Chapter 3. Characterization of the DBD APPJ
3.3 Optical Properties
3.3.1 Species Identification
3.3.1.1 Nitrogen-Based DBD APPJ
Figures 3-8(a)-(b) show the measured optical emission spectra in the range of 180-900 nm in the post-discharge region. In the discharge region, the addition of 0.06%
O2 reduced the production of N2
(
C3∏
u)
[N2+ →e N2(
C3∏
u)
+e] because of the reduced electron number density due to the high electron affinity of oxygen. Hence, in the post-discharge region, the emission line intensity for the N2 second positive [ N2(
C3∏
u)
→N2(
B3∏
g)
+hυ (337.1 nm) ] decreased accordingly (see Figure 3-8a).However, the addition of 0.06% O2 increased the amount of ground-state NO and, thus,
( )
NO A2
∑
+ and NO B(
2∏ )
[through NO+N2(
A3∑u+)
→NO A(
2∑+)
and(
u) ( )
NO+N2 A3
∑
+ →NO B2∏
]. Hence, in the post-discharge region, the NO-γ, the NO-β and the N2 first positive (Lewis-Rayleigh afterglow) emission line intensities increased through [ NO A( 2∑
+)→NO+hυ (180−260nm) ], [ NO B( 2∏
)→NO+hυ (260 380 − nm) ] and[N+N+N2→N2
(
B3∏g)
and N2(
B3∏g)
→N2(
A3∑u+)
+hυ (580 nm) ], respectively. In the above, the increase of the N2 first positive line intensity (580 nm) was mainly caused by the increase of N atoms through the reaction channels in the discharge region, such as1
2 2( u)
e+N →N a′∑− +e and N a2( ′1
∑
u−)+NO→N+O+N2 with 0.06% addition of O2. As more O2 was added, all line emission intensities decreased because of the reduced plasma (thus, electron) intensity in the discharge region [Golubovskii et al., 2002;Brandenburg et al., 2005; Golubovskii et al., 2004; Guerra et al., 2001; Dilecce et al., 2007].
In addition, the emission intensity of 236.6 nm was the highest among these emission lines, which was similar to earlier observations [Iwasaki et al., 2007]. For a clear picture of how the UV emission intensity (e.g. 236.6 nm) varied with its position in the post-discharge region and the concentration of oxygen addition into the nitrogen
discharge, we have summarized the measurements in Figure 3-9. Figure 3-9 shows that UV emission generally increased with increasing amounts of trace oxygen addition and reached a maximal value with only 0.06% oxygen addition. Note that the NO-γ emission was appreciable even with the “pure” nitrogen case since there are always impurities (oxygen) in commercial nitrogen bottles. Further addition of O2 (> 0.06%) greatly reduced the UV emission, which was probably caused by the very high electron affinity of oxygen that can appreciably reduce the amount of excited nitrogen [N2(A3
∑
u+)]. In addition, the UV emission generally decreased rapidly with increased downstream distance from the channel exit, probably because of dramatically decreasing amounts of excited nitrogen as it was quenched by entrained ambient air.Note detailed reaction channels related to nitrogen, oxygen or mixture of both are summarized in Table A-1 for reference. However, these could be clarified using some simulation techniques such as fluid modeling coupling with a flow solver, which is currently in progress in our group.
3.3.1.2 Air-Based DBD APPJ
3.3.1.2.1 Pure Air DBD
Figure 3-10 shows the optical emission spectra in the UV and VIS regions measured in the post-discharge region. UV emission was greatly reduced because of the strong electro-negativity of O2 gas, as mentioned earlier. Detailed plasma chemistry using more detailed measurements or simulations would be a worthy topic of study in the near future. In addition, in the range of 450-550nm, emission of the nitrogen discharge was much stronger than for either pure oxygen or compressed air, which caused the yellow-orange plume as observed in Figure 3-3.
3.3.1.2.2 Air/Carbon Fluorine Mixture DBD
Figure 3-11 shows the optical emission spectra in the 180-380 nm region measured in the discharge region with air and CF4/air (2%) DBDs. The 180-280 nm UV emission almost disappeared because of the strong electro-negativity of O2 gas for both cases. In addition, in the range of 300-380nm, the peak lines of emission were similar to the compressed air and CF4/air (2%) discharges. Obviously, there are no lines related to F atom and CFx (x=1-3) fragments in the OES data mainly because the excitation energy of F atoms is very high (between 14.37 and 14.75 eV) and it is impossible for the low-energy electrons to excite the F atoms. However, there have been some studies showing that it was possible to find CF2 fragment in the range of 240-360 nm if the argon and small amount of CF4 are used as the discharge gases of a DBD [Fanelli F, 2009]. This was probably caused by the abundant metastable argon having higher excitation energy (11.55-11.72 eV), which dissociated the CF4 more easily than by the electrons themselves. However, more studies are required to understand the observed phenomena.
3.3.2 Gas Temperature Measurements
Figure 3-12 shows the measured gas temperature distribution along the centerline (z=3-20 mm) (i.e., 8-25 mm from the end of the electrodes) in the post-discharge jet region for N2, O2 and compressed air discharges. Note gas temperatures were measured using the thermocouples, not by the OES technique as mentioned in Chapter 2. It is because the corresponding emission intensities in the post-discharge region were too weak for measuring the rotational gas temperatures. Also with addition of large quantity of oxygen, the related emission lines related to nitrogen diminished because of the high electron affinity of the oxygen. The results showed that the gas temperatures for all the
cases decreased rapidly in the downstream direction. For example, the temperature of the pure nitrogen case dropped from 71°C (z=3mm) to 27°C (z=20mm), which was essentially at room temperature. In addition, gas temperatures for the APPJ containing more nitrogen were higher than those containing more oxygen. This was attributed to the much higher power absorption of neutral nitrogen caused by the ohmic heating effect which positive nitrogen ions collide with neutral background gas molecules as compared to the oxygen case since the ion density is higher in the nitrogen case. In general, the low gas temperatures in the plasma jet region observed in this study were highly suitable for inactivating bacteria without the danger of damaging bio-medical devices, which are often made of polymer-like materials.