Cold atmospheric plasma jet (CAPJ) is one of the latest and most promising techniques for potential cancer treatment and other biomedical applications. Due to the direct contact of air, the jet is sensitive to the parameters of the local environment such as relative humidity (RH). In a RH controlled chamber, the electron density of a helium CAPJ is measured using the Rayleigh microwave scattering method, and its optical emission spectroscopy (OES) is recorded using a spectrometer. A decreasing electron density along with the increasing RH is thus revealed, while the humidity effect on OES at a high discharge voltage is increased. These trends imply a shift of electron energy distribution function (EEDF) due to extra attachments of electrons as the physics behind such a phenomenon. This hypothesis is supported by a computation of the mean electron temperature and EEDF based on the electron density we measured and a plasma chemistry model. Therefore, this report is a basis of future CAPJ stabilization development, which is a necessity of reliable biomedical applications, such as an active control loop to make CAPJ immune to any accident environmental disturbance during a plasma-based cancer surgery.

The biomedical application of the cold atmospheric plasma jet (CAPJ) is currently one of the hottest research topics, especially in cancer treatment,1–3 since its potential for such applications was introduced in 2005.4 However, the accuracy of these applications cannot be guaranteed without a sufficient understanding of CAPJ properties. In addition to its basic physics, earlier discoveries of the CAPJ performance include the following: the gas flow coupling with plasma,5–17 discharge modes,18–20 jet response to discharge voltage/waveform/frequency/power,21–37 jet response to external electric field,38–41 target properties/geometry during CAPJ treatment,42–44 and plasma jet array.45–47 

However, after a decade of study, there are still critical unknowns remaining, and some of them are common problems encountered during actual CAPJ use. Naidis and Walsh reported that the humidity and temperature of the air might cause random fluctuation on their “day by day” results.41 How the gas mixture of discharge gas can affect the plasma jet has been reported in many previous publications. For example, adding O2 can increase the concentration of O due to the dissociative collision, but a too high O2 percentage can reduce the electron energy, and thus, reduce the density of O while the power required for the ignition is increased.48 The same conclusion of plasma chemistry was also reported in a simulation work,49 and it was stated that the O2 mixture can decrease the electron density and electron temperature in the sheath but increase the temperature in the gas bulk area which leads to the concentration of O, O*, O2*, and O2 first increasing and then decreasing with the increase in O2 addition. A similar trend is also observed with optical emission intensities from a plasma jet array.46 The concentration of ozone can also be increased by adding O2.50,51 By adding N2, however, more NO3 and NO2 can be generated but less H2O2 compared with the effect of adding O2.43 Air is also a common addition of discharge gas, which can decrease the concentration of N2(C) and metastable He, NO3, and H2O2, but create more NO2.43,52 Adding H2O can increase the rotational temperature of OH and the 309 nm emission of OH will first increase but then decrease along with the increment of H2O, while some Ar emissions and the 777 nm O will decrease.53 The effect of OH density is confirmed with the observation using laser-induced fluorescence (LIF) diagnostics.54 

Considering these significant effects of adding gas mixture during the discharge, it is obvious that the chemical composition of the surrounding environment can also play a critical role in producing reactive species. Apparently, an inconsistent local humidity may result in an inconsistent plasma jet. Therefore, to make the plasma jet consistent, multiple research groups are currently using “shielding gas” to isolate the main discharge gas from the environment. How shielding gas can be used to protect and control the plasma jet has been studied both experimentally and in simulations. Reuter et al. reported that an O2 shielding could decrease the optical emissions from N2, while there will be no change to OH. Also, an N2 shielding can decrease the amount of NO2 and NO3 in a saline solution target.55 Another publication reported that an O2 shielding generates a heavy anion sheath which can focus and enhance the ionization wave propagation. Moreover, a reverse excitation wave is observed for O2 shielding while using N2, and the electrons can spread radially at the nozzle.56 

However, a shielding gas merely isolates the plasma jet at the side but not at the tip. At the vicinity of the target, specifically tissue and liquid targets, the humidity effect still determines the concentration of reactive species. In other words, the target feedback can also include the humidity effect. Therefore, a complete understanding of general humidity effects is a prerequisite for other CAPJ applications and research, especially actual surgeries in vivo in a humid environment. In this paper, we report on the effect of relative humidity (RH) in the environment on CAPJ. The general trend of the RH effect we discovered provides insights into how plasma jet performance differs from target to target due to the local H2O vaporization in addition to other plasma-target interactions. Together, these informative results will be a basis of future CAPJ control and adaptive optimization.

