Understanding and Controlling Edge Instabilities in Plasmas

Mitigation of Edge-Localized-Modes, ELMs, is critical for the success of ITER and other future burning plasma devices. Several methods of ELM mitigation are being explored such as: gas puffing to inject fuel or impurities in the plasma edge to control the plasma pedestal density, 3D magnetic perturbations to create an edge stochastic layer increasing the transport to lower the pedestal pressure gradient, lithium granule injections (LGI), etc. One method of reducing the negative effects on materials and plasma confinement from large ELMs is to induce high frequency, small ELMs, thus reducing transient heat pulses to the divertor to manageable levels. This method is known as ELM pacing. 

ELM pacing has been shown to reduce transient heat pulses to the diverter by inducing high frequency, small ELMs. Studies have predicted that ITER requires a frequency enhancement > 30 times the natural ELM frequency. One approach to ELM pacing is to inject small pellets of fuel or impurities into the edge of the plasmas, which can trigger an ELM at a rate exceeding the natural ELM frequency.