Page 7 - Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device

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FIG. 4. Top: Normalized excluded-flux radius evolutions in C-2 and C-2U experiments. Bottom: time evolutions of FRC-core electron temperatures under C-2 HPF14 (squares) and C- 2U w/ 10 MW NBI (circles) regimes; Te, measured by multipoint Thomson scattering system in the midplane, is averaged inside the separatrix as well as shot averaged for each data set.
between NB input power and FRC performance, particularly in the improved energy decay time and FRC plasma lifetime. Our in-house 1-D and 2-D transport simulations, Q1D and Q2D, also indicated a high probability of FRC plasma sustainment with an appropriate upgrade of the NB injector systems. This was the initial motivation for the C-2U project to demonstrate FRC plasma sustainment.
The C-2U experimental program commenced with various upgraded systems as previously described. The increased NB input power (higher current at reduced beam energy) and tilted beam-injection angle were the biggest changes from C-2. However, even with a reduced NBI power of ~5 MW the C-2U FRC performance already showed significant improvements in many aspects, in particular plasma lifetime as seen in Fig. 4. It implicitly indicates that other upgraded systems (e.g. edge biasing capability) and optimized operating conditions (e.g. external axial magnetic-field profile) have also contributed to this performance improvement. Furthermore, C-2U shots with ~10 MW NBI increased FRC performance even further and ultimately achieved sustainment of plasma radius and electron temperature in the first 5 milliseconds, as can be seen in Fig. 4; under the best/optimum operating condition, the plasma diamagnetism even reached record lifetimes of over 11 ms, timescales twice as long as C-2.
Both C-2 and C-2U experiments achieved great improvements in FRC performance, as evidenced by the temporal evolution of the excluded-flux radius and electron temperature in Fig. 4. The plasma radius in C-2U w/ ~10 MW NBI is essentially being kept constant for ~5+ ms, while there is instantaneous decay associated with all other traces from C-2 (w/ no- Gun/NBI, ~4 MW NBI) and C-2U (w/ ~5 MW NBI), although there is an indication of improved decay rate with higher NBI powers; ~5 ms sustainment of some of other critical plasma quantities such as plasma density and temperatures has also been observed in C-2U with ~10 MW NBI. The C-2U plasma performance, including the sustainment feature, has a strong correlation with NB pulse duration, with the diamagnetism persisting even several milliseconds after NB termination due to the accumulated fast ions. Note that the performance in C-2U is only limited by pulse-length constraints arising from finite stored energies in the power supplies of many critical systems, such as NB injectors (flat-top duration ~8 ms) and edge-biasing equipment (pulse duration ~5–7 ms depending on discharged current/energy from the plasma-gun electrode during a shot).
Under the well-confined FRC regimes, such as HPF and advance beam-driven FRC in C-2/C- 2U, and after careful 0-D global power-balance analysis [13,14], there appears to be a strong