In Sec. II, the hardware and setups of CAPJ generation and diagnostics are introduced. Section III includes how the electron density and the optical emission spectrum (OES) vary along with the discharge voltage and RH increment. In Sec. IV, we explain how these phenomena occur and how they are related to the electron temperature.

Figure 1 shows the schematic of the hardware setups. Using one of the typical designs of the CAPJ generator, one of our electrodes locates in a glass tube, while the other is a copper ring wrapped around the tube near its nozzle. When helium flow is passing through, a cathode directed ionization wave is generated and becomes a visible plume of the plasma jet. The well-known propagation details can be found in plasma textbooks and some early publications of CAPJ.5,21,57,58 In this work, the discharge gas is a 10 LPM helium, the nozzle diameter is 4 mm, and the electrodes are powered with a sinusoidal discharge voltage at 12.5 kHz. The voltage value varies from 4.5 kV to 7 kV (pk-pk) to analyze how discharge voltage is involved in RH effects. The resulting CAPJ length varies from 3.2 cm (4.5 kV) to 3.8 cm (7 kV), while RH has no significant effect on the jet length. The temporal profile of electron density is measured by the Rayleigh microwave scattering (RMS) system, in which the CAPJ is considered as a dipole antenna during a Rayleigh scattering of an incoming microwave and the power of the scattered wave is a function of electron density. Detailed introductions of the RMS method can be found in Refs. 59 and 60. In this work, the microwave setup is the same as that introduced in Ref. 59. At the same time, OES is recorded by a StellarNet spectrometer, and the probe (optical fiber) locates at 1 cm axial and 0.8 cm radial from the nozzle. The axial and the radial distance are chosen almost arbitrarily since this work reveals a general chemical effect of helium CAPJ with humid air. But the axial distance is intentionally chosen away from the jet tip to avoid an instable signal of turbulence, since helium at the jet tip starts to move upward due to the buoyance force, which crushes with the upper helium flow. The radial distance is also away from the CAPJ to make sure that the jet will not be disturbed by the probe, but a further distance reduces the signal more. Therefore, 0.8 cm is chosen. The probe position is fixed through the entire work to make the OES results effective, although the probe position is arbitrary. A 32 × 24 ×34 cm3 chamber is filled with humid air, and its RH is manipulated by a humidifier and a hygrometer at a range of 35%–80%.

FIG. 1.

Schematic of the hardware.

FIG. 1.

Schematic of the hardware.

Close modal

Figure 2(a) shows an example of the temporally resolved electron density measured using RMS at RH about 35%. The horizontal time axis is in a range from 0 μs to 80 μs which is a complete discharge period, while the vertical axis shows the discharge voltage. A peak of ne appears at around 10 μs which is also the time when the plasma bullet propagates.59 We define such a period as “main discharge” marked in the figure. Also, Fig. 2(a) shows that the ne peak appears earlier when the discharge voltage is increased, which agrees with the earlier appearance of plasma bullet velocity peaks and OES peaks.5,61Figure 2(b) shows the sinusoidal waveforms of the discharge voltage. Each ne peak appears roughly at the time when the voltage reaches its peak. However, Fig. 2 is merely an example of one RH case. The ne peak heights of all RH cases are summarized in Fig. 3, which shows how the electron density varies by the discharge voltage and RH. When the voltage is about 4500 V, the density is decreased from about 10 × 1011 to 4 × 1011 cm−3 along with the increment of RH. However, when the voltage is higher, such a humidity effect on electron density is weaker. This figure indicates two general trends: an increasing discharge voltage can increase the electron density, while generally, a rising RH has an opposite effect.

FIG. 2.

An example of temporally resolved electron density and discharge voltage. (a) The electron density is measured at around 35% relative humidity using the Rayleigh Microwave Scattering System. The horizontal axis shows a complete discharge period of 80 μs. (b) The discharge voltage waveform that shares the same vertical axis with subplot (a).

FIG. 2.

An example of temporally resolved electron density and discharge voltage. (a) The electron density is measured at around 35% relative humidity using the Rayleigh Microwave Scattering System. The horizontal axis shows a complete discharge period of 80 μs. (b) The discharge voltage waveform that shares the same vertical axis with subplot (a).

Close modal
FIG. 3.

Electron density peaks of a cold atmospheric plasma jet in the humid air.

FIG. 3.

