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Discussion on plasma instability of plasma cleaning machine manufacturers

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19 Mar,2021

Discussion on plasma instability of plasma cleaning machine manufacturers:
Plasma instability can be roughly divided into macroscopic instability and microscopic instability. Where the instability region develops on the micro scale, such as the cyclotron radius and Debye length of the particle, is collectively referred to as the macro instability. An instability that develops only on a microscopic scale is called a microscopic instability.
Macroscopic instability can cause a wide range of plasma disturbances, which can seriously damage the equilibrium. The main reason is that the excess energy bound to the magnetic field is stored in the plasma. In addition, the diamagnetism and other characteristics of the plasma also lead to macroscopic instability. This is a very important problem for confined plasmas in controlled thermonuclear fusion devices.

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There are many types of macro instability. In addition to the distortion instability, the exchange instability is more important, that is, the position of the plasma and the constrained magnetic lift exchange; Ripping modes, where the plasma is torn apart by a magnetic field into tiny beams, and so on. Magnetic fluid dynamics is a commonly used method to study macroscopic instability. In this case, the energy principle is a very effective method, which is to judge whether the equilibrium is stable or not according to the change of the system potential energy caused by the small displacement when the system deviates from the equilibrium. This method is especially suitable for magnetic fields with complex geometry.
In addition to the energy principle, the normal mode method is a commonly used analytical method. The method assumes that the perturbation quantity is dq(r,t)=dq(r)-e-iwt. W is usually found as a complex number: W = WR + Iwi. If Wi BBB 0 0, the amplitude of disturbance increases with t, that is, it is unstable; on the contrary, Wi <0, the system is stable.
There are many reasons for micro-instabilities. Spatial inhomogeneity, such as density, temperature, magnetic field gradient, etc., which can cause drift and potentially cause instability. Another reason is the inhomogeneity of velocity space, such as anisotropy of velocity, temperature, pressure, etc. In addition, wave to wave interaction and so on can also lead to micro instability. In summary, plasma deviating from thermal equilibrium has excess free energy that must be released to bring it toward equilibrium. The free energy released may cause microscopic instability.


Plasmas with micro instability are characterized by increasing fluctuation. This situation often leads to turbulence and anomalous transport. There are many types of micro-instabilities. The main reasons are: secondary instability, which is caused by two beams of particles flowing relative to each other; Drift instability is caused by drift motion caused by various gradients. Loss cone instability caused by anisotropy of velocity distribution, and parametric instability caused by wave-wave interaction, etc. The theory of micro-instability is based on dynamical theory, that is, it starts from the study of Vlasov equation.


For the study of instability, linear theory is generally adopted, which can only judge whether the system is stable or not, and sometimes can give the growth rate of instability at the initial moment of the system. The nonlinear theory is needed to study the evolution problem of the perturbation tending to saturation when the amplitude of the perturbation increases under appropriate conditions.

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