mhd

High temperature plasmas can be confined by strong magnetic fields (both externally applied and self-generated) in devices such as tokamak*.

(*tokamak: a doughnut-shaped device containing hot magnetized plasmas)

Magnetohydrodynamics (MHD) is the study of equilibrium states and instabilities in magnetically confined plasmas.

We investigate the nonlinear dynamics of MHD instabilities using our world-best imaging diagnostic tools at the KSTAR tokamak. 

(1)  Internal Instabilities driven by currents

Studies on MHD phenomena such as sawtooth instability and tearing mode have been conducted, with diagnostics on electron temperature fluctuation. Sawtooth oscillation found in the core region of tokamak plasma can be typified by relaxation of core electron temperature and density. Especially, the dynamics of sawtooth oscillation is accompanied by modulations in heating systems. As an example, the ECCD effect on the dynamics of the sawtooth is presented in figure 1 (*Published in Nuclear Fusion 58, 106038)

Tearing mode can be characterized by the formation of magnetic islands. The evolution of tearing mode, such as coupling or nonlinear growth, can lead to enhanced radial transport which triggers disruption. Figure 2 illustrates the visualization of tearing mode from ECEI diagnostics, along with the model of magnetic flux surface. (*Published in Plasma Physics and Controlled Fusion 61, 055001) 

(2)  Instabilities in the plasma edge 

ELMs are spontaneous, explosive instability involving degradation of plasma confinement, which typically occur in H-mode plasma. Their common features such as the formation of filamentary structures or their transition to the non-normal state are reported in several works. (*Published in PRL 107, 045004)

As illustrated in figure 3, solitary perturbations are observed before the pedestal collapse. These can be characterized by magnetic perturbations from mirnov coil signal and distinctive structures from ECE imaging diagnostics. (*Published in Scientific Reports 7, 45075)