A passive mitigation strategy of impurity deposition on the first mirrors using duct with baffles: A case study at a port of KSTAR with in-situ deposition monitoring

•The effect of the duct with baffles on the mirror deposition was investigated using test system installed at KSTAR tokamak.•Helium gas purging in the duct as a method of deposition mitigation was tested to check the effect on the plasma condition.•The impurity deposition rate was measured to be 0.6 – 40 ng/(h.cm2) or 2.7 × 10−3 – 0.18 nm/h (in case of graphite).•The most detrimental operation of high deposition was the glow discharge wall cleaning (GDC) under a baking condition.