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Nucl. Fusion 59 (2019) 112009 H. Gota et al
(a) (b)
Figure 6. (a) Shot-averaged electron temperature profiles from midplane Thomson scattering system measured during FRC collisional merging (t ~ 0.05 ms), compared with a typical shot-averaged C-2/2U Te result; typical error bars for each case are also included. (b) Time evolution of electron density radial profile from midplane FIR interferometer on C-2W.
and optimizing the best outcome/product after the FRC col- lisional-merging process remains to be obtained. We have spent a considerable amount of time to map out new oper- ating regimes in many subsystems as well as to optimize FRC performance inside the CV. The early experimental campaign was conducted without flaring magnetic field at inner-divertor regions to obtain C-2U like long-lived stable FRC plasmas with similar machine configuration to C-2U. In this early phase of machine/plasma multi-dimensional parameter map- ping, the optimization tool/algorithm previously developed with Google [40] was effectively used to find optimum oper- ating regimes and accelerated our experimental program and progress considerably.
Remarkably improved initial FRC formation and plasma states on C-2W have the following characteristics: forming and translating more energetic FRCs (CT plasmoids), much faster translation velocity (relative speed of the two-colliding FRCs gets up to ~1000 km s−1), and very flexible and wide range of operating parameters due to the upgraded pulsed- power and magnet systems. The high initial translational kinetic energy of the colliding FRCs yields high thermal energy post merging via shock heating (predominantly in the ion channel), as seen in C-2/2U [3, 23]; reconnection heating during the FRC-merging process may also contribute to the observed thermal energy increase, as seen and described in other CT-merging experiments [41, 42]. As anticipated by design and also in our simulations, the merged initial FRC state exhibits much higher plasma temperatures (in both elec- trons and ions), larger volume, and more trapped flux as com- pared to C-2U, providing a very attractive target for effective NBI in C-2W. The typical FRC plasma state right after the col- lisional-merging process has the following plasma properties: excluded-flux radius rΔφ ~ 0.45–0.5 m, length ls ~ 2.5–3.0 m, rigid-rotor poloidal flux φp ~ 10–12 mWb, electron temper- ature Te ~ 200–300 eV, total temperature (Ttot = Ti + Te, esti- mation based on pressure balance using density and magnetic
measurements [1]) up to ~1.5–2.0keV, and electron density ne ~ 1.5–3.0 × 1019 m−3. Figure 6(a) shows initial Te profiles, measured by multipoint Thomson scattering system during FRC collision/merging (t ~ 0.05 ms) in a typical shot-averaged C-2W discharge as compared to C-2/2U experiments. The ini- tial ~250eV flat Te profile (Ttot exceeding 1.5keV) in C-2W is testament to the improved initial FRC conditions produced by the upgraded formation pulsed-power systems. Time evo- lution of the Abel-inverted ne profile measured by 14-chord FIR interferometry is shown in figure 6(b) and clearly exhibits the expected hollowness of the radial density profile, corrobo- rating a typical FRC structure. Plasma behavior inside the CV is also visually monitored by side-on fast-framing camera as illustrated in figure 7(a), which has a wide field of view to see FRC plasma almost entirely. One example image (with a bandpass optical filter of oxygen 4 + line) of typical plasma discharge at t ~ 1 ms is shown in figure 7(b). A football-shaped plasma emission in the CV is clearly seen, and the edge of the emission (boundary of hot/cold plasmas) is consistent with the FRC shape/profile as estimated from the excluded- flux radius measurements, depicted with dashed lines in the camera image. Using this fast-framing camera, together with FIR interferometer and Mirnov probes, global MHD instabili- ties such as n = 1 wobble/shift and n = 2 elliptical/rotational modes are well diagnosed in C-2W.
In order to effectively inject fast ions into the FRC plasmas without causing too much charge-exchange losses due to background neutrals, titanium gettering systems are deployed inside the CV and in all four divertors for further impu- rity reduction and additional vacuum pumping capability. Reducing background neutrals outside of the FRC is one of the key elements to improve NB-to-FRC coupling and its effi- ciency. The gettering system inside the CV currently covers more than 80% of the total surface area of the CV inner wall, and has significantly reduced the neutral recycling at the wall and greatly reduced impurity content (e.g. oxygen, carbon,
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