Application Cases

Experiment Name: Characteristics of Glow Discharge and Principles of Wind Speed Measurement

Research Direction: Glow Discharge

Testing Equipment: Signal generator, ATA-8202 RF power amplifier, thermal imager, multimeter, plasma sensor

Experimental Procedure:
Based on the conditions for plasma formation and flow field response mechanisms, the main parameters affecting discharge stability and the performance of plasma wind speed measurement technology can be identified. These include the electrical parameters of the excitation device, electrode spacing, electrode width, electrode material, gas composition, and thermodynamic parameters. To study any of these parameters, it is necessary to ensure their controllability and measurability in experiments. Therefore, the construction of a glow discharge system and a flow field experimental setup was required first. For corona discharge to transition into glow discharge, the power supply needs to apply an effective voltage of about 1 kV across the electrodes. This breakdown voltage can easily cause the glow discharge to transition into spark discharge and burn the electrodes. Therefore, a protective resistor was added to the circuit to limit the current within a certain range. Studies indicate that when the discharge current reaches approximately 10 mA, normal glow discharge begins to transition into abnormal glow discharge. To ensure that the discharge mode remains in the normal glow discharge phase, a 100 kΩ resistor was connected in series with the discharge circuit to limit current changes.

Figure 2.7 shows a schematic diagram of the test system connections. The discharge voltage was monitored using a high-voltage probe with an attenuation ratio of 1000:1 connected to an oscilloscope, which recorded the discharge waveforms and data. The data were ultimately transmitted back to a computer for storage. Current was measured by calculating the voltage across a 1 kΩ standard resistor. During the discharge experiments, both a high-voltage DC power supply and an RF power supply were used. The high-voltage DC power was obtained by amplifying the output of a low-output signal generator via a power amplifier.

Figure: Schematic Diagram of the Test System Connections

To ensure impedance matching between the glow discharge and the subsequent circuit, and to achieve high-voltage output under modulated power, the power supply output was first connected to an impedance matcher and then amplified using a custom-designed high-frequency transformer.

An oscilloscope and a multi-function multimeter were used for signal testing. A thermal imager was used primarily to capture and record the plasma distribution during discharge and the electrode temperature at corresponding currents, ensuring that the discharge operation was conducted at appropriate currents to avoid electrode damage.

To investigate the effects of various variables on discharge and plasma wind speed measurement technology, single-variable control experiments were required. The constructed experimental setup needed to enable operations such as adjusting electrode spacing, alignment, rapid electrode replacement, and flow velocity testing. Its design and physical implementation are shown in Figure 2.8, and it mainly includes a standard anemometer, a 3D translation stage with micro-grippers, an industrial camera, and an air flow source. The standard pneumatic probes used were two Pitot tubes with different measurement ranges, both with an uncertainty of 1%. Their flow velocity measurement ranges were 0–120 m/s and 0–300 m/s, respectively. They were fixed above the discharge electrodes and simultaneously sensed the flow velocity along with the plasma generated by the glow discharge. The control precision of the 3D translation stage was 10 μm. The attached micro-grippers could hold electrode wires of 200 μm or more. With the assistance of an industrial camera, electrode spacing adjustment and coaxial alignment were completed. The flow field outlet was a stainless steel tube with a diameter of 20 mm, whose other end was connected to the air flow source via a pipeline. A high-power blower and a high-pressure air source were used as flow field generation devices, with flow velocity controlled by a power regulator or a precision pressure-regulating valve.

Figure: Glow Discharge Flow Field Test Platform

The regions of a DC glow discharge from the cathode to the anode can be divided into: the Aston dark space, the cathode glow layer, the cathode dark space, the negative glow, the Faraday dark space, the positive column, and the anode region, as shown in Figure 2.9. After the discharge stabilizes, distinct layered luminous regions exist within the gap. However, as the gap decreases, the positive column and the Faraday dark space disappear, and only a bright spot is observed in the discharge. Unlike the results obtained from low-pressure discharges in long gaps, the plasma generated by glow discharge in short gaps in air is more concentrated. Starting from the cathode, the region closest to the electrode surface is the Aston dark space. The energy of electrons in this region is insufficient to cause excitation or ionization, so no radiative emission occurs. Entering the cathode glow layer, the electron energy has reached the excitation energy required for ionization, and a weak luminous layer near the electrode surface can be observed during the discharge. In the cathode dark space, only some electrons can still undergo collisional ionization reactions with molecules, so the light intensity weakens. Upon entering the negative glow region, the electric potential remains essentially constant. Therefore, the electron and ion velocities are minimum in this region, forming a high-density charge region where electron-ion recombination is most frequent. The probability of slow electrons colliding with gas molecules to cause excitation increases, thus enhancing the luminosity, which reaches its maximum brightness within the short gap. Near the anode region, electrons are accelerated by the electric field, undergo collisional excitation with molecules, and emit weak light.