Electron density peaks of a cold atmospheric plasma jet in the humid air.

Close modal

For the OES, a typical example of helium CAPJ is plotted in Fig. 4. Most peaks appear in the region of UV and violet from 291 nm to 441 nm. These peaks indicate the existence of reactive nitrogen species (RNS) and OH, which are the critical chemistry elements of plasma medicine. The only optical helium peak observed is 706.52 nm, a triplet emission above the metastable state. The 777.2 nm emission proves the existence of dissociative of the oxygen molecule, such as the attachment collision: e + O2 => O + O. The humidity effects on these intensities are analyzed below.

FIG. 4.

A typical spectrum of a helium cold atmospheric plasma jet in the air. The humidity is 50% and the discharge voltage is about 5400 V pk-pk.

FIG. 4.

A typical spectrum of a helium cold atmospheric plasma jet in the air. The humidity is 50% and the discharge voltage is about 5400 V pk-pk.

Close modal

The intensities of four major N2 peaks are summarized in Fig. 5. For a voltage of about 4500 V (shown in blue), the 315.93 nm peak is decreased along the increment of RH, while the other three show no significant change. When the discharge voltage is close to 7000 V (shown in red), the intensities of all four N2 peaks are increased by the RH. A similar trend is also found in Fig. 6, which summarizes three peaks of N2+. However, as shown in Fig. 7, the intensities of 308.9 nm OH, 311.24 nm NO, 706.52 nm He, and 777.2 nm O have substantial decrements at a low discharge voltage (blue) when the RH increases. For a high voltage (red), the intensities are also proportional to the RH.

FIG. 5.

Optical emission intensities of excited N2. From blue to red: 4500 V to 7000 V.

FIG. 5.

Optical emission intensities of excited N2. From blue to red: 4500 V to 7000 V.

Close modal
FIG. 6.

Optical emission intensities of excited N2+. From blue to red: 4500 V to 7000 V.

FIG. 6.

Optical emission intensities of excited N2+. From blue to red: 4500 V to 7000 V.

Close modal
FIG. 7.

Optical emission intensities of excited OH, NO, He, and O. From blue to red: 4500 V to 7000 V.

FIG. 7.

Optical emission intensities of excited OH, NO, He, and O. From blue to red: 4500 V to 7000 V.

Close modal

All these behaviors of electron density and the OES intensities are summarized in Table I. The values are calculated as k(80–35)/b, where k is the trend line slope, b is the trend line intercept of the vertical axis, and “80” and “35” are the two ends of the RH axis. These values represent how much change is made from RH = 35% to 80%. At a low voltage around 4500 V, the electron density is lost about 40% and the OES peaks either decrease or there is no significant change. For a high voltage of around 7000 V, the electron density lost is about 7% and all OES peaks increase significantly. The reason why a high voltage can resist the humidity effect on electron density will be explained later in this paper after the energy distribution functions are calculated. Since the substantial humidity effects are shown in these results, to keep a constant CAPJ, the control of RH is necessary, such as using a chamber.

TABLE I.

Indicated by the trend lines: How the electron density and optical emission intensities are changed when the relative humidity is increased from 35% to 80%.

  At about 4500 V discharge voltage (%)  At about 7000 V discharge voltage (%) 
Electron density  −40.82  −7.25 
308.9 nm OH  −25.83  20.26 
311.24 nm NO  −23.72  21.04 
315.93 nm N2  −15.1  33.12 
337.13 nm N2  0.83  34.18 
353.67 nm N2  0.91  22.65 
358.21 nm N2+  −2.55  31.9 
380.49 nm N2  3.65  35.25 
391.44 nm N2+  −10.05  33.66 
427.81 nm N2+  2.4  35.84 
706.52 nm He  −21.37  17.04 
777.2 nm O  −15.88  30.36 
  At about 4500 V discharge voltage (%)  At about 7000 V discharge voltage (%) 
Electron density  −40.82  −7.25 
308.9 nm OH  −25.83  20.26 
311.24 nm NO  −23.72  21.04 
315.93 nm N2  −15.1  33.12 
337.13 nm N2  0.83  34.18 
353.67 nm N2  0.91  22.65 
358.21 nm N2+  −2.55  31.9 
380.49 nm N2  3.65  35.25 
391.44 nm N2+  −10.05  33.66 
427.81 nm N2+  2.4  35.84 
706.52 nm He  −21.37  17.04 
777.2 nm O  −15.88  30.36 

