Page 6 - Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device

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beginning of the plasma discharges in C-2U.
becomes comparable to the plasma pressure.
HPF and C-2U advanced beam-driven FRCs readily demonstrates the significant difference produced by the fast-ion pressure. Typical time evolutions of the radial electron density profiles in C-2/C-2U experiments are illustrated in Fig. 3. While the overall plasma radius in C-2U is not very different from C-2, there is a clear difference around the field-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 null field radius. This feature is indicative of the presence of the substantial fast-ion pressure in C-2U. The radial betatron oscillations of the fast ions lead to a broad fast-ion distribution that modifies the electron profiles 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-field-line / SOL plasma in terms of the particle and energy transport processes, improving confinement properties in the SOL is as important as in the core region, especially with the presence of large-orbit fast ions. A good example can be seen in Fig. 5 of Ref. 8 where the electron temperature in the FRC core was increased by 20– 30% (on average throughout the discharge) due to magnetic-field expansion in the divertor area. Field expansion can cause different field-lines / flux-surfaces to make contact with the end-on plasma guns, thereby creating stronger Er/r near the separatrix. The field expansion may also produce some thermal insulation for the SOL electron population. These improvements in the open-field-line region could explain the observed improvement in plasma confinement 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 mail goal of C-2U experiments was to demonstrate current 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 presence of turbulent fluctuations 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 confined, suppressing broadband magnetic turbulence as well as enhancing fusion reactivity via beam driven collective effects. In addition, density fluctuations near the separatrix and in the SOL have also been dramatically suppressed by a combination of NBI and E×B shearing via plasma-gun edge biasing, thereby improving global confinement properties [9].
Our previous experimental device, C-2, had initially no NBI or edge-biasing capabilities and produced ~1 ms FRC plasma lifetime, as shown in Fig. 4; the lifetime was limited by MHD instabilities such as n=1 and 2 modes. After extensive experimental runs with FRC / system optimization processes, C-2 produced HPF plasma regimes by combined effects of plasma- gun edge biasing and ~4 MW NBI; C-2’s best performing operating regime, HPF14, successfully demonstrated increasing plasma pressure and electron temperature, which indicates an accumulation of fast ions as well as plasma heating by NBI [4]. Under these well-confined, stable and long-lived HPF conditions in C-2, we observed a clear correlation
After a few milliseconds the fast-ion pressure
Comparing equilibrium density profiles in C-2