Page 5 - CHARACTERIZATION OF THE C-2W PLASMA GUNS

Nucl. Fusion 57 (2017) 116021
H. Gota et al
advanced beam-driven FRCs readily demonstrates the sig- ni cant difference produced by the fast-ion pressure. Typical time evolutions of the radial electron density pro les in C-2/ C-2U experiments are illustrated in gure 4. While the overall plasma radius in C-2U is not very different from C-2, there is a clear difference around the eld-null radius (rnull ~0.25 m) with the appearance of a ‘double-humped’ structure on top of the typical hollow center and steep separatrix gradients; the two density peaks are located on either side of the eld- null radius [19]. This feature is indicative of the presence of the substantial fast-ion pressure in C-2U, and Q2D simula- tions corroborate the ‘double-humped’ pro le due to the fast ions [16, 17]. The radial betatron oscillations of the fast ions lead to a broad fast-ion distribution that modi es the electron pro les accordingly. Together with other key elements of the beam-driven FRC regime and improvements described above, this upgraded NB system had a profound positive impact on C-2U performance: e.g. reduction of peripheral fast-ion losses, increased core heating, better NB-to-FRC coupling and reduced shine-through losses, and current drive.
The fast ions injected by the NBs travel both inside and outside of the FRC separatrix in large betatron orbits and slow down in a few milliseconds. Since there is a large interdependence between the FRC core and open- eld- line/SOL plasma in terms of the particle and energy trans- port processes, improving con nement properties in the SOL is as important as in the core region, especially with the presence of large-orbit fast ions. A good example and experimental observation can be seen in gure 5 where the electron temperature in the FRC core is increased by 20–30% (on average throughout the discharge) due to magnetic- eld expansion in the end-divertor area. As illus- trated in gures 5(a) and (b), turning the set of external divertor magnets ‘ON’ and ‘OFF’ produces either strongly- bundled or widely- ared magnetic ux lines inside the divertors. Field expansion can cause different eld-lines/ ux-surfaces to make contact with the end-on plasma guns, thereby creating stronger Er/r near the separatrix. The eld expansion may also produce some thermal insulation for the SOL electron population. These improvements in the open- eld-line region are well correlated with the observed improvement in plasma con nement and higher electron temperature in the FRC core.
3.2. Process and achievement of plasma sustainment
The primary goal of the C-2 experiments was to study and develop the physics of beam-driven FRC plasma states; while, the main goal of C-2U experiments was to demonstrate cur- rent drive and plasma sustainment by NBI in excess of all characteristics system timescales. Extensive experimental and computational evidence has shown that super-thermal ions slow down and diffuse nearly classically, even in the pres- ence of turbulent uctuations that drive anomalous transport of the thermal plasma. In C-2U’s advanced beam-driven FRC regime fast ions are well trapped and nearly classically con- ned, suppressing broadband magnetic turbulence as well as
Figure 4. Typical time evolutions of radial electron density pro le in (a) C-2 and (b) C-2U. The plots are obtained from
a large ensemble of similar plasma discharges using CO2/FIR interferometer system located in the machine midplane [18, 19].
Better plasma centering (less n = 1 wobble motion) is also obtained from line-tying to the plasma-gun electrodes. Hence, NBs are injected into near-axisymmetric FRC discharges, which improves beam trapping and fast-ion con nement inside the FRCs.
The C-2U NBs are injected tangentially to the FRC current (co-injection) and provide current drive. The fast ions, cre- ated primarily by charge exchange, have large betatron orbits that add to the FRC azimuthal current, and the strong fast-ion population signi cantly improves FRC stability and con ne- ment properties. Initial FRC parameters, obtained through dynamic CT-collisional-merging formation, are well suitable for NB capture (shine-through and rst orbit losses < 10%) and for fast-ion con nement; there is a signi cant and faster beam-ion build-up in C-2U due to the ~10 MW NBI (com- pared to C-2’s ~4 MW NBI) right from the beginning of the plasma discharges. After a few milliseconds the fast-ion pres- sure becomes comparable to the plasma pressure (example shown in gure 4 of [8]). Once the fast-ion pressure becomes dominant, it is the footprint of the fast ions, as determined by the beam-injection angle and externally applied magnetic eld, which determines the axial extent of the FRC [16, 17]. Comparing equilibrium density pro les in C-2 HPF and C-2U
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