Edge Localized Modes
The steep edge gradients (of density and temperature) associated with an H-mode lead to quasi-periodic violent relaxation phenomena, known as Edge Localized Modes (ELMs), which have a strong impact on the surrounding vessel. <ref>H. Zohm, Edge localized modes (ELMs), Plasma Phys. Control. Fusion 38 (1996) 105-128</ref> <ref>D.N. Hill, A review of ELMs in divertor tokamaks, Journal of Nuclear Materials 241-243 (1997) 182-198</ref>
Physical mechanism
The physical mechanism of ELMs has not been fully clarified. Several possible explanations have been put forward:
- Nonlinear interchange modes <ref>A. Takayama and M. Wakatani, ELM modelling based on the nonlinear interchange mode in edge plasma, Plasma Phys. Control. Fusion 38 (1996) 1411-1414</ref>
- Coupled peeling-ballooning modes <ref>J.W. Connor et al, Magnetohydrodynamic stability of tokamak edge plasmas, Phys. Plasmas 5 (1998) 2687</ref><ref>P.B. Snyder et al, Edge localized modes and the pedestal: A model based on coupled peeling–ballooning modes, Phys. Plasmas 9 (2002) 2037</ref><ref>J.-S. Lönnroth et al, Predictive transport modelling of type I ELMy H-mode dynamics using a theory-motivated combined ballooning–peeling model, Plasma Phys. Control. Fusion 46 (2004) 1197-1215</ref><ref>N. Hayashi et al, Integrated simulation of ELM energy loss and cycle in improved H-mode plasmas, Nucl. Fusion 49 (2009) 095015</ref>
- Peeling modes <ref>C.G. Gimblett, Peeling mode relaxation ELM model, AIP Conf. Proc. 871 (2006) 87-99</ref>
- Flux surface peeling <ref>E.R. Solano et al, ELMs and strike point jumps, Journal of Nuclear Materials 337-339 (2005) 747-750 </ref>
- Criticality of MHD equilibrium <ref> Emilia R. Solano, Criticality of the Grad–Shafranov equation: transport barriers and fragile equilibria, Plasma Phys. Control. Fusion 46 (2004) L7-L13 </ref>
- Self-Organised Criticality <ref>R. Sánchez et al, Modelling of ELM-like phenomena via mixed SOC-diffusive dynamics, Nucl. Fusion 43 (2003) 1031-1039 </ref>
ELMs and machine operation
The occurrence of an ELM leads to a significant expulsion of heat and particles, with deleterious consequences for the vessel wall and machine operation. Although Quiescent H-modes exist (without ELMs), <ref>K.H. Burrell et al, Advances in understanding quiescent H-mode plasmas in DIII-D, Phys. Plasmas 12 (2005) 056121</ref> they are generally considered not convenient due to the accumulation of impurities. To achieve steady state, an ELMy H-mode is preferred and this mode of operation is proposed as the standard operating scenario for ITER, thus converting ELM mitigation into a priority. <ref>M.R. Wade, Physics and engineering issues associated with edge localized mode control in ITER, Fusion Engineering and Design 84, Issues 2-6 (2009) 178-185</ref>
References
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