1 Introduction

In recent years, non-thermal plasma has received considerable attention in various industrial applications, such as biomedical treatment (Lloyd et al. 2010; Heinlin et al. 2011; Keidar 2015; Steinbeck et al. 2013) and decontamination(Pankaj and Keener 2017; Filipic et al. 2020; Cheng et al. 2006; Xiao et al. 2016; Hernandez-Diaz et al. 2021). It has been proven to be significant in the use of air purification and improving indoor air quality due to its advantages of high efficiency, short action time, and few side effects on the environment (Boudam et al. 2006; Yasuda et al. 2010; Lai et al. 2016; Zhou et al. 2016; Bourke et al. 2017). The non-thermal plasma generates reactive species, such as OH, N2+, and NO, through electric discharge under a high-intensity electric field. Such reactive species can further react with harmful bacteria and viruses to achieve sterilization and decontamination.

To obtain a better understanding of the interaction process between the non-thermal plasma and airborne microorganisms, direct imaging of the reactive species has been considered to be a non-intrusive method to obtain characteristics information of non-thermal plasma. Recently, imaging of non-thermal plasma has been widely investigated. For example, Adress and Graham imaged non-thermal oxygen plasma jets at atmospheric pressure (Adress and Graham 2021). They performed spatial and temporal records of images to investigate the interaction between the plasma and liquid surfaces. Jarrige et al. (2010) acquired images of a plasma bullet and showed its velocity and propagation in air. In addition, the imaging of the outflow from micro-plasma jets has been conducted by Bradley et al. (2011) and Oh et al. (2011) using Schlieren photography with an ICCD camera. Besides, Erfani et al. (2013) studied the velocity of the airflow caused by the multiple encapsulated electrode (MEE) plasma actuator using particle imaging velocimetry measurement. It is notable that these reported works focused on the imaging of plasma geometry and morphology but not the imaging of the single reactive species generated in the non-thermal plasma. Although the imaging of molecular species has been conducted in laser-induced plasma using narrow-bandpass filters (Glaus et al. 2015; Iqbal et al. 2017), such investigation of direct imaging in non-thermal plasma has not been reported, which is essential to understanding the physics and chemistry in the air purification process.

On the other hand, developing a suitable non-thermal plasma generation device is in great demand in plasma-based air purification technology. Plasma Shield Ltd. has manufactured a medical grade air treatment system that can be integrated into a building’s heating, ventilation, and air conditioning (HVAC) system (Vossoughi Khazaei 2018). This system has been confirmed to have disinfection capability to effectively destruct harmful microorganisms, eliminate volatile organic compounds (VOCs), and remove airborne particles (Whiley and Ross 2020; Whiley 2019; Crea and Tkaczuk 2018). As the reactive species produced by non-thermal plasma are important in sterilization and decontamination (Laroussi and Leipold 2004), investigations of such species are necessary to characterize the plasma generated by the PlasmaShield® system.

Exited N2 molecules are produced in all non-thermal plasma, operating in standard atmospheric conditions. This study aims to directly image the spontaneous emission of excited N2 molecules to resolve the size and location of the N2 molecules distribution. Two-dimensional UV images were recorded as a function of camera shutter time and electrical input power using a UV-sensitive charged coupled device (CCD). The recent development in optical technology has produced narrowband interference and edge filters with very high throughput as fabricated by Alluxa and Semrock, which may be used to transmit selective UV emission from excited N2 and blocks emission from other excited chemical species. The results from the direct imaging of excited N2 molecules can contribute to the understanding of plasma characteristics and evaluate the purification performance of the PlasmaShield® device. The technique reported in this work can also be applied to detect other individual reactive species in the non-thermal plasma.

