Page 5 - Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device

5 EX/P3-41 “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 flexible, 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-flux radius ~0.35 m, length ~3 m, rigid-rotor poloidal flux ~5–7 mWb, total temperature (Ti+Te) up to ~1 keV, and electron density ~2–3×1019 m-3.
In order to effectively inject beam particles into the FRC plasmas, a titanium gettering system has been deployed in the C-2U confinement 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 efficiency with mitigated charge-exchange losses. The gettering system covers over 80% of the total surface area of the inner vessel wall and has significantly reduced the neutral recycling by a factor of 4–5 compared to operation without wall conditioning.
Another key component for good FRC performance and further improvement of NBI effects is edge/boundary control. To this end, two plasma guns are mounted inside of each divertor and produce a hot (Te ~30–50 eV, Ti ~100 eV) tenuous (~1018 m-3) plasma stream. The guns also create an inward radial electric field (Er < 0) that counters the usual FRC spin-up in the ion diamagnetic direction and mitigates the n=2 rotational instability without applying quadrupole magnetic fields. Furthermore, we typically apply a negative potential (about –1 kV) on the central electrode of each plasma gun to enhance the edge-biasing capability for stability control. The electrically-biased plasma
guns also produce E×B velocity shear just outside of FRC separatrix, yielding improved FRC confinement properties and stability. 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 fast-ion confinement inside the FRCs.
The C-2U NBs (total injected power up to 10+ MW, 15 keV hydrogen) are injected tangentially to the FRC current (co-injection) at an angle of 70° (relative to the machine axis) and with an average radial impact parameter of 0.19 m, which permits current drive. The fast ions, created primarily by charge exchange, have large betatron orbits that add to the FRC azimuthal current, and the strong fast-ion population significantly improves FRC stability and confinement properties. Initial FRC parameters, obtained through dynamic CT-collisional- merging formation, are suitable for NB capture (shine-through and first orbit losses < 10%) and for fast-ion confinement; there is a significant and faster beam-ion build-up due to the ~10 MW NBI (compared to C-2’s ~4 MW NBI) right from the
(a) C-2 HPF regime
(b) C-2U advanced beam-driven FRC regime
FIG. 3. Typical time evolutions of radial electron density profile in (a) C-2 and (b) C-2U. The plots are obtained from a large ensemble of similar plasma discharges.