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L-H Transition and Edge Transport Barrier Formation on LHD

K. Toi, F. Watanabe, S. Ohdachi, S. Morita, X. Gao, K. Narihara, S. Sakakibara, K. Tanaka, T. Tokuzawa, H. Urano, A. Weller, I. Yamada, L. Yan, LHD Experiment Group

Fusion Science and Technology / Volume 58 / Number 1 / July/August 2010 / Pages 61-69

Chapter 3. Confinement and Transport / Special Issue on Large Helical Device (LHD) /

The L-H transition was observed in a unique helical divertor configuration where the core plasma is surrounded by ergodic layer, exhibiting rapid increase in edge electron density with sudden depression of H emission. Just after the transition, edge transport barrier (ETB) is formed at the plasma edge in the magnetic hill region, developing a steep density gradient. ETB region extends in ergodic layer beyond the last closed flux surface defined by the vacuum field. The transition occurs in relatively high beta plasmas when neutral beam absorbed power (Pabs) exceeds one to three times the ITER H-mode power threshold. Improvement of energy confinement time is modest (<1.1) for the ISS95 international stellarator scaling, whereas the particle confinement is clearly improved. The ETB width tends to increase with the increase in the toroidal beta at the ETB shoulder. ETB formation leads to destabilization of edge magnetohydrodynamic (MHD) modes with m/n = 2/3 or 1/2 (m and n being the poloidal and toroidal mode numbers) in ETB region of the inward-shifted configurations. Edge-localized modes (ELMs) are excited by these edge MHD modes through nonlinear evolution. Sometimes in outward-shifted plasmas, edge MHD modes are clearly suppressed in the H-phase and lead to an ELM-free H-mode. When large m/n = 1/1 resonant magnetic perturbations are applied to neutral beam injection-heated plasmas, the transition takes place at lower line-averaged electron density having the modest increase in electron temperature and small-amplitude ELMs.

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