2 Experimental setup

The PlasmaShield® air purification device consists of 12 cylindrical cells with multi-layered discharge arrays. Each cell is ~ 80 mm long and consists of round shape metal, and its external circumference has been engineered to form 60 sharp tips with ~ 2 mm long. Non-thermal plasma can be generated when a high voltage is applied between the outside of the cell and the discharge electrodes. Figure 1a shows a front image of the physical cell, where the front plate is applying 9.95 kV. The photograph was recorded with no air flowing inside the cell using a Tamron 90 mm macro-lens and digital photographic camera (Nikon D750, Aperture: f/3.8, Shutter Speed: 2.5 s, and ISO: 1000). It is clear from Fig. 1a, based on the visible inspection, that not all the discharge tips produce plasma. It was found that on average plasma can be seen on 90% of the discharge tips. Further, the intensity of the plasma slightly differs from tip to tip. Figure 1b shows a typical image of a non-thermal plasma generated on one of the sharp tips. The plasma appears to engulf the tip and extended to 250 µm below the tip. As shown in Fig. 1b, the region engulfing the tip, showing the highest emission intensity in the image, is identified as the near tip zone, while the circular region on the top of the tip is identified as the far from tip zone.

Fig. 1
figure 1

a Front visible image on the cylindrical cell showing individual plasma attached to the sharp tips, recorded with a Tamron 90 mm macro-lens by a digital photographic Nikon camera at the aperture, shutter speed, and ISO of f/3.8, 2.5 s, and 1000, respectively; b a typical image of a non-thermal plasma generated from one of the tips of the discharge electrode with the indication of the near tip zone and far from tip zone. The white line indicates the outfit of the tip. The size of the image is 800 µm × 800 µm; c transmission range of the optical filter. The transmission band is highlighted in blue

Full size image

Before the UV imaging can be done, the UV plasma emission was recorded by a 600 µm, 0.25 m long, solarization-resistant fiber (QP600-025-XSR, Ocean Optics). The fiber was connected to a Mayo2000-Pro (Ocean Optics) equipped with a 25-μm entrance slit. The spectrometer has a deep vacuum UV mirror and a UV holographic grating with a groove density of 1200 grooves per millimeter. The spectra were recorded with a windowless and back-illuminated detector (Hamamatsu S10420), having a quantum efficiency of 60% at 250 nm. Each spectrum was corrected for the nonlinearity of the sensor and recorded with 5 s integration time. Five spectra were averaged to obtain the spectra shown in Fig. 1c.

The imaging of N2 B3Π was recorded with UV capable CCD camera (PCO.ultraviolet). The camera resolution is 1392 × 1040 pixels, and the pixel size is 4.65 µm × 4.65 µm. The camera covers the spectral range from 190 to 1100 nm with a quantum efficiency of ~ 30% at 350 nm. To achieve UV imaging with enhanced spatial resolution, a combination of UV lens (Galaxy 23286) and a set of macro-extension tube (Pro maser 8819) was utilized. The spatial resolution achieved with this configuration was 3.21 µm per pixel. In addition, the power of the emitted UV radiation was measured with a calibrated UV-sensitive power sensor (Thorlabs, S120VC) and power meter (Thorlabs, PM320E). The UV sensor was placed facing the emitting plate to measure the UV power emitted from all the tips. The recorded values were then corrected taking into consideration the solid angle and view factor between the emitting plate and the UV sensor (Holman 2002).

An optical filter (Semrock FF01-390/SP-25) with 70% transmission in the 320–370 nm range and optical density > 5 in the range 390–1100 nm was selected to image N2 (C3Π–B3Π) emission. As shown in Fig. 1c, the filter blocks the emission from NO (A2Σ–X2Π) near 226 nm and emission from the N2+ (B2Σ–X2Σ) system, which is at wavelength > 390 nm (Machala et al. 2007; Lofthus and Krupenie 1977). Thus, imaging with this configuration captures the emission from N2 (C3Π–B3Π) (0–0), N2 (C3Π–B3Π) (0–1), N2 (C3Π–B3Π) (0–2), and N2 (C3Π–B3Π) (1–0), which lies within the transmission band of the filter.

