Experimental investigation of the electromagnetic effect and improvement of the plasma radial uniformity in a large-area,very-high frequency capacitive argon discharge

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency(LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency(VHF, e.g.60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band' effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.     

Magnetic probe diagnostics of nonlinear standing waves and bulk ohmic electron power absorption in capacitive discharges

It is recognized that standing wave effects appearing in large-area,very-high-frequency capacitively coupled plasma(CCP)reactors cause center-high plasma non-uniformity.Using a high-frequency magnetic probe,we present a direct experimental diagnostic of the nonlinear standing waves and bulk ohmic electron power absorption dynamics in low pressure CCP discharges for different driving frequencies of 13.56,30,and 60 MHz.The design,principle,calibration,and validation of the probe are described in detail.Spatial structures of the harmonics of the magnetic field,determined by the magnetic probe,were used to calculate the distributions of the harmonic current and the corresponding ohmic electron power deposition,providing insights into the behavior of nonlinear harmonics.At a low driving frequency,i.e.13.56 MHz,no remarkable nonlinear standing waves were identified and the bulk ohmic electron power absorption was observed to be negligible.The harmonic magnetic field/current was found to increase dramatically with the driving frequency,due to decreased sheath reactance and more remarkable nonlinear standing waves at a higher driving frequency,leading to the enhancements of the ohmic heating and the plasma density in the bulk,specifically at the electrode center.At a high driving frequency,i.e.60 MHz,the high-order harmonic current density and the corresponding ohmic electron power absorption exhibited a similar node structure,with the main peak on axis,and one or more minor peaks between the electrode center and the edge,contributing to the center-high profile of the plasma density.     

Analysis of the influence of sheath positions,flight parameters and incident wave parameters on the wave propagation in plasma sheath

The plasma sheath covering hypersonic vehicles has a significant effect on the propagation of electromagnetic waves. Based on the calculation of the flow field of a conical cylindrical, this work studies the propagation of electromagnetic waves in plasma sheath at L-band and Ku-band, and discusses the propagation characteristics in the head, side and tail of the sheath. The dielectric properties of plasma sheath are related to flight speed and altitude. A flight condition corresponds to a unique distribution of dielectric properties. For the conical cylindrical, the results show that flight speed is generally negatively correlated with the transmissivity of the plasma sheath. The reflection characteristics of electromagnetic waves at the L-band and Ku-band when obliquely incident to the plasma sheath show a downward trend. When the frequency is increased to Ku-band, the propagation characteristics of electromagnetic waves in the plasma sheath are related to the position of the sheath.     

Three-Dimensional Simulation of Plasma Deformation During Contact Opening in a Circuit Breaker,Including the Analysis of Kink Instability and Sausage Instability

A three-dimensional (3-D) transient model has been developed to investigate plasma deformation driven by a magnetic field and its influence on arc stability in a circuit breaker. The 3-D distribution of electric current density is obtained from a current continuity equation along with the generalized Ohm's law; while the magnetic field induced by the current flowing through the arc column is calculated by the magnetic vector potential equation. When gas interacts with an arc column, fundamental factors, such as Ampere's law, Ohm's law, the turbulence model, transport equations of mass, momentum and energy of plasma flow, have to be coupled for aria- lyzing the phenomenon. The coupled interactions between arc and plasma flow are described in the fl'amework of time-dependent magnetohydrodynamic (MHD) equations in conjunction with a K-~ turbulence model. Simulations have been focused on sausage and kink instabilities in plasma (these phenomena are related tO pinch effects and electromagnetic fields). The 3-D sjm- ulation reveals the relation between plasma deformation and instability phenomena, which affect arc stability during circuit breaker operation. Plasma deformation is the consequence of coupled interactions between the electromagnetic force and plasma flow described in simulations.