Page 2 - CHARACTERIZATION OF THE C-2W PLASMA GUNS

Nucl. Fusion 57 (2017) 116021 1. Introduction
A eld-reversed con guration (FRC) is a high-beta compact toroid (CT) which has closed- eld-line and open- eld-line regions of poloidal axisymmetric magnetic eld with no or small self-generated toroidal magnetic eld [1, 2]. The FRC topology is generated by the plasma’s own diamagnetic cur- rents, which are of suf cient strength to reverse the exterior magnetic eld, and only requires solenoidal coils located out- side of a simply connected vacuum vessel. The averaged beta value of FRCs is near unity: ⟨β⟩ = 2μ0⟨ p⟩/B2e ~90%, where μ0 is permeability of free space, ⟨ p⟩ is the average plasma pressure, and Be is the external magnetic eld. 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. Another attrac- tive feature of the FRC is its potential for a fusion reactor with lower-cost construction due to the simple geometry and high magnetic ef ciency. FRCs may also provide for the use of advanced, aneutronic fuels such as D-3He and p-11B.
Studying aspects of FRC plasma sustainment by neutral- beam (NB) injection (NBI) and additional particle fueling are the main goals of the C-2/C-2U experiments at Tri Alpha Energy. The world’s largest CT device, C-2 [3], was upgraded to C-2U [4] (illustrated in gure 1) to achieve sustainment of FRC plasmas by NBI and edge biasing. One of the key accomplishments of the C-2 experiments was the demonstra- tion of the high-performance FRC (HPF) regime, which is set apart by dramatic improvements in con nement and stability compared to prior FRC experiments [4–7]. C-2’s HPF plasma discharges have also demonstrated increasing plasma pressure and electron temperature, which indicates an accumulation of fast ions as well as plasma heating by NBI. Electrically biased end-on plasma guns and effective in-vessel wall-surface conditioning also played important roles in producing HPF plasmas, synergetically with NBI.
In order to enhance fast-ion effects and further improve FRC performance towards plasma sustainment, the C-2U experiment is characterized by the following key system upgrades: increased total NB input power from ~4 MW (20 keV hydrogen) to 10+ MW (15 keV hydrogen) with tilted injection angle as shown in gure 1, and enhanced edge- biasing capability inside of each end-divertor for boundary/ stability control. The upgraded NB system (higher NB input power with higher current at lower beam energy, angled and tangential co-current injection) alone has demonstrated signif- icant advantages and had a profound impact on C-2U perfor- mance: e.g. reduction of peripheral fast-ion losses; increased core heating; rapidly established dominant fast-ion pressure; better NB-to-FRC coupling and reduced shine-through losses; and current drive.
In fact, C-2U experiments with upgraded NBI and edge- biasing systems exhibit far better FRC performance than obtained in C-2 HPF regimes [8]. As anticipated, there are strong effects of the considerable fast-particle population (details can also be seen in [8]): (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
H. Gota et al
dimensions; (iii) double-humped electron density and temper- ature pro les (indicative of substantial fast-ion pressure); (iv) FRC lifetime and global plasma stability scale strongly with NB input power (examples shown in gures 11 and 18 of [4] for C-2, and C-2U experiments also show the same trend); and (v) plasma performance correlates with NB pulse dura- tion in which diamagnetism persists several milliseconds after NB termination due to accumulated fast ions. The key accom- plishment on C-2U is sustainment of advanced beam-driven FRCs with a macroscopically stable and hot plasma state for up to 5+ ms, limited by hardware and stored energy con- straints such as the NB’s pulse duration and current sourcing capability of end-on plasma guns. In this well-sustained FRC regime fast ions are almost classically con ned and then help to suppress broadband magnetic turbulence. A combination of NBI and E × B shearing via plasma-gun edge biasing reduces density uctuations near the separatrix and in the scrape-off layer (SOL), thereby improving con nement properties [9]. There also appears to be a strong positive correlation between Te and the energy con nement time. In addition, the particle con nement time is more than 10× greater than predicted by the conventional FRC scaling [10].
In this paper, the C-2U experimental apparatus and diag- nostic suite are described in section 2. Key system components and operational elements necessary to obtain HPF operating conditions as well as detailed characteristics of the newly- obtained advanced beam-driven FRCs are described in sec- tion 3; in addition, key C-2U experimental results including plasma sustainment are also discussed. Lastly, a summary is provided in section 4.
2. C-2U experimental device and diagnostic suite
The C-2U device, shown in gure 1(a), is a large theta- pinch, CT-merging system, built by Tri Alpha Energy to form relatively high ux, high temperature FRC plasmas [4, 8]. Figure 1(b) illustrates typical FRC magnetic ux and den- sitycontoursintheC-2Udevice.Thesecontoursareobtained from a 2D magnetohydrodynamic (MHD) numerical simu- lation performed with the LamyRidge equilibrium code [11]. The C-2U device has an overall length of ~20 m and consists of a central con nement region surrounded by two eld-reversed theta-pinch (FRTP) formation sources and two divertors. The stainless-steel con nement chamber (inner- wall radius, rw ~ 0.7 m) approximately conserves magnetic ux inside the vessel wall. However, the stainless-steel wall has a skin time of ~5ms so that for long-lived plasma dis- charges (lifetimes greater than 5ms) nite magnetic- ux leakage needs to be taken into account for accurate deter- mination of magnetic elds and other associated and post- processed plasma/physics parameters [12]. The formation tubes are made of quartz, which are approximately 3.5 m long and 0.6 m in diameter; the C-2U vacuum vessel accom- modates ultrahigh vacuum. A set of DC magnets generates a quasi-static axial magnetic eld, Bz, throughout the device, whereby the axial- eld pro le and amplitude can be con- trolled by particular coil/power-supply con gurations. The
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