velocity faster than the Alfvén speed. In which process, original magnetic structure composed of only poloidal field is violently disturbed in such a way that even toroidal component appears. And then, this highly disturbed state is re-organized to the tidy field reversed structure. This dynamic process is quite different from the quasi statically decaying process in which the Taylor relaxation takes place.
The simulation, which is based on the MHD assumption, reproduces approximately the same behaviour with the exper- imental results except for the dynamic collisional merging process with disturbed magnetic configuration. As the FRC has strong equilibrium toroidal flow which reaches the range of the Alfvén velocity, two-fluid theory may be required to describe the dynamic merging and relaxation process.
6. Summary
Drastic increase of the excluded flux leading to the improved confinement performance of the FRC, has been observed in the FAT-CM, as also seen in the C-2/C-2U. The internal magnetic diagnostics clearly shows the re-organizaion of field-reversed structure with larger magnetic flux than before the merging. This experimental evidence indicates the magnetic flux is not conserved in the super Alfvénic dynamic collisional merging process. The MHD simulation also indicates that the increased volume is due to an efficient conversion of the translating FRC kinetic energy into the thermal energy of the merged FRC.
Acknowledgments
The authors would like to acknowledge all of the Nihon University students who contributed to the FAT-CM experi- ments and discussions. We also thank all fellow TAE staff for
5