The trend of increasing OES intensities with decreasing electron density observed experimentally can be explained by a hypothesis based on a shift in the electron energy distribution function (EEDF) toward the low-energy region. Indeed, the shift of EEDF must relate to the increase of RH. The first possible relation can be related to the gas permittivity increase by RH considering the relative permittivity of water which is about 80. The following equation can be used to quantify the permittivity of humid air:62 

V.εair=ε01+211TP+48PsTRH106,

where ɛair and ɛ0 are the permittivity of humid air and vacuum, T is the gas bulk temperature in K, P is the pressure of air in mmHg, Ps is the pressure of saturated water vapor in mmHg at that temperature, and RH is the relative humidity. However, this equation provides a linear increment of permittivity from 1.00064 to 1.00078 along with the RH increment from 35% to 80%. Such a limited difference makes us exclude the permittivity as a major factor behind the phenomena reported in previous images.

On the other hand, the increment of humidity brings extra quenching collisions and attachments. For the quenching reactions, excited species are depopulated by energy transfers, and thus, the OES intensities are reduced. This is in contrast to the experimental observations in this work presented above. Therefore, the quenching collisions might not dominate the OES change. Considering attachments, in addition to direct collisions with H2O, the products of H2O reactions such as ozone also capture low-energy electrons.63 The further effect of attachment is the generation of negative ions. These ions, with the same scale of mobility as positive ions, create a negative electric field and weaken the field of the streamer head, which is known as a positive ion cloud. This increases the number of low-energy electron excitations and depopulates the high-energy electrons of ionization.

To verify this hypothesis, we calculate the average electron temperature of CAPJ using a model based on the temporally resolved electron density measured in RMS and the continuity equation as introduced in Ref. 35:

Net+vjdrift+jdiffedv=GainLoss,

where Ne is the total electron number in a control volume already measured; jdrift and jdiff are the drift and diffusion current of electrons which are functions of the electric field and the electron temperature accordingly. Departing from the original paper,35 in this work, the electric field was measured not by using the ring method39 but by the results of Boltzmann solver BOLSIG+64 which provides the relation between the reduced electric field (E/N) and the electron temperature (Te). The right-hand side includes the rates of electron gain and loss such as ionization, recombination, and attachments. Assuming a constant gas bulk temperature, the rate coefficients of these events are either constant or are functions of Te. Therefore, with the initial chemical composition acquired using FLUENT simulation (shown in Fig. 8), the only unknown in this equation is Te which can be solved. After this, the densities of all species can be updated using the rate equations for the next time step. In other words, this method solves for Te which can make the electron density vary like what is measured, and a detailed introduction can be found in Ref. 35. The chemical model in this work is summarized in Table II, where all temperatures are in Kelvin and the units of rate coefficients are cm3/s and cm6/s for two-body and three-body collisions accordingly. Figure 9 shows the resulting electron temperature peaks. A general trend is revealed, which is that a higher RH can reduce the Te, although such an effect is not substantial when the discharge voltage is from 5.2 kV to 5.9 kV. The general trend agrees with the initial assumption of the EEDF shift.

FIG. 8.

The mole fraction of a helium jet in a 50% relative humid air. (a) The mole fraction of He; (b) the mole fraction of N2; (c) the mole fraction of O2; and (d) the mole fraction of H2O.

FIG. 8.

The mole fraction of a helium jet in a 50% relative humid air. (a) The mole fraction of He; (b) the mole fraction of N2; (c) the mole fraction of O2; and (d) the mole fraction of H2O.

Close modal
FIG. 9.

The resulting electron temperature peaks.

FIG. 9.

The resulting electron temperature peaks.

Close modal
TABLE II.

The collisions and their rate coefficients. The units of all two-body collisions are cm3/s, and the units of all three-body collisions are cm6/s. Some rate coefficients are marked as “BOLSIG+”. This means that these values are acquired from the Boltzmann solver “BOLSIG+”. Details of this solver can be found in Ref. 64.

