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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