Figure: DC Glow Discharge

When a stable DC glow discharge was formed within the gap, oscillations with a certain period were generated in the circuit. The anode potential output waveform and its frequency spectrum recorded by the oscilloscope are shown in Figure 2.10. Because the ion migration velocity is less than the electron migration velocity, the ion density in the cathode fall region (the region between the cathode and the negative glow) becomes greater than the electron density. The accumulation of positive charge near the cathode weakens the electric field strength from anode to cathode under the original DC action, reducing the rate of collisional ionization reactions. Consequently, the number of charges within the gap decreases, and the anode potential increases. When the rate of ion replenishment in the cathode fall region becomes less than the rate at which ions are neutralized upon reaching the cathode, the number of positive charges in the space begins to decrease, reducing their ability to weaken the electric field. The electron avalanche reaction rate recovers, increasing the number of charges within the gap. Thus, the circuit current increases, and the anode potential will decrease. The positive charges generated by ionization will replenish the cathode fall region again, forming charge accumulation, and the process repeats. The experiment detected a certain repetition period for this process, with an oscillation frequency of approximately 23 kHz.

Figure: Anode Potential in DC Glow Discharge

Figure: Simulation Results of DC Glow Discharge

The AC glow discharge process is more complex than DC discharge. The discharge continuously undergoes extinction, maintenance, and re-breakdown processes. The AC glow discharge image and the voltage and current waveforms are shown in Figure 2.12. The discharge region is divided into two parts: the sheath and the plasma region.

Figure: AC Glow Discharge

In the experiment, a sinusoidal modulated signal was first generated and input into the signal amplifier. Similar to DC glow discharge, because the electron migration speed is much greater than that of ions, the flux of each species is unequal. The time-averaged charge distribution is mainly concentrated in the plasma region, with the electrode having a negative potential relative to the plasma region. This spatial region with high electric field strength and low charge density is called the sheath. Because the electric potential fluctuates under AC action, the plasma region oscillates back and forth within the gap.

To further analyze the discharge characteristics under AC drive, the power supply parameters in the simulation model described above were changed to the operating frequency of the subsequent experimental transformer (140 kHz). The voltage amplitude was set such that the effective discharge current was 15 mA. The resulting variations in the physical parameters of the AC glow discharge are shown in Figure 2.13. The waveforms of discharge voltage and current were roughly equivalent to those measured experimentally. At approximately 0.15T of each cycle, the voltage reached a peak, and the current began to increase. At this moment, the gas within the gap was broken down, forming a discharge. By approximately 0.47T, the charge density decreased rapidly, and the glow was extinguished until re-breakdown occurred in the next half-cycle. During the AC glow discharge process, the charge density within the gap varied in the range of 10⁰–10² m⁻³. Generated charges accumulated alternately near the anode and cathode, with both electrodes enduring charge bombardment in turn, resulting in their surfaces being eroded.

Figure: Simulation Results of AC Glow Discharge

Because the average velocity of ions reaching the electrode surface in an AC electric field is only 4000 m/s, which is one-quarter of that in a DC discharge, the sputtering damage to the electrodes is weaker than in DC discharge. From the distribution characteristics of charge velocity on the axis, it is known that under the influence of the flow field in AC glow discharge, the slow ions within the plasma region escape first. Since the density of slow ions is high, the sensitivity exhibited by glow discharge to flow velocity is relatively high. As the flow field velocity increases, the ion escape phenomenon extends to the sheath. Because the charge density in this region is relatively low, the exhibited sensitivity should also be reduced.

The average migration velocity of ions in the AC glow discharge space is approximately 10 m/s. The time required to traverse an 80 μm gap is about 10 μs. According to the analysis, the migration velocity of glow discharge ions determines the maximum frequency that can be applied to AC glow discharge. Therefore, the upper limit of the frequency response for wind speed measurement technology based on this principle can reach 15 MHz, fully meeting the requirements for testing unsteady flow fields in compressors.

To further confirm the conclusions drawn from the simulation analysis above, experiments were conducted using stainless steel electrode wires with a diameter of 250 μm. The electrode spacing was adjusted to 80 μm. A thermal imager was used to capture the electrode temperatures at different currents. The results are shown in Figure 2.14. Initially, the surface temperatures of the electrodes were roughly comparable. However, as the current increased, the difference in electrode temperatures between the two discharge types gradually grew. In DC glow discharge, a distinct red heat was generated at the cathode, while the anode was not significantly affected. In AC glow discharge, symmetrical erosion formed on both electrodes, with electrode temperatures slightly lower than those in DC glow discharge. At a current of 1.5 mA, the temperature rise induced by DC discharge was nearly 50°C higher than that induced by AC discharge. It is evident that the erosion capability of AC glow discharge on electrodes is indeed weaker than that of DC glow discharge to a certain extent, leading to a relatively longer electrode service life.

Figure: Comparison Between DC Glow Discharge and AC Glow Discharge

Experimental Results:
The variation of current change with flow velocity under both discharge modes described above tended to level off as flow velocity increased, consistent with the simulation expectations. It is evident that the sensitivity of glow discharge to electrode erosion, sputtering, and wind speed response patterns is closely related to the distribution of ion velocity and charge density within the gap. Although DC discharge is accompanied by strong asymmetric erosion and a lower frequency response, it can achieve a large measurement range at a relatively low discharge current. Therefore, DC discharge has more application potential in stable, high-speed flow fields. Switching to an AC power supply, although the measurement range is smaller at the same discharge current, the controllability of the carrier frequency provides a much higher upper frequency response limit than DC discharge. Additionally, AC discharge exhibits smaller output fluctuations and greater stability, making it more suitable for unsteady-state testing where a high-frequency response is required.