  Collision event  Rate coefficient  Reference 
R1  e + He ⇒ He(23S) + e  BOLSIG+  64   
R2  e + He ⇒ He+ + 2e  BOLSIG+  64   
R3  e + N2 ⇒ N2+ + 2e  BOLSIG+  64   
R4  e + N2+ ⇒ 2N  2.8 × 10−7 ×  (300/Te)0.5  65   
R5  e + O2 ⇒ O + O+ + 2e  BOLSIG+  64   
R6  e + O2 ⇒ O + O  BOLSIG+  64   
R7  e + O2 ⇒ O2+ + 2e  BOLSIG+  64   
R8  e + O2+ ⇒ 2O  2 × 10−7 × (300/Te 65   
R9  e + O3 ⇒ O2 + O  10−9  65   
R10  e + H2O ⇒ H2O+ + 2e  BOLSIG+  64   
R11  e + H2O ⇒ H2 + O  BOLSIG+  64   
R12  e + H2O ⇒ OH + H  BOLSIG+  64   
R13  e + H2O+ ⇒ O + 2H  1.37 × 10−6/Te0.5  66   
R14  e + H2O+ ⇒ O + H2  1.37 × 10−6/Te0.5  66   
R15  e + H2O+ ⇒ OH + H  2.73 × 10−6/Te0.5  66   
R16  He(23S) + N2 ⇒ N2+ + He + e  5 × 10−11  67   
R17  N2+ + H2O ⇒ H2O+ + N2  2.3 × 10−9  66   
R18  O + O2 + M ⇒ O3 + M  3.4 × 10−34 × (Tg/300)−1.2  24   
R19  O + N2+ ⇒ O+ + N2  1 × 10−11 × (300/Tg)0.5  66   
R20  O + O2 ⇒ O + O2  3.3 × 10−10  66   
R21  O + O ⇒ O2 + e  1.4 × 10−10  66   
R22  O + O3 ⇒ 2O2  8 × 10−12 × exp(−2060/Tg 66   
R23  O + OH ⇒ H + O2  2.2 × 10−11 × exp(−350/Tg 66   
R24  O + H2O+ ⇒ O + H2 2 × 10−7 × (300/Tg)0.5  66   
R25  O + O+ ⇒ 2O  2 × 10−7 × (300/Tg)0.5  65   
R26  O + O2+ ⇒ 3O  10−7  66   
R27  O + O2+ ⇒ O + O2  2 × 10−7 × (300/Tg)0.5  66   
R28  O + OH ⇒ 2O + H  10−7  66   
R29  O + O3 ⇒ O3 + O  8 × 10−10  66   
R30  O + H2O+ ⇒ O + OH + H  10−7  66   
R31  O + OH+ ⇒ O + OH  2 × 10−7 × (300/Tg)0.5  66   
R32  O + H2O ⇒ OH + OH  1.4 × 10−9  66   
R33  O + O3 ⇒ 2O2 + e  3 × 10−10  66   
R34  O+ + H2O ⇒ H2O+ + O  3.2 × 10−9  66   
R35  O+ + O2 ⇒ O2 + O  2 × 10−7 × (300/Tg)0.5  66   
R36  O+ + OH ⇒ OH+ + O  3.3 × 10−10  66   
R37  O+ + O3 ⇒ O3 + O  2 × 10−7 × (300/Tg)0.5  66   
R38  O+ + OH ⇒ O + OH  2 × 10−7 × (300/Tg)0.5  66   
R39  O+ + O3 ⇒ O2+ + O2  10−10  66   
R40  O+ + OH ⇒ O2+ + H  3.6 × 10−10  66   
R41  O2 + H2O+ ⇒ O2+ + H2 4.3 × 10−10  66   
R42  O2 + O2+ ⇒ 2O2  2 × 10−7 × (300/Tg)0.5  65   
R43  O2 + O ⇒ O3 + e  1.5 × 10−10  66   
R44  O2 + H2O+ ⇒ O2 + OH + H  10−7  66   
R45  2OH ⇒ O + H2 8.8 × 10−12 × exp(−503/Tg 66   
R46  2OH + M ⇒ H2O2 + M  6.9 × 10−31 × (Tg/300)−0.8  24   
R47  OH + O2+ ⇒ OH + 2O  10−7  66   
R48  OH + H2O+ ⇒ OH + H2 2 × 10−7 ×  (300/Tg)0.5  66   
R49  OH+ + O2 ⇒ O2 + O + H  10−7  66   
R50  OH+ + OH ⇒ H2O+ + O  7 × 10−10  66   
R51  OH+ + OH ⇒ OH + O + H  10−7  66   
R52  H + OH ⇒ H2O + e  1.8 × 10−9  66   
R53  N + O ⇒ NO + e  2.6 × 10−10  65   
R54  N + O2 ⇒ NO2 + e  5 × 10−10  65   
R55  H2 + O ⇒ H2O + e  7 × 10−10  66   
