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beam-driven FRC plasma state since it typically takes ~1 ms for the injected fast ions to accumulate and develop suffi- cient pressure inside the FRC. An HPF equilibrium state was first obtained in the C-2 device, and then further improved/ advanced in C-2U to a beam-driven FRC plasma state via increased injection power of NBs and effective edge biasing/ control in which FRC plasma was successfully maintained for 5 + ms. To achieve both HPF and beam-driven FRC operating conditions in C-2/2U, the following key elements/ approaches were identified as necessary: (i) dynamically colliding and merging two oppositely-directed CTs for a robust FRC formation; (ii) effective wall conditioning inside vacuum vessels, e.g. using a titanium gettering system, for background neutral and impurity reduction; (iii) effective edge/boundary control around the FRC separatrix via end-on plasma guns and concentric annular electrodes inside end divertors; and (iv) effective NBI into FRCs for current drive and heating. The main feature/characteristics of those high- performance beam-driven FRC regimes are: macroscopically stable plasma discharges, dramatically reduced transport rates (up to an order of magnitude lower than the non-high- performance FRC regime), high fast-ion population/pressure inside the FRC, long-lived plasma/diamagnetism lifetimes, and emerging global energy confinement scaling with strongly favorable temperature dependence.
The key elements specified above for C-2/2U experiments are still important and critical to the C-2W experimental pro- gram. In order to enhance fast-ion effects by NBI as well as to further improve FRC performance towards the program goals, those key elements (i)–(iv) have been significantly upgraded in C-2W as described in section 2. Furthermore, two more key elements were added in C-2W: (v) adequate particle refueling into FRC core and edge for plasma density and particle con- trol, and (vi) actively controllable external magnetic field for plasma shape/position control. These additional elements are critical, particularly in operations phase 2, for the expected FRC plasma ramp-up with increased NB input power of up to ~21 MW. Under no plasma ramp-up experimental condi- tions in C-2W, such as in operations phase 1, the key elements to produce a decent (stable, long lived, and hot) FRC plasma state/condition are fundamentally the same as in C-2/2U experiments even with C-2W’s slightly different machine configuration (e.g. presence of inner divertors, larger diameter of the CV). However, significantly upgraded NBs and edge biasing/control systems, as well as extensive FRC/system optimization processes, have led to further improved FRC performance, ultimately showcasing an ‘advanced beam- driven FRC’ equilibrium state in C-2W.
3.2. Robust initial FRC formation and translation
FRCs are produced/formed by colliding and merging two oppositely-directed CTs using the FRTP method in the forma- tion sections. This flexible, well controllable dynamic FRC formation technique [3] allows formation of various initial target FRC states for effective NBI study and plasma per- formance optimization. This formation technique was well- established initially on C-2, and also used in C-2U without a
H. Gota et al
major upgrade in the pulsed-power systems. In C-2W the for- mation pulsed-power system is significantly upgraded to form more robust FRCs as well as better targets for NBI. Another important change from C-2/2U to C-2W is the new large divertor placed between the formation section and the CV as illustrated in figure 1. This requires that FRCs generated in the formation section must be robust enough to translate through the inner divertor area (~1 m gap without a conducting wall/ shell) without too much degradation.
To test and verify an appropriate FRC formation and trans- lation with the inner-divertor vessel, the C-2W experimental program commenced early as part of subsystem commis- sioning using only one side of the device. One side of the formation pulsed-power system, inner-divertor fast-switching coils, mirror-plug coil, and confinement equilibrium/mirror coils were thoroughly tested for functionality as well as to characterize/optimize FRC formation and translation during this early phase of experiments. Relatively good FRC plasmas were formed in the formation section and then successfully translated through the inner divertor with adequate guide magnetic field applied by the in-vacuum fast-switching coils and confinement mirror coils in the single-sided machine con- figuration. In this early experiment, the translated FRC plasmas had an axial speed of 150–200 km s−1 entering into the CV, even with slightly reduced formation pulsed-power voltages, and reached all the way to the other end of the CV where they reflected off the strong confinement-mirror magnetic field. After some dedicated tuning/conditioning of the formation pulsed-power systems with higher voltages on the MR mod- ules, axial speed of the translated FRC got increased signifi- cantly, roughly twice as fast (sometimes up to ~500 km s−1); an example of successful FRC translation entering the CV is shown in figure 4 where the diamagnetic signal contour indi- cates that FRC enters the CV at around 20 μs, bounced back- and-forth by the strong confinement-mirror fields at both ends of the CV, and finally settling down near the midplane (z = 0). This successful experimental test of the FRC formation and translation was also simulated and verified using the Lamy Ridge 2D resistive MHD code [38] using the actual exper- imental machine settings as its input parameters, at which the simulation result shows a consistent picture of the FRC for- mation/translation process as observed in the C-2W single- sided experiment; note that an example of good agreement between experiment and Lamy Ridge simulation can be seen in figures 2 and 4 of [23] for C-2 experiment. Figure 5 shows an example of the single-sided FRC simulation on C-2W, depicting a time sequence of the simulated FRC plasma (field lines and density contour) at three phases of the formation, translation through inner divertor, and near the end of transla- tion in the CV. During this FRC formation/translation study in both experiment and simulation, it was clearly observed and verified that having an adequate guide magnetic field (using in-vacuum fast-switching coils) inside inner divertor is critical for FRC translation. Additionally, a proper balance of magn- etic field amplitudes and axial profile generated by the fast- switching coils and confinement mirror coils was found to be necessary; otherwise, FRCs do not properly penetrate through the inner divertor and mirror region and can either reflect off
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