Page 6 - CHARACTERIZATION OF THE C-2W PLASMA GUNS

Nucl. Fusion 57 (2017) 116021
H. Gota et al
temperature, which indicates an accumulation of fast ions as well as plasma heating by NBI [4]. Under these well-con ned, stable and long-lived HPF conditions in C-2, we observed a clear correlation between NB input power and FRC perfor- mance, particularly in the improved energy decay time and FRC plasma lifetime. 1 and 2D transport simulations, Q1D [22] and Q2D [16, 17], 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 with upgraded NB systems.
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 at ~5 MW the C-2U FRC performance already showed signi cant improvements in many aspects, in particular plasma decay rate and FRC lifetime as seen in gure 6. It implicitly indi- cates that other upgraded systems (e.g. edge-biasing capa- bility) and optimized operating conditions (e.g. external axial magnetic- eld pro le) have also contributed to this perfor- mance improvement in C-2U. Furthermore, C-2U shots with ~10 MW NBI increased FRC performance even further and ultimately achieved sustainment of plasma radius and elec- tron temperature in the rst 5 milliseconds, as can be seen in gure 6; 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 improve- ments in FRC performance, as evidenced by the temporal evolution of the excluded- ux radius and electron temper- ature in gure 6. 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 and C-2U w/~5 MW NBI, although there is an indication of improved decay rate with higher NBI powers. Sustainment of ~5+ ms of the other critical plasma quantities such as plasma density, temperatures and magnetic ux has also been observed in C-2U with ~10 MW NBI. As shown in gure 7, the C-2U plasma performance, including the sustainment fea- ture, has a strong correlation with NB pulse duration, with the diamagnetism persisting even several milliseconds after NB termination due to the accumulated fast-ion population. Note that the sustainment in C-2U is limited by pulse-length constraints arising from nite stored energy in the power sup- plies of many critical systems, such as NB injectors ( at-top duration ~8ms) and edge-biasing equipment (pulse duration ~5–7ms depending on discharged current/energy from the plasma-gun electrode during a shot).
Under the well-con ned FRC regimes, such as HPF and advanced beam-driven FRC in C-2/C-2U, and after careful global power-balance analysis [23–25], there appears to be a strong positive correlation between electron temperature and energy con nement time; i.e. the electron energy con nement time in C-2U scales strongly with a positive power of Te [4]. The power-balance analysis, detailing loss channel character- istics and plasma timescales, shows substantial improvements
Figure 5. Magnetic topology of (a) regular and (b) ared magnetic elds in C-2U end divertors; (c) time evolution of core electron temperature under ared (circle) and regular (square) magnetic elds in the divertors. Te, measured by multipoint Thomson scattering system in the midplane [20], is averaged inside the separatrix as well as shot averaged for each data set.
enhancing fusion reactivity via beam driven collective effects. In the FRC core, ion-scale turbulence is absent, and only weak electron-scale modes have been detected (0.04 ⩽ kθρe ⩽ 0.4, 5 ⩽ kθρs ⩽ 50, where kθ is toroidal wavenumber, ρe and ρs are the electron gyroradius and the ion sound gyroradius, respec- tively) by multichannel Doppler Backscattering (DBS) re ec- tometry [9, 21]; while, in the surrounding boundary layer of the SOL plasma, ion- and electron-scale turbulence is observed once a critical density gradient is exceeded. However, den- sity uctuations near the separatrix and in the SOL have been dramatically suppressed by a combination of NBI and E × B shearing via plasma-gun/electrode edge biasing, thereby improving global con nement properties.
Our previous experimental device, C-2, had initially no NBI or edge-biasing capabilities and could produce only ~1ms FRC plasma lifetime, as seen in gure 6; the lifetime was then 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, success- fully demonstrated increasing plasma pressure and electron
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