  Collision event  Rate coefficient  Reference 
R1  e + He ⇒ He(23S) + e  BOLSIG+  64   
R2  e + He ⇒ He+ + 2e  BOLSIG+  64   
R3  e + N2 ⇒ N2+ + 2e  BOLSIG+  64   
R4  e + N2+ ⇒ 2N  2.8 × 10−7 ×  (300/Te)0.5  65   
R5  e + O2 ⇒ O + O+ + 2e  BOLSIG+  64   
R6  e + O2 ⇒ O + O  BOLSIG+  64   
R7  e + O2 ⇒ O2+ + 2e  BOLSIG+  64   
R8  e + O2+ ⇒ 2O  2 × 10−7 × (300/Te 65   
R9  e + O3 ⇒ O2 + O  10−9  65   
R10  e + H2O ⇒ H2O+ + 2e  BOLSIG+  64   
R11  e + H2O ⇒ H2 + O  BOLSIG+  64   
R12  e + H2O ⇒ OH + H  BOLSIG+  64   
R13  e + H2O+ ⇒ O + 2H  1.37 × 10−6/Te0.5  66   
R14  e + H2O+ ⇒ O + H2  1.37 × 10−6/Te0.5  66   
R15  e + H2O+ ⇒ OH + H  2.73 × 10−6/Te0.5  66   
R16  He(23S) + N2 ⇒ N2+ + He + e  5 × 10−11  67   
R17  N2+ + H2O ⇒ H2O+ + N2  2.3 × 10−9  66   
R18  O + O2 + M ⇒ O3 + M  3.4 × 10−34 × (Tg/300)−1.2  24   
R19  O + N2+ ⇒ O+ + N2  1 × 10−11 × (300/Tg)0.5  66   
R20  O + O2 ⇒ O + O2  3.3 × 10−10  66   
R21  O + O ⇒ O2 + e  1.4 × 10−10  66   
R22  O + O3 ⇒ 2O2  8 × 10−12 × exp(−2060/Tg 66   
R23  O + OH ⇒ H + O2  2.2 × 10−11 × exp(−350/Tg 66   
R24  O + H2O+ ⇒ O + H2 2 × 10−7 × (300/Tg)0.5  66   
R25  O + O+ ⇒ 2O  2 × 10−7 × (300/Tg)0.5  65   
R26  O + O2+ ⇒ 3O  10−7  66   
R27  O + O2+ ⇒ O + O2  2 × 10−7 × (300/Tg)0.5  66   
R28  O + OH ⇒ 2O + H  10−7  66   
R29  O + O3 ⇒ O3 + O  8 × 10−10  66   
R30  O + H2O+ ⇒ O + OH + H  10−7  66   
R31  O + OH+ ⇒ O + OH  2 × 10−7 × (300/Tg)0.5  66   
R32  O + H2O ⇒ OH + OH  1.4 × 10−9  66   
R33  O + O3 ⇒ 2O2 + e  3 × 10−10  66   
R34  O+ + H2O ⇒ H2O+ + O  3.2 × 10−9  66   
R35  O+ + O2 ⇒ O2 + O  2 × 10−7 × (300/Tg)0.5  66   
R36  O+ + OH ⇒ OH+ + O  3.3 × 10−10  66   
R37  O+ + O3 ⇒ O3 + O  2 × 10−7 × (300/Tg)0.5  66   
R38  O+ + OH ⇒ O + OH  2 × 10−7 × (300/Tg)0.5  66   
R39  O+ + O3 ⇒ O2+ + O2  10−10  66   
R40  O+ + OH ⇒ O2+ + H  3.6 × 10−10  66   
R41  O2 + H2O+ ⇒ O2+ + H2 4.3 × 10−10  66   
R42  O2 + O2+ ⇒ 2O2  2 × 10−7 × (300/Tg)0.5  65   
R43  O2 + O ⇒ O3 + e  1.5 × 10−10  66   
R44  O2 + H2O+ ⇒ O2 + OH + H  10−7  66   
R45  2OH ⇒ O + H2 8.8 × 10−12 × exp(−503/Tg 66   
R46  2OH + M ⇒ H2O2 + M  6.9 × 10−31 × (Tg/300)−0.8  24   
R47  OH + O2+ ⇒ OH + 2O  10−7  66   
R48  OH + H2O+ ⇒ OH + H2 2 × 10−7 ×  (300/Tg)0.5  66   
R49  OH+ + O2 ⇒ O2 + O + H  10−7  66   
R50  OH+ + OH ⇒ H2O+ + O  7 × 10−10  66   
R51  OH+ + OH ⇒ OH + O + H  10−7  66   
R52  H + OH ⇒ H2O + e  1.8 × 10−9  66   
R53  N + O ⇒ NO + e  2.6 × 10−10  65   
R54  N + O2 ⇒ NO2 + e  5 × 10−10  65   
R55  H2 + O ⇒ H2O + e  7 × 10−10  66   

