Publication

Dependence of the L-Mode scrape-off layer power fall-off length on the upper triangularity in TCV

Abstract

This paper reports on experimental observations on TCV with a scan in upper triangularity δup{\delta }_{\mathrm{up}}, including negative triangularity, focusing on the power fall-off length λq{\lambda }_{{\rm{q}}} in L-Mode. The upper triangularity is scanned from −0.28 to 0.47. Smaller λqout{\lambda }_{{\rm{q}}}^{\mathrm{out}} is measured at the outer divertor target for decreasing δup{\delta }_{\mathrm{up}} together with higher edge temperature Te,edge{T}_{{\rm{e}},\mathrm{edge}} leading to increased confinement. This effect is observed for both magnetic drift directions for discharges in deuterium and helium. In helium larger λq{\lambda }_{{\rm{q}}} values are observed compared to deuterium. The power fall-off length at the inner divertor target λqin{\lambda }_{{\rm{q}}}^{\mathrm{in}} has a non-monotonic behaviour with changing triangularity. The largest values are around δup=0{\delta }_{\mathrm{up}}=0. The ratio λqin/λqout{\lambda }_{{\rm{q}}}^{\mathrm{in}}/{\lambda }_{{\rm{q}}}^{\mathrm{out}} increases for decreasing δup{\delta }_{\mathrm{up}} for positive triangularity and is approximately constant for negative triangularity. λqout{\lambda }_{{\rm{q}}}^{\mathrm{out}} is compared to available scaling laws. Partial agreement is only observed for a scaling law containing a proxy for Te,edge{T}_{{\rm{e}},\mathrm{edge}} at ASDEX Upgrade (Sieglin 2016 Plasma Phys. Control. Fusion 58 055015). Extending this scaling to TCV and using Te,edge{T}_{{\rm{e}},\mathrm{edge}} at ρpol=0.95{\rho }_{\mathrm{pol}}=0.95 suggests that λqout{\lambda }_{{\rm{q}}}^{\mathrm{out}} is independent of machine size λqL\mboxMode(mm)={\lambda }_{{\rm{q}}}^{{\rm{L}} \mbox{-} \mathrm{Mode}}\,(\mathrm{mm})\,= 165Bpol(T)0.66165\cdot {B}_{\mathrm{pol}}{({\rm{T}})}^{-0.66}\cdot A(u)0.15A{({\rm{u}})}^{-0.15}\cdot Te,edge(eV)0.93{T}_{{\rm{e}},\mathrm{edge}}{(\mathrm{eV})}^{-0.93}\cdot R(m)0.03R{({\rm{m}})}^{-0.03}. Possible explanations for smaller λqout{\lambda }_{{\rm{q}}}^{\mathrm{out}} for decreasing δup{\delta }_{\mathrm{up}} is a reduction in turbulence or a direct effect of increasing Te,edge{T}_{{\rm{e}},\mathrm{edge}}.

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