  A hallmark of unconventional superconductors is their complex electronic
phase diagrams where "intertwined orders" of charge-spin-lattice degrees of
freedom compete and coexist as in copper oxides and iron pnictides. While the
electronic phase diagram of kagome lattice superconductor such as CsV3Sb5 also
exhibits complex behavior involving coexisting and competing charge density
wave order and superconductivity, much is unclear about the microscopic origin
of superconductivity. Here, we study the vortex lattice (VL) in superconducting
state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and
enhances superconductivity. Using small-angle neutron scattering, a strictly
bulk probe, we show that the VL exhibits a strikingly conventional behavior.
This includes a triangular VL with a period consistent with 2e-pairing, a field
dependent scattering intensity that follows a London model, and a temperature
dependence consistent with a uniform superconducting gap expected for s-wave
pairing. These results suggest that optimal bulk superconductivity in
Cs(V1-xTax)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer
electron-lattice coupling, different from spin fluctuation mediated
unconventional copper and iron based superconductors.