For each Te value shown in Fig. 9, a unique electron energy probability function (EEPF) can be found using BOLSIG+, which is defined as EEDF divided by the electron energy in square root.68 To analyze the general trend of RH, four Te values of four most extreme cases are used to plot the EEPF, as shown in Fig. 10. The shift of EEPF, is thus, revealed: for both RH values, a higher discharge voltage U results in less low-energy electrons but more high-energy ones, which is logical; for the same discharge voltage, a higher RH produces more low-energy electrons but less high-energy electrons. Such deformations also agree with the Te decrement effect introduced in Fig. 9.

FIG. 10.

The resulting Electron Energy Probability Functions.

FIG. 10.

The resulting Electron Energy Probability Functions.

Close modal

It should be pointed out that Fig. 10 is not adequate to explain how RH manipulates electron density and OES signals. To compare the population of electrons involved in excitations and ionizations, EEPFs must be integrated over three energy regions which are defined as “Low Energy” from 0 eV to 11.03 eV, “Excitation” from 11.03 eV to 24.5 eV, and “Ionization” from 24.5 eV to the infinity. Generally, 11.03 eV is the energy required for an electron to excite N2 from its ground state to C3Πu which provides the second positive system,69 a major part of the OES of CAPJ in the air. Also, 24.5 eV is the first ionization energy of helium.70 To directly observe the RH effect, as shown in Fig. 11, the final integrated EEPF values of 80% RH are subtracted by the ones of 35% that ε1ε2fp(ε)|RH=80%dεε1ε2fp(ε)|RH=35%dε, where fp is the EEPF, and ɛ, ɛ1, and ɛ2 are the electron energy and the boundaries of the integration. In Fig. 11, both the “Low-Energy” bars are positive, which means that 80% RH causes more electrons in this region than 35% RH does. For the “Excitation” region, the 4500 V bar is negative, which means 80% RH causes less electrons to excite than 35% RH does. This agrees with the slight decrement of OES intensities at 4500 V when the RH is increasing, as shown in Figs. 57. However, the 7000 V bar is positive with a higher value. This means that 80% RH causes more electrons to excite than 35% RH does at 7000 V, which agrees with the significant increment of OES intensities at 7000 V, as shown in Figs. 57. For the “Ionization” bars, both are negative, which indicates that 80% RH leads to less energetic electrons to ionize than 35% RH does. These two negative bars explain the reduction of electron density when the RH is increasing, as shown in Fig. 3. Figure 11 also implies that the humidity effect of the EEPF shift is generally stronger when the discharge voltage is lower. For U = 4500 V, more electrons in the “Ionization” region are moved to the “Low-Energy” region, while for U = 7000 V, less electrons in the “Ionization” region are moved to the “Excitation” and “Low-Energy” regions. This phenomenon also agrees with the idea that a higher discharge voltage generates a higher mean Te playing against the RH effect of decreasing Te.

FIG. 11.

The effect of relative humidity on the electron energy.

FIG. 11.

The effect of relative humidity on the electron energy.

Close modal

To discover how exactly EEPF is shifted by humidity, those collisions with H2O should be analyzed. In Table II, R10 is the electron-impact ionization that provides a higher electron temperature when the H2O density is higher. This is a tendency that is opposite to that of the main RH effect. Thus, it might not play a key role. R11 and R12 are dissociative attachments that generate negative ions. These two reactions cause the loss of electrons during the avalanche, which is the primary process of the streamer propagation. As a result of the weakened avalanche, the density of the positive ion cloud is lowered and a weaker electric field is provided to accelerate electrons, which finally leads to an electron energy decrease. R13-R15 are an electron-H2O+ recombination representing an additional type of electron loss. However, as shown in Fig. 12, the recombination rate coefficients are high only before and after the main discharge and differ from the attachments occurring during the streamer propagation. Note that the recombination rate coefficients are about 103 times higher than the ones of attachments, which agrees with Ref. 71, but the density of H2O+ is about 105 times lower than the density of H2O, as shown in Fig. 13. Therefore, the contributions of recombination are not comparable to the effect of attachments and R13-R15 are not critical collisions behind the observed RH effect. R17, R32, R34, and R41 are charge transfer collisions that have a minimal contribution of shifting EEPF. R24, R30, R44, and R48 are an ion recombination of H2O+. However, the loss of H2O+ is no match for the loss of N2+ due to the increment of RH, as shown in Fig. 14. Therefore, these ion recombination collisions are also not critical factors behind the RH effect.

