Experimental scalings of scrape-off layer particle flux width by outboard divertor Langmuir probes for deuterium and helium plasmas on EAST

Abstract

The scrape-off layer (SOL) power width ( λ _q ) is an important parameter for predicting the heat load on divertor targets for future magnetically confined devices. Currently, the underlying physics for λ _q scaling is not fully understood. This paper extends the previous SOL particle flux width ( λ _js , λ _q ≈ λ _js is assumed, j _s is the measured ion current density used as a proxy for particle flux) scaling measured by the inboard divertor Langmuir probes (Div-LPs) in deuterium plasmas (Liu et al 2019 Plasma Phys. Control. Fusion 61 045001) to the λ _js scalings measured by the outboard Div-LPs in both deuterium and helium plasmas on EAST. A systematic method has been used to correct the upper-outer Div-LP measurements to reduce the measurement uncertainty of λ _js . About 520 discharges are selected for the construction of six databases (H-mode, L-mode, and Ohmic for deuterium and helium plasmas) to scale λ _js . Since the published λ _q scalings do not fit EAST λ _js well in the databases, four separate λ _js scalings are proposed using statistical method to minimize the scaling parameters. It is found that λ _js has a robust scaling dependence on the plasma line-averaged density ( ${\bar n_{\text{e}}}$ ) for both deuterium and helium plasmas and a strong positive scaling dependence on the divertor leg length ( L _div,leg ). The scaling exponent of the stored energy agrees well with the previous λ _q / λ _js scalings for deuterium plasmas. The near unity scaling exponents of edge safety factor and L _div,leg reveal the importance of parallel connection length in determining λ _js . The positive scaling dependence of λ _js on ${\bar n_{\text{e}}}$ and the 2 ∼ 4 times larger scaling amplitude compared with the published λ _q scalings probably indicate the turbulence broadening of λ _js on EAST. The comparison of λ _js for deuterium and helium plasmas using the constructed databases shows no significant difference. Statistical comparison of the normalized λ _js with different ELM types suggests that the grassy-ELM λ _js is slightly larger than the inter large-ELM (type-I ELM) λ _js for both deuterium and helium plasmas.

Keywords

• scrape-off layer particle flux width
• divertor Langmuir probe
• helium plasma
• EAST