IFS Seminar with Sergei Sharapov

Abstract:

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 last 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 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 neutron rate was maximised then. Figure 1 shows the sequence of MAST-U discharges with such TAE-free time window at the end of every discharge, where the neutron rates started from the reference ~1014 s-1, but then achieving ~2.5x1014 s-1 , ~2.8x1014 s-1, and ~3x1014 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] 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 although plasma density was limited by ~4x1019 m-3 very suitable for the beam-plasma fusion. Similar IREs were 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 ~8x1019 m-3. 

Sergei Sharapov is Principal Research Scientist at Culham Campus of UKAEA, UK. He graduated in experimental nuclear physics from Moscow Physical Technical

Institute in 1985 and did his PhD in physics and chemistry of plasmas at Kurchatov Institute of Atomic Energy, Moscow. Subsequently, he worked at Kurchatov IAE on the theory of nonlinear waves and energetic particle- driven Alfvén waves. In 1993, he moved to work on JET and on the spherical tokamaks START and MAST, located at the Culham Centre for Fusion Energy, UK. Dr. Sharapov’s areas of interest and expertise lie in the theory, experiment, and diagnosis of energetic particles and energetic particle- driven instabilities in magnetic fusion devices.