FIG. 12.

An example of the rate coefficients of e-ion recombination and e-H2O attachments of RH at 35%.

FIG. 12.

An example of the rate coefficients of e-ion recombination and e-H2O attachments of RH at 35%.

Close modal
FIG. 13.

An example of the density of H2O and H2O+ at RH is 35%.

FIG. 13.

An example of the density of H2O and H2O+ at RH is 35%.

Close modal
FIG. 14.

The density comparison of N2+ and H2O+ between RH is 35% and 80%. Similar to the electrons, these ions are short-life species that have density peaks only during the main discharge.

FIG. 14.

The density comparison of N2+ and H2O+ between RH is 35% and 80%. Similar to the electrons, these ions are short-life species that have density peaks only during the main discharge.

Close modal

Considering the analysis above, only the attachments of electrons are critical for the RH effect during the streamer propagation. Indeed, in this case, a high discharge voltage should resist the RH effect. In fact, such a trend can be observed in Fig. 11 showing that the blue bars for a low discharge voltage suggest that more electrons are transferred toward a lower energy area. The electron temperature is recalculated without R11 and R12. The new results (removed R11 and R12 from the chemical model) are compared with the old ones with a full chemical model in Fig. 15. The comparison reveals that the loss of H2O attachments reduced the inclination of Te decrement. In other words, without the H2O attachments, the humidity effect is reduced. Moreover, at a high discharge voltage, the reduction of the humidity effect is not much sensitive to the H2O attachment, while at around 5267 V, the humidity effect is completely eliminated when R11 and R12 are turned off.

FIG. 15.

Comparisons of the computed electron temperature with a full chemical model and without H2O attachments.

FIG. 15.

Comparisons of the computed electron temperature with a full chemical model and without H2O attachments.

Close modal

In addition, by making a comparison with a simulation work recently published by Lietz et al.,72 some interesting observations should be made. It should be pointed out that their simulations display a Te decrease with an increase of H2O density, which is in agreement with our experimental observation. However, it is emphasized in Ref. 23 that it is the e-H2O vibrational excitation that plays a major part, while e-H2O attachments contribute to the Te attenuation in a limited way.

In this work, the humidity effect on the electron density and OES intensities of helium CAPJ with a variety of discharge voltages is studied. For a high discharge voltage, the electron density decreases, but all the OES intensities observed increase along with the increment of RH from 35% to 80%. However, for a low discharge voltage of around 4500 V, the electron density still decreases, while the OES intensities either decrease or there is no substantial change. This phenomenon can be explained by hypothesizing that the shifting of EEDF is a result of the increment of RH.

The hypothesis is supported by a Te computation and a BOLSIG+ calculation of EEPF based on the electron density and gas composition under different RH conditions. The calculation results in apparent EEPF shifts. The shift between 11.03 eV and 24.5 eV successfully explains the humidity effect on OES intensities, while the shift above 24.5 eV explains the decrement of the electron density. Also, in agreement with the hypothesis, the discharge voltage increases by canceling the RH effect. To discover the mechanism of how the humidity shifts EEPF, the chemical model is analyzed, and the possible chemical reactions in which H2O is involved are excluded, except the e-H2O attachments. To prove this, the Te computation without H2O attachments is compared with the original one, which shows that the RH effect is attenuated when those attachments are removed from the model.

The result of this work is a basis for the stabilization of CAPJ which will make the plasma jet immune to humidity disturbance, and thus, ensure repeatable CAPJ results in the future. The stabilization is an urgent project since a stable CAPJ is a prerequisite for its biomedical applications, considering the inconsistent CAPJ performance reported by other researchers.41 Moreover, the humidity effect can also be a part of a novel self-adaptive plasma jet control system that will optimize the jet for a variety of situations.2,3,73,74

This work is supported by the USPI Inc. under Plasma Medicine Initiative program and in part by the National Science Foundation (Grant Nos. 1465061 and 1747760).

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