High Frequency Discharge in Vacuum System
High frequency discharge is the form of discharge caused by the presence of high frequency alternating electric field between two electrodes. When the alternating voltage is applied at both ends of the discharge tube, the electrons and positive ions in the gas will produce additional resonance under the action of alternating electric field. Because the mass of positive ions is much larger than that of electrons, the resonance amplitude is very small. When the frequency is low and the amplitude of the resonance is much larger than the distance between the two poles, the electrons go through the whole process of collapse, discharge and extinction in each half cycle. At this time, the discharge situation is the same as that of DC. When the frequency is high and the amplitude of the resonance is far less than the distance between the two poles, the ionization ability of the electrons will be greatly enhanced due to the continuous back and forth motion of the electrons. Because of the small amplitude of the electronic resonance, the number of electrons entering the human electrode will be greatly reduced. This ensures that the electrons generated by the self-sustaining discharge of gases will not be supplied by secondary electrons generated by the electrodes, but by electrons generated by ionization when the electrons move back and forth. At this time, although a small number of positive ions and photons bombarded the electrodes to produce secondary electrons, the vibration direction of secondary electrons is sometimes the same as the direction of the flow of electrons into the electrodes, sometimes contrary to the direction of the polarity of the two electrodes. Therefore, it is not conducive to ensuring self-sustainment conditions.
This kind of high-frequency discharge, as long as there is a high-frequency electric field can be formed, not necessarily remote electrodes, so it is generally known as electrodeless discharge. 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 generated by residual ionization in the gas 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 axis to tube wall. Because the electron diffuses faster than the ion, the positive potential appears at the axis and the negative potential appears at the wall of the tube, which in turn generates the electrostatic field pointing from the axis to the wall. Therefore, there are two kinds of electric fields in the discharge tube, one is the eddy electric field around the magnetic line, and the other is the electrostatic field pointing from the axis to the wall of the tube. Under the simultaneous action of these two electric fields, electrons move around the axis and expand to the tube wall, resulting in a series of concentric halos. Because the energy of electrons is different in different radius and the number and level of excitation are different, the halos with different radius have different colors.
The ignition voltage of high frequency discharge is the amplitude of the alternating potential difference added to the electrode when 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 pressure, the stronger the breakdown electric field and the higher the breakdown frequency. High frequency discharge is widely used in radar and pulse technology.
