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

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