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Nucl. Fusion 59 (2019) 112009
same dynamic FRTP formation technique utilized in C-2 [3]. The formation pulsed-power systems consist of Bias modules for negative-bias magnetic field, main-reversal (MR) modules for main theta-pinch magnetic field, and rotating magnetic field modules for deuterium gas pre-ionization in the formation section. The previously used ringing pre-ionization (PI) system is no longer used in C-2W; the load sharing between the PI and MR capacitor banks led to mis-firing/discharging issues which limited stored energy and reliability. Note that we define ‘t = 0’ in our FRC experiment as the time when the first MR module gets discharged/triggered. Each pulsed-power module has substantial system upgrades from the previous C-2/2U pulsed-power systems; the stored energy of each system has increased significantly, triggering efficiency has increased, and the overall system reliability and operating performance has improved considerably. Due to such improved overall pulsed- power system performance, the generated initial FRC plasma in the formation section has more magnetic and kinetic ener- gies which enables a path to higher thermal energy in the CT collisional-merging process as observed in C-2/2U [3, 23].
To control open field-line plasmas as well as to provide sufficient radial electric field for E × B shear flow around the FRC separatrix, coaxial plasma guns and concentric annular electrodes are installed inside of each outer divertor as illus- trated in figure 1(b). This edge-biasing/control configuration with a capability of magnetic field flaring at end divertors is essentially the same as C-2U [5, 6], but the C-2W edge-biasing system has more functionality and flexibility in terms of its operations such as higher voltage/potential that can be applied on the electrodes with longer pulse duration up to ~30ms. Furthermore, similarly designed/developed annular electrodes (with a large hole in the central electrode, required for FRC translation) as well as funnel limiters are installed inside inner divertors as shown in figure 1(c). Electrical potentials on those inner-divertor electrodes/limiters as well as on outer-divertor electrodes can be controlled independently by power supplies. Control of magnetic field topology and boundary potentials in the divertors is the key to flexible C-2W experiments, enabling effective edge/boundary control of FRCs via open field-lines/scrape-off layer (SOL). The role of the SOL and divertors is not only to provide a favorable boundary condition for the core FRC plasma but also to handle energy and particle exhaust from the core. There is also a halo region, outside of the SOL, with open field-lines contacting to the limiters and wall of the CV that produces secondary electron emis- sion/wall recycling; the region contains low temperature, low density partially ionized plasma sustained by power flow from the plasma core, beam ions, and warm neutrals. Therefore, adequate wall conditioning with sufficient pumping capability is important and required in order to effectively inject NBs into FRC with small charge-exchange losses.
Eight newly upgraded NB injectors are installed on the CV as shown in figure 1(a) for plasma heating, current drive, and partial particle refueling. The C-2W NBI system has the following key features: NB’s input power and pulse duration increased from ~10 MW (15keV hydrogen, co-current injec- tion)/~8 ms in C-2U to 13 + MW (fixed energy of 15 keV hydrogen)/up to ~30ms in C-2W phase 1 operations that
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is being further increased up to ~21 MW with tunable beam energy of 15–40keV (four out of eight NBs will eventually have tunable energy capability) in phase 2 operations; tilted NBI angle in a range of 65°–75° (presently fixed at 70°) rela- tive to the machine axis with average NBI impact parameter at ~19 cm to enable sufficient coupling between the beams and the target FRC plasma. The NBs provide energetic particles with a large orbit size crossing inside and outside of the FRC separa- trix that stabilize global magnetohydrodynamic (MHD) modes; they also provide a significant amount of fast ion population and pressure inside the core, thus producing an advanced beam- driven FRC plasma. Ramping up CV fields in phase 2 opera- tions will be matched by increased beam energy so as to match orbits; additionally, charge-exchange losses will be reduced at the higher particle energy, total NBI power will increase, and the fast-ion and plasma pressure will increase as well.
Plasma particle inventory must be controlled to main- tain proper densities for NB capture, which is required in the presence of particle losses from the core that is unavoidable. A plasma refueling system must be capable of matching the particle losses as well as increasing the total particle inventory if desired. Diamagnetic current within the FRC flows across magnetic field lines and thus sustainment of total pressure gra- dient in the core is essential for sustainment of trapped flux and FRC magnetic configuration. Without central refueling or heating the trapped magnetic flux decays due to finite plasma resistivity across the magnetic field. To that end, there are three main particle refueling systems deployed on C-2W: multi- pulsed CT injector systems near the CV midplane [24, 25], a cryogenic pellet injector system [26], and gas puffing systems at both ends of the CV (near confinement mirror regions) as well as near mirror-plug regions for open-field-line plasmas. Contrary to the cryogenic pellet injection (over dense and cold gas), CT-injection refueling system can supply hotter plasma particles with controllable density level, resulting in less plasma cooling [25]. Gas puffing in the CV cannot provide effective core refueling but can be used for the edge density control.
2.2. Plasma diagnostic suite
The C-2W device is planned eventually to have more than 50 plasma diagnostic systems installed on the CV, inner/outer divertors, and formation sections. The role of the plasma diag- nostic suite is to investigate and characterize not only core FRC plasma performance but also open field-line plasmas at various areas such as SOL/Jet regions and inside divertors. Figure 3 illustrates a schematic view of C-2W showing four distinct zones (core, SOL/Jet, divertors, and formation) of diagnostic interest with abridged lists of diagnostics deployed in each zone. To support C-2W experiments towards the pro- gram goals, the diagnostic suite has been significantly upgraded from the previous diagnostic suite in C-2U [27]. Much of the expansion and improvements were driven by a highly increased interest in the open-field-line plasma, which has a large impact on the core FRC and overall system performance. Furthermore, some key plasma parameters (e.g. temperatures, density/pressure, and magnetic flux) are expected to evolve in time as external magnetic field and NB input power are
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