Cai Shidong has been engaged in plasma theory research for more than 30 years. His research scope is wide, and he has published more than 50 papers/kloc-0. His research work is widely cited by colleagues. He has made systematic and creative contributions in some important frontier fields. 1. The elimination identities of off-diagonal elements of heat flow and diamagnetic convection term are derived, which are later called "Chua's equation" by colleagues in Princeton Laboratory. From then on, the expressions of stress and heat flux in plasma macroscopic equation were fixed. In the 1960' s, the collision drift wave instability in static plasma (Q-machine plasma) was extensively studied experimentally and theoretically. Although their research results can qualitatively explain some experimental results, there are still many unexplained phenomena. Based on the analysis of previous work, Cai Shidong considered the interaction of thermal fluctuation and boundary conditions, as well as collision viscosity and heat transport. The calculated results greatly improved the agreement with the experimental results, and predicted the existence of entropy mode instability, which was later confirmed by experiments.

Cai Shidong's paper on the study of drift wave and its instability is one of the important documents on the study of drift wave.

2. A thermodynamic method for solving the nonlinear saturation limit of anisotropic electromagnetic oscillation is established. Cai Shidong extended the original isotropic electrostatic method to the case of nonlinear saturation limit of anisotropic electromagnetic oscillation, thus establishing a thermodynamic method to solve the nonlinear saturation limit of anisotropic electromagnetic oscillation.

3. In the study of large amplitude Langmuir waves, a hybrid calculation method is established. A hybrid analytical and simulation algorithm based on boundary condition matching is established in the study of spatial evolution of finite amplitude plasma waves in collision-free plasma. The algorithm can be applied to large-scale asymptotic expansion or small-scale disturbance, and the calculation amount is greatly reduced. This is especially beneficial when the computer capacity and speed were not very large at that time.

4. The general dispersion relation of weakly relativistic plasma is deduced, and the analytical calculation method of dispersion function of drift plasma is established. Under the uniform and non-uniform WKB approximation, the general dispersion relation of weakly relativistic plasma is deduced in cooperation with Professor Wu Jingsheng of the University of Maryland, and the L function is extended to the complex plane, and the analytical calculation method of dispersion function of drift plasma is established. Therefore, the microscopic instability problem driven by inhomogeneity and relativistic effect, which can only be calculated by model or numerical value for many years, can be strictly analyzed, and a calculation method is provided for many problems of celestial bodies, space and laboratory plasma. This method is widely cited.

5. The qualitative relationship between ion temperature gradient instability and plasma confinement is explained. According to the experimental results of PLT in Princeton Tokamak, the unstable mode structure caused by ion temperature gradient in Tokamak plasma is derived for the first time, and the scaling rate of abnormal transport is deduced from this. Firstly, the relationship between constraint time and current is qualitatively explained. 1. The cyclotron dynamics equation of relativistic plasma with arbitrary frequency and arbitrary magnetic field configuration is established.

Whether the plasma can be confined in the fusion device for a long time is one of the important conditions for thermonuclear fusion to realize positive energy output as an energy source. Cyclotron dynamics equation is an important basic tool to study the stability and confinement of plasma, especially high temperature plasma.

Cai Shidong cooperated with Professor Ai Chen of the University of California, USA, and extended the cyclotron dynamics equation which can only deal with low frequency to the plasma with relativity, arbitrary frequency and arbitrary magnetic field configuration, and obtained a set of simple microscopic theoretical equations of high-energy plasma. This work and the concept and method of calculating the composition of high-energy particles are widely cited and included in graduate textbooks. Cai Shidong and his collaborators used it to calculate various instabilities and extended it to calculate non-local heating and transmission. He has been invited to give special reports and comment reports in this field for many times. Together with his students, he also extended this concept to inverse field and neutral plasma, which have been considered unsuitable for many years. It is expected that this work will promote the research of plasma physics of magnetic tail neutral sheet.

2. A new concept of magnetohydrodynamic model of high energy particle stabilized plasma is proposed.

1983, Professor M.N. Rosenbluth of the University of Texas invited him to engage in cooperative research. In cooperation with Professor Van Dam, they put forward a new idea. Adding high-energy particles to the tokamak plasma can suppress the balloon mode instability and make the plasma directly enter the second stable region with high specific pressure (high specific pressure refers to the ratio of fusion energy generated to total energy input). This work specifically proves that properly distributed high-energy particle components can suppress balloon mode instability which plays a huge destructive role in quasi-steady-state devices such as tokamak and enhance plasma stability. Later, Cai Shidong and his team extended this stability effect to the internal distortion mode instability of tokamak plasma, which may play an important role in suppressing sawtooth instability and internal fracture mode instability. In addition, they also considered the possibility of increasing resistance instability after the ideal magnetic fluid mode was suppressed, and further discussed the possibility of stabilizing high-energy particles from the perspective of heating and transportation.

Although Tokamak is the most promising device to realize controlled thermonuclear reaction by magnetic confinement, Cai Shidong saw the importance of satellite simulator for nuclear fusion energy. He led graduate students to prove that high-energy component particles may also suppress the instability of balloon modes and exchange modes in satellite simulators and dipole fields.

3. It is predicted that fusion products (high-energy α particles) will cause some new instabilities, and the design of fusion reactors will encounter new problems.

The balloon mode instability of dynamic beams excited by high-energy ions and high-energy α particles predicted by Cai Shidong and Chen E in 1993 has been widely observed in the laboratory in recent years. The instability of high-energy particles may qualitatively affect the fusion effect of ITER, an international fusion reaction experimental device.

Generally speaking, the tokamak device is the most likely device to realize controlled thermonuclear reaction by magnetic confinement. However, the instability of balloon mode, internal torsion mode and internal rupture mode and sawtooth instability in tokamak plasma make the plasma unstable. Therefore, the new idea of stabilizing tokamak plasma with high-energy component particles immediately aroused the response of many scientists in the world. With the progress of experiments, high-energy particle composition plasma has developed into a hot field.

Cai Shidong's contribution to the theory of high-energy particle composition plasma has been highly praised internationally, and he thinks that "Cai Shidong is one of the world's leading experts in this field". Cai Shidong, an official of the US Department of Energy and director of fusion theory, invited him to the United States and suggested that he study fusion products (α particles) at ITER. The editor-in-chief of Physical Science of Oxford University Press invited Cai Shidong to write a monograph on this subject. Cai Shidong's research covers a wide range, involving many important aspects of plasma. Besides, he has done a lot of weighty work in cooperation with his colleagues. For example, he discovered the influence of plasma sheath on drift mode, which was later confirmed by experiments; The linear and nonlinear behaviors of resistance problems and resistance modes are studied. Nonlinear parametric excitation; A new concept of electromagnetic combined fusion research device is proposed. Instability and collision-free shock waves passing through magnetic fields; Random process and heating; Space ion ring problem; Tokamak plasma enhanced radiation and polar kilometer radiation; Magnetic tail neutral sheet and inverse field problem; Weak theory of turbulence, simulation of strong turbulence and supercavity collapse: strongly coupled plasma. He trained theoretical research talents in plasma physics in his own group. He also helped Fudan University, Fuzhou University and other universities to carry out plasma research, which also played a certain role in promoting plasma research in developing countries in Asia and Africa.