Although (EDM Filters) discharges take place in a dielectric liquid, the first section of this chapterwill deal with discharges in gases. They have been extensively studied [47–49], and have common features with discharges in liquids. After that, a brief review about the specific characteristics of discharges in liquids will be given in the second section. Finally, other plasmas similar to the (EDM Filters) plasmas are presented in the last section. 2.1 Discharges in gases 2.1.1 Spark, arc, glow & co.
Depending on the gas pressure, the electrode gap and the electrode configuration, several discharge regimes can be distinguished. They can be classified according to their currentvoltage characteristics, as shown in figure 2.1.
Four main types of steady or quasi-steady processes exist:
- the Townsend’s dark discharge, characterized by a very weak current (» 10¡8 A);
- the glow discharge, widely used in many industrial processes, operating at lowcurrent (» 10¡2 A), fairly high voltage (» 1 kV) and low pressure (» mbar). The glow plasma is weakly ionized and in a non-equilibrium state, and is visible as a uniform glowing column. As in the Townsend’s discharge, electrons are emitted by ion impacts on the cold cathode;
- the corona discharge, also at low current (» 10¡6 A) but at atmospheric pressure. Corona discharges develop locally (typically around sharp ends of wires) in stronglynon-uniform electric field;
- the arc discharge, characterized by high current (» 100 A), low voltage (» 10 V) and a bright light emission. The arc discharge differs from the glow discharge in the electron emission mechanism. In arcs, electrons are emitted by thermionic processes, due to the heating of the cathode. The plasma of high pressure arcs can be considered to be in a state of thermodynamic equilibrium.
On the contrary to Townsend, glow, corona and arc discharges, the spark discharge is not a steady process but a transient process, i.e. an unstable transition state of limited lifetime towards a more stable regime (see figure 2.1). To be perfectly rigorous, it would be more proper to say “spark breakdown” than “spark discharge”, since it is a transition mechanism and not a state resulting from a transition [48]. However, we will keep (abusively but for clarity) the term spark discharge or simply discharge when speaking about the period during which the (EDM Filters) plasma exists (part c in figure 1.2), and the term breakdown will refer to the transition from the no-plasma situation to the plasma situation (part b in figure 1.2).