Page 23 - Demo

exhibited the best plasma performance in C-2, shots with a particle confinement time exceeding 2 ms were observed.
While strong fluctuations were observed in the SOL region outside of the FRC (separatrix), the fluctuation level inside the separatrix was greatly reduced as evidenced by measurements of a Doppler back scattering (DBS) reflectometry system [14]. Compared with the case where edge/boundary control by plasma gun or NBI is not performed, fluctuation levels are dramatically suppressed in the HPF state. Since the fluctuation amplitude is closely related to the transport mechanism of particles and energy, its observed reduction aligns with the improvement of the confinement characteristic of the FRC plasma.
As an example of plasma performance improvements, Fig. 3 shows two different operating conditions of C-2 (no Gun/NB and HPF 14) and C-2U plasma discharges (~5 MW and 10 MW NBI power levels). Depicted are the time evolution of the normalized plasma radius (upper panel) and the electron temperatures (lower panel, one trace each for C-2 / C-2U). As can be seen in Figs. 2 and 3, the influence and effect of the plasma guns and NBI on overall FRC plasma performance are very remarkable. The FRC plasma radius and configuration lifetimes being compared in Fig. 3 show clear improvements. C-2U plasma performance exceeds that of C-2 HPF14, even at the reduced NBI power of Pbeam~5 MW. These achievements demonstrate the level of improvement and operational optimization of the NBI equipment and other C-2U subsystems mentioned above; furthermore, in the C-2U shot with maximum NBI power (Pbeam~10 MW), the flat-top of the plasma radius successfully exceeded 5 ms and the plasma diamagnetism was commensurately extended up to 10+ ms. In addition, the temporal evolution of the electron temperature also illustrates the comparative improvement in FRC plasma performance in C-2U where the average electron temperature of ~120 eV is maintained for a relatively long time (up to ~5 ms). Looking at the radial electron temperature profile, it was also found that the higher temperature was maintained over a wider radial range, extending from inside of the FRC separatrix to the open magnetic field line region or SOL. This is likely due to the high-energy beam ions traversing the FRC and open-field line / SOL regions, thereby effectively heating both the core and edge of the FRC plasma. In the advanced beam-driven FRCs of C-2U effective NBI improves confinement and sustains higher and wider radial temperature profiles
When considering the improved confinement characteristics of the FRC plasma in the C-2/C-2U experiments, the thermal energy variation in the plasma can be analyzed using a zero dimensional model (0-D power balance analysis) [15, 16]. In this model electrons and ions are handled separately so as to maintain the overall power balance between inputs (heating terms) and outputs (loss terms) to each channel. The respective thermal energy (Eth) confinement characteristics, τE,e and τE,i, can then be calculated. Figure 4 shows the relationship between the electron energy confinement time τE,e obtained from the C-2 experiments and the measured electron temperature Te from the
Thomson scattering system. It can be seen in the figure that the electron energy confinement time clearly improves as the electron temperature rises [10]. Moreover, it was also confirmed that this relationship continues at the higher temperature range based on the recent C-2U results and their associated power balance analysis. This is a very important and encouraging result in promoting FRC experiments and research at TAE. In addition, this scaling law will be useful for development of further experimental devices and hopefully the design of FRC-based fusion reactors in the future.
3.3 Future Challenges and Perspectives
Achieving the FRC plasma performance in the C-2/C-2U experiments using NBI at TAE, which have not been achieved by other FRC experiments thus far, is very significant. As a result of dramatically improving plasma performance metrics, such as improved FRC plasma confinement and extension of the configuration lifetime, it has now become possible to further understand important physics topics/mechanisms (e.g., FRC equilibrium state, turbulence, particle and energy transport, etc.) that experiments and associated analysis were not able to address in past FRC research. In addition, experimental demonstration of auxiliary heating, particle refueling, and current drive has also commenced, aiming to further improve FRC plasma performance. The emerging confinement scaling, whereby electron energy confinement time is proportional to a positive power of the electron temperature, is also very attractive from the viewpoint
Fig. 4. C-2 correlation between electron energy confinement time and electron temperature (scaling law) obtained in C-2 HPF12 and HPF14 conditions.
Fig. 5. Schematic of TAE’s next-generation experimental device, C-2W. 4