Page 4 - CHARACTERIZATION OF THE C-2W PLASMA GUNS

Nucl. Fusion 57 (2017) 116021
H. Gota et al
Figure 2. Key approaches to obtain HPF and advanced beam- driven FRC regimes in C-2/C-2U experiments.
3. C-2U experimental results
3.1. Advanced beam-driven FRC regime
A high-performance FRC equilibrium state was rstly obtained/achieved in the C-2 device [5]. To achieve HPF operating conditions the following key approaches, as illus- trated in gure 2, are necessary: (i) dynamically colliding and merging two oppositely directed CTs for robust FRC forma- tion; (ii) active vessel-wall conditioning using titanium and/or lithium gettering systems for background neutral and impurity control; (iii) effective control and edge plasma biasing near the FRC separatrix via end-on plasma guns and concentric ring electrodes inside the end divertors; and (iv) NB injec- tion into FRCs for current drive and plasma heating. The main characteristics of the C-2 HPF regime include: macroscopi- cally stable plasma discharges, dramatically reduced transport rates (up to an order of magnitude lower than the non-HPF regime), long-lived and record diamagnetism lifetimes, and emerging global energy con nement scaling with strongly favorable temperature dependence [4]. While C-2U inherited key systems/elements for HPF operating conditions from C-2, the signi cantly upgraded NB and edge-biasing systems as well as extensive FRC/system optimization processes led to further improved FRC performance, ultimately showcasing an ‘advanced beam-driven FRC’ equilibrium state in C-2U.
In C-2/C-2U experiments FRCs are produced/formed by colliding and merging two oppositely-directed CTs using FRTP scheme in the formation sections; this exible, well controllable dynamic FRC formation technique [3] allows to form various initial target FRC plasma states for performance characterizations, including NBI optimization. Typical FRC plasma states right after the CT-collisional-merging process have the following plasma properties: excluded- ux radius (rΔφ) ~0.35 m, length ~3 m, rigid-rotor poloidal ux ~5–7 mWb, total temperature (Ti + Te) up to ~1 keV, and electron den- sity ~2–3 × 1019 m−3. The merged FRC state was directly measured and veri ed by probing the internal magnetic eld structure using a multi-channel magnetic probe array located in the machine midplane (z = 0) [15]. Although the internal
Figure 3. Radial pro les of axial and azimuthal magnetic elds, Bz(r) and Bt(r), measured by internal magnetic probe array mounted in the midplane of the con nement vessel.
probe array strongly degrades the FRC performance in terms of its con guration lifetime and radial displacement (offset from the z-axis), the measured Bz radial pro le clearly shows a eld-reversed structure after the CT collisional merging (time around 100 μs), as shown in gure 3. Each of the two translated plasmoids exhibits signi cant toroidal elds with opposite helicity, and a residual toroidal magnetic eld (Bt) appears to at least transiently persist inside the separatrix after merging.
In order to effectively inject beam particles into the FRC plasmas, a titanium gettering system has been deployed in the C-2U con nement chamber as well as in the divertors for further impurity reduction and additional vacuum pumping. Reducing background neutrals outside of the FRC is one of the key elements for better NB injection ef ciency with mitigated charge-exchange losses. The gettering system covers over 80% of the total surface area of the inner vessel wall. In typical C-2U FRCs the dominant impurities without wall conditioning are oxygen, carbon and nitrogen, mainly coming out from the chamber walls and small virtual leaks from trapped volumes inside the vessel. The gettering has sig- ni cantly reduced the neutral recycling based on deuterium Balmer-alpha emissions (λ ~ 656 nm) by a factor of 4–5 com- pared to operation without wall conditioning. Most dominant impurity concentrations such as oxygen and carbon are also reduced by orders of magnitude, based on survey spectro- meter measurements.
Another key component for good FRC performance and further improvement of NBI effects is edge/boundary con- trol. To this end, two plasma guns are mounted inside of each divertor, as illustrated in gure 1(b) and produce a hot (Te ~ 30–50eV, Ti ~ 100eV) tenuous (~1018 m−3) plasma stream. The guns also create an inward radial electric eld (Er < 0) that counters the usual FRC spin-up in the ion dia- magnetic direction and suppresses the toroidal mode n = 2 rotational instability (without applying a quadrupole magn- etic eld which breaks the FRC azimuthal symmetry and so causes rapid stochastic diffusion of the NB fast-ion orbits). Electrically-biasing of plasma guns and electrodes also pro- duces E × B velocity shear just outside of the FRC separatrix, yielding improved FRC con nement properties and stability.
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