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Nucl. Fusion 59 (2019) 112009 H. Gota et al
Figure 10. Examples of typical plasma discharges under different machine configurations: shot #104989 in OP1.1—without inner-divertor magnetic-field flaring and with outer-divertor edge biasing, C-2U like configuration; shot #107322 in OP1.2—with field flaring and outer- divertor biasing (positively). From the top panel to bottom: excluded-flux radius, electron density, electron temperature (averaged inside FRC separatrix), and total temperature estimated from pressure balance. Thomson scattering system can operate two modes together: (i) a 30-pulse chain at 1 kHz; and (ii) six pulses at 13 kHz or four pulses at 20 kHz burst with selectable start time of interest [31]. Note that NBs are injected for longer than FRC plasma lifetime in both discharges.
Figure 11. Time evolutions of electron temperature radial profile on (a) shot #104989 in OP1.1 (C-2U like configuration; edge biasing from outer divertors) and (b) shot #107322 in OP1.2 (with inner-divertor magnetic field flaring and outer-divertor edge biasing), measured by midplane Thomson scattering system. Note that low temperature regions (color in dark blue) are mostly outside of FRC separatrix.
4. Summary
The C-2W device was newly constructed with substantial subsystem upgrades from C-2U. C-2W initially demon- strated a robust FRC formation and translation through the newly installed inner divertor with adequate guide magnetic field, and then produced improved initial FRC plasma states after merging of the two colliding FRCs (relative speed of collision up to ~1000 km s−1) as expected from the system upgrades. The merged initial FRC state exhibited higher plasma temperatures (Te ~ 250 + eV, Ttot exceeding 1.5 keV) than C-2U. Under outer-divertor operating condition in OP1.1
(without flaring magnetic field at inner divertor regions) like C-2U configuration, stable and long-lived FRC plasma state was obtained by effective edge biasing and NBI; FRC achieved ~9 ms plasma lifetime which is equivalent to C-2U performance. Under inner-divertor operating condition with magnetic field flaring in OP1.2, electron temperature inside FRC appeared to stay hot and increase for a short period of time. The C-2W NB system is being further upgraded to be able to ramp up beam injection energy, at which the total injection power will go up to ~21 MW and then FRC plasma pressure is expected to ramp up as well in the next operation phase (OP2).
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