3 Results and discussion

Figure 2a–d shows typical two-dimensional UV imaging results at a certain power setting used to investigate two parameters, i.e., the total emitting UV power and the radius of excited N2 (C3Π–B3Π) distribution. The total emitting UV power is recorded by a power meter. From the figures, two plasma zones, i.e., near tip zone and far from tip zone, are clearly identified. At the near tip zone, the region with relatively high intensities is responsible for the shape of the sharp tip, while at the far from tip zone, there is a circular plasma shape with the highest local intensity at the center of the circle. The radius of excited N2 (C3Π–B3Π) is defined as the distance from the projected centroid of plasma to the bounding edge, assuming the projection of plasma is a circle.

Fig. 2
figure 2

Typical examples of two-dimensional UV imaging of the N2 (C3Π–B3Π) system at supplied electrical power of a 0.10, b 0.50, c 1.25, and d 2.45 W. The axes show the dimension of the images along with x (horizontal) and y (vertical) directions in the unit of µm. The size of the images is 1000 µm × 1000 µm; d the total emitting UV power from all the tips in one plate and the radius of the plasma from a single tip at various supplied electrical power up to 5.65 W

Full size image

The variation of the total emitting UV power and the radius of the excited N2 (C3Π–B3Π) zone with supplied electrical power is given in Fig. 2e. It is observed that the total emitting UV power increases linearly in the range of the supplied electrical power from 0.50 to 5.65 W, corresponding to the emitting UV power from 0.7 to 3.2 µW which is integral over the total volume. It is worth noting that the method used provides an approximate total UV radiation. A more accurate approach is to use an integration sphere suitable for UV radiation. This corresponds to the imaging result that the central area gradually becomes saturated as the supplied power increases. The total emitting UV power is positively correlated with the emission intensity. For the radius of the excited N2 (C3Π–B3Π) zone, it indicates a logarithmic growth as the supplied power increases. The radius starts with 160 μm at the supplied electrical power of 0.10 W. When higher electrical power is applied, the radius quickly rises to 285 μm. After that, the change of the radius enters a plateau phase, and the maximum radius remains around 375 μm at 2.5 W.

The effect of exposure time on image intensity is studied to indicate the structure of non-thermal plasma. Figure 3 shows typical two-dimensional UV images of the N2 (C3Π–B3Π) system at various exposure times from 1 to 100 ms in mesh plots. According to Fig. 3d, the regions with higher intensity are illustrated as three peaks. Peak 1 and Peak 2 refer to the left and right peaks in the near tip zone, while Peak 3 refers to the peak in the far tip zone. The emission intensities of the three peaks increase with the exposure time. The rate change for excited N2 molecules at the near tip zone is 3 times more than at the far tip zone.

Fig. 3
figure 3

Emission images of N2 (C3Π–B3Π) recorded at different exposure times of a 1 ms, b 20 ms, c 60 ms, and d 100 ms in mesh plots. The locations of the three peaks are indicated in d. The location of the tip is indicated by black lines

Full size image

When reducing the exposure time from 100 to 1 ms, the weak part of plasma disappears, while the strong part of plasma remains in the images. This indicates that the inside region has a stronger N2 concentration than the outside region, and the strongest part of plasma is at the near tip zone. Further, the gradually increased intensity along with the exposure time also indicates there is no self-absorption inside the plasma. This is because the N2 (C3Π–B3Π) does not couple with the electronic ground state.

In addition, the intensity profile of the N2 (C3Π–B3Π) at the far zone appears to be symmetrical and spherical. Within this far zone, the intensity at the center is much more intense than at the edge. This is because of the line-of-sight problem associated with the applied technique, which deals with the presence of longer path length with the presence of exited N2 (C3Π–B3Π). As the geometry at the far zone is spherical, the Abel inversion procedure can be employed to reconstruct the images to recover the qualitative intensity of N2 B3Π (Abel 1826).

