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plasmas, thereby improving the overall FRC performance.
(3) Edge/Boundary Control: One of the most disruptive and dangerous MHD instabilities in FRCs is the n=2 rotational instability described above. In order to suppress this instability and prevent FRC collapse, the C-2 device initially employed quadrupole antennas in the confinement region. The application of this multipole magnetic field stabilized the FRC plasma to a certain extent, but the effect of NBI was significantly reduced due to the resultant azimuthal magnetic field asymmetry. Therefore, an approach to plasma stabilization was introduced that utilizes a coaxial plasma gun inside each divertor (Fig. 1, Plasma gun) and without applying multipole fields; the end-on plasma guns communicate with the core FRC plasma through open magnetic field lines for stability control. The guns each generate inward radial electric fields (–Er), which, when coupled with the axial magnetic field (Bz), create an E×B shear flow outside of the FRC core; its direction is counter to the direction of natural FRC rotation (ion diamagnetic direction); thereby dissipating sufficient free energy to suppress the n=2 rotational mode. In addition, line-tying/freezing of open magnetic field lines onto the plasma gun electrode surfaces leads to a reduction of the global n=1 wobble instability, which otherwise causes radial shifts of the FRC’s position. Since suppressing these two macroscopic instabilities with the help of the plasma guns, we have succeeded in further enhancing the beam injection effect on the FRC plasma in the C-2/C-2U devices; the plasma stabilizing effect of the beam ions (finite Larmor radius effect) synergistically leads to further FRC stabilization and longer-lived
discharges.
(4) NB Injection: All of the above technologies (1)–(3) are
important elements towards the effective implementation of NBI into FRC plasma. As previously mentioned, sustainment of the FRC plasma by the injected beam ions was the main objective of the C-2/C-2U experiments, and TAE has been conducting years of experiments and theoretical research in order to realize it. In C-2, the NB system operated with a beam energy (Ebeam) of 20 keV, total injected beam power (Pbeam) of ~4 MW, and co-current injection into the FRC with an injection angle of 90° with respect to the machine axis. Based on the results of C-2 experiments and associated numerical simulations, we examined and studied the optimum conditions for NBI applicable to the follow-on C-2U experiments; we then developed brand new NBI systems that feature Ebeam~15 keV, Pbeam~10+ MW, and variable injection angles in the range of 65°–75° with respect to the machine axis, as illustrated in Fig. 1. In addition to the all-new NB system, optimization of the external magnetic field profile in the confinement section and active control of the magnetic field soak-through time of the metal chamber were also performed, thereby allowing for control of the shape of the FRC plasma and the density distribution of the beam ions.
3.2.3 Improvement of plasma confinement characteristics and sustainment of FRC plasma
Improvement of confinement characteristics of FRC plasma in
the C-2/C-2U experiments was achieved by operational optimizations and the above summarized synergistic effects of key technologies. It was found that those effects together with the control of external magnetic field profiles (Fig. 1, Confinement/Formation/Mirror-plug sections) also led to improvements of the plasma characteristics in the open magnetic field line region (Scrape-off layer (SOL)/Jet region). Figure 2 shows the particle confinement time (τN) of the FRC plasma obtained in various stages of C-2 experiments and contrasted with the conventional FRC scaling behavior obtained in other FRC experiments [13]. The results of the C-2 experiment were obtained under various experimental conditions, in the early stage of the experiment (Fig. 2, No Gun/NB), where the instability suppression using the plasma gun or NB was not performed, the particle confinement properties appear to be very close to those observed elsewhere. However, the later experimental results, obtain by using plasma guns and NBI, greatly deviate from the prior scaling law, and it can be seen that the confinement characteristic of the FRC plasma in C-2 is remarkably improved. In the HPF14 plasma regime, which
Fig. 2. Comparison of particle confinement time τN of FRCs obtained in C-2 experiments with the scaling law [13] obtained in other FRC experiments. C-2 data (⚫) is the average value over 10 shots under each condition.
Fig. 3. Temporal evolution of plasma parameters obtained in C-2/C-2U experiments: normalized plasma radius (top), electron temperature (bottom) as measured by a Thomson scattering system located at the midplane of the device.
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