Application of RF power amplifier in glow discharge characteristics and wind speed measurement principle

Experiment name: Glow discharge characteristics and wind speed measurement principle

Research direction: glow discharge

Test equipment: signal generator, ATA-8202 RF power amplifier, thermal imager, multimeter, plasma sensor

experiment procedure:

Based on the plasma formation conditions and flow field response mechanism, the main parameters that affect the discharge stability and plasma wind speed test technical performance include: electrical parameters of the excitation device, electrode spacing, electrode width, electrode materials, and gas composition and its thermodynamic parameters. The study of any of the above parameters must ensure its controllability and measurability in the experiment. Therefore, it is first necessary to complete the construction of the glow discharge system and flow field experimental device. The transition from corona discharge to glow discharge requires a power supply to load an effective voltage of about 1kV at both ends of the electrode. This breakdown voltage can easily cause the glow to transform into spark discharge and burn the electrode. Therefore, a protective resistor needs to be added to the circuit to control the current within a certain range. It can be seen from the research that when the discharge current reaches about 10mA, normal glow discharge begins to transform into abnormal glow discharge. In order to ensure that the discharge mode is maintained in the normal glow discharge stage, a 100kΩ resistor was selected in series with the discharge circuit to limit the change of current.

Figure 2.7 is a schematic diagram of the test system connection. The discharge voltage is monitored by connecting a high-voltage probe with an attenuation ratio of 1000:1 to an oscilloscope, recording the discharge waveform and data, and finally transmitting them back to the computer for storage. The current is monitored by measuring the voltage across the 1kΩ standard resistor and obtaining it through calculation. In the discharge experiment, high-voltage DC power supply and radio frequency power supply were used respectively. Among them, high-voltage direct current is obtained by amplifying the voltage of the signal generator with a smaller output and amplifying the power amplifier.

Figure: Test system connection diagram

In order to ensure the impedance matching between the glow discharge and the back-end circuit and the high-voltage output under modulated power, the power output is first connected to an impedance matcher and then amplified through a self-designed high-frequency transformer.

The signal test end uses an oscilloscope and a multi-function multimeter respectively. The thermal imager is mainly used to photograph and record the plasma distribution during discharge and the electrode temperature under the corresponding current to ensure that the discharge operation is under the appropriate current to avoid damaging the electrode.

In order to explore the influence of various variables on discharge and plasma wind speed measurement technology, a single variable control experiment needs to be carried out. The experimental bench built must be able to realize the adjustment, alignment, rapid replacement and flow rate testing of electrode spacing. Its design and physical As shown in Figure 2.8, it mainly includes a standard anemometer, a three-dimensional mobile platform micro fixture, an industrial camera and an airflow source. The standard pneumatic probes used are two pitot tubes with different measuring ranges. The uncertainties are both 1%. The measurement flow speed ranges are 0~120m/s and 0300m/s respectively. They are fixed above the discharge electrode and are connected with the glow. The plasma generated by the discharge senses the incoming flow velocity at the same time. The control accuracy of the three-dimensional mobile platform is 10um. The micro-clamp equipped can hold electrode wires of more than 200um. With the assistance of industrial cameras, the electrode spacing adjustment and coaxial alignment can be completed. The flow field outlet shown is a stainless steel pipe with a diameter of 20 mm, and the other end is connected to the air flow source via a pipe. In the experiment, high-power blowers and high-pressure air sources were used as flow field generating devices, and the flow rate was controlled by a power regulator or a precision pressure regulating valve.

