Study of electromagnetic mode contributing inward particle pinch in the scrape-off layer during H-mode discharge

Abstract

With the application of a four-tip Langmuir probe, the anomalous transport contributed by the turbulence and its impact on particle decay length in the scrape-off layer (SOL) has been studied on HL-2A tokamak. A high-frequency electromagnetic mode (HFM, 100 kHz < fHFM < 400 kHz) is reported, which can both be observed in the pedestal region and the SOL during the type-III ELMy H-mode discharge. In SOL, the HFM propagates in the electron diamagnetic direction in both the laboratory frame (22 km s−1) and plasma frame (24 km s−1) with m ≃ 28, n ∼ 8 and kθ ρi ∼ 0.04. Direct measurement shows that the HFM can drive inward particle flux, which can be explained by the electron diamagnetic rotation of the HFM. The time evolutions of particle flux driven by the low frequency turbulence (LFT, ‘blobby’ structures) and the HFM as well as the typical decay length of divertor particle flux suggest that the decay length in SOL can both be modified by the outward particle transport of the ‘blobby’ structures (Γr,LFT, <40 kHz) and inward particle transport by the HFM (Γr,HFM). The decay length of the particle flux is mostly suppressed when a significant inward flux by the HFM bursts under the condition of a low-level outward flux by ‘blobby’ events, suggesting the important role of anomalous transport on the decay length in the SOL.