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Nucl. Fusion 59 (2019) 112009
have been tried via open field-lines, in which concentric electrodes located in both inner and outer divertors as well as end-on plasma guns are electrically biased independently. As a result of effective outer-divertor electrode biasing alone, FRC plasma is well stabilized and diamagnetism duration has reached up to ~9 ms which is equivalent to C-2U plasma duration. Magnetic field flaring/expansion in both inner and outer divertors plays an important role in creating a thermal insulation on open field-lines to reduce a loss rate of electrons, which leads to improvement of the edge and core FRC confinement properties. An experimental campaign with inner-divertor magnetic-field flaring has just commenced and early result indicates that electron temperature of the merged FRC stays relatively high and increases for a short period of time, presumably by NBI and E × B heating.
Keywords: field-reversed configuration, compact toroid, neutral beam injection, edge control, collisional merging
H. Gota et al
(Some figures may appear in colour only in the online journal)
1. Introduction
A field-reversed configuration (FRC) is a prolate, high-beta compact toroid (CT) that has closed magnetic field-lines inside the separatrix and open field-line regions of poloidal axisym- metric magnetic field with zero or small self-generated toroidal magnetic field [1, 2]. The FRC topology is generated by the plasma’s own diamagnetic currents, of sufficient strength to reverse the exterior magnetic field, and the FRC only requires external solenoidal coils to hold the structure inside a confine- ment vessel (CV). The plasma current typically decays away in a resistive timescale but can be maintained by a current drive such as neutral beam (NB) injection (NBI). The averaged beta value of FRCs is near unity: ⟨β⟩ = 2μ0 ⟨ p⟩ /B2e ∼ 90%, where μ0 is the permeability of free space, ⟨ p⟩ is the average plasma pressure, and Be is the external magnetic field. The edge layer outside of the FRC separatrix coalesces into axial jets beyond each end of the FRC, providing a natural divertor, which may allow extraction of energy without restriction via direct energy conversion. The FRC has potential for a fusion reactor with low-cost construction and minimal maintenance due to its simple geometry. FRCs may also allow the use of advanced, aneutronic fuels such as D-3He and p-11B.
In the past C-2 [3, 4] and C-2U [5, 6] FRC experiments at TAE Technologies, studying FRC behavior, as well as demonstration of the FRC plasma sustainment by NBI and edge biasing/control, were the primary goals. One of the key accomplishments in the C-2 experiments was demonstration/ creation of the high-performance FRC (HPF) regime, which was set apart by dramatic improvements in confinement and stability compared to other past FRC devices [4, 7–10]. HPF plasma discharges have also demonstrated increasing plasma pressure and electron temperature Te, indicating an accumula- tion of fast ions as well as plasma heating by NBI. Electrically biased end-on plasma guns and effective wall/surface con- ditioning inside the vacuum vessel also played important roles in producing HPF plasmas, synergistically with NBI. The HPF plasmas are macroscopically stable and microsta- bility properties appear to be different from typical regimes in toroidal confinement devices. Ion-scale fluctuations are found
to be absent or strongly suppressed in the plasma core, mainly due to the large FRC ion orbits (finite Larmor radius effect [11]), resulting in near-classical thermal ion confinement [10, 12]. In order to further improve HPF plasma parameters, the C-2U experimental program commenced after various key system upgrades from C-2, including increased total NB input power from ~4 MW (20 keV hydrogen) to ~10 + MW (15keV hydrogen, higher current at reduced beam energy) with tilted injection angle, and enhanced edge biasing capa- bility for boundary/stability control. The upgraded NBI and edge biasing systems enabled significant plasma performance advances and had a profound impact on C-2U performance such as: (i) rapid accumulation of fast ions (about half of the initial thermal pressure replaced by fast-ion pressure); (ii) fast-ion footprint largely determines FRC dimensions; (iii) double-humped electron density and temperature pro- files (indicative of substantial fast-ion pressure) [13, 14]; (iv) FRC lifetime and global plasma stability scale strongly with NB input power; and (v) plasma performance correlates with NB pulse duration in which diamagnetism persists several milliseconds after NB termination due to accumulated fast ions. Therefore, this improved plasma state created via effec- tive NBI is called a beam-driven FRC. Under the optimum C-2U operating conditions, plasma sustainment for ~5 + ms, as well as long-lived decaying plasma discharges of up to 10 + ms, were successfully achieved [6] and performance was mostly limited by hardware and stored energy constraints such as the NB’s pulse duration and the current sourcing capa- bility of the end-on plasma guns. Furthermore, with careful 0D global power-balance analysis [15, 16], there appeared to be a strong positive correlation between electron temperature Te and energy confinement time; i.e. the electron energy con- finement time τE,e in C-2U FRC discharges scales strongly with a positive power of Te [6, 16], which is basically the same characteristics/trend as observed in C-2 [4]. This posi- tive confinement scaling is very attractive, and similar features of temperature dependence have also been observed in other high-beta devices such as NSTX [17, 18] and MAST [19], whereby the energy confinement time scales nearly inversely with collisionality.
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