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This research studies a microhollow cathode discharge (MHCD) based plasma microjet operated at direct current with air, helium or helium/oxygen mixture as working gas and its interaction with water (or contaminated water).

The MHCD based PMJ device comprises two metal tubes separated from each other by a third insulating tube. The key dimensions are the inter-electrode distance and the diameter of the exit nozzle, which are 0.5 mm and 0.4 mm (or 0.8 mm), respectively. The electrode embedded in the device is connected to a DC high voltage power supply while the outer electrode is grounded for safety considerations. A ballast resistor, followed by a current monitoring resistor is inserted between the powered electrode and the DC power supply.  The inner tube also serves as the channel for the gas flow to the discharge area.  Gas flow rate usually ranges from 0.3 to 5 standard liters per minute. The discharge sustaining voltage varies in the range of 230 ¨C 600 V (depending on the working gas used) with an operating current in the 3 ¨C 40 mA range.  A schematic diagram of the essential part of device is shown in figure 1 (a). 

(a)                                               (b)

Figure 1. (a) A schematic diagram of MHCD plasma microjet device; and (b) air PMJ working in water

There are two ways the PMJ can interact with liquid media: (1) PMJ sustained in air, in close contact with liquid; and (2) PMJ sustained in quasi-steady gas cavity in water (A picture of this situation is shown in figure 1 (b). Nevertheless, either way, a stable PMJ has to be generated in air before the interaction with liquid happens. Initially, room air was compressed, dried and used as working gas. Figure 2 shows two examples of the PMJ working in air. The PMJ produced varies in length and diameter, depending on the diameter of the exit nozzle and the gas flow rate. Both laminar (figure 2, left top) and turbulent (figure 2, left middle) mode of the jet were observed. The length of the jet is ranging from 0.5 ¨C 1 cm. Not to my surprise, a considerable amount of ozone (tens to a few hundred ppm, measured by an ozone analyzer) was produced since all the conditions are right for the production of ozone with air present. Besides, the PMJ can get quite hot. The surface of the outer electrode reaches about 70 ^oC, although at 1 cm away from the exit nozzle, the temperature is closer to room temperature due to the forced flow cools the system down. Helium and helium/oxygen are used in some cases to replace air to generate different type of reactive species as well as to reduce downstream temperature. Figure 2 also shows three pictures of the helium or helium/oxygen PMJ.  When running with pure helium, the visible part of the PMJ is very faint, a short bluish jet was observed (figure 2, right top). Upon mixing oxygen in the gas, the PMJ becomes much better defined. The visible jet length increases tremendously when oxygen is added into the helium stream, reaching 30 mm in the laminar mode (figure 2, right bottom). At higher flow rate, turbulent mode (figure 2, right middle) is observed. At higher operating currents (I > 25 mA), the temperature of the jet at 1 cm away from the exit nozzle as well as the temperature of the outer metal tube remained approximately the same as the air as working gas situation, but can be decreased to a lower value by lowering the discharge current. At a current of 5 mA (average power consumption around 2-3 W), with an oxygen concentration above 0.1%, a visible jet can still be achieved, rather with a shorter jet length (around 1 cm).  In a particular condition, bifurcation of the jet occurred (figure 2, left bottom)

Figure 2. Pictures of PMJ working in air and in helium or helium/oxygen mixture

Current-voltage curve of the PMJ exhibits a self-pulsing mode (characterized by a negative differential resistance, NDR) followed by a normal glow mode. Figure 3 shows an example of the I-V characteristics when helium/oxygen (0-4%).

Figure 3. I-V characteristics of PMJ with helium/oxygen (0-4%) as working gas

When the PMJ was injected into water, a set of fixed parameters were chosen to maintain a stable operation (Current: 30 mA, flow rate: 2 slm, water volume: 20 ml).  Basic water properties such as pH and overall temperature were monitored.  When air is used as working gas, pH of water (distilled) drops to and stabilizes at around 3.5 in 2 minutes. Overall water temperature increases from room temperature to approximately 42 ^oC in 6 minutes. Figure 4 shows the time evolution of the pH and water temperature. When helium or helium/oxygen (2%) is used as working gas, the pH barely changes to about 6, probably due to the mixing-in of air above the water. Water temperature is slightly lower than in air case, reaching around 37 ^oC.

Figure 4. PH and overall temperature change of 20 ml distilled water with air PMJ treatment

Optical emission spectra of the air and helium/oxygen were taken with an intensified CCD through a monochromator. In air case, majority of peaks are from nitrogen emissions with weak oxygen emission at 777 nm. However, when helium/oxygen is used as working gas, helium emissions dominate the visible spectrum. However, strong oxygen emissions at 777 nm and 844 nm are observed. When varying the oxygen concentration in the working gas, it is found that the oxygen emissions peak at an oxygen concentration of 0.1-0.2%. This is attributed the penning ionization of oxygen by helium metastables followed by an electron impact dissociation. Figure 5 shows the emission spectra of PMJ with (a) air and (b) helium/oxygen (2%) as working gas.

                                    (a)                                                                                      (b)

Figure 5. Optical emission spectra of PMJ with (a) air and (b) helium/oxygen (2%) as working gas

Future work includes detailed study of the PMJ interaction with contaminated water, analysis of radicals and ions generated in the water.