核聚变Nuclear fusion

轻核的比结合能小，聚合成 ⁴He 时比结合能大幅增加。氘氚反应 D + T → ⁴He + n 放出 17.6 MeV，按质量计算约为铀裂变的 4 倍。要让原子核足够接近以发生量子隧穿，需要约 1 亿度以上的等离子体。劳森判据规定，等离子体密度、温度与能量约束时间的乘积必须超过一定值，聚变才能产生净能量。

Light nuclei have low binding energy per nucleon, which rises sharply on forming ⁴He. The D + T → ⁴He + n reaction releases 17.6 MeV, about four times as much per unit mass as uranium fission. Getting nuclei close enough to tunnel together needs a plasma above about 100 million kelvin. The Lawson criterion requires the product of plasma density, temperature and energy confinement time to exceed a threshold before fusion yields net energy.

1920 年爱丁顿提出太阳能量可能来自氢聚合为氦。1932 年奥利芬特在实验室实现氘-氘聚变。1938–1939 年贝特和冯·魏茨泽克阐明了恒星中的质子-质子链和碳氮氧循环，贝特获 1967 年诺贝尔物理学奖。1952 年美国试爆第一颗氢弹“常春藤麦克”。

In 1920 Eddington suggested the Sun's energy might come from hydrogen fusing into helium. Oliphant achieved deuterium–deuterium fusion in the laboratory in 1932. In 1938–39 Bethe and von Weizsäcker worked out the proton–proton chain and the CNO cycle in stars; Bethe received the 1967 Nobel Prize in Physics. The United States tested the first hydrogen bomb, Ivy Mike, in 1952.

磁约束以苏联 1950 年代发明的托卡马克为主流，国际热核聚变实验堆（ITER）正在法国建设；中国 EAST、HL-3 等装置不断刷新长脉冲高约束运行纪录。惯性约束用激光压缩燃料靶丸，2022 年美国国家点火装置首次实现聚变输出能量大于激光输入能量。聚变燃料氘可从海水提取，产物无长寿命放射性废物，但工程挑战巨大。

Magnetic confinement is led by the tokamak, invented in the Soviet Union in the 1950s; ITER is under construction in France, and China's EAST and HL-3 keep setting records for long, high-confinement pulses. Inertial confinement compresses fuel pellets with lasers, and in 2022 the US National Ignition Facility first obtained more fusion energy out than laser energy in. Deuterium fuel can be extracted from seawater and fusion leaves no long-lived high-level waste, but the engineering challenges are immense.