Impact of pedestal density gradient and collisionality on ELM dynamics

TL;DR

This study uses BOUT++ turbulence simulations to investigate how pedestal density gradient and collisionality influence the dynamics and size of ELMs in tokamak plasmas, revealing conditions that lead to small ELMs.

Contribution

It provides new insights into the physics of small ELMs by analyzing the effects of pedestal density gradient and collisionality through comprehensive linear and nonlinear simulations.

Findings

Small ELMs are associated with high pedestal density and collisionality.

Linear growth rates decrease as collisionality increases, leading to smaller ELMs.

Small ELMs can be triggered by peeling-ballooning or local pedestal foot instabilities.

Abstract

BOUT++ turbulence simulations are conducted to capture the underlying physics of the small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT ++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to the intermediate-n peeling-ballooning modes and eventually to peeling-ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The elm size decreases with increasing the separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling-ballooning instability near the peak pressure gradient position inside pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and elm size decrease with collisionality increasing. While the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by high-n ballooning mode or by low-n peeling mode in low collisionality region 0.04~0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.