The Abel inversion for the integrated intensity along the line of the optical path at different locations (x, y) can be calculated by Abel (1826), Cheskis and Goldman (2009):

$$A\left( y \right) \propto \mathop \int \limits_{0}^{R} \left[ {(x^{2} + y^{2} )^{1/2} } \right]{\text{d}}x$$
(1)

where R is the plasma radius and \({{(x}^{2}+{y}^{2})}^{1/2}\) equals the distance from the center of the plasma. However, the direct application of this equation may cause noise in the results. Therefore, polynomials are used to fit and smooth the data. The Abel inversion function is modified to:

$$F\left( r \right) = - \frac{1}{\pi }\mathop \int \limits_{r}^{{r_{c} }} \frac{{\frac{{{\text{d}}I\left( y \right)}}{{{\text{d}}y}}}}{{(y^{2} - r^{2} )^{1/2} }}{\text{d}}y$$
(2)

where dI(y) is the observed total intensity from the plasma of width dy, and r is the cutoff radius, while rc is at the edge of the plasma.

Figure 4 shows the emission image of N2 (C3Π–B3Π) in the far from tip zone before and after the Abel inversion procedure at the exposure times of 20 and 100 ms. The shape of the emission distribution in the far tip zone is assumed to be spherical symmetry. The total intensity value measured at each pixel is the integrated intensity of the emission originating along the line of the optical path. The profile on the top of each image is used to indicate the effect of the Abel inversion in one direction, where the intensity values are obtained at the location of 50 μm away from the plasma center. Figure  4 shows the profiles are similar before and after the Abel inversion. This is expected as the plasma radius of each tip is very small, ~ 500 μm.

Fig. 4
figure 4

Images of N2 B3Π in the far zone before (left) and after (right) Abel inversion at the exposure times of a 20 and b 100 ms. The axes show the dimension of the images along with x (horizontal) and y (vertical) directions in the units of µm. The size of each image is 650 µm × 500 µm. The profile on the top of each image indicates the intensity before (left) and after (right) Abel inversion obtained at the location of 50 μm away from the plasma center

Full size image

Furthermore, from Fig. 4, it can be seen that more excited N2 molecules are present at the center of the far zone and the distribution at the edge is uniform. When the exposure time is increased, more counts of the excited N2 molecules were captured by the CCD camera, especially in the center. In addition, taking the images recorded at 100 ms exposure time as an example, the qualitative N2 B3Π molecule at the center is 15 times higher than that at the edge due to the distribution over a larger volume.

4 Conclusion

Spontaneous emissions of single chemical species, excited N2 molecules, are imaged and investigated to resolve the distribution of excited N2 species in the non-thermal plasma. Two-dimensional UV images of the N2 (C3Π–B3Π) system are recorded as a function of camera shutter time and electrical input power using a UV-sensitive charged-coupled device (CCD). The dependence of exposure time on the images of emission intensity is used to indicate the structure of non-thermal plasma. Three local peaks, which are the peaks at left and right in the near tip zone and the peak in the far from tip zone, are identified from the images. The near tip zone has three times higher excited N2 intensity than the far from tip zone at the same exposure time. The supplied electrical power is used to determine the radius of the excited N2 (C3Π–B3Π) zone. When the supplied power increase to 5.65 W, the maximum radius achieves 375 μm with the total emitting UV power of 3.2 µW. The qualitative intensity of the excited N2 (C3Π–B3Π) in the plasma has been evaluated using Abel inversion. For 100 ms exposure time, the intensity of the N2 B3Π molecule at the plasma center is 15 times higher than that at the edge due to the distribution over a larger volume. To the best of the authors’ knowledge, this is the first demonstration of the two-dimensional imaging of single chemical species based on two-dimensional spectral discrimination in non-thermal plasma. The distribution of reactive N2 molecules can help to understand the plasma characteristics and evaluate the purification performance of the PlasmaShield® device. The imaging technique reported in this work can also be used to detect other individual reactive species in the non-thermal plasma.