UKAEA-CCFE-PR(25)300
An explanation for hard MHD stability limits in low-q95 diverted tokamak plasmas
This paper proposes a model attempting to explain the appearance of fast growing global disruptive instabilities in diverted configurations when the safety factor near the magnetic separatrix approaches two. We show that if edge density and pressure gradients are strong enough, although external kink modes are stable, poloidal harmonics may couple allowing for a global fluid perturbation to become unstable. A narrow window of instability characterised by rather large growth rates even with modest gradients is identified when the mode resonance occurs in the edge region.