Supplementary information
From short pulses to short breaks: exotic plasma bullets
via residual electron control
YuBin Xian1, Peng Zhang2, XinPei Lu1, XueKai Pei1, ShuQun Wu1, Qing Xiong1,
and Kostya (Ken) Ostrikov3,4,1,*
1State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China
2 Institute 601, Shenyang 110035, People’s Republic of China
3CSIRO Materials Science and Engineering, PO Box 218, Lindfield NSW 2070, Australia
4School of Physics, The University of Sydney, Sydney NSW 2006, Australia
Correspondence and requests for materials should be addressed to X. L.
(E-mail:)
This Supplementary information file includes:
S1. Atmospheric-pressure plasma jet device
S2. High-speed photographs of the plasma plumes generated during the voltage fall phase
S3. Optical emission spectroscopy
S4. Numerical model of residual electron density
Figures S1-S7
TableS1
References R1-R14
S1. Atmospheric-pressure plasma jet device
The plasma jet device is driven by a sub-microsecond pulsed DC voltage. It can generate a non-equilibrium plasma plume with a length typically up to 4 cm long. Figure S1 and Figure S2show a schematic and a photograph of the device, respectively. The high-voltage (HV) wire electrode, which is made of a copper wire with a diameter of 2 mm, is inserted into a 4 cm long quartz tube with one end closed. The inner and outer diameters of the quartz tube are 2 mm and 4 mm, respectively. The quartz tube along with the HV electrode is inserted into a hollow barrel of a syringe. The diameter of the hollow barrel is about 6 mm and the diameter of the syringe nozzle is about 1.2 mm. The distance between the tip of the HV electrode and the nozzle is 1 cm.
When helium with a flow rate of 1 l/min is injected into the hollow barrel and the HV pulsed DC voltage (amplitude of 8 kV, pulse repetition rate of 1 khz, and pulse width variable from 900 ns to 999.1 s were used in this work) is applied to the HV electrodes, a homogeneous plasma is generated in front of the end of the quartz tube, along the nozzle, and in the surrounding air as shown in Fig. S2. The length of the plasma plume can be adjusted by the gas flow rate and the applied voltage (amplitude, frequency, and pulse width). Because the device has only one electrode, the discharge is generated between the HV electrode and the surrounding air. High speed ICCD camera (Princeton Instruments, Model: PIMAX2, exposure time down to 2ns) is used to capture the dynamics of the plasma plume. The applied voltages are measured by a P6015 Tektronix high voltage probe and the currents by a TCP202 Tektronix current probe. For more information (including applications) about this and similar types of atmospheric-pressure plasma jets please refer to Refs. [R1-R5].
Figure S1 | A schematic of the atmospheric-pressure plasma jet device
Figure S2 | A photograph of the atmospheric-pressure plasma jet device
S2. High-speed photographs of the plasma plumes generated during the voltage fall phase
Figure S3 | High-speed photographs of the plasma plumes generated during the voltage fall phase of: (a) 2 s pulses and (b) 2s DC interruptions. The time labeled on each photograph corresponds to the time in Figs. 1(b) and 1(c), respectively.
Figure S4 | The quantum efficiency of the intensifier used in the CCD-camera.It represents the spectral efficiency of the CCD-camera.
S3. Optical emission spectroscopy
FigureS5 | Emission spectra of the plasma plume from (a)200–500 nm and (b) 500–800 nm.The discharge conditions are the same as in the photograph of Figure2(b). It shows that the spectra ofthe plasma jet are dominated by the excited OH, N2, N2+, He, andO species.
S4. Numerical model of residual electron density
FigureS6 | (a) is the two-dimensional distribution of air due to diffusion and (b) is the axial distribution of mole fraction of O2 at the radial positions r=0 and 0.4 mm.He (purity of 99.99%) with a flow rate of 1 L/min is used as working gas. The internal diameter of the nozzle is about 1.2 mm.
The temporal distribution of electron density ne is evaluated. According to Martens et al.,R6when the minimum concentration of air in the He flow stream is above 100 ppm, the N4+ ions would be the main positive species.In the model, He, N2 and O2, the electrons, the N4+ ions, and the O2- ions are included. The chemical reactions in the model are composed using the results of relevant publications.R6-R10The key chemical reactions are shown in Table S1 below.The method used to calculate the electron density is the same as in [R11].
TableS1 | Key reactions with their reaction coefficients
Reaction / Rate constantThe gas temperature of the plasma plume is about 300 K,R12 and assuming that the electron temperature equals to 1 eV.According to the numerical simulations,R13-R15immediately afterthe discharge, the electron density of plasma jets is typically of the order of ~1012 cm-3. Therefore, after the discharges at the rising edge or the falling edge, assuming that the initial spatial distribution of the number densities of electrons and positive ions in plasma plumes are taken to be uniform along the symmetry axis (r=0 mm), ne=ni~ 1012cm-3.
FigureS7 | The numerically simulated spatial distribution of the seed electron densitybefore the discharge at the rising edge of the voltage pulse for different pulse widths.
It is shown that when the pulse width is 2μs, the seed electron densitybefore the discharge at the rising edge of the voltage pulse decreases with the distance from the nozzle due to the air diffusion into the helium flow. When the pulse width is extremely long, the seed electron densitychanges abruptly at about 8-10mm away from the nozzle. This is because the discharge at the falling edge of the voltage pulse only reaches about 8 – 10 mm (As can be seen from Fig. S3(b)). The seed electron density is of the order of ~1011 cm-3 before the abruptly changing point. This is due to the discharge at the falling edge of the previous voltage pulse. After the discharge at the falling edge of the previous voltage pulse, it decays. However, the time between the discharge at the falling edge of the previous voltage pulse and the current discharge at the rising edge of the voltage pulse is only about 2 s for the pulse width of 998 s and 0.9s for the pulse width of 999.1 s. Therefore, the decay time is quite short and the seed electron density maintains very high. On the other hand, for the plasma morethan 8-10 mm away from the nozzle, as mentioned above, the discharge at the falling edge of the previous voltage pulse could not reach this position, so the seed electronsoriginate from the discharge at the rising edge of the previous voltage pulse. Thus the density of the seed electrons decays for about 1 ms. That is why it is much lower and close to the case of the pulse width of 2 s (decay time of about 0.998 ms).
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