A pump includes one or more stages, each stage including a tubular housing and a mandrel disposed in the housing. The mandrel includes a rotor rotatable relative to the housing and having: an impeller, a shaft, and one or more helicoidal blades on an outer surface of the impeller. The mandrel further includes a diffuser: connected to the housing, having the rotor shaft extending therethrough and having a plurality of vanes on an outer surface thereof. Each vane has an arcuate portion and a longitudinal portion. The diffuser further has a channel formed between an adjacent pair of vanes. The pump further includes a fluid passage formed between the housing and the mandrel.