Radial propagation of ELM-filaments and boundary turbulence characteristics with divertor heat flux mitigation in the small ELM regime on HL-2A

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Abstract This paper investigates the radial propagation of ELM-filaments and boundary turbulence characteristics with divertor heat flux mitigation in the small ELM regime on HL-2A. Divertor nitrogen seeding mitigates divertor heat flux while maintaining high plasma performance (H_98>1.0 and β_N>2.0) and negligible degradation of total stored energy and energy confinement time. In the pedestal, the pressure gradient and ELM amplitudes decrease after divertor heat flux mitigation. The magnetic perturbation diminishes while electrostatic turbulence increases, with poloidal velocity shear remaining unchanged in the pedestal. Turbulent particle flux and perpendicular heat flux decrease in the far SOL region. The ELM-filament burst rates and conditionally averaged amplitudes from the pedestal to the near SOL and the far SOL are systematically investigated before and after heat flux mitigation. ELM-filaments burst rates increase and ELM-filaments amplitudes decrease in the pedestal after heat flux mitigation. The evolution of near-SOL filaments could be partially reflected by that at the outer divertor target (ρ≈0.99-1.09). At the divertor target, the filaments burst rates are larger at ρ≈1.01 but become smaller at ρ≈1.09 The burst rates of filaments in the far SOL (ρ≈1.16) decrease dramatically, resulting in a significant reduction in filament transport after divertor heat flux mitigation. These results provide improved insight into the radial propagation of ELM-filament, and benefit the understanding of power deposition on the first wall in the small ELM regime.

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