Kēxuéjiā men zhìlìyú yánjiū kěkòng jùbiàn, yǐ shíxiàn qīngjié néngyuán.
Scientists are working on controlled fusion to achieve clean energy.
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聚 = to gather together; 变 = to change/transform. Fusion is what happens when atoms 'gather and transform' - light nuclei fuse into heavier ones. Contrast with 裂变 (fission) where atoms split apart.