A new scheme is proposed in which high-ux and high-density positrons are generated via ultra-intense laserpulses colliding in a cylinder lled with near-critical-density plasmas. Comparing with the only cylinder caseand the only gas case, positron generation is enhanced with a higher density. When discussing the inuenceof plasma density in positron generation, an optimal gas density is found around the near-critical density. Thescheme will facilitate the realization of positron generation in labs and further applications of positrons.
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