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Nucl. Fusion 59 (2019) 112009 H. Gota et al
Figure 7. (a) Illustration of the C-2W CV with fast-framing camera setup and its field of view from near the top of the CV. (b) Camera image of FRC plasma emission with O 4+ bandpass optical filter (λ ~ 650.0 ± 0.5 nm) at t ~ 1 ms in a typical plasma discharge; fitted excluded-flux radius profile measured by magnetic probes is also overlaid (dashed lines) in the image.
and nitrogen). Newly installed cryo-panels and a titanium arc gettering system with a liquid N2 cooling system inside all four divertors are working effectively as designed; divertor pumping tests indicate that system pump speed has achieved up to ~2000 m3 s−1 (for hydrogen) per each divertor. The tita- nium-arc gettering system can also be operated in between plasma shots/discharges as desired to improve/recover the divertor pumping capability. Overall FRC plasma perfor- mance, in particular lifetime and temperatures, has consider- ably advanced with improving wall conditioning by extensive titanium gettering in the CV and divertors [43].
As for edge biasing and boundary control of the FRC and open-field-line plasmas, there are several techniques imple- mented in the C-2W divertor areas as described in section 2.1: applying negative or positive voltages on the plasma guns, outer-divertor electrodes, inner-divertor electrodes, and/or funnel limiters relative to the machine ground or even to other
biasing systems/points. There are so many different biasing options/configurations using those systems, that finding and mastering the most effective biasing schemes for plasma edge control is currently one of the most important objec- tives to achieve in this operations phase 1. Here, we present two examples of different schemes and effects to illustrate the challenges of balancing different effects on FRC performance against each other. As the first example of edge biasing scheme, as routinely used in C-2/2U experiments and well described in [7], applying negative radial electric field (−Er) in the open field-lines around the FRC separatrix produces E × B shear flow that counters a natural FRC rotation in the ion diamagn- etic direction, thus n = 2 rotational mode can be effectively stabilized in C-2W. Figure 8 shows an example of the effect of edge biasing on FRC stabilization using −Er applied edge- biasing systems in outer divertors, where the central elec- trode was charged up at around −1.5 kV right after t = 0. In
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