Ideal Magnetohydrodynamic Stability Analysis of H-mode Edge Plasmas in KSTAR and ITER

Abstract

After the first discovery in 1982 at the ASDEX tokamak, the H-mode is of great interest due its high fusion advanced plasma performance. There is a transport barrier in the plasma edge region resulting in high plasma confinement. The edge region with a transport barrier is called pedestal. Because of the steep pressure profiles in the pedestal region, an instability called edge-localized-mode(ELM) is occurred which was also discovered at ASDEX in 1982. As the plasma pressure gradient and the plasma current of the pedestal region is over the critical value, the instability occurs and pedestal structure is crashed. Since the first discovery of the ELMs, crash and recovery of the pedestal structure during an ELM cycle is widely studied in many experiments. The ELMs can be explained by ideal magentohydrodynamic (MHD) theory. The peeling-ballooning model has successfully explained the ELM activities, especially type-I ELMs. The high energy of plasmas released by ELMs is expelled to the divertor plate following the magnetic field line. Among many types of ELMs, type-I ELM forces the highest heat flux onto the divertor plate so that materials to endure the heat flux from the type-I ELM are critical concern in ITER. In KSTAR, the H-mode plasmas were achieved firstly in 2010 campaign and more experiments were carried out in 2011 campaign with more diagnostics such as electron cyclotron emission spectroscopy and charge exchange recombination spectroscopy which enables analyses of the type-I like ELMs using the peeling-ballooning model. A typical H-mode discharge with type-I like ELM equipping with reliable diagnostic data is selected and analyzed on the current density–pressure gradient plane. It is identified that plasma is near the peeling-ballooning boundary during the ELM cycle which could strongly support that the ELM is in type-I. After the validation of the peeling-ballooning model with KSTAR experimental data, same analysis was adopted to the ITER H-mode plasmas. To predict the performance of ITER plasmas, accurate prediction of the pedestal structure is very important since the fusion performance highly depends on the pedestal. Stability analysis of four H-mode plasmas is carried out and the pedestal pressure which could exhibit highest performance is identified on the stability boundary. It is shown that the plasma becomes unstable when the pedestal temperature approaches 5.5 keV. Various pedestal width and height are evaluated which can offer a guideline of optimized pedestal shape at highest performance in ITER reference scenario.