Cheng Kaijia

Cheng Kaijia the Chinese nuclear physicist and explosive expert (1918-2018) who was a principal figure in the development of China's atomic and hydrogen bombs, designing the experimental apparatus and the diagnostic techniques that made the first nuclear tests possible.

Early Life and Education

Cheng Kaijia was born on 3 August 1918 in Wuxi, Jiangsu province, into a family with scholarly traditions. His name, meaning to open the armour, was chosen by his relatives and later seemed strangely appropriate for a man who devoted his life to understanding explosions and protective materials. He grew up amid the political turbulence of the early republic and the approach of war.

He studied at Zhejiang University in Hangzhou, where he was taught by several of the leading Chinese physicists of the day and graduated in physics in 1941. The wartime conditions of his student years forced the university to move repeatedly, and he completed his education under difficult circumstances that instilled in him a habit of improvisation and endurance.

In 1946 he travelled to Britain on a scholarship and entered the University of Edinburgh, where he studied under the physicist Max Born and worked on experimental problems in physics. He earned his doctorate in 1948 and then spent further time in Britain, acquiring expertise in measurement techniques and instrumentation that would prove decisive for the work that lay ahead.

Return and the Nuclear Programme

Cheng Kaijia returned to China in 1950, soon after the founding of the People's Republic, and took up a teaching position at Zhejiang University and later at Nanjing University. He worked on nuclear physics and instrument development, and he began to train students in the experimental methods he had learned abroad. For several years his research concerned the detection of radiation and the design of apparatus.

In the late 1950s he was recruited into the secret programme to develop a Chinese atomic bomb, joining a team of scientists and engineers assembled under the direction of senior leaders. He was placed in charge of the development of the explosive assembly that would capture and compress the nuclear core, the apparatus that surrounded the plutonium and drove it to criticality.

Working in remote testing grounds in the northwest, he and his colleagues devised the precise lenses of high explosive that would focus the detonation into a converging shock. This was among the most demanding engineering problems of the project, requiring exacting fabrication and measurement, and the successful method he helped to create was essential to the first test.

Diagnostic Techniques and the First Test

Cheng Kaijia specialised in measurement, and he designed the diagnostic systems that recorded what happened inside the device during detonation. He developed fast electronic instruments and photographic methods capable of capturing events that lasted only a fraction of a microsecond, allowing the scientists to determine whether the implosion had proceeded as intended.

He also built a distinctive array of detectors and oscilloscopes shielded against blast and radiation, and he reasoned carefully about how to interpret the signals they produced. His work turned the test from an act of faith into an experiment whose outcome could be measured, analysed and used to improve the next design.

China carried out its first atomic test in October 1964 at Lop Nur and its first hydrogen bomb test in October 1967. Cheng Kaijia participated in both, and he continues to be associated with the diagnostic methods that made the results intelligible to the scientists. His contributions earned him the status of a key figure in the programme.

Later Career and the Hydrogen Bomb

After the initial successes Cheng Kaijia remained centrally involved in the testing programme, working on both fission and thermonuclear designs. He helped to develop the methods by which a small fission device could ignite a much larger thermonuclear reaction and collaborated in the analysis of the tests that validated the concept.

He also turned his attention to problems of protection and to the study of materials under extreme conditions, fields in which his expertise in explosion and shock was directly applicable. He led research groups that investigated the behaviour of matter under high pressure and the effects of nuclear explosions on structures and equipment.

In the 1980s he was elected a member of the Chinese Academy of Sciences and continued to advise on defence science and to supervise doctoral students. He held a succession of senior academic posts, including a vice-presidency of the academy, and he was honoured with national awards for his scientific and technical achievements.

Influence and Legacy

Cheng Kaijia was one of the small group of scientists whose work gave China the ability to conduct nuclear tests. His combination of theoretical understanding and practical skill in instrumentation marked him out, and his insistence on measuring precisely what had occurred set a standard for the whole programme.

His legacy includes the training of a later generation of researchers in shock physics, detonics and nuclear diagnostics. Institutions and laboratories in which he worked continued his methods for decades, and his name is associated with the rigorous experimental tradition that underpinned Chinese nuclear defence science.

He lived to be almost a hundred years old, dying on 17 November 2018 in Beijing. He received many of the highest honours his country could confer, and he is remembered as a scientist of great modesty and persistence whose contributions were for many years unknown to the wider public.

See also

• Qian Sanqiang
• Lop Nur
• Nuclear weapon design
• Chinese Academy of Sciences