Speaker
Description
The MAST-U spherical tokamak [1] with aspect ratio R/a [m] = 0.8/0.5, max B=0.72 T at 0.8 m, elongation up to 2.5, operated at 1 MA currents recent two years and delivered high D-D fusion beam-plasma performance. This performance was achieved with two deuterium NBI sources, one of which was on-axis, and the other - off-axis (shifted by 65 cm from the magnetic axis up). Of particular interest for maximising the neutron yield was the scenario where the beam-driven fishbones and TAEs could be suppressed by combining on-axis and off-axis beams with opposite radial gradients at flat/ weakly non-monotonic central q-profile. A quiescent TAE-free time window was established in such scenario, and the beam-plasma neutron rate was maximised then. The sequence of MAST-U discharges with such TAE-free time window at the end of every discharge, was performed with and without the uplift of the plasma, and the neutron rates, which started from the reference ~10^14 s-1, then achieved ~2.5x10^14 s-1, ~2.8x10^14 s-1, and ~3x10^14 s-1.
Plasmas in these discharges were of the hot ion type, with ion temperatures ~3 keV and electron temperatures ~1.5 keV, with a hollow electron density profile and hollow toroidal rotation profile. Internal Reconnection Events (IREs) [2, 3] appeared to be common in such MAST-U plasmas and these exhibited many properties similar to the disruptions: rapid increase in plasma current, negative spike in loop voltage, increase in elongation, and reduction in plasma energy. However, no terminations of the current were caused by IREs. The plasma density was limited by ~4x10^19 m-3 making the beam slowing-down time long enough for high yield beam-plasma fusion. Similar IREs were recently investigated with neural networks on the ST-40 tokamak concluding that IREs were less likely at higher q(95%) and β_pol. [3]. Based on this knowledge, we proposed and tested a “current ramp-down” technique used just before an IRE. Such technique did mitigate the IRE successfully followed by an increase in plasma density up to ~8x10^19 m-3.
[1] J.R. Harrison et al., 2019 Nucl. Fusion 59 112011; [2] R. Buttery et al., 1996, Proceed. of 23rd EPS, Kiev, Part I, p.416; [3] I. Semenov et al., 2003 Phys. of Plasmas 10 664; [4] C. Windsor et al., 2026, (to be submitted for publication).