Properties of high-pressure discharge plasmas

Extract

Plasma properties during and immediately after electrical breakdown in a high-pressure gas are investigated. A simple scaling law of electrical discharge at high pressure is obtained. Introducing the normalized net ionization rate ξ, the electron temperature at breakdown is described uniquely in terms of the ionization properties of the gas, the second ionization coefficient γ at the cathode, and the parameter pd, where p is the gas pressure and d is the system dimension. The electron attachment process plays a decisive role in the breakdown phenomenon for a high-pressure gas, whereas it is not important in low-pressure discharges. An analytical expression for the high-pressure plasma density is obtained by making use of the electron rate equation. A simple analytical expression for plasma generation in a high-pressure gas provides important scaling laws in the d.c. electrical discharge system, where the electron attachment process is negligible. It is found that the logarithm of the electron density at breakdown is proportional to the discharge time τ_b, and is inversely proportional to the pulse risetime and the gap distance d between the two electrodes. The plasma density at breakdown is also an increasing function of the gas ionization energy ε_i.