The surface charge distribution in a surface dielectric barrier discharge driven by repetitive pulse bias superimposed on AC voltage is measured using the Pockels effect of an electro-optic crystal. The impact of surface charge on surface-breakdown characteristics is investigated by varying the phase of the pulse superimposition. It is demonstrated that the surface charge accumulation varies at different superimposition phases depending on the potential difference between the two electrodes. The accumulated positive/negative surface charge will facilitate the following surface discharge when the AC voltage polarity changes. In addition, different spatiotemporal characteristics of the surface charge distribution are presented when changing the polarity of superimposed pulses. Positive surface discharges are usually easier to develop than negative surface discharges due to their lower breakdown voltage caused by the accumulation of negative surface charges near the edges of exposed electrodes. The decay of positive surface charge is dominated by neutralization of negative surface charge and negatively charged particles (free electrons and negative ions) from the volume above the dielectric. There are two decay modes of positive surface charge: exponential decay and linear decay.

The aspect ratio (AR) of discharge geometry is an important parameter in view of inductively coupled plasma (ICP) source design. AR is defined as the ratio of chamber radius (R) and chamber height (L). The effects of the AR on plasma parameters and uniformity are investigated in a 2 MHz ICP source. The argon discharge is performed in two chambers of AR = 0.72 and 0.35 with different heights, and the effects of AR on electron loss mechanisms are studied using a global model. The results show a tendency for generation of higher density plasma with better uniformity in the case of AR = 0.72 at 0.5–2 Pa. The difference in electron density is caused by the differences in the volume. The observations on plasma uniformity are explained by the study of the electron loss mechanism. The accumulation of electrons is weakened by axial diffusion at the chamber center in the case of AR = 0.72. The difference in plasma uniformity between the two chambers diminishes with the increase in the gas pressure from 0.5 to 2 Pa, which results from the fact that the electron diffusion along the axial direction becomes more difficult. At higher pressures from 5–10 Pa, the ambipolar diffusion loss of electrons to the chamber wall becomes more difficult with the increase in pressure due to frequent collisions. Therefore, the electron accumulation in the discharge center is more evident in the chamber with AR = 0.72, which deteriorates the plasma uniformity. The above study can give a reference to the design of cylindrical ICP sources for practical applications.

The influence of the cylindrical exposed electrode diameter on distribution and attenuation of the surface charge in surface dielectric barrier discharges is experimentally investigated using the Pockels effect of the Bi₁₂SiO₂₀ crystal. For the actuator with the smaller exposed electrode diameter, more positive charges accumulate closer to the exposed electrode after positive half periodic discharge, while the quantity of positive charges deposited on the surface in a discharge period at the same distance away from the exposed electrode is smaller. At the same time, the smaller the exposed electrode diameter is, the shorter decay time the positive charge filament needs. Combined with thrust generation of actuators, it can be indicated that a positive residual surface charge closer to the exposed electrode may play a more crucial role in the discharge and consequent thrust than one far away from the exposed electrode does. Also, the effect on the discharge of the positive residual surface charge may be greater than that of the negative residual surface charge. More explorations of the residual surface charge have been done by selectively enhancing the discharge of the actuator with a 10 μm exposed electrode using the pulse-AC dual power supply, drawing the same conclusions.

The dynamics of plasma and ejection characteristics of spheromaks produced by a magnetized coaxial plasma gun are studied. By placing three magnetic probes at various axial positions, the distribution of current paths in the gun is found to vary in two distinct discharge modes. During the first half-period of a discharge, the plasma moves forward in the form of a current sheet, while the diffuse distribution of current paths in the second half-period indicates a deflagration mode. The evolution images and photodiode signals of the plasma show that only a single spheromak is ejected during the entire discharge. This is because the diffuse current paths reduce the $\mathit{J}\times \mathit{B}$ Lorentz force on the leading-edge plasma, which cannot be ejected from the gun. In addition, the existence of kinks in the plasma flow in two discharge modes proves that the instability is driven by ${\mathit{E}}_{\mathrm{r}}\mathbf{\times }{\mathit{B}}_{\mathrm{z}}$ drift, which causes rotation of the central column. Spheromak velocities increase linearly with discharge current amplitude but are inversely proportional to the gas puff mass. In ejected spheromaks, both toroidal and poloidal magnetic fields are axisymmetric, with field strength increasing with discharge current. During magnetic reconnection events, the toroidal electric field ${\mathit{V}}_{\mathrm{z}}\mathbf{\times }{\mathit{B}}_{\mathrm{r}}$ drives toroidal current that generates an additional poloidal field that amplifies the starting magnetic field in the spheromak plasma. This study clarifies the relationship between the formation of a single spheromak and the current distribution, and also provides a new way to optimize the spheromak's injection performance.

The effects of bias magnetic field on the discharge characteristics of a coaxial gun and the parameters of ejected plasma are studied. At constant initial conditions (charging voltage, gas-puffed), magnetic probe data at different axial positions reveal the influence of the external bias magnetic field on the current distribution within the annular flow channel. Both photodiode signals and plasma radiation images show the plasma ejection characteristics. It is found that either applying or not applying the bias magnetic field results in two types of plasma with distinct ejection characteristics: spheromak or jet. By comparing the plasma parameters with and without bias magnetic field, it is found that the variation of jets velocity and momentum with discharge conditions are consistent with the snowplow model, whereas the spheromak plasma is subjected to a retarding force − J ϕ B r z ⇀ during the ejection, which decelerates the spheromak. Irrespective of the bias magnetic field, an increase in the discharge current amplitude leads to an elevation in electron density and temperature. The density of the spheromak is lower than that of the jet, but the temperature is higher. This is due to the fact that the electromagnetic focusing mechanism in the jet leads to a further increase in density, while the magnetic field lines stretched during the formation of the spheromak inhibit the focusing effect and expand freely after ejection. In addition, the magnetic field stretching process consumes part of the axial kinetic energy from the plasma and converts it into magnetic energy. The subsequent magnetic reconnection event causes the magnetic energy to be dissipated resistively by heating the plasma and then be converted into thermal energy. These results give us a better understanding in the interaction between plasma and magnetic field.