# Carbon Microstructures Synthesis in Low Temperature Plasma Generated by Microdischarges
## 3 Results  
### Discharge Modes  
Five main modes of discharge in a gas at atmospheric pressure can be identified from the following current-voltage characteristics of electric discharges: onset streamer, glow corona, pulsed spark discharge and atmospheric pressure glow discharge [24; 25]. The relationship between the voltage between the electrodes and the discharge current flowing through the gas depends on the kind of gas, the gas pressure, the distance between the electrodes, the geometry of the electrodes, and the electrode material. The current-voltage characteristics of electrical discharge in a needle-plate electrode system for a mixture of argon with cyclohexane at atmospheric pressure for different inter-electrode distances for a positive polarity and a cyclohexane concentration of 5% are shown in Figure 2.  
For a given magnitude of supply voltage, the time averaged discharge current was measured, and the mean value of the voltage drop between the electrodes was calculated using the following formula (see Figure 2):  
\[\text{Ud}=\text{Us}-\text{Id}\cdot\text{R}, \tag{1}\]  
where Ud is the voltage drop between the electrodes, Id is the discharge current, and R is the resistance of the external ballast resistor in the circuit. Figure 3 shows photographs of various forms of electrical discharges in a mixture of argon and cyclohexane (95:5 by volume) for a positive polarity. All of the photographs  
Figure 2: Current–voltage characteristics of electric discharge in Ar+C\({}_{6}\)H\({}_{12}\) (95:5) mixture at atmospheric pressure for positive polarity of needle electrode, for three inter-electrode distances (2.5, 3.7 and 15 mm). were taken at the same optical magnification but with various exposures. The photographs show different stages of the development of the discharge with increasing voltage. For voltages lower than the corona onset voltage, only dark discharge is generated. The discharge current is very low (<0.1 \(\mu\)A) because the electrons and ions flowing toward the electrodes, due to an applied electric field, are generated only by natural ionization. The ionization and excitation processes are so weak that the discharge is not visible to the \"naked eye\". This discharge was also not visible in the photos taken at a long exposure time of 15 s.  
Figure 3: Photographs of various forms of corona discharge (**a**), onset streamers (**b**) corona glow (Hermstein glow), (**c**) pre-breakdown streamers and (**d**) atmospheric pressure glow discharge in a mixture of Ar+C\({}_{6}\)H\({}_{12}\) (95:5) for different supply voltages, for positive needle polarity (see [24]). With the voltage increasing, the Townsend discharge appears in the needle region, and the current-voltage characteristics fulfil the quadratic Townsend law, for voltages \(\mathrm{U_{d}>U_{d0}}\), until the pre-breakdown streamers begin, as follows:  
\[\mathrm{I_{d}=aU_{d}(U_{d}-U_{d0})}, \tag{2}\]  
where a is a constant depending on the configuration of electrodes, temperature, pressure and the kind of gas; \(\mathrm{I_{d}}\) is the discharge current; \(\mathrm{U_{d}}\) is the voltage between the electrodes, and \(\mathrm{U_{d0}}\) is the corona onset voltage. A corona discharge of positive polarity was visible as a faint glow at the anode tip, the area and intensity of which increased with an increasing supply voltage. Outside the luminous layer, the ionization processes are negligible--the number of electrons is insignificant, their kinetic energy is too low and only an ionic current flows toward the counter electrode (see [26,27]). This area is called the drift region. The kinetic energy of electrons is too low to sustain the excitation or ionization process in the drift region. With the supply voltage increasing to about 7-8 kV, the pre-breakdown streamers appeared. The streamers are a spatially inhomogeneous phenomenon that occur over a period of several hundreds of nanoseconds. The value of the discharge current temporarily increased up to tens of microamperes from the lowest value for a given supply voltage.