After decades of research, the scientist finally attained his goal of developing a sustainable energy source.

attained 是动词 attain 的过去式，attain 由前缀 at-（表示"朝向"）和词根 tain（来自拉丁语 tenere，意为"持有、达到"）组成，意思是"达到、获得、实现"。在这里指科学家经过长期努力后终于"达成"了研究目标。