Figure: Glow discharge current field test platform

DC glow discharge can be divided from cathode to anode into: Aston dark area, cathode light layer, cathode dark area, negative glow area, Faraday dark area, positive column area, and anode area, as shown in Figure 2.9, the discharge is stable Finally, there is an obvious stratified light area in the gap, but as the gap decreases, the positive column area and Faraday dark area disappear, and only a dazzling light spot is observed in the discharge. Different from the results obtained by long gap low-voltage discharge, the plasma generated by glow discharge under short gap in air is more concentrated. Starting from the cathode, the position closest to the electrode surface is the Aston dark zone. The energy of the electrons in this region is not enough to cause excited ionization, so there is no radiative luminescence. Entering the cathode light layer, the electron energy has reached the excitation energy required for ionization, and a weak light layer close to the electrode surface can be observed during the discharge process. In the dark area of ​​the cathode, only some electrons can still undergo collision ionization reactions with molecules, so the light intensity is weakened. After entering the negative glow area, the electric potential basically does not change, so the speed of electrons and ions in this area is minimum, and a high-density charge area is formed, where electrons and ions recombine most frequently, and the probability of excitation and collision between slow electrons and gas molecules increases, so The luminescence is enhanced, creating the highest brightness within a short gap. Near the anode area, electrons are accelerated by the electric field, collide with molecules and are excited, and emit weak light.

Figure: DC glow discharge

When a stable DC glow discharge is formed in the gap, an oscillation with a certain period is generated in the circuit. The anode potential output waveform and spectrum recorded by the oscilloscope are shown in Figure 2.10. Because the migration speed of ions is smaller than the migration speed of electrons, the ion density in the cathode drop zone (the area between the cathode and the negative glow zone) is greater than the electron density. The accumulation of positive charges near the cathode will weaken the original anode to cathode connection under the action of direct current. The intensity of the electric field reduces the impact ionization reaction rate, so the number of charges in the gap decreases and the anode potential increases. When the replenishment speed of ions in the cathode drop zone is less than the speed of ions being neutralized after reaching the cathode, the number of positive ions in the space begins to decrease, the ability to weaken the electric field is reduced, the electron flow reaction rate is restored, and the amount of positive ions in the gap is increased. The amount of charge, so the circuit current increases, the potential of the anode will decrease. The positive charges obtained by ionization will be replenished to the cathode drop zone to form charge accumulation, and the above process will be repeated. Experiments have detected that there is a certain repetition period in this process, and the oscillation frequency is about 23kHz.

Figure: DC glow discharge anode potential

Figure: DC glow discharge simulation results

The AC glow discharge process is more complex than DC discharge, and the discharge continuously goes through the process of extinguishing, sustaining and re-breakdown. The AC glow discharge image and voltage and current waveforms are shown in Figure 2.12. The discharge interval is divided into two parts: the sheath and the plasma region.

Figure: AC glow discharge

In the experiment, the generated sinusoidal modulation signal is first input into the signal amplifier, which is similar to the DC glow discharge. Since the migration speed of electrons is much greater than that of ions, the flux of each substance is not equal, and the time-averaged charge distribution is mainly concentrated in plasma. In the body region, the electrode has a negative potential relative to the plasma region. This spatial region with high electric field intensity and low charge density is called the sheath. Because the potential is in a floating state under the action of alternating current, the plasma region oscillates back and forth in the gap.

In order to further analyze the discharge characteristics under AC driving, the power supply parameters of the above simulation model were changed to the subsequent experimental transformer operating frequency of 140kHz, and the voltage amplitude was such that the effective discharge power was 15mAms. The changes in the physical parameters of the AC glow discharge were obtained as shown in Figure 2.13. The waveforms of the discharge voltage and current are roughly consistent with the experimentally measured results. The voltage peaks at 0.15T in each cycle, and the current begins to increase. At this time, the gas in the gap will breakdown and form a discharge: to the 0.47T position, The charge density drops rapidly and the glow is extinguished until it breaks down again in the second half of the cycle. During the AC glow discharge process, the charge density in the gap ranges from 10° to 102m. The generated charges converge and accumulate near the anode and cathode. The two electrodes take turns to be bombarded by charges, so their surfaces will be eroded.

Figure: AC glow discharge simulation results

Because the average speed of ions reaching the electrode surface in the AC electric field is only 4000m/s, which is one-quarter of that of DC discharge, the sputtering damage to the electrode is weaker than that of DC discharge. It can be seen from the distribution characteristics of the charge velocity on the axis that under the action of the flow field in the AC glow discharge, the slow ions in the plasma region escape first. Because the density of slow ions is large, the sensitivity of the glow discharge to the flow rate is also limited. relatively bigger. As the flow field velocity increases, the ion escape phenomenon develops into the sheath, and the charge density of this part is relatively small, so the sensitivity should also be reduced.