High frequency discharge is the discharge form caused by the existence of high frequency alternating electric field between two electrodes. When alternating voltage is applied to both ends of the discharge tube, the electrons and positive ions in the gas will generate additional harmonic vibration under the action of alternating electric field. Because the mass of positive ion is much larger than that of electron, the amplitude of harmonic vibration is very small. When the frequency is low and the amplitude of harmonic vibration is far greater than the distance between the two poles, the electron goes through the whole process of collapse, discharge and extinction in each half cycle, and the discharge condition is the same as that of DC. When the frequency is high, the amplitude of the harmonic vibration is much smaller than the distance between the two poles, and the ionization ability of the electron will be greatly enhanced because of the constant back and forth motion of the electron. Because of the small amplitude of the electronic harmonic vibration, the number of electrons entering the electrode will be greatly reduced. In this way, the electrons of gas self-sustaining discharge will not be provided by the secondary electrons produced by the electrode, but by the electrons produced by ionization when the electrons move back and forth. At this time, although a small number of positive ions and photons bombard the electrode to produce secondary electrons, the vibration direction of the secondary electrons is sometimes the same as the direction of the electron flow into the electrode, sometimes the opposite, because the polarity of the two electrodes is constantly changing. Therefore, it is not conducive to ensuring self-sustaining conditions.
This kind of high-frequency discharge, as long as there is a high-frequency electric field, can be formed. It is not necessary to use electrodes, so it is also called electrodeless discharge. The electrodeless discharge can be formed not only in alternating electric field, but also in alternating magnetic field. Because the alternating magnetic field can produce a disastrous electric field, the electrons in the gas produced by the residual ionization accelerate around the magnetic field line under the action of the eddy electric field, resulting in a large number of ionization. When the magnetic field direction is along the axis of the discharge tube, the eddy electric field will gradually weaken along the radius direction, and the ionization ability of electrons will also weaken along the radius direction, thus forming the concentration gradient of electrons and ions along the radius direction. Under the action of concentration gradient, electrons and ions diffuse from the axis to the tube wall. Because the electron diffusion is faster than the ion diffusion, the positive potential appears at the axis and the negative potential appears at the tube wall, which in turn produces the electrostatic field from the axis to the tube wall. Therefore, there are two kinds of electric fields in the discharge tube, one is the eddy electric field around the magnetic field line, the other is the electrostatic field from the axis to the tube wall. Under the simultaneous action of these two electric fields, electrons move around the axis and expand towards the tube wall, resulting in a series of concentric rings. Due to the different energy levels and numbers of electrons excited on different radii, rings with different radii have different colors.
The ignition voltage of high frequency discharge is the amplitude of the alternating potential difference on the electrode when the discharge occurs. The amplitude of this voltage is lower than the ignition voltage of DC discharge. The electric field strength of high frequency breakdown is related to air pressure. The higher the air pressure is, the stronger the breakdown electric field is and the higher the breakdown frequency is. High frequency discharge is widely used in radar and pulse technology.
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